IL97675A - Chaff fiber having evanescent electromagnetic detection signature and method of making and using the same - Google Patents

Description

CHAFF FIBER HAVING EVANESCENT ELECTROMAGNETIC DETECTION SIGNATURE AND METHOD OF MAKING AND USING THE SAME DH3 en n»0m 76 ABSTRACT An article comprising a non-conductive substrate having a sub-micron thickness of an oxidizable metal coating thereon, and at least one feature selected from the group consisting of: (i) an oxidation enhancingly effective amount of a salt, present on the oxidizable metal coating; (ii) a second (promoter) metal which is galvanically effective to promote the corrosion of the oxidizable metal coating, discontinuously coated on the oxidizable metal coating; (iii) the oxidizable metal coating being sulfurized; and (iv) the oxidizable metal coating being overcoated with a microporous layer of an electrically insulative material. Also disclosed are related methods of forming such articles, comprising chemical vapor depositing the oxidizable metal coating on the substrate. When utilized in a form comprising fine-diameter substrate elements such as glass or ceramic filaments, the resulting product may be usefully employed as an "evanescent" chaff. In the presence of atmospheric moisture, such evanescent chaff undergoes oxidization of the oxidizable metal coating, so that the radar signature of the chaff transiently decays. c:\gail\chaff.apl CHAFF FIBER HAVING AN EVANESCENT ELECTROMAGNETIC DETECTION SIGNATURE, AND METHOD OF MAKING AND USING THE SAME Description Field of the Invention This invention relates to chaff with a transient radar reflectance characteristic, having utility as an electromagnetic warfare countermeasure useful as an electromagnetic detection decoy or for anti-detection masking of an offensive attack.

Description of the Related Art In modern warfare, a wide variety of weapons systems are employed which operate across the electromagnetic spectrum, including radio waves, microwaves, infrared signals, ultraviolet signals, x-rays, and gamma rays.

To counter such weapons systems, smoke and other obcurants have been deployed In the past, smoke has been variously employed as a means of protection of ground-based military vehicles and personnel during conflict, to blind enemy forces, to camouflage friendly forces, and to serve as decoys to divert hostile forces away from the positions of friendly force. With the evolution of radar guided missiles and increasing use of radar systems for battlefield surveillance and target acquisition, the obscurant medium must provide signal response in the millimeter wavelengths of the electromagnetic spectrum.

The use of "chaff", viz., strips, fibers, particles, and other discontinuous-form, metal-containing media to provide a signal response to radar, began during World War II. The first use of chaff involved, metal strips about 300 millimeters long and 15 millimeters wide, which were deployed in units of about 1,000 2 strips . These chaff units were manually dispersed into the air from flying aircraft, to form chaff "clouds" which functioned as decoys against radars operating in the frequency range of 490-570 Megahertz .

Chaff in the form of aluminum foil strips has been widely used since World War II. More recent developments in chaff technology include the use of aluminum-coated glass filament and silver-coated nylon filament.

In use, chaff elements are formed with dimensional characteristics creating dipoles of roughly one-half the wavelength of the hostile electromagnetic system. The chaff is dispersed into a hostile radar target zone, so that the hostile radar "locks onto" the signature of the chaff dispersion. The chaff is suitably dispersed into the air from airborne aircraft, rockets or warheads, or from ground-based deployment systems.

The chaff materials which have -been developed to date function effectively when deployed at moderate to high altitudes, but are generally unsatisfactory as obscuration media in proximity to the ground due to their high settling rates.

Filament-type chaff composed of metal-coated fibers may theoretically be fashioned with properties superior to metal strip chaff materials, but historically the "hang time" (time aloft before final settling of the chaff to the ground) is unfortunately still too short to accommodate low altitude use of such chaff. This high settling rate is a result of large substrate diameters necessary for standard processes, typically on the order of 25 microns, as well as thick metal coatings which increase overall density. A further problem with metallized filaments is that typical metal coatings, such as aluminum, remain present and pose a continuing electrical hazard to electrical and electronic systems after the useful life of the chaff is over.

It would therefore be a substantial advance in the art to provide a chaff material which is characterized by a reduced settling rate and increased hang time, as compared with conventional chaff materials, and which overcome the persistence of adverse electrical characteristics which is a major disadvantage of conventional chaff materials.

Accordingly, it is an object of the present invention to provide an improved chaff material which overcomes such difficulties.

It is another object of the present invention to provide a chaff material having a metal component with an evanescent electromagnetic detection signature.

It is another object of the present invention to provide a chaff material whose electronic signature may be selectively adjusted so that the chaff material is transiently active for a predetermined time, consistent with its purpose and its locus of use.

Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.

Summary of The Invention the present invention in its various aspects relates to an article comprising a non-conductive substrate having a sub-micron thickness of an oxidizable metal coating thereon.

The oxidizable metal may suitably be any metal species or combination of metal species which is compatible with the substrate and materials employed with the oxidizable metal coating (as described hereinafter) , and which is appropriate to the end use application of the coated product article. Suitable metals may for example be selected from the group consisting of iron, copper, zinc, tin, nickel, and combinations thereof.

The non-conductive substrate may be formed of any of a wide variety of materials, including glasses, polymers, preoxidized carbon, non-conductive carbon, and ceramics, with glasses, particularly oxide glasses and specifically silicate glasses, generally being preferred. For chaff applications, the substrate preferably is in the form of a filament, which may for example be on the order of 0.5 to about 25 microns in diameter, and preferably from about 2 to about 15 microns in diameter.

In one aspect, the metal-coated substrate may have an oxidation enhancingly effective amount of salt on the oxidizable metal coating.

The salt may for example comprise from about 0.0005 to about 25% by weight, based on the weight of oxidizable metal, of a metal salt or organic salt on the oxidizable metal coating, the specific amount employed being enhancingly effective for oxidization of the oxidizable metal coating. The salt provided on the oxidizable metal coating may be constituted by any of various suitable salts, including metal halide, metal sulfate, metal nitrate, and organic salts. Preferably, the salt is a metal halide salt, whose halide constituent is chlorine.

A related method aspect of the invention pertains to a method of forming an evanescently conductive coating on a non-conductive substrate, comprising: (a) depositing on the substrate a sub-micron thickness of oxidizable metal, to form an oxidizable metal-coated substrate, wherein the oxidizable metal preferably is selected from the group consisting of iron, nickel, copper, zinc, tin, and combinations thereof; and (b) providing on the oxidizable metal-coated substrate a salt which is enhancingly effective for the oxidation of the oxidizable metal deposited on the substrate, wherein the salt preferably is present at a concentration of from about 0.000% to about 25%, more preferably from about 0.1% to about 20%, and most preferably from about 0.5% to about 15% by weight of salt, based on the weight of oxidizable metal in the oxidizable metal coating on the substrate, and as dictated by the desired corrosion rate.

In another aspect, the present invention relates to an article comprising a non-conductive substrate having a sub-micron thickness of an oxidizable conductive first metal coated thereon, and a second metal which promotes galvanic corrosion of the first metal, discontinuously applied on the first metal coating. The first metal coating preferably is substantially continuous in character. A salt may be overcoated on the article, as previously described.

The second metal discontinuously coated on the first metal coating may comprise any of various suitable metals, depending on the character of the first metal coating. Illustrative second metal species which may be potentially suitable in the broad practice of the present invention include cadmium, cobalt, nickel,, tin, lead, copper, mercury, silver, and gold, with copper being generally preferred due to its low toxicity, low cost, and low oxidation potential. It is to be recognized, of course, that the second metal species is selected to provide a galvanically active combination for purposes of achieving corrosion of the conductive first metal coating, to yield non-conductive corrosion products therefrom. Accordingly, the second metal is different from the first metal.

In a corresponding method aspect, the present invention relates to a method of forming an evanescent conductive coating on a non-conductive substrate, comprising: (a) depositing on the substrate a sub-micron thickness of an oxidizable first metal, to form a first metal coated substrate; and (b) applying to the first metal-coated substrate a discontinuous coating of a second metal which promotes galvanic corrosion of the first metal, and optionally further treating the resulting second metal-doped first metal-coated substrate by application of a surface coating of a salt thereon.

Another aspect of the invention relates to an article comprising a non-conductive substrate having a sub-micron thickness of a sulfurized, oxidizable metal coating thereon. VThe sulfurized, oxidizable metal coating may, for example, comprise from about 0.01 to about 10.0% by weight, based on the weight of oxidizable metal, of sulfur associated with an oxidizable metal coating on the substrate of the article. This article may further comprise (i) a promoter metal which is galvanically effective to promote the corrosion of the oxidizable metal, discontinuously coated on the sulfurized, oxidizable metal coating, and/or (ii) a salt on the sulfurized, oxidizable metal coating.

The sulfur constituent associated with the oxidizable metal coating may be present on and/or within the oxidizable metal coating,, in any suitable form which is efficacious to promote the corrosion of the oxidizable metal under the exposure conditions 7 applicable thereto. Thus, the sulfur constituent is present in an oxidation-enhancing amount for the conductive metal, whereby the oxidation of the coating under exposure conditions takes place at a rate which is higher than would be the case in the absence of the sulfur constituent.

As used herein, the term "sulfur" is intended to be broadly construed to include sulfur, sulfur compounds, sulfur complexes, and any other forms of sulfur which are oxidation-enhancing in character, relative to the oxidizable metal.

A corresponding method aspect of the invention pertains to a method of forming an evanescently conductive coating on a non-conductive substrate, comprising: (a) depositing on the substrate a sub-micron thickness of oxidizable metal, to form a conductive oxidizable metal coated substrate, wherein the oxidizable metal may suitably comprise a metal constituent selected from the group consisting of iron, nickel, copper, zinc, and tin, and combinations thereof; and (b) sulfurizing the oxidizable metal coating deposited on the substrate, as for example with from about 0.01 to about 10% by weight sulfur, based on the weight of conductive oxidizable metal coated on the substrate.

In a further method aspect, the sulfurized, oxidizable metal-coated substrate formed as described above, may be further treated by applying thereto a promoter metal and/or salt, to further enhance the oxidation of the oxidizable metal coating on the substrate.

In still another aspect, the invention relates to an article comprising a non-conductive substrate which is coated with a sub- 8 micron thickness of an oxidizable metal and overcoated with a microporous layer of an inorganic electrically insulative material .

The inorganic electrically insulative material may, for example, comprise a glass or ceramic, and preferably is selected from the group consisting of polysilicate, titania, and alumina, and combinations thereof. The polysilicate, titania, and/or alumina layer may suitably be formed by a sol gel formation technique.

Optionally, the oxidizable metal coating on the non-conductive substrate may be sulfurized to enhance the oxidizability thereof. The sulfurized oxidizable metal coating may, for example, comprise from about 0.01 to about 10% by weight, based on the weight of oxidizable metal, of sulfur associated with an oxidizable metal coating. The coated article comprising the inorganic insulative coating may also optionally comprise (i) a promoter metal which is galvanically effective to promote the corrosion of the oxidizable metal, discontinuously coated on the oxidizable metal coating, and/or (iii) a salt on the oxidizable metal coating, wherein the microporous layer of inorganic electrically insulative material is overcoated on the applied promoter metal and/or salt on the oxidizable metal coating.

The preferred polysilicate, titania, and/or alumina microporous layer materials suitably may have a porous microstructure characterized by an average pore size of from about 50. to about 1000 Angstroms. Preferably such overcoat layer is formed by a sol gel layer formation technique of the type disclosed in U.S. Patent 4,738,896 issued April 19, 1988 to W. C. Stevens. 9 In a related method aspect, the invention relates to a method of forming an evanescently conductive coating on a non-conductive substrate, comprising: (a) depositing on the substrate a sub-micron thickness of oxidizable metal, to form a conductive metal coated substrate, wherein the oxidizable metal may for example comprise a metal constituent selected from the group consisting of iron, nickel, copper, zinc, and tin, and combinations thereof; and (b) overcoating the oxidizable metal coating deposited on the substrate with a microporous layer of an inorganic electrically insulative material, which as indicated preferably is a glass or ceramic material, and most preferably is a material selected from the group consisting of polysilicate, titania, alumina, and combinations thereof.

In a further method aspect, the oxidizable metal-coated substrate, formed as described above, may, prior to overcoating with a microporous layer of inorganic electrically insulative material, be further treated by one or more of the following steps: (i) sulfurizing the oxidizable metal film, (ii) coating the oxidizable metal coating with a discontinuous film of a promoter metal which is galvanically effective to promote corrosion of the oxidizable metal coating; and (iii) coating the oxidizable metal coating with a salt, all of such optional treatment steps being selectively employable to further enhance the oxidization of the continuous metal coating on the substrate.

In chaff applications, where the chaff article may suitably include a filamentous or other fine-diameter substrate element, the oxidizable metal coating of the invention is characterized by a radar signature which in the presence of moisture, e.g., 10 atmospheric humidity, decays as a result of progressive oxidation of the continuous metal coating.

Other aspects and features of the invention will be more fully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an electron photomicrograph, at magnification of 5,000 times, of salt-doped, iron-coated glass filaments according to one embodiment of the present invention, with iron (III) chloride as the deposited salt species.

Figure 2 is a graph of tow resistance, in Megaohms, as a function of exposure time, for a tow of iron-coated glass fibers devoid of any salt coating, and for corresponding tows with 0.4% and 0.5% by weight lithium chloride deposited thereon, respectively, in a 56% relative humidity environment.

Figure 3 is a graph of tow resistance, in Megaohms, as a function of exposure time, for a tow of iron-coated glass filaments, devoid of any salt coating, and for corresponding filament tows with 0.04% and 0.5% by weight iron (III) chloride deposited thereon, respectively, in a 58% relative humidity environment.

Figure 4 is an electron photomicrograph, at magnification of 10,000 times, of an iron-coated glass filament having '•islands'' of copper deposited thereon.

Figure 5 is an electron photomicrograph, at magnification of 30,000 times, of a tow of salt-doped, copper-coated iron-coated glass fibers in accordance with one embodiment of the present invention. 11 Figure 6 is an enlargement of the demarcated rectangular area shown in the left central portion of the electron photomicrograph of Figure 5.

Figure.7 is an electron photomicrograph, at a magnification of 1500 times, of a tow of fibers of the type shown in Figure 2, after exposure to 52% relative humidity conditions at 25°C for 20 hours.

Figure 8 is an enlargement of the rectangular demarcated area of the Figure 7 photomicrograph.

Figure 9 is a graph of tow resistance, in Megaohms/cm. , as a function of exposure time, in hours, for a tow of iron-coated glass fibers discontinuously coated with copper, in 11%, 52% and 98% relative humidity environments.

Figure 10 is a graph of tow resistance, in Megaohms/cm. , as a function of exposure time, in hours, for a salt-doped, copper on iron-coated glass fiber, at 52% relative humidity conditions.

Figure 11 is a graph of tow resistance, in Megaohms/cm. , as a function of exposure time, in hours, for a tow of copper on iron-coated glass fibers, and for a corresponding tow having iron (III) chloride salt doped thereon, in a 52% relative humidity environment.

Figure 12 is a graph of tow resistance, in Megaohms/cm. , as a function of exposure time, in hours, for a tow of salt-doped, copper on iron-coated glass filaments, and for a corresponding filament tow devoid of any copper thereon, in a 52% relative humidity environment.

Figure 13 is an electron photomicrograph, at magnification of 5000 times, of sulfurized iron-coated glass filaments. 12 Figure 14 is a photomicrograph, at magnification of 2000 times, of a tow of sulfurized iron-coated glass filaments, as oxidized after 500 hours exposure at 35°C and 11% relative humidity conditions.

Figure 15 is an electron photomicrograph, at magnification of 2000 times, of a tow of iron-coated glass filaments similar to the tow shown in Figure 14, but not subjected to sulfurization treatment, after 500 hours exposure at 35°C and 11% relative humidity.

Figure 16 is a graph of tow resistance, in ohms/cm. , as a function of relative humidity, at 11%, 52%, and 98% relative humidity exposure values, for iron-coated glass filaments devoid of any sulfurization ("STANDARD") and for a tow of corresponding sulfurized iron-coated glass fibers ("H2S DOPED").

Figure 17 is a graph of tow resistance, in Megaohms/cm. , as a function of exposure time, in hours, for a tow of iron-coated glass fibers devoid of any sulfurization (-"Standard") and a corresponding tow of a sulfurized iron-coated glass fibers ("H2S Doped") , at 25°C and 98% relative humidity exposure conditions.

Figure 18 is a chart of change in tow resistance over a ten hour exposure time for a tow of iron-coated glass fibers ("H2S") and a corresponding tow of sulfurized iron-coated glass fibers ("STND") as a function of relative humidity and exposure temperatures. The chart indicates less dependence upon environmental conditions for oxidation of the conductive coating following sulfurization in the practice of the present invention.

Figure 19 is an electron photomicrograph, at magnification of 3000 times, of a tow of silica-overcoated, iron-coated glass filaments. 13 Figure 20 is a photomicrograph, at magnification of 4000 times, of discrete fibers of silica-overcoated, iron-coated filaments, of the type shown in Figure 19.

Figure 21 is an enlargement of the portion of the electron photomicrograph of Figure 19 which is demarcated by the rectangular boundary in the central portion thereof.

Figure 22 is a graph of tow resistance, in ohms/cm. , as a function of exposure time, at 52% relative humidity conditions, for iron-coated glass filaments devoid of any silica-overcoating ("STANDARD") and for a tow of corresponding silica overcoated, iron-coated glass fibers ("Sol Gel Coat") .

Figure 23 is a bar graph of tow resistance, in ohms/cm. , as a function of weight percent of silica overcoated on iron-coated glass filaments, based on the weight of such filaments.

Figure 24 is a graph of current, in amperes, as a function of voltage, for tows of iron-coated glass fibers (".075 Fe/GL") , a tow of silica-overcoated, iron-coated glass fibers in which the weight of the silica overcoating was 0.7 weight percent of the weight of the fibers ("SG/Fe/GL") , and a tow of silica-overcoated, iron-coated glass fibers, wherein the weight of the silica coating was 2.6% of the weight of the fibers ("4x SG/Fe/GL") .

DETAILED DESCRIPTION OF THE INVENTION. AND PREFERRED EMBODIMENTS THEREOF The present invention relates broadly to an article comprising a non-conductive substrate having a sub-micron thickness of an oxidizable metal coating thereon. 14 Preferably, the oxidizable metal is selected from the group consisting of iron, copper, zinc, tin, nickel, and combinations thereof.

Although discussed primarily in the ensuing discussion in terms of chaff article applications, wherein the substrate element preferably is a small-diameter filament, the utility of the present invention is not thus limited, but rather extends to any other applications in which a temporary conductive coating is desired on a substrate.

Examples of other illustrative applications include moisture sensors, corrosivity monitors, moisture barrier devices, and the like.

Accordingly, the substrate may have any compositions and may take any form which is suitable to the manufacturing conditions and end use environment of the product article.

For chaff applications, it is preferred that the substrate be in filamentous (i.e., fiber) form, however, other substrate forms, such as microbeads, microballoons, hollow fibers, powders, flakes, ribbons, and the like may be employed.

For applications other than chaff, it may be necessary or desirable to provide the substrate element in bulk physical form, or alternatively in a finely divided, filamentous, or particulate form, of the types illustratively described above in connection with chaff articles according to the invention. irrespective of its physical form, the substrate element is non-conductive in character, and may be formed of any material which is appropriate to the processing conditions and end use applications of the product article. Illustrative substrate element materials of construction include glass, polymeric, 15 ceramic, pre-oxidized carbon, and non-conductive carbon materials.

By "pre-oxidized carbon" is meant polyacrylonitrile fibers which have been heat stabilized.

Among the foregoing group of materials, the classes of glasses and ceramics are preferred in most instances, due to their low cost and light weight. Oxide materials such as boria (B203) may be usefully employed in some applications. For chaff usage, boria has the advantage of being water soluble, whereby it can be dissipated by moisture.

Illustrative examples of potentially useful polymeric materials of construction for substrate elements include fibers of polyethylene, polyester, polyacrylonitrile, and polymeric fibers commercially available under the trademarks Kevlar® and Kynol®. ~* In chaff applications, the density of the substrate element material of construction preferably is less than 2.9 grams per cubic centimeter, and most preferably is on the order of from about 1.3 to about 2.9 per grams per cubic centimeter.

The most preferred materials of construction for chaff articles of the present invention are glasses, particularly oxide glasses, and more specifically silicate glasses. Silicate glasses have been advantageously employed in filamentous substrate elements in the practice of the present invention, and sodium borosilicate, calcium silicate, sodium silicate, aluminosilicate, and aluminoborosilicate glasses may also be used to*.advantage. In general, the glasses useful for substrate elements in chaff applications have a density on the order of from about 2.3 to about 2.7 grams per cubic centimeter. 16 When filamentous glass substrate elements are employed to form chaff articles in accordance with the present invention, the fiber diameter of the substrate element preferably is on the order of from about 0.5 to about 25 microns, and more preferably from about 2 to about 15 microns. It is believed that if the fiber diameter is decreased substantially below about 0.5 microns, the coated chaff fibers tend to become readily respirable, with a corresponding adverse effect on the health, safety, and welfare of persons exposed to such chaff. If, on the other hand, the diameter of the glass chaff fiber is increased substantially above 25 microns, the fiber tends to exhibit poor hang times, dropping too rapidly for effective utilization. These size constraints are dictated by the properties of the substrate material. Lower density fibers may be successfully employed at large diameters.

Deposited on the substrate is a sub-micron thickness of an oxidizable conductive metal coating, which may be formed of any suitable metal-containing composition which includes a metal which is oxidizable in character. Preferably, the oxidizable metal coating is formed of a metal selected from the group consisting of iron, nickel, copper, zinc, tin, and combinations (i.e., alloys, mixtures, eutectics, etc.) of such metals with each other or with other (metallic or non-metallic) constituents.

By "sub-micron thickness" is meant that the oxidizable metal coating has an applied thickness of less than 1.0 micron.

Consistent with the objective of the invention to provide a conductive coating on the substrate which is rapidly rendered non-conductive by oxidization thereof, the thickness of the oxidizable metal coating does not exceed 1.0 mil. Further, it has been found that at oxidizable metal coating thicknesses above about 1.0 micron, metal coated filaments in chaff applications tend to stick or adhere to one another, particularly when the chaff is provided in the form of multifilament tows, which 17 typically may contain on the order of from about 200 to about 50,000 filaments per tow, and preferably from about 1,000 to about 12,000 filaments per tow. Additionally, it has been found that at oxidizable metal coating thicknesses significantly above 1.0 micron, differential thermal effects and/or deposition stresses tend to adversely affect the adhesion of the metal film to the substrate element, with consequent increase in the tendency of the oxidizable metal film on the coated article to chip or otherwise decouple.

In chaff applications utilizing filamentous substrate elements, the oxidizable metal coating thickness may suitably be on the order of 0.002 micron to about 0.25 micron, with a thickness range of from about 0.025 micron to about 0.10 micron being generally preferred. Disproportionately lower film thicknesses of the oxidizable metal coating result in discontinuities which adversely affect the desired conductivity characteristics of the applied oxidizable metal coating. In chaff applications, the oxidizable metal preferably is iron, although other metal species such as nickel, copper, zinc, and tin may potentially advantageously be employed, as well as combinations of such metals.

To achieve the desired sub-micron thicknesses of the oxidizable metal coating on the substrate, it is preferred in practice to utilize chemical vapor deposition processes to deposit elemental oxidizable metal on the substrate from an organometal precursor material, although any other process techniques or methods which are suitable to deposit the oxidizable metal coating in the desired thickness may be usefully employed.

It will be recognized, however, that the specific substrate element material of construction must be selected to retain the substrate element's desired end-use characteristics during the 18 coating operation, as well as during the subsequent treatment steps. Accordingly, when chemical vapor deposition is employed to deposit an oxidizable metal coating, e.g., of iron, on the substrate, temperatures in the range of 90°C-800°C can be involved in respective steps of the coating process. Oxidizable metal application temperatures are dictated by the thermal carrying properties and thermal stability of the substrate.

Thus, these properties of the substrate can determine the properties of the deposited film. Accordingly, a substrate material accommodating a range of processing temperatures is preferred, e.g., glass or ceramics.

As an example of the utilization of chemical vapor deposition to deposit the elemental oxidizable metal coating on the substrate material, the substrate element may be a borosilicate glass fiber with a diameter on the order of 3-8 microns. Such fibers may be processed in a multizone chemical vapor depositio (CVD) system including a first stage in which the substrate filament is desized to remove epoxy or starch size coatings, at a temperature which may be on the order of 650°C-800°C and under an inert or oxidizing atmosphere. Following desizing, the clean filament may be conducted at a temperature of 450°C-600°C into a coating chamber of the CVD system. In the coating chamber, the hot filament is exposed to an organoiron precursor gas mixture, which may comprise iron pentacarbonyl as the iron precursor compound at a concentration of 5-50% by weight in a carrier gas such as hydrogen. This source gas mixture may be at a temperature on the order of 75°C-150°C in the coating chamber, whereby elemental iron is deposited on the substrate element from the carbonyl precursor compound. The coating operation may be carried out with repetition of the heating and coating steps in sequence, to achieve a desired film thickness of the applied iron coating. 19 It will be appreciated that the foregoing description of coating of the non-conductive substrate with iron is intended to be illustrative only, and that in the broad practice of the present invention, other CVD iron precursor compound gas mixtures may be employed, e.g., ferrocene in a hydrogen carrier gas.

Alternatively, other non-CVD techniques may be employed for depositing the oxidizable metal on the substrate, such as solution plating.

In a first embodiment, the article may further comprise an oxidation enhancingly effective amount, e.g., form about 0.0005% to about 25% by weight, based on the weight of oxidizable metal in the oxidizable metal coating, of a salt (e.g., a metal salt or an organic salt) on the oxidizable metal coating.

Subsequent to application to the substrate of an oxidizable metal coating of the desired thickness, the external surface of the oxidizable metal coating on the substrate in this first embodiment, has a salt applied thereto.

The salt may comprise any suitable salt species, such as for example metal salts (e.g., halides, nitrates, sulfates, etc.) and organic salts (e.g., citrates, stearates, acetates, etc.), the choice of a specific salt being readily determinable by simple corrosion tests without undue experimentation. It will likewise be appreciated that the type and amount, or "loading," of the salt on the oxidizable metal coating may be widely varied as necessary or desirable to correlatively provide a predetermined service life for the oxidizable metal under corrosion conditions in the specific end use environment in which the product article is to be deployed.

Since it is desired that the oxidizable metal coating be retained in an oxidizable state, the oxidizable metal-coated substrate suitably is processed in the salt application or 20 formation ("doping"), and succeeding steps, under an inert or other non-oxidizing atmosphere.

The salt doping of the oxidizable metal-coated substrate advantageously may be carried out by passage of the oxidizable metal-coated substrate through a reaction zone in which the oxidizable metal coating is exposed to halogen gas, such as chlorine, to form a metal salt on the oxidizable metal surface, or by contacting of the oxidizable metal-coated substrate with a solution of a salt, e.g., metal salt or organic salt, or in any other suitable manner, effecting the application of the salt to the external surface of the iron coating.

Generally, however, solution bath application of the salt is preferred, and for such purpose the bath may contain a low concentration solution of salt in any suitable solvent.

Preferably, the solvent is anhydrous in character, to minimize premature oxidation of the oxidizable metal coating. Alkanolic solvents are generally suitable, such as methanol, ethanol, and propanol, and such solvents are, as indicated, preferably anhydrous in character. The salt may be present in the solution at any suitable concentration, however it generally is satisfactory to utilize a maximum of about 25% by weight of the salt, based on the total weight of the salt solution.

In a preferred salt solution formation of a salt coating on the oxidizable metal surface, any suitable salt may be employed in the salt solution bath, although metal halide salts and metal sulfate salts are preferred. Among metal halide salts, the halogen species may be utilized to advantage. Examples of suitable metal halide salts include lithium chloride, sodium chloride, zinc chloride, and iron (III) chloride. A preferred metal sulfate species is copper sulfate, CuSO. Typically from about 0.005 to about 25% by weight of salt, based on the weight 21 of oxidizable metal, is applied to the oxidizable metal coating, with from about 0.1 to about 20% by weight being preferred, and from about 0.5% to about 15% being most preferred (all percentages of salt being based on the weight of oxidizable metal in the oxidizable metal coating on the substrate element) .

Among the aforementioned illustrative metal chlorides, iron (III) chloride is a preferred salt. It is highly hygroscopic in character, binding six molecules of water for each molecule of iron chloride in its most stable form. Iron (III) chloride has the further advantage that it adds Fe (III) to the metal-coated fiber to facilitate the ionization of the oxidizable metal. For example, in the case of iron as the oxidizable metal on the non-metallic substrate, the presence of Fe (III) facilitates the ionization of Fe (0) to Fe (III) . Additionally, iron (III) chloride. is non-toxic in character. Copper sulfate is also a preferred salt dopant material since the copper cation functions to galvanically facilitate the ionization of iron, enhancing the rate of dissolution of the iron film, when iron, the preferred oxidizable metal, is employed in the metal coating on the non-metallic substrate.

When the salt dopant is applied from a solution bath, or otherwise from a salt solution, the coated substrate after salt solution coating is dried, such as by passage through a drying oven, to remove solvent from the applied salt solution coating, and yield a dried salt coating on the exterior surface of the oxidizable metal film. The temperature and drying time employed in the solvent removal operation may be readily determined by those skilled in the art without undue experimentation, as appropriate to yield a dry salt coating on the oxidizable metal-coated substrate article. When alkanolic solvents are employed, the drying temperature generally may be on the order of 100°C. 22 After salt coating of the oxidizable metal-coated substrate, and drying to effect solvent removal from the applied salt coating when the salt is applied from a solvent solution, the resulting salt-doped oxidizable metal-coated substrate product article may be packaged for subsequent use.

As indicated, during the processing of the substrate subsequent to application of the oxidizable metal coating thereon, the resulting oxidizable metal-coated substrate preferably is processed under an inert or otherwise non-oxidizing atmosphere, to preserve the oxidizable character of the oxidizable metal film. Thus, the salt coating, drying, and packaging steps may be carried out under a non-oxidizing atmosphere such as nitrogen. In the final^packaging step, the salt-doped, oxidizable metal-coated substrate may be disposed in a package, chamber, housing, or other end use containment means, for storage, pending use thereof, with a non-oxidizing environment being provided in such containment means. Thus, the final product article may be stored in the containment means under nitrogen, hydrogen or other non-oxidizing atmosphere, or in a vacuum, or otherwise in an environment substantially devoid of oxygen or other oxidizing species of constituents which may degrade the oxidizable metal coating or otherwise adversely affect its utility for its intended end use.

Depending on the type and character of the substrate element, it may be desirable to treat the substrate article in order' to enhance the adhesion thereto of the oxidizable metal coating. For example, as described above regarding the usage of glass filament as the substrate element, it may be necessary or desirable to desize the glass filament when same is initially provided-with a size or other protective coating, such as an epoxy, silane, or amine size coating, by heat treatment of the filament. More generally, it may be desirable to chemically or thermally etch the substrate surface, such as by acid exposure or flame spray treatment. It may also be desirable to employ a primer or adhesion promoter coating or other interlayer on the substrate to facilitate or enhance the adhesion of the oxidizable metal coating to the substrate. Specifically, it may be desirable in some instances, particularly when the substrate element is formed of materials such as glasses, ceramics, or hydroxy-functionalized materials, to form an interlayer on the substrate surface comprising a material such as polysilicate, titania, and/or alumina, using a sol gel application technique, as is disclosed and claimed in U.S. Patent 4,738,896 issued April 19, 1988 to W. C. Stevens for "SOL GEL FORMATION OF POLYSILICATE, TITANIA, AND ALUMINA INTERLAYERS FOR ENHANCED ADHESION OF METAL FILMS ON SUBSTRATES." Referring now to the drawings, Figure 1 shows an array of salt-doped, iron-coated glass filaments from a tow of such filaments. Each of these coated filaments comprises a glass fiber core having on an exterior surface thereof a sub-micron iron coating. On the exterior surface of the respective iron coatings of these filaments is a salt coating comprising localized salt crystalline formations. Although the localized salt deposits or polycrystalline formations are present as gross deposits of crystalline salt, it is to be recognized that microcrystals of salt also are present on the exterior surface of the iron coating, intermediate such gross crystal formations.

This distribution of gross crystallite formations and scattered microcrystals on the intermediate surface areas is produced by the solution bath application method for applying salt as illustratively described hereinabove.

It is to be recognized, however, that other methods of salt coating may be employed in the broad practice of the present invention, which will result in different distributions or morphologies of salt being formed on the surface. In this respect, it is to be appreciated that the salt may be present on 24 the exterior surface of the iron, or other oxidizable metal coating, solely in the form of scattered crystallite formations, or as a more continuous distribution on the surface of microcrystals, or a combination of such salt formations, as shown in Figure l, or in still other distributions or morphologies.

The photomicrograph of Figure 1 shows the salt-doped, iron-coated glass filament at a magnification of 5,000 times. This electron micrograph was taken at a voltage of 20 kv, and the scale of the photograph is shown by the line in the right central portion at the bottom of the photograph, representing a distance of two microns.

The glass filaments employed in the coated fibers shown in Figure 1 were of lime aluminoborosilicate composition, commercially available as E-glass ((Owens-Corning D filament) 54% Si02; 14.0% A1203; 10.0% B203; 4.5% MgO; and 17.5% CaO) ) having a measured diameter of 4.8 microns, and were coated with an iron coating of 0.075 micron thickness. The salt coating was formed of iron (III) chloride, and was present on the iron coating in an amount of from about 1 to about 5% by weight of salt, based on the weight of iron present in the iron coating.

Figure 2 is a graph of resistance, in Megaohms, as a function of exposure time, in minutes, for fiber tows of the type shown in Figure 1, but which were salt doped, in a first sample, with lithium chloride salt coatings formed by coating the iron film with a 0.04% lithium chloride by weight solvent solution, and, in a second sample, with 0.5% lithium chloride solvent solution. A control tow of fibers was utilized as a basis for comparison, in which the fibers included an iron coating of the same thickness as the two salt-doped fiber tows, but did not include any salt coating.

In order to measure the tow resistance of the respective fiber tows, each tow was mounted on a copper contact circuit board with a known spacing, in either a two-point or four-point arrangement. Electrical contact was assured through use of conductive silver paint. Fiber tows were analyzed by use of digital multimeter. A known voltage was applied across the fiber circuit. The resulting current was metered and the resistance computed. This measurement was repeated periodically over the fiber lifetime of interest, with voltage applied, during each interval, for a duration just long enough to allow measurement to be made.

The resistance of each of the respective fiber tows was measured as a function of time of exposure to 56% relative humidity conditions. As shown in the graph, the control tow, comprising fibers devoid of any salt coating thereon, exhibited a constant resistance over an exposure time of 1,000 minutes. The second tow, comprising fibers doped with 0.5% lithium chloride, maintained a constant resistance for approximately 150 minutes and then exhibited a rapid increase in resistance over the next 150 minutes, indicating that the oxidizable iron coatings on the glass filaments in that tow were being rapidly oxidized during the latter time period, with the conductive iron coating being transferred to non-conductive iron oxide. The third fiber tow, comprising fibers doped with 0.04% lithium chloride, maintained a constant resistance for 600 minutes and then exhibited a rapid increase in resistance over the next 200 minutes of exposure, indicating that oxidation of the iron coating was rapidly taking place in the latter time period.

The foregoing results show that the life of the conductive oxidizable metal coating may be controllably adjusted by selective doping levels of salt(s) on the surface of the oxidizable metal coating. Thus, for example, in chaff applications, such selective doping levels may be utilized to,-.- 26 correspondingly adjust the service life of the oxidizable metal- coated chaff fibers, consistent with the desired retention of the initial radar signature characteristic thereof for a given length of time, followed by rapid dissipation of the radar signature characteristic of such "evanescent chaff" material.

Figure 3 is a graph of resistance, in Megaohms, as a function exposure time, in minutes, for salt-doped, iron-coated glass fibers of the type described hereinabove in connection with Figure 1, including a first tow having iron-coated fibers doped with salt by solution coating thereof with a 0.04% by weight iron (III) chloride solution, and a second tow with a coating of the same salt material derived from a 0.5% by weight solution thereof. A corresponding control, devoid of any salt coating thereon, was employed for comparison purposes.

As shown by the graph of Figure 3 , the control, having no salt coating on the iron film, exhibited a constant resistance over the full 1,000 minute exposure to 58% relative humidity conditions. The tow containing fibers coated with 0.04% iron (III) chloride solution exhibited a constant resistance for the initial 400 minutes of exposure, followed by a steady increase in the resistance over the succeeding 600 minutes of the 1,000 minute . exposure. The third tow, comprising fibers coated with 0.5% iron (III) chloride solution, exhibited a constant resistance value for the initial 200 minutes of exposure, followed by exponentially increasing resistance indicating extremely rapid oxidation of the iron coating. By contras ^ the tow comprising fibers coated with the 0.04% iron (III) chloride solution exhibited a substantially linear increase in resistance during oxidation, indicative of uniformly progressing oxidation of the iron coating. These data show that salt doping of the fiber may be employed to selectively adjust the useful life and conductivity decay characteristics of the oxidizable metal film coated on the substrate element. 27 As used herein, the term "oxidizable metal" is intended to be broadly construed to include elemental oxidizable metals per se. and combinations of any of such elemental metals with each other and/or with other metals, and including any and all metals, alloys, eutectics, and intermetallic materials containing one or more of such elemental oxidizable metals, and which are depositable in sub-micron thickness on a substrate and subsequent to such deposition are oxidizable in character.

Although iron is a preferred oxidizable metal in the practice of the present invention, and the invention has been primarily described herein with reference to iron-coated glass filaments, it will be recognized that other metals such as nickel, copper, zinc, and tin may be potentially usefully employed in similar fashion. It will also be recognized that the substrate element may be widely varied, to comprise the use of other substrate element conformations and materials of construction.

In the use of nickel, copper, zinc, and tin as oxidizable metal constituents, preferred salt species may vary from those described above, which are disclosed as being applicable to the invention and preferred in application to iron, but in the -context of the broad range of preferred oxidizable metal constituents (iron, nickel, copper, zinc, and/or tin) of the present invention, metal ha1 des, particularly those in which the halide moiety is chlorine, are considered to be a preferred class of salt materials.

The present invention in a second embodiment relates to an article comprising a non-conductive substrate having a sub-micron thickness of an oxidizable conductive first metal coating thereon^ and a second metal which is galvanically effective to promote the corrosion of the first metal, discontinuously applied on the first metal coating. Preferably, the second (promoter) 28 metal will be present on the first metal coating, in an amount of from about 0.1 to about 10% by weight of the total coating (first metal and second metal) .

The oxidizable conductive first metal coating may comprise any of various suitable metals as described above in connection with the first embodiment.

Subsequent to application to the substrate of a conductive first metal coating of the desired thickness, the first metal-coated substrate is coated or "doped" with a discontinuous coating of a second metal, sometimes hereinafter referred to as a "promoter metal," which is galvanically effective to promote the corrosion of the oxidizable first metal coating. The second metal coating is discontinuous in character, in that the second metal coating does not fully cover or occlude the conductive first metal coating on the non-conductive substrate. As a result of the exposure of the oxidizable first metal coating "through" the discontinuous second metal coating to the ambient environment, the conductive first metal coating is converted by atmospheric moisture to a non-conductive metal oxide film.

Such oxidation or corrosion of the conductive first metal film is galvanically .assisted and accelerated by the discontinuous coating of the second metal which is superposed on the oxidizable first metal coating.

The second metal discontinuously coated on the oxidizable, conductive first metal coating in the broad practice of the present invention may include any suitable metal which is galvanically effective to promote the corrosion of the first metal in the oxidizable conductive first metal coating on the non-conductive substrate. As used in such context, the term "metal" is to be broadly construed to include elemental metal, as well as alloys, intermetallics, composites, or other materials 29 containing a corrosion promotingly effective second metal constituent.

In order for the second metal to effectively promote galvanic corrosion of a conductive first metal film, and assist in the oxidation of the first metal film, the second metal must have a lower standard oxidization potential than the first metal, thereby enabling the second metal to act as a cathodic constituent in the galvanic corrosion reaction. Illustrative of elemental second metals which may be potentially usefully employed in the broad practice of the present invention are cadmium, cobalt, nickel, tin, lead, copper, mercury, silver, and gold. In general, the lower the oxidation potential, E°, the faster the reduction-oxidation corrosion reaction.

Of the above-listed exemplary elemental metals useful in the broad practice of the present invention, and with preference to iron as the oxidizable conductive first metal species, copper is typically a preferred elemental second metal, due to its low toxicity, low cost, and low oxidation potential.

The application or formation of the discontinuous coating of second metal on the oxidizable conductive first metal coating may be carried out in any suitable manner, such as flame spraying, low rate precipitation in a plating bath, or other surface application methods. It is also within the broad purview of the present invention to provide a continuous coating of the second metal on the substrate first metal film, and to thereafter preferentially etch or attack the continuous second metal film to render same discontinuous in character. Further, it is possible to form the discontinuous second metal coating on the oxidizable conductive first metal film by in situ chemical reaction, wherein the reaction product comprises a second metal species which is effective to galvanically accelerate the corrosion of the 30 oxidizable first metal film under ambient exposure conditions in the presence of atmospheric moisture.

In general, however, it is preferred to achieve a discontinuous deposition of the second metal on the first metal-coated substrate by chemical vapor deposition techniques, utilizing as the precursor material for the second metal an organometal compound whose metallic moiety is the second metal. The specific concentrations and concentration ranges which are suitable to form discontinuous second metal films from a given organometal precursor material will be readily determinable by those of ordinary skill in the art, without undue experimentation.

As indicated, for iron-coated substrates, copper is typically a most preferred second metal species, in the broad practice of the present invention. Tin is also preferred and, to a lesser extent, nickel, although nickel may be unsatisfactory in some applications due to toxicity considerations, depending On the ultimate end use.

For the aforementioned most preferred copper second metal species, when iron is the first metal species, application of the discontinuous coating of copper to the iron-coated substrate by chemical vapor deposition techniques may Utilize copper hexafluoroacetylacetonate as an organocopper precursor compound for elemental copper deposition. In the chemical vapor deposition process, the gas-phase concentration of this organocopper precursor compound is maintained at a suitably low level, e.g., not exceeding about 200 grams per cubic centimeter of the vapor (carrier gas and volatile organometal precursor compound), and typically much lower, such as for example 0.001 gram per cc. By maintaining the vapor-phase concentration of the second metal precursor compound suitably low, the discontinuous coating of the second metal is achieved. For example, at the aforementioned concentration of 0.001 gram of copper hexafluoroacetylacetonate per cubic centimeter of vapor mixture in the chemical vapor deposition chamber, it is possible to form localize discrete deposits, e.g., "islands," of the second metal derived from the organometal precursor compound.

The choice of a specific organometallic precursor compound for the second metal may be suitably varied, depending on the chemical vapor deposition process conditions, metal constituents, character of the oxidizable first metal-coated substrate, etc. , as will be apparent to those skilled in the art. In the case of tin as a second metal species, a suitable organometallic precursor compound is tetraethyl tin.

Subsequent to application to the conductive first metal-coated substrate of a discontinuous film of second ("promoter") metal, the second metal-doped, first metal-coated substrate may optionally be further coated or "doped" with a suitable amount, for example from about 0.005% to about 25% by weight, based on the weight of first metal in the oxidizable conductive first metal coating, of a salt on the external surface of the oxidizable first metal coating, as described for the first embodiment.

Since it is desired that the conductive first metal coating be retained in an oxidizable state, the first metal-coated substrate suitably is processed in the second metal application, optional salt application, and any succeeding treatment steps, under an inert or other non-oxidizing atmosphere.

It is to be recognized that the salt modification of the second metal-doped, first metal-coated substrate is not required in the broad practice of the present invention, but is an optional additional coating treatment which may be carried out to further enhance the oxidation of the conductive first metal film 32 on the substrate during the galvanically accelerated corrosion of the first metal coating resulting from the presence of the second metal thereon.

As a final packaging step, the second metal-doped, first metal-coated substrate may be disposed in a package, chamber, housing, or other end use containment means, for storage pending use thereof, with a non-oxidizing environment being provided in such containment means. Accordingly, the final product article may be stored in the containment means under nitrogen, hydrogen or other non-oxidizing atmosphere, or in a vacuum, or otherwise in an environment substantially devoid of oxygen or other oxidizing species or constituents which may degrade the oxidizable conductive first metal coating or otherwise affect its utility. for its intended end use.

It may be necessary or desirable in the broad practice of the present invention to treat or process the first metal-coated substrate to enhance the" adhesion of the discontinuous coating of the second metal to the conductive first metal coating on the substrate.

Referring now to the drawings, Figure 4 is an electron photomicrograph, at a magnification of 30,000 times, of a copper-coated, iron-coated glass filament. The coated article comprises an oxidizable iron coating on the exterior surface of the substrate glass filament, with a discontinuous coating of copper on the oxidizable iron coating. The discontinuous copper coating, as shown, has the form of "islands" on the iron coating.

The scale of the electron photomicrograph of Figure 4 is shown by the line in the right central portion at the bottom of the photomicrograph, representing a distance of 1 micron.

The glass filament employed in the coated fiber shown in Figure 4 was of lime aluminoborosilicate composition, commercially available as E-glass (Owens-Corning D filament (54% Si02; 14.0% A1203; 10.0% B203; 4.5% MgO; and 17.5% CaO) ) having a measured diameter of 4.8 microns. This glass filament was coated with an iron coating at a thickness of about 0.075 micron, and as shown in Figure 4, the copper islands on the iron film had dimensions in the range of 1-10 microns, as measured along the surface of the iron coating on which the islands were deposited. Both the iron coating and the copper islands on the coated fiber shown in Figure 4 were applied by chemical vapor deposition techniques.

Figure 5 shows a tow of fibers of copper-coated, iron-coated glass filaments similar to the coated filaments shown in Figure 4, but on which the copper coating was relatively more continuous than the copper "islands" of the coated filament shown in Figure 4. The tow shown in Figure 5 comprised filaments of copper-coated iron-coated glass fibers, which are doped with salt by depositing approximately 1.8% by weight iron (III) chloride (based on the weight of iron in the oxidizable film) on the copper-coated, iron-coated glass fibers, from a 0.25% by weight solution of iron (III) chloride in methanol.

Figure 6 is an enlargement of the demarcated rectangular portion of the electron photomicrograph of Figure 5, showing the presence of salt crystallites on the copper-coated, iron-coated glass fibers.

Figure 7 is a an electron photomicrograph, at magnification of 1500 times, of a tow of fibers corresponding to those shown in Figure 5, after exposure of the tow to 52% relative humidity conditions a 25°C for 20 minutes. The corrosion of the iron coating on the fibers is dramatically evident from this photograph, an enlargement of the demarcated rectangular portion of which is shown in Figure 8.

Figure 9 is a graph of resistance, in Megaohms, as a function of exposure time, in hours, for fiber tows which comprised 6 micron nominal diameter (4.8 micron measured diameter) glass filaments as the substrate elements, on which were coated a 0.075 micron thickness of iron film, and then a relatively continuous coating of copper.

As indicated in Figure 9, corresponding tows were exposed at 11%, 52%, and 98% relative humidity exposure conditions, and the resistance of the tow, in ohms/cm. , was measured during the time of exposure. The results shown in Figure 9 demonstrate that tow resistance remained substantially constant with time, when the copper coating was substantially continuous in character.

Figure 10 is a graph of a tow resistance, in Megaohms/cm. , as a function of exposure time, in hours, for a tow of fibers comprising 6 micron nominal diameter (4.8 microns measured diameter) glass filaments having a 0.075 micron thick iron coating deposited thereon, and coated with a discontinuous film of copper, and doped with iron (III) chloride salt.

The data plotted in Figure 10 show that tow resistance remained negligible for approximately five hours, followed by a raid exponential increase in resistance, indicative of rapid oxidation of the oxidizable iron coating. The conductivity of this fiber tow sample was fully decayed in about 15 hours.

. Figure 11 is a graph of tow resistance, in Megaohms/cm. , as a function of exposure time, in hours, to 52% relative humidity conditions, for a fiber tow of the type employed to generate the data of Figure 9 ("Cu Doped"), and a corresponding fiber tow of the type employed to generate the data of Figure 10 ("FeC13 Coat 35 Cu D") . The data of Figure 11 show that the copper-coated, iron- coated fibers on which the copper coating was substantially continuous in character, exhibited a substantially negligible resistance over the full exposure period, while the corresponding salt-doped fiber tow exhibited substantially constant resistance for about eight hours, after which its resistance rapidly increased. These data show that even where the copper coating on the iron coating is substantially continuous, and would otherwise prevent significant oxidization of the iron coating, the presence of the metal salt, which acts as an electrolyte, nonetheless initiates corrosion of the underlying iron film.

Figure 12 is a graph of tow resistance, in Megahohms/cm. , as a function of exposure time, in hours, at 52% relative humidity exposure conditions. The tows which were evaluated comprised fihers of 6 micron nominal diameter (4.8 measured diameter) coated with a 0.075 micron thickness of iron thereon. A first tow was doped with iron (III) chloride salt; this tow was designated "FeC13 Coat." The other tow utilized a same iron-coated fiber, on which was coated copper and iron (III) chloride salt; this tow was designated as "FeCl3 Coat CuD." The results in Figure 12 show that the salt-doped, iron-coated glass fiber tow began to rapidly oxidize within thirty minutes or so of initial exposure to 52% relative humidity conditions. The corresponding salt-doped, copper-coated, iron-coated fiber tow exhibited negligible resistance for approximately 8 hours, followed by rapidly increasing resistance, indicative of high rate oxidation of the iron film.

From the foregoing, it is seen that the rate of oxidation of an iron film coated with a discontinuous coating of promoter metal, and optionally with a metal salt coating, may be selectively adjusted over a wide range to achieve a predetermined conductive life and a selected rate of decay of such conductivity. Where the copper coating is relatively continuous in character, it is highly desirable to utilize a further coating of metal salt to accelerate the galvanic corrosion reaction by which the iron film on the substrate fiber is oxidized and rendered non-conductive in character.

In the above-described tow resistance tests, the data from which are shown in Figures 9-12, the tow resistance was determined by the following method.

In order to measure the tow resistance of the respective fiber tows, each tow was mounted on a copper contact circuit board with a known spacing, in either a two-point or four-point arrangement. Electrical contact was assured through use of conductive silver paint. Fiber tows were analyzed by the use of a digital multimeter. A known voltage was applied across the fiber circuit. The resulting current was metered and the resistance computed. This measurement was repeated periodically over the fiber lifetime of interest, with voltage applied, during each interval, for a duration just long enough to allow measurement to be made. The increase in resistance over time then was plotted as an indicator of decay rate and conductive lifetime.

Thus, the life of the conductive first metal coating may be controllably adjusted by the discontinuous coating of a second ("promoter") metal and optionally by selectively doping salt on the surface of the promoter metal-doped first metal coating. In chaff applications, the respective coating levels may be utilized to correspondingly adjust the service life of the first metal-coated chaff fibers, consistent with a desired retention of the initial radar signature characteristic thereof for a given length of time, followed by rapid dissipation of the radar signature character of such "evanescent chaff" material. 37 In some instances in which the promoter metal-doped, first metal-coated substrate is subjected to contact with other coated articles or otherwise to abrasion prior to actual deployment, it may be desirable to overcoat the promoter metal-doped, first metal-coated substrate with a material serving as a fixative for the promoter metal (and optional salt coating) , to prevent damage to the promoter metal and/or salt coating as a result of abrasion or other contacts which would otherwise serve to remove the applied promoter metal and/or salt coatings. For example, a porous gel coating or binder material may be applied to the promoter metal-coated, oxidizable first metal-doped film, for the purpose of adheringly retaining the promoter metal coating in position on the conductive first metal film. The overcoat may generally be of any suitable material which does not adversely affect the respective promoter metal and conductive first metal coatings for the intended purpose of the coated product article. A preferred overcoat material comprises polysilicate, titania, and/or alumina formed on the promoter-doped, conductive first metal film from a sol gel dispersion of polysilicate, titania, and/or alumina material, as more fully described hereinafter in connection with the so-called "fourth embodiment of the invention.

In a third embodiment, the invention relates to an article comprising a non-conductive substrate having a continuous sub-micron thickness of a conductive oxidizable metal coating thereon, wherein the metal coating has been sulfurized, i.e., associated with an oxidation-enhancing amount of sulfur.

Preferably, the amount of sulfur associated with the sulfurized, oxidizable metal coating on the substrate is from about 0.01 to about 10% by weight of sulfur, based on the weight of oxidizable metal in the oxidizable metal coating on the non-conductive substrate. More preferably, the amount of sulfur associated with the oxidizable metal coating is from about 0.02 38 to about 5% by weight, and most preferably from about 0.05 to about 2.0% by weight, on the same oxidizable metal weight basis. As used in such quantitative ranges of concentration, the amount of sulfur refers to the amount of elemental sulfur. It is to be appreciated that the sulfur constituent associated with the oxidizable metal coating may take any of a wide variety of forms, including elemental sulfur, compounds of sulfur such as iron sulfide, hydrogen sulfide, and sulfur oxides, as well as any other sulfur-containing compositions which provide sulfur in a form which is effective to enhance the rate and/or extent of corrosion of the oxidizable metal coating on the substrate.

The sulfur constituent is associated with the oxidizable metal coating on the substrate, e.g., within the oxidizable metal coating and/or on a surface of the oxidizable metal coating, and/or otherwise in sufficient proximity to the oxidizable metal coating to render the sulfur in the sulfur constituent enhancingly effective for the oxidation of the oxidizable metal coating. Preferably the sulfur constituent is associated with the oxidizable metal coating, by being present in the oxidizable metal coating itself and/or on a surface of the oxidizable metal coating.

In the practice of the third embodiment of the invention, the oxidizable coating formed on the non-conductive substrate may be "sulfurized, " i.e., have sulfur associated therewith, before, during, and/or after the application of the oxidizable metal coating to the substrate. For example, a sulfur-containing material may be applied to the substrate prior to application of the oxidizable metal coating thereon, or the sulfur constituent may be co-deposited with the oxidizable metal coating, or serially applied between successive applications of oxidizable metal film to yield the final oxidizable metal coating, or the sulfur constituent may be applied to an external surface of the applied oxidizable metal coating, or by any combinations of such steps, or selected ones thereof, with or without other steps, for associating sulfur with the oxidizable metal.

As indicated hereinabove, it is generally preferred to deposit the oxidizable metal coating on the substrate material by chemical vapor deposition techniques, when the substrate element is glass or ceramic, utilizing an organometallic precursor compound as a source material for the deposited oxidizable metal. The chemical vapor deposition process may involve repetition of successive heating and coating steps for deposition of the oxidizable metal film at a desired thickness, and in such case it generally is preferred to deposit the sulfur constituent in the heating zones between successive coating zones of the process system.

In such system, the sulfur-containing material may be introduced in the heating zone(s) to deposit a sulfur constituent on the substrate, with the deposited sulfur constituent then being overlaid with a film of applied oxidizable metal coating in the next succeeding oxidizable metal coating zone. In this manner, the sulfur material may be deposited on an initial and succeeding film of applied oxidizable metal which in the aggregate make up the oxidizable metal coating on the substrate.

For ease of description in the ensuing discussion, each constituent application of oxidizable metal to a substrate in a multi-zone metal coating process system will be referred to as a "pass", so that for example a "five-pass system" entails five discrete applications of oxidizable metal film to the substrate to yield the overall oxidizable metal coating. In such five-pass system, sulfur-containing material may be applied to the oxidizable metal film after the first pass and/or any succeeding pass(es) including the final pass. 40 Although any suitable application scheme for associating sulfur constituent (s) with the oxidizable metal coating may be employed in a multi-pass system, it generally is desirable to apply the sulfur constituent (s) to the oxidizable metal coating in at least the outer portion of the applied oxidizable metal film, so that sulfur availability in the outer portion of the film is provided for, consistent with the objective of enhancing the corrosion rate of the oxidizable metal film with a sulfur constituent. Typically it is preferred not to deposit the sulfur constituent in an initial filament desizing step, but rather in at least some of the subsequent preheating zones upstream of the corresponding chemical vapor deposition reaction chambers.

In the preheat zone(s) , sulfur may for example be introduced in the form of a sulfur compound such as hydrogen sulfide, in a carrier gas such as nitrogen or hydrogen. When hydrogen sulfide is used as the sulfur-containing material for deposition, it generally is suitable to operate the coating process system with a concentration of from about 0.01 to about 20% by weight, based on the total weight of hydrogen sulfide and carrier gas, of hydrogen sulfide in the carrier gas. For example, a 10% by weight hydrogen sulfide in hydrogen carrier gas mixture has been used to good advantage.

The heating zone during the deposition of the sulfur material may be maintained at a temperature in the range of from about 450°C to about 600°C for the aforementioned hydrogen sulfide/carrier gas mixture, although the specific temperatures, sulfur-containing material, and other process conditions may be widely varied depending on the nature of the application system and the desired final product article.

Generally, hydrogen is preferred as a carrier species for the sulfur-containing material, since hydrogen aids in reducing the previously applied oxidizable metal coating, and opposing the 41 oxidation thereof. Hydrogen sulfide is a preferred sulfur-containing material for use in the aforementioned illustrative chemical vapor deposition system, and when employed in a hydrogen carrier gas, results in the formation of metal sulfide in the previously applied oxidizable metal film, along with the formation of inclusions of hydrogen sulfide oxide, and elemental sulfur, in the resulting "sulfurized" coating of oxidizable metal.

It will be appreciated that the method of association of the sulfur material with the oxidizable metal coating may be carried out in a wide variety of methods, and with a wide variety of suitable sulfur-containing materials. For example, it may be advantageous in some applications to sulfurize the oxidizable metal coating by application thereof of a coating of a solvent solution of a suitable sulfur-containing material. As an illustration, it may be desirable in some instances to coat the oxidizable metal coating with a solvent solution of a sulfur-containing compound, such as thiophene, whereby subsequent drying of the solution coating will yield the sulfur-containing compound on the oxidizable metal coating.

This third embodiment of the invention is based on the substantial and unexpected discovery that very low quantities of sulfur may be associated with a oxidizable metal coating on a non-conductive substrate, to markedly increase the rate of corrosion of the oxidizable metal coating on the substrate element, so that the conductive oxidizable metal coating is oxidatively converted to non-conductive metal oxide.

Further> the enhancement of the corrosion reaction involving the oxidizable metal coating has been found to take place at an accelerated rate when the oxidizable metal coating is sulfurized, even at relatively low humidity exposure conditions, e.g., 11% relative humidity. Thus, the sulfur functions to reduce the 42 amount of atmospheric moisture (water) otherwise required to oxidize the oxidizable metal coating to the corresponding metal oxide reaction product.

The specific loading of sulfur associated with the oxidizable metal coating in the article of the present invention may be readily determined by those skilled in the art without undue experimentation, by the simple expedient of varying the sulfur loading and/or metal oxidation (corrosion) conditions, to determine the sulfur loading which is necessary or desirable in a given end use application.

As an example of the oxidation characteristics of articles of the present invention, it has been found that sulfurization of an iron coating in a chemical vapor deposition process system, of the type previously illustratively described, to provide a 0.1% by weight loading of sulfur in an iron coating of 0.075 micron thickness on a 4.8 micron diameter of the iron coating after about 10 hours at 98% relative humidity exposure conditions. "" It will likewise be appreciated that it is feasible in the broad practice of the present invention to selectively vary the sulfur loading associated with the oxidizable metal coating, to achieve a predetermined corrosion rate and service life of the conductive oxidizable metal coating, in chaff or other oxidizable metal coating conductivity dissipation applications.

Subsequent to application to the substrate of the oxidizable metal coating of the desired thickness, and sulfurization thereof, the oxidizable metal-coated substrate may optionally be coated or "doped" with a discontinuous coating of a "promoter metal" which is galvanically effective to promote the corrosion of the oxidizable metal, on the external of the oxidizable metal coating, as described hereinabove in connection with the second embodiment of the invention. 43 As a further optional treatment of the sulfurized oxidizable metal-coated substrate, which may be employed with or without the aforementioned optional application of a promoter metal, the sulfurized oxidizable metal-coated substrate may be further coated or "doped" with a suitable amount, for example from about 0.005 to about 25% by weight, based on the weight of oxidizable metal in the oxidizable metal coating, of a salt, e.g., a metal salt or organic salt, on the external surface of the oxidizable metal coating, as described hereinabove in connection with the first embodiment of the invention.

After salt coating of the sulfurized oxidizable metal-coated substrate, and drying to effect solvent removal from the applied salt coating when the salt is applied from a solvent solution, the resulting salt-doped, sulfurized oxidizable metal substrate product article is hermetically sealed for subsequent use.

It is to be recognized that salt coating or promoter metal coating of the sulfurized oxidizable metal-coated substrate is not required in the broad practice of the present invention, but represent optional additional coating treatments which may be carried out to further enhance the oxidization of the oxidizable metal film on the substrate during the accelerated corrosion of the oxidizable metal coating resulting from the presence of the sulfur in association therewith.

As indicated, during the processing of the substrate by application of the oxidizable metal-coating thereto, and sulfurization of such oxidizable metal coating, the coated article is processed under an inert or otherwise non-oxidizing atmosphere to preserve the oxidizable character of the oxidizable metal .film. Thus, the coating, sulfurization, and optional promoter metal and/or salt doping and packaging steps may be carried out under a non-oxidizing atmosphere such as nitrogen. 44 Referring now to the drawings, Figure 13 is an electron photomicrograph, at a magnification of 5000 times, of sulfurized iron-coated glass filaments. Each of the coated filaments comprises an oxidizable iron coating on the exterior surface of the substrate glass filament, with the iron coating having been sulfurized by hydrogen sulfite contacting between successive depositions of iron in a multizone heating/coating chemical vapor deposition system.

The scale of the electron photomicrograph in Figure 13 is shown by the line in the right central portion at the bottom of the photograph, representing a distance of two microns.

The glass filaments employed in the coated fibers shown in Figure 13 were of lime aluminoborosilicate composition, commercially available as E-glass (Owens-Corning D filaments) (54% SiQ2; 14.0% "A1203; 10.0% B203; 4.5% MgO, and 17.5% CaO) having a measured diameter of 4.8 microns, and were coated with an iron coating of 0.075 micron thickness. The iron coating contained about 0.1% by weight sulfur (measured as elemental sulfur) , based on the weight of iron in the oxidizable iron coating on the substrate.

Figure 14 shows the corrosion product of a tow of sulfurized iron-coated glass filaments of the type shown in Figure 13, after 500 hours exposure at 35°C and 11% relative humidity conditions. As shown, the corrosion of the fibers is substantial. The magnification of the photomicrograph of Figure 14 is 2000 times, with the scale of the photograph being shown by the line in the right-hand central region of the photograph, at the bottom thereof, representing a distance of 5 microns.

Figure 15 is a. photomicrograph, at the same magnification as Figure 14, of a tow of fibers corresponding to those of Figure 45 14, but in which the iron coatings were not sulfurized, after 500 hours exposure at 35°C to 11% relative humidity conditions. As shown, the Figure 15 tow of fibers exhibited relatively negligible corrosion after the same exposure which produced a high degree of corrosion in the tow of sulfurized iron-coated glass filaments shown in Figure 14. These respective photographs clearly show the advantages of the sulfurizing treatment of the oxidizable metal coating in the articles of the present invention, with respect to corrosion of the oxidizable metal coatings on the substrate.

Figure 16 is a graph of tow resistance, in ohms/cm. , as a function of percent relative humidity for respective fiber tows of the type shown in Figures 14 and 15, respectively. The tows comprising unsulfurized iron-coated glass filaments ("STANDARD") are identified in the graph by solid bars, and the corresponding sulfurized iron-coated filament tows ("H2S DOPED") are denoted by the diagonally striated bars.

The data in Figure 16 show that the sulfurization of the iron coating on the tow filaments lowered initial conductivity at low relative humidity conditions relative to the corresponding unsulfurized filament tows, while at higher humidity conditions, the tow resistance was less than that of the corresponding unsulfurized filament tows. The sulfurization process appears to have evened out the effect of relative humidity upon initial conductivity.

When the filament tows comprising sulfurized iron coatings which provided the initial conductivity data of Figure 16 were exposed overnight to ambient atmospheric conditions, the metal coatings became very discolored as rust, and subsequent testing of conductivity showed nearly full decay of current carrying properties. 46 Figure 17 is a graph of resistance, in Megaohms/cm. , as a function of exposure time, in hours, for fiber tows which comprised approximately 4.8 microns diameter glass filaments as the substrate elements, on which were coated 0.075 micron thicknesses of iron. One tow was sulfurized by exposure to hydrogen sulfide ("H2S Doped") and the other was retained in an unsulfurized condition ("Standard") .

In order to measure the tow resistance of the respective fiber tows, each tow was mounted on a copper contact circuit board with a known spacing in either a two-point or four-point arrangement. Electrical contact was assured through the use of conductive silver paint. Fiber tows were analyzed by use of a digital multimeter. A known voltage was applied across the fiber circuit. The resulting current was metered and the resistance computed. This measurement was repeated periodically over the fiber lifetime of interest, with voltage being applied during each interval for duration just long enough to allow measurement to be made.

The resistance of each of the respective fiber tows for which data is shown in Figure 17 was measured as a f nction of time of exposure to 98% relative humidity conditions, at 25°G. As shown in the graph, the control tow, comprising fibers devoid of any sulfur content, exhibited a constant resistance over an exposure time of approximately 115 hours. The second tow, which comprised sulfurized iron-coated filaments, showed a rapid increase in resistance beginning at about 8 hours of cumulative exposure, indicating that the oxidizable iron coatirig on the glass filaments in that tow were being rapidly oxidized, with the conductive iron coating being transformed to non-conductive iron oxide.

Figure 18 is a chart again comparing an iron-coated glass fiber tow devoid of sulfur ("STND") with a corresponding 47 sulfurized tow ("H2S") . The percentage of change in resistance oyer an initial 10 hours of exposure is plotted (as an indicator of corrosion rate) as a function of temperature and humidity. The standard non-sulfurized material represented by solid and cross hatched patterns exhibits a strong dependence upon temperature and humidity of exposure. The sulfurized sample represented by the diagonal pattern shows a more even response across the conditions tested along with a dramatic enhancement of corrosion rate.

Thus, the life of the conductive oxidizable metal coating may be controllably adjusted by selectively varying the sulfurization of the conductive oxidizable metal coating, and optionally by selectively coating a promoter metal and/or providing a salt on the surface of the sulfurized oxidizable metal coating. In chaff applications, such selective sulfurization and optional salt/promoter metal coating of the oxidizable metal coating may be utilized to correspondingly adjust the service life of the oxidizable metal-coated chaff fibers, consistent with a desired retention of the initial radar signature characteristic thereof for a given length of time, followed by rapid dissipation of the radar signature of such "evanescent chaff" material.

In some instances in which the sulfurized oxidizable metal-coated substrate is subjected to contact with other coated articles, or otherwise to abrasion prior to actual deployment, it may be desirable to overcoat the sulfurized oxidizable metal-coated substrate, particularly if a salt coating and/or promoter metal coating is/are employed, to prevent damage to the coated article as a result of abrasion or other contacts, as more fully described hereinafter with reference to the fourth embodiment of the invention. 48 The fourth embodiment of the invention relates to an article comprising a non-conductive substrate which is coated with a sub^ micron thickness of an oxidizable metal and overcoated with a microporous layer of an inorganic electrically insulative material.

The microporous layer of inorganic electrically insulative material preferably is formed from materials such as glasses and/or ceramics, and most preferably such layer is formed of a material selected from the group consisting of polysilicate, titania, and alumina, and combinations thereof. The preferred polysilicate, titania, and/or alumina microporous layers may suitably be formed by sol gel formation techniques, as described more fully hereinafter.

The microporous insulative layer has two functions. Being electrically insulative, it serves to attenuate direct contact between the oxidizable metal coating and sensitive electrical or electronic devices, which may result in damage to circuitry or components therein, or otherwise adversely affect the function of such devices.

In addition, although the porosity of the insulative layer accommodates penetration of atmospheric moisture (relative humidity) to the oxidizable metal coating, to effect corrosion thereof and thereby dissipate the metal coating's conductive characteristics, it has surprisingly and unexpected been found that the morphology of the microporous insulative layer serves to assist in retaining moisture in proximity to the oxidizable metal coating. Such moisture "fixing" may substantially increase the rate of oxidation of the oxidizable metal coating, with the specific magnitude of such enhancement depending on the morphology and composition of the insulative layer, and the exposure (relative humidity) conditions to which the coated, article is exposed. 49 Most preferably, the insulative layer is formed of a material selected from the group consisting of polysilicate, titania, alumina, and combinations thereof. By "combination" is meant that any two or more of such polysilicate, titania, and alumina materials may be utilized with one another, interspersed with one another, or otherwise concurrently present in a microporous composite matrix layer. When titania is employed as a microporous layer material of construction, it is preferred that such material be essentially completely free of palladium.

A suitable porous micrbstructure in the insulative layer may for example have an average pore size, i.e., pore diameter, on the order of from about 50 to about 1000 Angstroms, preferably from about 100 to about 500 Angstroms. Insulative layers comprising polysilicate materials, having an average pore size of from about 100 to about 500 Angstroms, are particularly usefully employed in the practice of the present invention.

The most preferred polysilicate, titania, and/or alumina microporous layers may be formed with the characteristics and by the formation methods described in the aforementioned U.S. Patent 4,738,896.

Generally, the microporous insulative layer may be formed on the oxidizable metal coating in any suitable manner, e.g., by electrolytic methods, chemical vapor deposition, etc., however it is preferred, to form the insulative layer on the oxidizable metal coating by applying over the metal coating a sol gel dispersion, which then is dried, under ambient or elevated temperature conditions, as required, to form the product overcoat insulative layer.

For insulative layers comprising a polysilicate material, as formed on the oxidizable metal coating from a sol gel dispersion of polysilicate, a suitable polysilicate starting material may 50 comprise a tetraalkyorthosilicate, such as tetraethyl-orthosilicate, or tetramethylorthosilicate.

The tetraalkyorthosilicate suitably is hydrolyzed in a solvent medium comprising an aqueous solution of an organic alcohol, such as a C^Ce alcohol. Following the hydrolysis in which the tetraalkylorthosilicate reacts to form the corresponding silanol, the silanol product is condensed to form polysilicate as a dispersed phase component of the resulting sol gel dispersion.

For sol gel formation of titania or alumina overlayers, the sol gel may be formed as a dispersion of titanium alkoxide or aluminum alkoxide, respectively, in solvent solutions such as those described above with respect to polysilicate sol gel dispersions.

Once applied to the oxidizable metal coating, by any suitable method, such as for example dipping (tub sizing) , spraying, roller coating, brushing, and the like, the sol gel dispersion is dried to remove the organic and aqueous solvents (along with any volatile products of the condensation reaction, in the case of the aforementioned polysilicate sol gel dispersion) therefrom, to yield the insulative layer as a dry coating layer on the substrate.

It will be appreciated that the thickness of the respective oxidizable metal layer and insulative layer may be varied widely and independently of one another, subject of course to the requirement that the oxidizable metal coating is present at a sub-micron thickness on the non-rconductive substrate, to provide respective layers most appropriately dimensioned to the end use application intended for the coated product article. 51 In general, it will be satisfactory to provide the insulative layer at a thickness of from about 200 to about 2500 Angstroms, with insulative layer thicknesses of from about 200 to about 1000 Angstroms being generally satisfactory in chaff applications. The preferred insulative layer formation by sol gel techniques may be widely varied in character, as known to those skilled in the art, to produce an insulative layer of a desired composition, morphology, and physical characteristics.

In the case of the preferred polysilicate, titania, and/or alumina materials, the sol gel dispersion may suitably comprise the insulative material constituent (or a precursor thereof) in an aqueous solution of an alkanol such as ethanol, as the solvent component of the sol gel mixture. After the sol gel dispersion is coated on the oxidizable metal coating, the coated article may be passed through a dehydration furnace to effect drying of the sol gel coating.

The dried sol gel coating has a porous microstructure. The temperature of the drying step, and the other drying conditions, may be appropriately selected to partially collapse the pores of the coating to control its hardness and other physical and performance properties. Thus, temperatures sufficiently high to cause microstructural changes such as pore collapse can be achieved by appropriate drying conditions, to tailor the morphology of the insulative layer so that an overcoat layer of the desired characteristics is achieved. The porosity of the insulative layer is readily determinable by standard porosimetry techniques, so that one of ordinary skill may easily determine the sol pH, drying, and any heat treatment conditions necessary to obtain a desired porosity, without undue experimentation.

It is within the purview of the present invention to modify the chemical composition of the sol gel dispersion to provide 52 covalent or associate bonding of the oxidizable metal coating to the insulative layer.

In the broad practice of the present invention, the oxidizable coating formed on the non-conductive substrate may optionally be "sulfurized, ·* i.e., have sulfur associated therewith, before, during, and/or after the application of the oxidizable metal coating to the substrate, as described in connection with the third embodiment of the invention.

Subsequent to application to the substrate of the oxidizable metal coating of the desired thickness, and optional sulfurization thereof, but prior to application of the insulative layer overcoat thereon, the oxidizable metal-coated substrate may optionally be coated or "doped" with a discontinuous coating of a "porous metal" which is galvanically effective to promote the corrosion of the oxidizable metal, on the external surface of the oxidizable metal coating, as described hereinabove with reference to the second embodiment of the invention.

As a further optional treatment of the oxidizable metal-coated substrate, which may be employed with or without the aforementioned optional sulfurization of the oxidizable metal coating, and with or without the aforementioned optional application of a promoter metal, the oxidizable metal-coated substrate may be further coated or "doped" with a suitable amount, for example from about 0.005 to about 25% by weight, based on the weight of oxidizable metal in the oxidizable metal coating, of a salt, as described hereinabove in connection with the first embodiment of the invention.

After the oxidizable metal-coated substrate article is overcoated with the microporous insulative layer, the resulting pvercoated article then may be hermetically sealed for subsequent use. 53 It is to be recognized that the sulfurization of the oxidizable metal coating, the salt coating, and the promoter metal coating, are each optional treatment steps, one or more of which may be carried out as desired in a given application. None of these optional steps are required in the broad practice of the fourth embodiment, but merely represent additional coating treatments which may be carried out prior to insulative layer overcoating, to further enhance the oxidization of the oxidizable metal film on the substrate under corrosion-producing conditions.

Referring now to the drawings, Figure 19 is an electron photomicrograph, at a magnification of 3000 times, of a tow of sulfurized iron-coated glass filaments. Each of the coated filaments comprises an oxidizable iron coating on the exterior surface of the substrate glass filament, with the iron coating having been sulfurized by hydrogen sulfide contacting between successive depositions of iron in a multizone heating/coating chemical vapor deposition system.

The scale of the electron photomicrograph in Figure 1 is shown by the line in the right central portion at the bottom of the photograph, representing a distance of 3.33 microns.

The glass filaments employed in the tow of coated fiber shown in Figure 19 were of lime aluminoborosilicate composition, commercially available as E-glass (Owens-Corning D filament) 54% Si02; 14.0% A1203; 10.0% B203; 4.5% MgO; and 17.5% CaO) having a measured diameter of 4.8 microns, and were coated with an iron coating of 0.075 micron thickness.

The iron-coated filaments then were overcoated with a film of polysxlicate representing approximately 0.7% by weight, based on the total weight of the fiber. The polysilicate was applied from a 1% solution of hydrolyzed tetraethylorthosilicate in an 54 aqueous ethanol solution. The thickness range of the polysilicate overcoat was in the range of about 0.02 to about 0.1 micron, with microporosity in the range of from about 0.005 to about 0.10 micron.

Figure 20 is an electron photomicrograph of discrete fibers of the type shown in Figure 19, at a magnification of 4000 times, and Figure 21 is an enlarged view of the portion of the Figure 20 electron photomicrograph demarcated by the rectangular boundary in the left central portion thereof. As shown in Figures 20 and 21, the polysilicate coatings are smooth, adherent, and continuous in appearance, while being microporous.

Figure 22 is a graph of tow resistance, in ohms/cm, as a function of time of exposure, in hours, to 50% relative humidity conditions, for fiber tows which comprised approximately 4.8 diameter glass filaments as the substrate elements, on which were coated 0*075 micron thicknesses of iron. One such tow was overcoated with a sol gel-applied layer of polysilicate ("Sol Gel Coat") , while the other tow was retained in a non-overcoated condition ("Standard") .

The data of Figure 22 show that the non-overcoated metallized filaments ("Standard") maintained a relatively constant resistance over the full 100 hour period of exposure. By contrast, the polysilicate-overcoated metallized filaments ("Sol-Gel Coat") exhibited an increase in resistance of approximately 73% over the 100 hour exposure period.

Figure 23 is a bar graph of initial tow resistance, in ohms/cm, for a tow of polysilicate-overcoated iron-coated glass filaments of the type previously described in connection with Figure 19 (0.7 weight percent polysilicate overcoated iron-coated glass filaments, wherein the percent weight of polysilicate is based on total coated fiber weight) , and a corresponding second 55 tow in which the overcoating thickness was increased to provide 2.6 weight percent polysilicate on the iron-coated glass filaments. These overcoated filament tows were compared against a corresponding tow of iron-coated fibers, devoid of any overcoating layer thereon ("0 WT% SG on Fe/GL") .

The initial resistance of these respective fiber tows was measured, with the value being shown by the bars in Figure 24. The non-overcoated filament tow had 500 ohms/cm initial resistance, while the 0.7 weight percent polysilicate-over-coated metallized filament tow had a resistance on the order of about 3000 ohms/cm, and the 2.6% polysilicate-overcoated metallized filament tow had an initial resistance of approximately 15,000 ohms/cm.

Figure 24 is a graph of current, in amperes, as a function of voltage, for three fiber tows. The first fiber tow (".075 Fe/GL") comprised approximately 4.8 micron diameter glass filaments as the substrate elements, on which were coated 0.075 micron thicknesses of iron, but these filaments were not overcoated with any insulating material layers. The second tow ("SG/Fe/GL") comprised filaments coated with iron, of the same type as the first tow, but which additionally were overcoated with a polysilicate coating, at 0.7% by weight polysilicate coating, based on the total weight of the coated fiber. The third tow ("4x SG/Fe/GL") comprised iron-coated filaments of the same type of the first tow, but which were overcoated with polysilicate at 2.6% by weight of polysilicate, based on the total weight of the coated fiber.

The data in Figure 24 sho that the more heavily overcoated tow of metallized filaments had a higher resistance than the corresponding fiber tow ("SG/Fe/GL") with a low polysilicate overcoat thickness (resistance being the slope of the current versus voltage curve) , but event at the higher insulated coating 56 thickness, a small amount of current still passed through the tow. This is possibly due to the absorbed surface moisture acting as a means of conduction between metal coating areas exposed through pores of the overcoating.

Attempts to determine break-down voltage under atmospheric conditions of these polysilicate overcoated samples indicated slight insulating character. Inspection of low voltage data in Figure 24 shows that potentials of greater than 3 volts were required to create an ohmic response, i.e., a linear relationship between current and voltage. The deviation from linearity in the non-overcoated sample (".075 Fe/GL") at high voltages in Figure 24 is hypothesized to be due to oxidation caused by ohmic heating. The microporous overcoat layer provided a coating of higher, but measurable, resistance. The passage of current through this microporous layer may be controlled by the concentration of ionic conductors and the moisture content of the coating. The thinner overcoat ("SG/Fe/GL") shows some evidence of breakdown at about 13 volts, as evidenced by the change in slope for the*appertaining curve. No point of breakdown is seen for the more heavily insulated sample at the voltage studied.

Thus, to control the oxidizable metal coating exposure and its rate of oxidation, the porosity of the inorganic insulating layer on the oxidizable metal coating is controllable. The use of sol-gel overcoated layers may be an effective method for providing an insulative layer on the oxidizable metal coating, if a modest increase in the density of the overall product article is acceptable. The presence of the insulating layer may protect electrical and electronic equipment while corrosion of the oxidizable metal coating takes place.

The microporous overcoat layers discussed above with reference to Figures 22-24, although insulative in character, did not fully preclude conductivity of the coated fibers in tow form, 57 but did accommodate accelerated corrosion of the oxidizable metal coating on the product article, at high relative humidity. While not wishing to be bound by any theory as regards the nature and efficacy of the overcoated metallized articles of the present invention, it is believed that microporously absorbed water played a key role in the conductivity and corrosion characteristics which were observed. Densification of the overcoat layer may be employed to selectively inhibit corrosion of the oxidizable metal coating and more fully insulate the conductive fiber.

In order to measure the tow resistance of the respective fibers, as employed to generate the data plotted in the graphs of Figures 22-24 hereof, each tow under evaluation was mounted on a copper contact circuit board with a known spacing, in either a two-point or four-point arrangement. Electrical contact was assured through use of conductive silver paint. Fiber tows were analyzed by use of a digital multimeter. A known voltage was applied across the fiber circuit. The resulting current was metered and the resistance computed.

This measurement was repeated periodically over the fiber lifetime of interest, with voltage being applied during each interval for a duration just long enough to allow measurement to be made.

Thus, the life of the conductive oxidizable metal coating may be controllably adjusted by selectively varying the thickness, density, composition, and porosity characteristics of the inorganic overcoating layer, and optionally by sulfurizing the conductive oxidizable metal coating, and/or providing a discontinuous coating of a promoter metal on the oxidizable metal film, and/or doping the oxidizable metal coating with a salt. In chaff applications, such selective overcoating, and optional sulfurization, salt doping, and/or promoter metal coating of the 58 oxidizable metal film may be utilized to correspondingly adjust the service life of the oxidizable metal-coated chaff fibers, consistent with a desired retention of the initial radar signature characteristic thereof for a given length of time, followed by rapid dissipation of the radar signature of such "evanescent chaff" material.

The features and advantages of the present invention are more fully shown with reference to the following non-limiting examples, wherein all parts and percentages are by weight, unless otherwise expressly stated.

Example I A calcium aluminoborosilicate fiberglass roving material (E-glass, Owens-Corning D filament) , comprising glass filaments of approximately 4.8 microns measured diameter and a density of approximately 2.6 grams per cubic centimeter, was desized under nitrogen atmosphere to remove the size coating therefrom, at a temperature of approximately 700°C. Following desizing, the filament roving at a temperature of approximately 500°C was passed through a chemical vapor deposition chamber maintained at a temperature of 110°C. The chemical vapor deposition chamber contained 10% iron pentacarbonyl in a hydrogen carrier gas. The fiber roving was passed through heating and coating deposition zones in sequence, for a sufficient number of times to deposit a coating of elemental iron at approximately 0.075 micron thickness on the fiber substrate of the roving filaments.

Subsequent to iron coating formation, the roving was passed through a solution bath containing 2% by weight of iron (III) chloride in methanol solution, under nitrogen atmosphere. The roving then was passed through a drying oven at a temperature of approximately 100°C under nitrogen atmosphere, to remove the methanol solvent and leave a salt coating of iron (III) chloride 59 on the iron film. The salt-doped, iron-coated roving then was packaged under nitrogen atmosphere in a moisture-proof package.

Example II A calcium aluminoborosilicate fiberglass roving material (E-glass, Owens-Corning D filament) comprising glass filaments having a measured diameter of 4.8 microns and a density of 2.6 grams per cubic centimeter, was desized under nitrogen atmosphere to remove the size coating therefrom, at a temperature of approximately 700°C. Following desizing, the filament roving at a temperature of approximately 500°C was passed through a chemical vapor deposition chamber maintained at a temperature of 110°C. The chemical vapor deposition chamber contained 10% iron pentacarbonyl in at hydrogen carrier gas. The fiber roving was passed through heating and coating deposition zones in sequence, for a sufficient number of times to deposit a coating of elemental iron a approximately 0.075 micron thickness on the fiber substrate of the roving filaments.

Subsequent to iron coating, the roving was passed through a chemical vapor deposition chamber to which a gas stream of approximately 50-80% by weight copper hexafluoroacetylacetonate in hydrogen carrier gas was supplied, resulting in deposition of copper islands whose dimensional size characteristics, as measured along the surface of the iron coating, were in the range of from about 0.5 to about 10 microns. The resulting copper-coated, iron-coated roving then was packaged under nitrogen atmosphere in a moisture-proof package.

Example III In this example, an oxidizable iron coating was applied to a silicate fiberglass roving material, and then coated with a discontinuous coating of copper, as described in Example II. 60 Subsequent to the formation of deposited copper islands on the iron coating, the roving was passed through a solution bath containing 2% by weight of iron (III) chloride in methanol solution, under nitrogen atmosphere. The roving then was passed through a drying oven at a temperature of approximately 100°C under nitrogen atmosphere, to remove the methanol solvent and leave a salt coating of iron (III) chloride on the copper-coated, iron-coated substrate. The salt-doped, copper-coated, iron-coated roving then was packaged under nitrogen atmosphere in a moisture-proof package.

Example IV A calcium aluminoborosilicate fiberglass roving material (E-glass, Owens-Corning D filament) , comprising glass filaments having a measured diameter of approximately 4.8 microns and a density of approximately 2.6 grams per cubic centimeter, was desized under nitrogen atmosphere to remove the size coating therefrom, at a temperature of approximately 700°C. Following desizing, the filament roving at a temperature of approximately 500°C was passed through a chemical vapor deposition chamber maintained at a temperature of 110°C. The chemical vapor deposition chamber contained 10% iron pentacarbonyl in hydrogen carrier gas. The fiber roving was passed through heating and coating deposition zones in sequence, comprising five coating deposition zones, to deposit a coating of elemental iron of approximately 0.075 micron thickness on the fiber substrate of the roving filaments.

In the heating zone upstream of the second and succeeding chemical vapor deposition coating zones in the process system, the fiber coated with iron film in the preceding coating chamber was exposed to 10% hydrogen sulfide in hydrogen carrier gas mixture (the percentage being based on the total weight of hydrogen sulfide and hydrogen) , at a temperature of 450°C-600°C, 61 to reduce the previously applied iron film and incorporate sulfur-containing material in the film. As a result, the sulfur loading of the oxidizable iron film was about 0.1% by weight sulfur (measured as elemental sulfur) , based on the weight of elemental iron in the oxidizable iron coating on the glass filament substrate.

Example V The sulfurized iron-coated filament roving of Example IV was passed through a chemical vapor deposition chamber to which a gas stream of approximately 50% to 80% by weight copper hexa-fluoroacetylacetonate in carrier gas was supplied, resulting in deposition of copper islands whose dimensional size characteristics, as measured along the surface of the iron coating, were in the range of from about 0.5 to about 10 microns. The resulting copper-coated, sulfurized iron-coated roving then was packaged under nitrogen atmosphere in a moisture-proof package.

Example VI In this Example, an oxidizable iron coating was applied to a glass filament roving material, which was sulfurized during the iron coating process, and then coated with a discontinuous coating of copper, as described in Example V. Subsequent to the formation of deposited copper islands on the iron coating, the roving was passed through a solution bath containing 2% by weight of iron (III) chloride in methanol solution, under nitrogen atmosphere. The roving then was passed through a drying oven at a temperature of approximately 100°C under nitrogen atmosphere, to remove the methanol solvent and leave a salt coating of iron (III) chloride on the copper-coated, sulfurized iron-coated substrate. The salt-doped, copper-coated, sulfurized iron-coated 62 roving then was packaged under nitrogen atmosphere in a moisture-proof package.

Example VII An aluminoborosilicate fiberglass roving material (E-glass, Owens-Corning D filament) , comprising glass filaments having a measured diameter of approximately 4.8 microns and a density of approximately 2.6 grams per cubic centimeter, was desized under nitrogen atmosphere to remove the size coating therefrom, at a temperature of approximately 700°C. Following desizing, the filament roving at a temperature of approximately 500°c was passed through a chemical vapor deposition chamber maintained at a temperature of 110°C. The chemical vapor deposition chamber contained 10% iron pentacarbonyl in a hydrogen carrier gas. The fiber roving was passed through heating and coating deposition zones in sequence, comprising five coating deposition zones, to deposit a coating of elemental iron of approximately 0.075 micron thickness on the fiber substrate of the roving filaments.

The roving material was sulfurized during the iron coating process as in Example IV, and then coated with a discontinuous coating of copper as described in Example II. Subsequent to the formation of deposited copper islands on the iron coating, the roving was passed through a solution bath containing 2% by weight of iron (III) chloride in methanol solution, under nitrogen atmosphere. The roving then was passed through a drying oven at a temperature of approximately 100°C under nitrogen atmosphere, to remove the methanol solvent and leave a salt coating of iron (III) chloride on the copper-coated, sulfurized -iron-coated substrate. The salt-doped, copper-coated, sulfurized iron-coated roving then was packaged under nitrogen atmosphere in a moisture-proof package. 63 Example VIII In Brinker, et al, J. Non-Cryst. Solidsf Vol. 48, 1982, pages 47-64, methods are described for making gels which result in various microstructures, using a two-step hydrolysis procedure in which relative rates of hydrolysis and condensation are varied. Microstructure development by these methods is related to gel formation which depends on (a) hydrolysis of alkoxide groups to form silanols, (b) condensation of silanols to form silicate polymers, and (c) linking of polymers to form gels.

The relative rates of these steps (a) -(c) depend on the concentration of water and the tetraalkylorthosilicate in the reaction system, and the pH of-the reaction volume.

A sol gel dispersion was prepared according to the formulation set out in Table I below, to duplicate Sample A3 described in the Brinker, et al article.

Table I Component Concentration. Mole % Tetraethylorthosi1icate 6.1 Water 75.5 N-propanol 18.4 HC1 0.005 Following the procedure in the Brinker, et al article, the silicate starting material, alcohol, water and acid were initially mixed in the mole ratio of 1:3:1:0.007, as a mixture of 22 grams propanol, 22.4 grams silicate, 1.9 grams water, and 0.0026 gram acid. 64 This initial mixture was stirred for 1.5 hours at approximately 60°C. 16.5 milliliters of water were added and the mixture was stirred at room temperature for approximately 5 hours.

The resulting sol gel dispersion was contacted with a fiber roving of iron-coated glass filament prepared as in Example I, with the fiber roving being dipped into a container of the sol gel dispersion. The wetting of the iron coating with the sol gel dispersion appeared good, and the coated fiber roving was dried overnight at 200°C under nitrogen atmosphere. The polysilicate overcoated metallized roving of glass filaments then is packaged under nitrogen atmosphere in a moisture-proof package.

Example IX A sulfurized iron-coated filament roving is prepared as in Example IV, and then overcoated with a polysilicate layer according to the procedure of Example VIII. The resulting roving then is packaged under nitrogen atmosphere in a moisture-proof package.

Example X A copper-coated, sulfurized iron-coated roving overcoated with a polysilicate layer is prepared in accordance with Example II and Example VIII, with respect to the metallization and insulative coating thereof. The resulting polysilicate overcoated, copper-coated, sulfurized iron-coated roving then is packaged under nitrogen atmosphere in a moisture-proof package.

Example XI In this Example, a salt-doped, copper-coated, sulfurized iron-coated roving formed by the method of Example VII is coated 65 with a sol gel dispersion of polysilicate and dried as in Example VIII to form a polysilicate-overcoated, salt-doped, copper-coated, sulfurized iron-coated roving, which then is packaged under nitrogen atmosphere in a moisture-proof package.

Best Mode for Carrying Out The Invention The best mode of carrying out the invention, regardless of which of the first, second, third, and/or fourth embodiments thereof is employed, utilizes glass filament as a chaff substrate element on which a sub-micron thickness of iron is coated, wherein the filament is from about 2 to about 15 microns in diameter.

In the first embodiment, the salt preferably is a metal halide salt wHose halide constituent is chlorine, e.g., sodium chloride, present at a weight of from about 0.5% to about 15% by weight, based on the weight of the oxidizable iron coating.

In the second embodiment, the second metal preferably is copper and the discontinuous copper coating is present on the iron coating in an amount of from about 0.1% to about 10% by weight of the total coating (iron coating and discontinuous copper coating) .

In the third embodiment, the sulfurized iron coating preferably comprises from about 0.05% to about 2% by weight of sulfur, based on the weight of oxidizable iron in the oxidizable iron coating on the glass filament substrate element.

In the fourth embodiment, the microporous insulative layer preferably is formed of a material selected from the group consisting of polysilicate, titania, alumina, and combinations thereof, and is characterized by an average pore size of from 66 about 100 to about 500 Angstroms, and an insulative layer thickness of from about 200 to about 1000 Angstroms.

Each of the respective first, second, third, and fourth embodiment "best modes" may be used independently, as well as in combinations of two or more enhancements from these embodiments (i.e., the iron-coated glass filament may be treated to enhance the oxidization of the iron coating by using the oxidation enhancement features of two or more of these embodiments, on the filament.

Industrial Applicability of The Invention The present invention has particular industrial applicability, when utilized in a form comprising fine diameter substrate elements such as glass or ceramic filaments, as an evanescent chaff. In the presence of atmospheric moisture, such evanescent chaff undergoes oxidation of the oxidizable metal coating so that the radar signature of the chaff transiently decays. The chaff may thus be used as an electronic warfare countermeasure useful as an electromagnetic detection decoy or for anti-detection masking of an offensive attack.

In addition to chaff applications, the utility of the present invention extends to other applications in which a temporary conductive coating on a substrate is desired. Examples of such other applications include moisture sensors, corrosivity monitors, moisture barrier devices, and the like.

While preferred and illustrative embodiments of the invention have been described, it will be appreciated that numerous modifications, variations, and other embodiments are possible, and accordingly, all such modifications, variations, and embodiments are to be regarded as being within the spirit and scope of the present invention.

Claims (46)

CLAIMS ; -67- 97675/2

1. An article comprising a non-conductive substrate having a sub-micron thickness of an oxidizable metal i coating thereon, i) a second metal which is galvanically effective to promote corrosion of the oxidizable metal coating, discontinuously coated on the oxidizable metal coating; and optionally one, two or all of: ii) an oxidation-enhancingly effective amount of salt on the oxidizable metal coating; and iii) the oxidizable metal coating being sulfurized; and iv) the oxidizable metal coating being overcoated with a microporous layer of an electrically insulative material.

2. An article according to claim 1, wherein the non-conductive substrate is formed of a material selected from the group consisting of glasses, polymers, pre-oxidized carbon, non-conductive carbon, and ceramic materials.

3. An article according to claim 1, wherein the non-conductive substrate is formed of a glass material.

4. An article according to claim 1, wherein the non-conductive substrate is in the form of a filament.

5. An article according to claim 4, wherein the filament has a diameter of from about 0.5 to about 25 microns. 68

6. An article according to claim 1, wherein the oxidizable metal coating is selected from the group consisting of iron, nickel, copper, tin, zinc, and oxidizable alloys thereof.

7. An article according to claim 1, wherein the oxidizable metal coating is formed of iron or ferrous metal.

8. An article according to claim l, wherein the oxidizable metal coating has a thickness of from about 2 x 10"3 to about 0.25 micron.

9. An article comprising a non-conductive substrate having a sub- micron thickness of an oxidizable metal coating thereon, and an oxidation enhancingly effective amount of salt on the oxidizable "metal coating.

10. An article according to claim 9, wherein the salt is present on the oxidizable metal coating at a loading of from about 0.005% to about 25% by weight, based on the weight of oxidizable metal coated on the non-conductive substrate.

11. An article according to claim 9, wherein the salt is selected from the group consisting of metal salts and organic salts.

12. An article according to claim 9, wherein the salt is a metal salt selected from the group consisting of metal halides, metal nitrates, and metal sulfates.

13. An article according to claim 9, wherein the salt is an organic salt selected from the group consisting of citrate, acetate, and stearate salts. 69

14. An article according to claim 9, wherein the salt is selected from the group consisting of lithium chloride, iron (III) chloride, zinc chloride, sodium chloride, and copper sulfate.

15. An article according to claim 9, comprising from about 0.01% to about 20% by weight of salt, based on the weight of oxidizable metal, on the oxidizable metal coating.

16. An article comprising a non-conductive substrate having a sub-micron thickness of an oxidizable, continuous conductive first metal coating thereon, and a second metal which is galvanically effective to promote the corrosion of the first metal, discontinuously coated on the first metal coating. - ■■■·-■

17. An article according to claim 16, overcoated with a salt.

18. An article according to claim 16, wherein the second metal is selected from the group consisting of cadmium, cobalt, nickel, tin, lead, copper, mercury, silver, and gold.

19. An article according to claim 16, wherein the second metal is present at a concentration of from about 0.1 to about 10% by weight, based on the weight of first metal coated on the substrate.

20. An article according to claim 16, wherein the second metal is copper.

21. An article comprising a non-conductive substrate having a sub-micron thickness of a sulfurized oxidizable conductive metal coating thereon.

22. An article according to claim 21, wherein the sulfurized oxidizable metal coating has a salt coated thereon.

23. An article according to claim 21, wherein the sulfurized oxidizable metal coating is sulfurized with from about 0.01 to about 10% by weight, based on the weight of oxidizable metal in the oxidizable metal coating, of a sulfur-containing material.

24. An article comprising a non-conductive substrate which is coated with a sub-micron thickness of an oxidizable conductive metal and overcoated with a microporous layer of an inorganic electrically insulative material.

25. An article according to claim 24, wherein the oxidizable metal coating has a salt coated thereon.

26. An article according to claim 24, wherein the oxidizable metal coating is sulfurized.

27. An article according to claim 24, wherein the microporous layer of inorganic electrically insulative material is formed of a material selected from the group consisting of glasses, ceramics, and combinations thereof.

28. An article according to claim 24, wherein the microporous layer of inorganic electrically insulative material is formed of a material selected from the group consisting of polysilicate, titania, alumina, and combinations thereof.

29. An article according to claim 24, wherein the oxidizable metal 'coating is discontinuously coated with a second metal which is galvanically effective for corrosion of the oxidizable metal coating.

30. ; An article comprising a boria substrate having coated thereon a sub-micron thickness of an oxidizable metal coating. -71- 97675/2

31. A method of forming an evanescent conductive coating on a non-conductive substrate, comprising: i) depositing on the substrate a sub-micron thickness of oxidizable metal; ii) applying to the oxidizable metal-coated substrate a discontinuous coating of a second metal which is galvanically effective to promote the corrosion of the oxidizable melt coating on the substrate; iii) optionally performing one, two or all of the following steps: A) providing on the oxidizable metal-coated substrate an oxidation enhancingly effective amount of a salt; B) sulfurizing the oxidizable metal coating deposited on the substrate; and C) overcoating the oxidizable metal coating deposited on the substrated with a microporous layer of an inorganic electrically insulative material.

32. A method of forming an evanescent conductive coating on a non-conductive substrate, comprising: (a) depositing on the substrate a sub-micron thickness of oxidizable metal, to form an oxidizable metal-coated substrate; and (b) providing on the oxidizable metal-coated substrate an oxidation enhancingly effective amount of a salt for enhancing the oxidization of the oxidizable metal, to yield a salt-doped, oxidizable metal-coated substrate.

33. A method according to claim 32, wherein the salt is provided on the oxidizable metal-coated substrate by contacting of the oxidizable metal-coated substrate with a solvent solution of the salt, to form a salt solution coated, oxidizable metal-coated substrate, and drying the salt solution coated, oxidizable metal-coated substrate to remove the solvent from the salt solution, to yield the salt-doped, oxidizable metal-coated substrate article.

34. A method of forming an evanescently conductive coating on a non-conductive substrate, comprising: (a) depositing on the substrate a sub-micron thickness of an oxidizable conductive first metal, to form a first metal-coated substrate; and (b) applying to the first metal-coated substrate a discontinuous coating of a second metal which is galvanically effective to promote the corrosion of the first metal coated on the substrate, thereby forming a second metal-doped, first metal-coated substrate.

35. A method according to claim 34, comprising the further step of: (c) applying to the second metal-doped, first metal-coated substrate a salt at a concentration of from about 0.005% to about 25% by weight of salt, based on the weight of first metal on the substrate, to form a salt- modified, second metal-doped, first metal-coated substrate. 73

36. A method according to claim 34, wherein the promoter metal is selected, based upon galvanic properties, from the group consisting of cadmium, cobalt, nickel, tin, lead, copper, mercury, silver, and gold.

37. A method of forming an evanescent conductive coating on a non-conductive substrate, comprising: (a) depositing on the substrate a sub-micron thickness of oxidizable metal, to form an oxidizable metal-coated substrate; and (b) sulfurizing the oxidizable metal coating deposited on the substrate.

38. A method according to claim 37, wherein said sulfurization is effected by exposure of the oxidizable metal coating to hydrogen sulfide.

39. A method according to claim 37, wherein said sulfurization of the oxidizable metal coating is effected by application of a sulfur-containing compound to the oxidizable metal coating.

40. A method according to claim 37, further comprising applying to the sulfurized oxidizable metal-coated substrate a discontinuous coating of a promoter metal which is galvanically effective to promote the corrosion of oxidizable metal in the oxidizable metal coating.

41. A method according to claim 37, comprising the further ste of: (c) applying to the sulfurized oxidizable metal-coated substrate a salt at a concentration of from about 0.005 to about 25% by weight of salt, based on the weight of 74 oxidizable metal in the oxidizable metal coating on the substrate, to form a salt-doped, sulfurized oxidizable metal-coated substrate.

42. A method of forming an evanescent conductive coating on a non-conductive substrate, comprising: (a) depositing on the substrate a sub-micron thickness of oxidizable metal, to form an oxidizable metal-coated substrate; and (b) overcoating the oxidizable metal coating deposited on the substrate with a microporous layer of an inorganic electrically insulative material.

43. A method according to claim 42, wherein the oxidizable metal coating deposited on the substrate is sulfurized prior to overcoating with said microporous layer of inorganic electrically insulative material.

44. A method according to claim 42, further comprising applying to the oxidizable metal-coated substrate a discontinuous coating of a promoter metal which is galvanically effective to promote the corrosion of oxidizable metal in the oxidizable metal coating.

45. A method according to claim 42, comprising the further step of: (c) applying to the oxidizable metal-coated substrate a salt at a concentration of from about 0.005 to about 25% by weight of salt, based on the weight of oxidizable metal in the oxidizable metal coating on the substrate, to form a salt-doped, oxidizable metal- coated substrate. 75

46. A method of reflecting radar for a selected time interval comprising dispersing in effective proximity to a radar source an effective amount of a chaff comprising a plurality of metal-coated fiber articles according to claim 1. COHEN ZEDEK & RAPAPORT p 0. Box 3311* , Tel-Av iv Attoreets tor kffUcAi