US3356525A

US3356525A - Metal carbide formation on carbon fibers

Info

Publication number: US3356525A
Application number: US324477A
Authority: US
Inventors: Carlos L Gutzeit
Original assignee: HITCO CORP
Current assignee: HITCO Corp
Priority date: 1963-11-18
Filing date: 1963-11-18
Publication date: 1967-12-05
Anticipated expiration: 1984-12-05
Also published as: GB1025499A; DE1469489A1

Description

Dec. 5, 1967 c. 1.. GUTZEIT 3,356,525

METAL CARBIDE FORMATION ON CARBON FIBERS Filed Nov. 18, 1963 900 /000 /292 I472 I652 I832 ma'c 200 2/2; 392

INVENTOR. Carlos A. uizeif l/r ornays United States Patent 3,356,525 METAL CARBIDE FORMATION 0N CARBON FIBERS Carlos L. Gntzeit, Long Beach, Calif., assignor to Hitco Corporation, a corporation of California Filed Nov. 18, 1963, Ser. No. 324,477 12 Claims. (Cl. 117-46) The present invention generally relates to improvements in fibers and more particularly relates to improved oxidation-resistant carbon fibers and to a method of improving the oxidation resistance of carbon fibers.

Amorphous carbon fibers offer improved properties for various uses, in contrast to some older types of fibers. Thus, carbon fibers have been widely accepted in the missile propulsion field inasmuch as they combine the excellent ablation characteristics of graphite with a lower thermal conductivity than graphite.

Carbon fibers are particularly suitable for high temperature use in that they sublime at an extremely high temperature, in excess of 6600 F., and because their strength increases with temperature. Moreover, the carbon resists chemical attack except highly oxidizing substances at elevated temperatures, for example, above 800 F. In addition, the electrical resistance of carbon fibers can vary according to their diameter, surface area, etc., so that such carbon fibers can act either as electrical insulators or as electrical conductors for special applications.

Since carbon fibers are flexible and readily formable, and since they do not smear or rub oif as do graphite fiber and cloth, they can be readily and conveniently fabricated into a wide variety of products, including cloth or fabric, sleeving, roving, tape, felt, batts and bulk fiber products, using fibers of uniform or of varying lengths. The fibers are available in diameters ranging up to about 0.005 inch or more and in high purity. For example, a typical amorphous carbon fiber analysis is the following:

Carbon-95.2% by weight; hydrogenl.0% by weight; oxygen3.3% by weight; impurities0.5% by weight (composed mainly of nickel, copper and tin, with trace quantities of calcium, iron, silicon, magnesium and titanium).

Carbon fibers can be prepared, for example, in accordance with the teachings of copending US. patent application, Ser. No. 160,605, now Patent No. 3,294,489, filed Dec. 19, 1961 and entitled Process for Preparing Carbon Fibers, of which Richard D. Millington et al. are the inventors, said application having been assigned to the assignee of the present invention. As described in that patent application, non-thermoplastic cellulosic fiber, such as rayon, wool, etc., is scoured to remove any finishing material, rinsed free of scouring solution, dried and then fired in a closed atmosphere at controlled successively higher temperatures to drive ofi volatiles and carbonize the same without destroying their fibrous nature. During such treatment, the temperature increases from about 300 F. to less than about 1000 F. in increments of 20- 50 F. at 8-30 hour intervals. Thereafter, a flash firing operation under non-oxidizing conditions is carried out at, for example about 1800-2000 F. to drive off remaining volatiles and heat shrink the fibers, after which the fibers are cooled to below 300 F. under non-oxidizing conditions. Without such controlled sequential heating and flash firing of the fibers during processing, the carbon fibers would tend to disintegrate into separate masses, and laminated structures subsequently built therefrom would be much more likely to delaminate during use at elevated temperatures. Furthermore, the flash firing decreases the reactivity of the carbon fibers.

3,356,525 Patented Dec. 5, 1967 Although carbon fibers produced as per the foregoing description are resistant to most kinds of chemical attack, erosion and change of structure, except at extremely high temperatures at which the fibers sublimate, such fibers still are subject to pronounced deteriorations and rapid Weight loss when exposed to elevated temperatures in excess of about 800 F. in the presence of an oxidizing medium. Under such circumstances, the carbon of the fibers tends to react with the oxygen of the oxidizing medium (e.g.

atmosphere) to form the carbon oxides, carbon monoxide and carbon dioxide. Accordingly, the basic structure of the fiber volatilizes and is lost.

Accordingly, it is a principal object of the present invention to provide improved oxidation-resistant carbon fibers.

It is also an object ofthe present invention to provide a method of improving the oxidation resistance of amorphous carbon fibers.

It is still a further object of the present invention to provide a simple, effective, low cost method of improving the oxidation resistance of amorphous carbon fibers at temperatures in excess of 800 F.

These and other objects are accomplished, in accordance with the present invention, by providing an improved method which substantially increases the oxidation resistance of amorphous carbon fibers at elevated temperatures, for example, in excess of about 800 F. It Will be understood that the term car-bon fibers herein is directed only to amorphous or essentially amorphous carbon fibers, as contrasted with graphite fibers. The latter fibers have characteristics distinctly different from those of carbon fibers and treatment thereof is not a part of the present invention. The method is simple, inexpensive, rapid and effective.

In accordance with the method, carbon fibers are used which have previously been subjected to controlled sequential firing operations to remove volatiles and which also have been previously subjected to flash firing operations at temperatures of, for example, about 2000 F. or the like, so that remaining volatiles have been removed and the fibers have been heat shrunk. These car-bon fibers are-coated with suitable organic carbon-containing material which also contains metal selected from the group consisting of zirconium, chromium, titanium, nickel and mixtures thereof. The coating step is carried out so that the fibers are substantially completely enclosed in the material, after which the fibers are treated in a manner to form refractory carbides of the metal substantially completely from the carbon and metal of the material.

The carbide is, in accordance with the present method, uniformly distributed over substantially the entire surface of each of the fibers, and acts to protect the same. Moreover, the carbide impregnates the fiber surface, i.e., is disposed in the surface irregularities of the fibers and is permanently bonded to the fibers. When the carbide encased fibers are exposed to an oxidizing environment, such as air at above about 800 F., the carbide of each fiber is converted to the corresponding refractory oxide. The refractory oxide, in turn, does not react with the atmosphere and, accordingly, provides a strong, protective, continuous oxidation-resistant covering over the carbon fibers. Rapid deterioration of the carbon fiber in such environment is thereby avoided. Moreover, the refractory oxide not only functions as a chemical barrier, resisting oxidative deterioration, but also acts as a thermal shield, improving the overall durability of the carbon fiber.

As a specific example, car-bon cloth, which has been manufactured by scouring, rinsing and drying rayon cloth, heating the cloth in a closed atmosphere from 300 up to 1000 F. in 50 increments at 8 hour intervals, and then flash firing the cloth at about 2200 F. for 10 seconds, is treated according to the following procedure:

The resulting flash fired carbon cloth is immersed in an impregnating solution containing 175 gm. of chromium acetate, i.e., suflicient to provide approximately weight percent concentration of chromium carbide in the finished cloth, the weight being based on the metal itself. The carbon cloth absorbs about 0.8 ml. (milliliter) .of impregnating solution per gm. (gram) of the cloth. The carbon cloth is maintained in the solution for 20 minutes, then withdrawn from the solution, drained, air dried at about 250 F. and calcined in a non-oxidizing atmosphere, in this instance nitrogen, although an inert gas such as argon, krypton, neon or the like can be used, for 15-20 seconds at about 2200 F., a temperature sufliciently high to decompose the acetate to chromium carbide. The in situ formed carbide wholly and tightly encloses each carbon fiber, is bonded to the surface thereof and fills the surface irregularities and pores of the carbon fiber. Thereafter, the treated cloth is cooled to ambient temperature in the non-oxidizing atmosphere. Further objects and advantages of the present invention will be apparent from a study of the following detailed description and the accompanying drawings, of which:

The single figure is a graph depicting oxidation resistance of carbon fibers treated by the method of the invention.

Now referring more particularly to the steps of the method of the present invention, carbon fibers in any suitable form, for example, in the form of a mat, batt, felt, fabric, sleeving, roving, tape, or as bulk fibers or the like are treated so as to increase their oxidation resistance.

The carbon fibers to be treated can be produced in any suitable manner, such as that more particularly set forth in the previously described copending United States patent application, Ser. No. 160,605, Millington et al. In any event, the carbon fibers to be treated should already be in finished form, that is, they should have been subjected previously to a flash firing or comparable operation at, for example, about 1800-2000 P. so as to have had essentially all volatiles removed therefrom and so as to have been heat shrunk.

This is a requirement inasmuch as if the treatment were applied to carbon fibers which still contained some moisture or other comparable volatiles, release of such moisture or other deleterious volatiles from the carbon fibers during the calcining step in the present method would have the effect of hydrolyzing (in the case of moisture) or of otherwise adversely affecting the carbide being formed on the surfaces of the fiber so as to cause the loss of carbide from such surfaces. Accordingly, only carbon fibers which are already in the flash fired, heat shrunk, substantially completely devolatilized condition are used in the present method.

Such carbon fibers are contacted in the present method with a selected agent consisting essentially of heat decomposable material containing carbon and metal selected from the group consisting of nickel, chromium, zirconium, titanium and mixtures thereof. Only these metals have been found to be fully effective in preparing the desired carbide coating at relatively low temperature on the surface of the carbon fibers, in accordance with the present method.

It is important to note that heat decomposable material is used which yields a carbon-containing residue at calcining temperature. Thus, the material itself serves as the source of carbon for carbide formation so that the relatively small diameter carbon fibers need not and do not materially contribute carbon to the carbide-forming reaction. Accordingly, the integrity of the fine diameter carbon fibers is retained while still providing in situ carbide formation on the surfaces thereof. If, instead, the source of carbon for the in situ carbide formation were 4 the carbon of the carbon fibers, then such in situ carbide formation would result in a depreciation in the integrity and mass of the carbon fibers. Inasmuch as the carbon fibers usually are of relatively small diameter and, accordingly, large surface area, in such instance an appreciable portion of the total mass of the carbon fibers would be involved in carbide formation from the substance thereof. Upon subsequent use of such fibers in an oxidizing environment at elevated temperature, for example at above 800 F., a substantial proportion of the substance of the carbon fibers would be lost by conversion of the carbide to the corresponding refractory oxide and release of the carbon from the carbide in the form of carbon oxide. Accordingly, the main object in coating the carbon fibers would be defeated, i.e. to protect the substance or mass thereof against oxidation and loss from the fibers in the form of volatile carbon oxides.

In view of the above, the present method provides a source of carbon external of the carbon fibers themselves and disposed in intimate contact with the selected metal in the treating agent so as to assure effective carbide formation from the material of the treating agent.

The selected metals, nickel, titanium, zirconium and chromium, are those which will readily react to form protective carbides by decomposition of organic compounds containing the same at relatively low temperature and which will form carbides which do not detract from desired characteristics of the carbon fibers. With the heat decomposable organic compounds containing the selected metals, decomposition can take place so as to provide the carbide in situ without debilitating the carbon fibers. Moreover, the relatively low temperatures represent a saving in processing costs. Thus, when organic compounds in accordance with the invention are used, decomposition temperatures of the order of about 1000 C. can be effectively employed in contrast to the usual carbonizing temperatures of at least about 1500 C.2000 C.

Any suitable carbon-containing compound which also contains metal selected from the group consisting of chromium, zirconium, titanium, nickel and mixtures thereof can be used. However, those compounds which are readily soluble in a suitable inexpensive solvent or other medium, such as water, are preferred. So also are those compositions which can be concentrated to viscous syrups which dry to glass-like, substantially continuous smooth coatings in contrast to those compounds which readily crystallize and, accordingly, have a tendency to provide a discontinuous island configuration coating, especially over the surfaces of small diameter fibers, such as carbon fibers.

Thus, it is preferred to use an organic material or compound such as an acetate or formate, for example chromium acetate, zirconium acetate, zirconium formate, nickel acetate, nickel formate, titanium. oxide hydrosol, stabilized by acetic acid, and the like.

Chromium acetate and zirconium acetate generally are considered to be acetate complexes rather than true compounds. They can be concentrated to viscous syrups which can be applied to smoothly, uniformly and evenly coat the entire exposed area of the carbon fibers to be treated and can easily penetrate pores in the carbon fiber and fill surface irregularities, so as to improve bondability of the compound to the fiber. In other words, such compounds readily wet and adhere to the carbon fibers. So also do carbides formed in situ from these compounds.

Nickel acetate is usually crystalline but again can be dissolved in a suitable solvent, such as Water, and can be used in that form, Titanium oxide hydrosol stabilized by acetic acid is a colloid which is stable over a limited range of conditions and which sets to an all embracing hydrogel when heated. Selected formates are also preferred, for example, nickel formate which has the formula is a dark green tanning liquid which can also be used in the present invention. It will be understood that any other suitable heat decomposable organic compound which yields carbide of nickel, chromium, zirconium, titanium or mixtures thereof upon heating, can also be used in accordance with the present invention, for example, selected organo-metallic compounds such as metal chelating agents.

The treating agent is usually disposed in a suitable medium, such as a dispersant, solvent or the like so that it can'efiectively uniformly contact and penetrate into the carbon fibers to be treated. For example, zirconium acetate, chromium acetate, nickel acetate, nickel formate, chromium formate and certain other organic compounds are Water soluble. The water can be subsequently removed by evaporation, as by drying the impregnated coated fibers. In the event another solvent or dispersant is used, to which the carbon is inert, for example, ethyl alcohol, etc., depending on the treating agent, a drying step can also be used to remove such solvent from the coating.

The treating agent is present in the medium in any suitable concentration. In this regard, the minimum concentration used is that which provides an essentially continuous refractory carbide coating over the entire exposed surface area of the carbon fibers to be treated upon one or a series of immersions. It has been found that, depending upon the extent of surface area of the fibers, the irregularities in the surface of the fibers and other factors, the minimum concentration of the treating agent will necessarily vary so as to vary the amount of carbide coating enclosing the carbon fibers. Usually, a carbide coating, the metal of which weighs at least about 0.5 percent that of the final product (carbide plus the carbon fibers) will be sufiiciently thick and continuous to effectively protect the underlying carbon fiber against oxidative deterioration at temperatures in excess of 800 F. Carbide concentrations of up to Weight percent or more can be used, based upon the weight of the metal of the carbide in the finished product. However, concentrations greater than about 10 weight percent may result in a substantial change in one or more properties of the carbon fibers, for example, the general appearance and texture of carbon cloth. Moreover, concentrations of carbide of about 2-4 percent, based on the metal thereof, are preferred for maximum efficiency of oxidation resistance and minimum difliculties.

Carbide concentrations of over about 6 weight percent of the finished product offer little improvement of oxidation resistance over concentrations of somewhat less than 6 weight percent. Moreover, there is some small tendency of the product during processing to exhibit increasing brittleness as the carbide concentration increases above 6 weight percent. The increased brittleness and stiffness of the carbon fibers is due, apparently, at least in some instances to the glassy nature of the selected dehydrated metal complex used as the treating agent. The brittleness disappears from the fiber when the metal complex is thermally decomposed to the carbide. However, it is simpler, easier and more economical to use lower concentrations of the metal and avoid the inconvenience and possibility of mechanical rupture due to the stiffness and brittleness of the carbon fibers containing concentrations of the metal complex in excess of about 6 weight percent. Accordingly, for most purposes, the maximum concentration of the carbide in the finished product will be about 6 weight percent, based on the weight of the metal of the carbide.

' In the present method, the carbon cloth is contacted with the treating agent in any suitable manenr, e.g. immersion in the treating agent or a mixture, dispersion or solution, etc. of the treating agent and a suitable medium. Alternatively, the fibers can be sprayed with the treating agent or treating agent-medium or can be passed through 6 a zone wherein the treating agent or treating agent-medium mixture has been vaporized, etc. Still alternatively, the treating agent can be painted on, poured on or otherwise contacted with the carbon fibers. The contacting is 3 carried out so as to impregnate the carbon fibers and coat,

cover or encapsulate essentially all exposed surfaces thereof with the treating agent.

After the desired concentration of treating agent has been absorbed by the carbon fibers, the carbon fibers are removed from contact with the remaining treating agent, as by withdrawing the carbon fibers from an aqueous solution or bath of the treating agent. The carbon fibers are then treated to remove excess treating agent, as by draining the same, and thereafter the carbon fibers are dried or otherwise freed of treating agent medium, if any, and treated so as to bond the treating agent to the surface of the fibers as an enclosing protective cover and so as to set the treating agent. For example, water solvent for zirconium acetate treating agent is removed from the treating agent contained on the surfaces of the carbon fibers, as by air drying at, for example, about 250 F. for a period of about one-half hour in circulating air. The treating agent thereupon sets to a solid glassy cover comprising zirconium acetate.

Carbon fibers thus impregnated, coated and wholly enclosed within the treating agent are then heat treated (cal cined) to convert the treating agent to the desired carbide or carbides by decomposition of the treating agent to a carbon and metal-containing carbide-forming residue. In this regard, the dried carbon fibers containing a uniform coating of the treating agent disposed over the surface thereof are calcined under non-oxidizing conditions, e.g. nitrogen, hydrogen, helium, argon, etc. at a temperature in excess of about 1000 C. (about 1800 F.) for a sufficient period of time to substantially completely decompose the carbon-containing treating agent and form the desired selected carbide or carbides in situ on the fibers. The calcining temperature can be substantially lower than that temperature which would be required to react the pure refractory met-a1, or metal hydride or oxide with carbon to form the carbide (carburizing temperatures). It will be noted that the calcining temperature at which the carbide is formed in situ in the present method should be maintained below that at which the carbon is converted into graphite fiber (graphitizing temperature), because the thermal conductivity of graphite fiber is substantially higher than that of carbon fiber and, for high temperature applications, less desirable.

The thus prepared selected carbide coated carbon fiber product is then cooled to ambient temperature and is then ready for use. The cooling step can take place under any suitable conditions, for example, non-oxidizing conditions or alternatively, oxidizing conditions, inasmuch as the carbide layer effectively protects the carbon fiber against oxidation at elevated temperatures. Thus, the cooling step can either be carried out in the presence of hydrogen, nitrogen, argon, krypton, xenon, helium or the like, or in a vacuum or in the presence of oxygen or an oxygen-containing atmosphere, such as air.

The finished carbide coated carbon fibers are now ready for use. They are particularly suitable as thermal insulation, and can be exposed to high temperatures under oxidizing conditions wherein unprotected carbon fibers would rapidly be oxidized and rapidly lose mass and utility. The continuous carbide coating bonded to the surface of the carbon fibers and filling the surface pores and irregularities thereof is effective both as an oxidation-resistant barrier and as a heat shield for the carbon fibers. In this regard, the carbides exhibit improved insulating qualities over the carbon fibers themselves. Moreover, zirconium carbide, titanium carbide, chromium carbide and nickel carbide are converted to the respective oxides upon exert to. oxidizing conditions are, formed in situ and tightly.

Example I Carbon cloth is treated according to the present method. The carbon cloth has the following chemical analysis:

Percent by wt.

Carbon 95.2 Hydrogen 1.0

Oxygen 3.3 Ash 0.5

An aqueous solution containing a sufficient concentration of zirconium acetate to provide 4 weight percent of zirconium carbide, calculated as zirconium, after calcining carbon cloth impregnated with the solution, is used as the treating solution. The carbon cloth is immersed therein and the cloth absorbs approximately 0.8 m1. of solution per gram of carbon cloth over a 30 minute period. Thus, every 800 ml. of the solution contains approximately 192 grams of zirconium acetate having the formula The cloth is then Withdrawn from the solution, drained of excess aqueous zirconium acetate solution and then is passed through a dryer operating at about 275 F. until dry, in about 20 minutes. Thus, excess Water is removed from the zirconium acetate and the compound is dried into a glassy solid state as a continuous smooth layer bonded to exposed surfaces of the carbon fibers and impregnating and filling all surface pores and surface irregularities.

The dry, coated cloth is then passed to a furnace containing a nitrogen atmosphere and is heated slowly over a period of about 5 hours to a temperature of 1000 C. and maintained at this temperature for a period of about 8 hours. At the end of this time the heating unit of the furnace is shut otf and the cloth is allowed to cool to ambient temperature in the nitrogen atmosphere. During such firing treatment at 1000 C. in the furnace, the volatiles of the zirconium acetate are removed and the zirconium acetate is decomposed to zirconium carbide. The zirconium carbide formed in situ is bonded to the surfaces of the carbon fibers, filling the surface pores and irregularities and effectively protecting the fibers against erosion due to oxidation. The carbon of the carbide is substantially completely derived from the carbon of the acetate rather than the carbon of the carbon fibers. Accordingly, the mass of the carbon fibers is retained substantially completely intact during the in situ carbide formation.

Upon examination, the finished fibers are found to have substantially improved oxidation resistance and somewhat lowered thermal conductivity, in contrast to untreated carbon fibers of the same type. In all other respects, the treated carbon fibers are substantially identical to the untreated carbon fibers.

Example 11 Carbon cloth is treated in accordance with the method set forth in Example 1, except that an aqueous solution of zirconium formate is used instead of the aqueous solution of zirconium acetate. Moreover, the concentration of zirconium formate is such as to provide in the finished carbon cloth a concentration of about 2 percent, by weight, of zirconium in the form of the desired zirconium car- In a parallel series of tests, a chromium carbide coating is formed on flash-fired carbon fibers in accordance with the method of Example I, but utilizing an aqueous solution of chromium acetate in a concentration to provide in the finished product a chromium carbide concentration of about 2 percent (based on the chromium) by weight of the finished product.

The zirconium carbide coated carbon fibers (2 wt.-percent zirconium) and also the chromium carbide coated fibers (2 wt.-percent chromium) exhibit the same general appearance and other characteristics as the uncoated carbon fibers, except for substantially improved oxidation resistance at temperatures in excess of 800 C.

The single figure of the accompanying drawings graphically depicts the results of oxidation resistance tests performed on fibers prepared by the present method. In each instance, the temperature of one gram samples of the fibers was raised at the rate of 10 C. per minute to approximately 900 C.l000 C. While the fibers were ex posed to air flowing at the rate of 1 s.c.f. (Standard cubic foot) per hour. The weight loss for the fibers was measured at various times during the tests. Line A indicates the progressive weight loss during the test period of uncoated flash-fired carbon cloth, while Line B indicates the progressive weight loss during the test period for the zirconium carbide coated carbon fibers, and Line C indicates the same type of data for the chromium carbide coated carbon fibers. As clearly indicated in the single figure, during the test period the zirconium carbide treated carbon fibers (Line B) exhibited substantially improved oxidation resistance in contrast with the uncoated carbon fibers (Line A). This substantially improved oxidation resistance became particularly apparent at temperatures in excess of about 900 C. Moreover, the chromium carbide coated carbon fibers (Line C) have even more improved oxidation resistance throughout substantially the entire test temperature range. Accordingly, the single figure clearly indicates that substantially improved oxidation resistance is afforded to carbon fibers by providing the same with a coating of selected carbide.

Similar tests have been run on nickel carbide coated carbon fibers containing approximately 5 weight percent nickel, chromium carbide carbon fibers containing approximately 6 weight percent chromium, titanium carbide coated carbon fibers containing 2 weight percent titanium and untreated carbon cloth, i.e. carbon cloth containing uncoated carbon fibers. The tests have been run in accordance with the preceding criteria, i.e. by exposing the one gram samples of the various fibers to an air flow of 1 s.c.f. per hour and a temperature rise of 10 C. per minute over a temperature range of from about ambient temperature to about 1000 C. At the end of the test period, the weight percent residue, based upon the initial weight concentration of each of the types of samples, was determined. It was found that the carbon cloth which had not been protected by a carbide layer had decreased in mass to only 2.15 weight percent that of the original mass. The titanium carbide coated fibers (containing about 2.8 percent of titanium, based on the total fiber weight) exhibited substantially improved oxidation resistance in that the weight of the residue was approximately 33.2 weight percent that of the initial fibers. The

while the chromium carbide coated carbon fibers were present in the residue in a concentration of 37.4 weight percent that of the initial fibers.

Example II] An aqueous solution of nickel formate is prepared into which solution flash fired carbon cloth is immersed. The solution has a concentration of nickel formate sufii- 'cient to provide inthe finished dried and calcined carbon cloth product a nickel concentration of approximately weight percent. Thus the nickel formate solution contains approximately 264 grams of nickel formate per liter of solution. A 30 liter volume of the solution is used and a 10,000 gram sample of carbon cloth is disposed therein. The carbon cloth is left immersed in the nickel formate solution for a period of about one-half hour, after which it is withdrawn from the solution, drained, dried at about 290 F. and then calcined at about 1200 C. for about 2 hours in a nitrogen atmosphere. It is then cooled to ambient temperature in the nitrogen atmosphere over a period of about 3 hours.

The nickel carbide coated carbon cloth is examined and found to have the carbide coating wholly enclosing and tightly bonded to all of the exposed surfaces of the carbon fibers of the cloth. Moreover, the carbide fills the pores, depressions, and surface irregularities of the carbon fibers. When the carbide-containing cloth is exposed to an oxygen-containing atmosphere, such as air, at a temperature in excess of 800 F. the carbide is found to have been converted to nickel oxide, and it is further found that the nickel oxide coating is continuous and fully protective over the surfaces of the carbon fibers. The oxide coating is essentially inert at such elevated temperature in the oxygen-containing environment and also acts as a thermal shield.

In a parallel series of tests, samples of titanium carbide coated carbon fibers are prepared and tested. In this regard, flash fired carbon fibers are first immersed in cloth form in a bath containing titanium oxide hydrosol, stabilized by acetic acid, as the treating agent.

The titanium oxide hydrosol is obtained by addition of ethyl orthotitanate to an agitated, cold, 25 percent by weight, aqueous acetic acid solution to obtain an approximately percent colloidal solution of titanium oxide. The solution forms an all-embracing gel if warmed appreciably above room temperature. The solution is analyzed for titanium oxide concentration by drying and igniting an aliquot sample thereof to form titanium oxide. The solution is then diluted with 25 percent aqueous acetic acid solution to 4.25 weight percent Ti0 content, which is equivalent to 2.5 weight percent metallic titanium. When subsequently used to impregnate carbon cloth, this treating agent produces a coating of 2 weight percent Ti as the acetate stabilized oxide.

The cloth is withdrawn from immersion in the solution after about one hour, drained, and then dried at a temperature of about 250 F. On drying this treating agent on the cloth produces a gelatinous coating which retains acetic acid until it is calcined. The calcining is carried out in hydrogen at a temperature in excess of 1000 C. but below about 1100 C. for a period of approximately 8 hours, that is, until substantially all of the treating agent decomposes into and forms titanium carbide as a continuous protective layer over the surface of the carbon fibers of the cloth. This finished product is then cooled to ambient temperature in hydrogen and it is found that the titanium carbide coating (2 percent titanium, by weight of the finished product) is uniform and tightly bonded to the carbon fiber surface and that it protects all surfaces of the car-bon fibers of the cloth. Such carbide coating effectively increases the oxidation resistance of the cloth.

The foregoing examples clearly illustrate that the method of the present invention results in improved carbon fibers having increased oxidation resistance, particularly at elevated temperatures. The fibers can be prepared in a simple, effective and rapid manner utilizing readily available materials. The finished fibers have a bonded outer coating of carbide selected from a group consisting of titanium carbide, zirconium carbide, chromium carbide, nickel carbide and mixtures thereof, which carbide effectively protects the underlying carbon fibers against loss of mass when the fibers are expose-d to oxidizing con ditions. Upon such exposure, the carbide forms the cor- 10 responding oxide which is then stable against oxidation and forms a heat shield around the fibers.

Accordingly, a new improved product and a new method are provided. The present method is specifically directed to treatment of carbon fibers and is contrasted to other possible ways of forming carbides, in that the integrity of the small diameter carbon fibers is maintained during carbide formation in situ by the present method by providing a source of carbon other than the fibers. Moreover, the carbide-forming reaction takes place merely by decomposition of an organic compound at relatively low temperature. Other advantages of the present invention are as set forth in the foregoing.

Various modifications, changes, rearrangements and alterations can be made in the present method and in the product provided thereby. All such modifications, changes, rearrangements and alterations as are within the scope of the appended claims form a part of the present invention.

What is claimed is:

1. A method of improving the oxidation resistance of amorphous carbon fibers, which method comprises covering the surfaces of carbon fibers with and bonding to said surfaces a layer of heat decomposable organic material containing carbon and metal selected from the group consisting of zirconium, chromium, nickel, titanium, and mixtures thereof, said carbon fibers having been heat shrunk at about 1800-2200 F., and thereafter decomposing said material under non-oxidizing conditions and at elevated temperatures to carbide of said metal while substantially preserving the carbon of said fiber, said temperature being below that at which graphitization occurs, whereby said fibers have improved oxidation resistance at temperatures in excess of 800 F.

2. A method of improving the oxidation resistance of amorphous carbon fibers, which method comprises substantially wholly enclosing amorphous carbon fibers within a layer of heat decomposable organic material containing carbon and metal selected from the group consisting of zirconium, chromium, nickel, titanium and mixtures thereof, said carbon fibers having been heat treated at below about 1000 F. and then heat shrunk at about 1800-2200 F., bonding said layer to the surface of said fibers and heat decomposing said layer to carbide of said metal under non-oxidizing conditions at a temperature in excess of about 1000 C. but below graphitization temperature while maintaining the carbon of said fiber in unaltered form, whereby said fibers have improved oxidation resistance at temperatures in excess of 800 F.

3. The method of claim 2 wherein the metal of said carbide is present in said fibers in a concentration of between about 0.5 and about 10 percent, by Weight of said fibers.

4. The method of claim 3 wherein the metal of said carbide is present in said fibers in a concentration of lgtween about 2 and about 4 percent by weight of said ers.

5. A method of improving the oxidation resistance of amorphous carbon fibers, which method comprises impregnating and substantially wholly enclosing the exposed surfaces of amorphous carbon fibers with heat decomposable organic material containing carbon and metal selected from the group consisting of zirconium, chronuum, titanium, nickel and mixtures thereof, said material disposed in a suitable medium in a suificient concentration to provide, upon subsequent decomposition thereof, carbide of metal on the surface of said fibers in an oxidation resistant-enhancing amount, removing said medium and bonding said material to said fibers, and thereafter calcining said fibers at a temperature in excess of about 1000 C. but below graphitization temperature under non-oxidizing conditions while maintaining the carbon of said fibers in unaltered form, whereby said material is decomposed to carbide of said metal and whereby the oxidation resistance of said fibers at temperatures in excess of about 800 'F. is increased.

6. The method of claim wherein the metal of said carbide is present in a concentration of between about 0.5

and about percent, by weight of said fibers.

7. The method of claim 6 wherein said metal of s-aid carbide is present in a concentration of between about 2 and about 4 percent, by weight of said fibers.

8. A method of improving the oxidation resistance of amorphous carbon fibers, which method comprises impregnating and wholly enclosing the exposed surfaces of amorphous carbon fibers in an organic compound heat decomposable to a carbonaceous residue containing metal selected from the group consisting of zirconium, chromium, nickel, titanium and mixtures thereof, said compound being disposed in a suitable medium, evaporating said medium from said compound and setting said compound on said fibers so as to bond the same into a continuous adherent covering on the surface of said fibers, heating said fibers to above about 1000 C. but below graphitizing temperature under non-oxidizing conditions and maintaining said fibers at said temperature under said conditions until carbide of said metal is substantially completely formed from said compound while the carbon of said fibers remains unaltered, whereby the oxidation resistance of said fibers is increased.

9. The method of claim .8 wherein an aqueous solution of zirconium acetate is used to impregnate said fibers and wherein said impregnated fibers are dried below about 300 F. and then calcined to provide a zirconium carbide protective layer bonded to the surface of and Wholly enclosing said fibers, the Zirconium of said carbide being present in a concentration of between about 0.5 and about 10 percent, by weight of said fibers.

10. The method of claim 8 wherein an aqueous solution of chromium acetate is used to impregnate said fibers and wherein said impregnated fibers are dried below about 300 F. and then calcined to provide a chromium carbide protective layer bonded to the surface of and wholly enclosing said fibers, the chromium of said carbide being present in a concentration of between about 0.5 and about 10 percent, by weight of said fibers.

11. The method of claim 8 wherein an aqueous solution of nickel formate is used to impregnate said fibers and wherein said impregnated fibers are dried below about 300 F. and then calcined to provide a nickel carbide protective layer bonded to the surface of and wholly enclosing said fibers, the nickel of said carbide being present in a concentration of between about 0.5 and about 10 percent, by weight of said fibers.

12. The method of claim 8 wherein hydrosol of titanium oxide stabilized with acetic acid is used to impregnate said fibers and wherein said impregnated fibers are dried below about 300 F. and then calcined to provide a titanium carbide protective layer bonded to the surface of and wholly enclosing said fibers, the titanium of said carbide being present in a concentration of between about 0.5 and about 10 percent, by weight of said fibers.

References Cited UNITED STATES PATENTS 679,926 8/1901 Voelker 117-228 1,000,761 8/1911 Snyder. 2,282,098 10/1940 Taylor. 2,587,036 2/1952 Gemmer et al. 117-106 2,587,523 2/1952 Prescott 117-46 2,597,963 5/1952 Winter 117-169 2,615,932 10/1952 Marko et a1. 2,703,334 3/1955 Clough et a1 117-228 X 2,898,235 8/ 1959 Bullotf 117-106 3,011,981 12/1961 Soltes 23-20920 X 3,053,775 9/1962 Abbott 252-421 3,061,465 10/1962 Norman et al 1l7-107.2 3,073,717 1/1963 Pyle et a1. 117-106 ALFRED L. LEAVITT, Primary Examiner.

RICHARD D. NEVIUS, Examiner.

A. GOLIAN, Assistant Examiner,

Claims

1. A METHOD OF IMPROVING THE OXIDATION RESISTANCE OF AMORPHOUS CARBON FIBERS, WHICH METHOD COMPRISES COVERING THE SURFACES OF CARBON FIBERS WITH AND BONDING TO SAID SURFACES A LAYER OF HEAT DECOMPOSABLE ORGANIC MATERIAL CONTAINING CARBON AND METAL SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM, CHROMIUM, NICKEL, TITANIUM, AND MIXTURES THEREOF, SAID CARBON FIBERS HAVING BEEN HEAR SHRUNK AT ABOUT 1800-2200* F., AND THEREAFTER DECOMPOSING SAID MATERIAL UNDER NON-OXIDIZING CONDITIONS AND AT ELEVATED TEMPERATURES TO CARBINE OF SAID METAL WHILE SUBSTANTIALLY PERSERVING THE CARBON OF SAID FIBER, SAID TEMPERATURE BEING BELOW THAT AT WHICH GRAPHITIZATION OCCURS, WHEREBY SAID FIBERS HAVE IMPROVED OXIDATION RESISTANCE AT TEMPERATURES IN EXCESS OF 800* F.