EP0505561A1

EP0505561A1 - A low temperature process of applying high strength metal coatings to a substrate and article produced thereby

Info

Publication number: EP0505561A1
Application number: EP92900032A
Authority: EP
Inventors: Scott A. Ploger; Lloyd D. Watson
Original assignee: US Department of Energy
Current assignee: US Department of Energy
Priority date: 1990-10-18
Filing date: 1991-10-15
Publication date: 1992-09-30
Also published as: JPH05503249A; EP0505561A4; WO1992006797A1; CA2071492A1

Abstract

Procédé d'application d'un revêtement métallique dense (99 % environ de densité théorique) sur un substrat, au moyen d'une buse convergente/divergente pour nébuliser le métal en colonne comportant une masse et des flux thermiques uniformes et une répartition de la dimension des gouttelettes métalliques située entre 5 et 15 microns environ. On peut déposer les revêtements sur des substrats qui se dégradent thermiquement à des températures très inférieures au point de fusion des métaux déposés. L'invention décrit également des articles pourvus d'un revêtement et possédant des caractéristiques de liaison mécanique importantes.Method for applying a dense metallic coating (approximately 99% of theoretical density) on a substrate, by means of a converging / diverging nozzle for nebulizing the metal in a column comprising a mass and uniform heat fluxes and a distribution of the size of the metallic droplets between 5 and 15 microns approximately. The coatings can be deposited on substrates which degrade thermally at temperatures much lower than the melting point of the deposited metals. The invention also describes articles provided with a coating and having important mechanical bonding characteristics.

Description

A LOW TEMPERATURE PROCESS OF APPLYING HIGH STRENGTH METAL COATINGS TO A SUBSTRATE AND ARTICLE PRODUCED THEREBY

CONTRACTUAL ORIGIN OF THE INVENTION The United States Government has rights in this invention pursuant to Contract No. DE-AC07-76ID01570 the U.S. Department of Energy and Idaho National Engineering Laboratory.

Background Of The Invention The present invention relates to a method for spraying a thin, uniformly dense metal coating on a substrate and an article of manufacture produced thereby.

This invention relates to a low temperature spray coating process which has been developed at the Idaho National Engineering Laboratory (INEL) , the process is now referred to as the Controlled Aspiration Process (CAP) . The CAP process is set forth in some detail in U.S. patent no. 4,919,853 issued to Alvarez and Watson, April 24, 1990, for Apparatus and Method For Spraying Liquid Materials, the disclosure of which is herein incorporated by reference. The nozzle herein identified as a converging/diverging nozzle is the nozzle disclosed in the '853 patent. The CAP process using the converging/diverging nozzle of the '853 patent can be manipulated provide a process or method of mechanically adhering thin dense metal coatings of uniform thickness on substrates. These coatings have superior mechanical properties to coatings applied by other processes and because of the CAP process parameters permit metal coatings to be deposited on substrates which thermally degrade at temperatures far below the melting point of the metal being deposited thereon.

The CAP process of spray forming metals aspirates a molten metal into the throat of a converging/diverging gas nozzle, where the liquid is nebulized into a directed spray of rapidly cooling droplets. The gas flow (usually an inert gas such as argon) accelerates the droplets toward the substrate, against which the droplets impact before completely solidifying. Under appropriate operating conditions, the incident metal consolidates into a strong deposit with negligible porosity at the interface between the coating and the substrate surface. Rapid cooling occurs in flight by a variety of thermodynamic mechanisms including convection and radiation as well as by convection and conduction upon arrival at the substrate surface. Rapid solidification of the nebulized metal droplets in the deposit produced by the CAP process enhances the metallurgical properties by limiting grain sizes, by preserving constituent homogeneity, by preventing impurities from segregating into inclusion defects and by freezing metastable metallurgical phases that would otherwise decompose during cooling to room temperature. The homogeneous dispersion of impurities is particularly important since by preventing the segregation of impurities as inclusion defects, the structural integrity of the metal coating is improved.

The CAP coating capabilities provide precise control of the spray forming process and the coatings which result therefrom have better mechanical properties due to the low gas pressure used in the CAP process with a low droplet velocity having low momentum permitting the gas in the plume to deflect away from the dynamic deposition area. This results in less gas entrapment in the deposit, and thus less bulk porosity in the coating layer. Particularly, using pressures approximately 10 psi above atmospheric pressure or in the range of about 20-25 psi absolute, low droplet velocity results in gentle droplet impact conditions and complete consolidation at the substrate surface. Adherent coatings having tensile strengths greater than 3000 psi have been prepared using the CAP process.

The most important aspect of the CAP process for obtaining low interfacial porosity and improved adhesion strength is the spatial uniformity of the mass and thermal fluxes in the plume which promotes complete consolidation of the first droplets to impact the substrate, thereby preventing "chill zone" formation.

Another feature of the CAP process is that the mass and thermal fluxes of the plume produced from the converging/diverging nozzle are uniform across the plume and can be varied as can the gas to metal mass ratio without changing the droplet size in order to control the in-flight cooling rates. However, since coating adherence to the substrate is also a function of the substrate surface treatment, it is important that the surface be grit blasted, ball peened, or otherwise roughened, as well as cleaned or degreased. Near perfect interfacial interlinkage has been achieved between sprayed tin coatings on grit blasted steel coupons, as coatings having virtually no interfacial porosity have been deposited.

A metal spraying technique known as the Osprey Process is theoretically capable of depositing metal coatings but the Osprey Process deposits coatings having relatively high porosity at the substrate surface and also produces plumes in which neither the mass flux nor the thermal flux is uniform across the plume. The low mass and thermal fluxes at the leading plume edge in the Osprey Process form an interfacial chill zone with up to 10% porosity which is aggravated by the high fraction of small, completely solidified droplets produced at an Osprey plume periphery. In addition, the Osprey nebulizing gas is pressurized from 100-200 psi absolute to shear its thick liquid metal stream, compared to the 18 to 25 psi absolute used with the CAP process.

In addition to the Osprey process, coatings may be flame sprayed by mixing oxygen and a reactive gas such as acetylene and igniting the gas mixture to inject the liquid solids by combustion. In the flame spraying method, flame temperatures are normally from 3000^*C to 5000"C. The process suffers from adverse reactions that may occur when metal particles are introduced into such a high temperature atmosphere and byproducts of the combustion process which accumulate on the substrate surface and degrade adhesion of the metallic coatings thus formed. For these reasons, flame spraying has typically been confined to ceramic coatings, where the combustion byproducts are a minor concern.

Plasma spraying is accomplished by using two electrodes, one a pointed tip and the other an annular ring to form a circular opening between the two electrodes where the gases are ionized into a plasma. Both metal and ceramic powders may be introduced via a secondary gas stream into the resistant heated plasma which can reach temperatures of 15,000"C. Plasma sprayed metallic coatings are lower in porosity than flame sprayed layers and they are naturally free of combustion byproducts.

The CAP process provides coatings which are substantially different from thermal, that is flame-and plasma-sprayed coatings. The choice of melt feedstock for sprayed-forming coatings is very flexible since the metal is nebulized directly from a molten reservoir. In thermal spraying, the particles need considerable attention to size distribution which are rarely uniform even from the same commercial lot which results in a difficulty in tailoring compositions of an alloy by mixing metal powders before plasma injection. Because there is a wide size and shape distribution of injected particles in the thermal processes, the cooling rates as well as the droplet aspiration rates all vary. In thermal spraying, unlike the CAP process, high plume temperatures preclude achieving the rapid solidification benefits of the CAP process and induce metallurgical coating-substrate bonding.

Accordingly, in order to provide superior thin metal coatings capable of being deposited on substrates wherein the substrates degrade at lower temperatures than the melting point of the metal deposited thereon, there has been provided a new process which provides uniform mass and thermal fluxes across the plume resulting in uniform nearly theoretically dense coating displaying benefits of rapid solidification, that is small grain sizes, homogeneously dispersed impurities, high strength bonding and the other advantages previously described. It is an object of the present invention to provide a method of forming a uniform metal coating on a substrate surface and the article produced thereby which is accomplished at relatively low temperatures, with low droplet velocities and controllable cooling rates wherein the particle size distribution of the nebulized metal droplets is narrowly maintained in the range of from about 5 to about 15 microns.

Another object is of the invention is to provide a method of applying a dense metal coating to a substrate comprising, providing a substrate to be coated having a clean and roughened surface, and directing a plume of nebulized metal droplets toward the substrate surface wherein the droplet size distribution is in the range of from about 5 microns to about 15 microns for a time sufficient to produce a coating having a thickness not less than about 3 mils and a density not less than about 98% of theoretical density.

Another object of the invention it to provide a method of applying a dense metal coating to substrate comprising, providing a substrate to be coated having a clean and roughened surface, directing a plume of nebulized metal droplets toward the substrate from a converging/diverging nozzle having a throat at which the nebulized metal is introduced and an exit from which the metal droplets leave entrained in a carrier gas, wherein the mass flux at any point in the plume at and after the nozzle exit is substantially the same to provide a coating of uniform thickness having a density not less than about 98% of theoretical density.

Yet a further object of the invention is to provide a method of applying a dense metal coating of uniform thickness to substrate comprising, providing a substrate to be coated having a clean and roughened surface, directing a plume of nebulized metal droplets toward the substrate from a converging/diverging nozzle having a throat at which nebulized metal is introduced and an exit from which the metal droplets leave entrained in a carrier gas, wherein the mass flux at any point in the plume after the nozzle exit is substantially the same to provide a coating of uniform thickness.

Still another object of the invention is to provide a substrate having a rapidly solidified metal coating layer mechanically adhered thereto wherein the metal coating is uniform throughout in density, the coating having a thickness greater than about 3 mils and a density not less than about 98% of theoretical density. A final object of the invention is to provide an article of manufacture produced by the process of providing a substrate having a clean and roughened surface, forming a plume of nebulized metal in a carrier gas wherein the metal droplet size distribution is in the range of from about 5 to about 15 microns and directing the plume toward the substrate for a time sufficient to provide a metal coating having a thickness of not less than about 3 mils on the substrate.

The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

Detailed Description of the Invention A variety of experiments were conducted using the converging/diverging nozzle of the '853 patent. A 100 gram crucible charge was more than sufficient for all of the experiments conducted. This amount of tin was melted and brought up to a typical 400"C temperature within a few minutes while the other spray system components were warming up, as governed by their temperature controllers. The gas supply heater had a design requirement of raising inert gas temperatures to 600^βC between inlet pressures of 12.4 and 50 psi over an argon flow rate from 0-10 scfm. The performance of the nozzle/tundish assembly disclosed in the '853 patent was without difficulty and produced high quality coating consistently. It has been found that preheating the substrate to temperatures in the range of from 100°C to about 225°C produced superior coatings in that adhesion strength of the metal coatings on the substrate surface which had previously been cleaned and roughened were improved over the coatings deposited on substrates which were not preheated, due to superior interfacial wetting and reduced differential thermal stress during cooling.

In the experiments an integral gun type preheater was attached to the fixture back which held the substrate to be coated and in this manner no interference occurred with the nozzle plume so that coatings could be sprayed at any distance from the nozzle. The experimental setup used in testing the invention permitted the base metal or substrate metal speed to be monitored directly on a computer screen thereby permitting this variable to be changed in the course of a single experiment. Accordingly, experiments were run wherein the substrate speed with respect to the nozzle were varied during the experimental run.

The most straightforward fashion for evaluating process responses to different spraying parameters and base metal conditions is systematically reviewing coating quality data. Quantitative results include measurements of bend deflections, adhesion strengths, porous areas in tin layers, and roughness on external coating surfaces. The sequence of experiments is set forth below, all of which were conducted at Idaho Falls.

(1) 150 RDL Positioning a steel sleeve 3 inches from a converging/diverging nozzle exit produced a l- inch wide deposit, while the 18.5 inch/minute base metal speed yielded a thickness of .010 inch. Nozzle operating pressure ranged between 23 and 24 psia. The coating was separated into three segments over different grit-blasted bands, and the temperature of the base metal increased from 105 to 125"C.

(2) 153 RDL The nozzle-to-sleeve separation was still 3 inches, but the speed was lowered to 7.3 inches/minute to increase coating thickness to .025 inch. The coating occurred in two circumferential passes, with the first straddling bands roughened with glass beads and fine silica grit and the second over bands blasted with coarse alundu . Operating pressure was held at 23.7 psia, and the base metal temperature was steady at 130"C. (3) 160-0 HOR The preheater was still employed on the cylindrical sleeve with the horizontal preparation pattern, where one 120 degree segment was blasted with fine No. 240 silica and the other two were blasted with coarse No. 46 and No. 30 alundum. The coating remained 1 inch wide, but the 15.7-inch/minute speed lowered thickness to .012 inch. The temperature of the steel sleeve was a constant 125°C. The nozzle operating pressure- was held at 23.9 psia. The overall intent was to reproduce the 150 RDL coating as a control experiment on the new base metal preparation pattern.

(4) 160-2A HOR The preheater tube was removed so the sleeve could be moved to 2 inches from the nozzle exit plane, which produced a .7 inch wide deposit. Increasing the base metal speed to 40 inches/minute reduced the coating thickness to .004 inch. Operating pressure was a constant 23.9 psia. Despite the more concentrated spray plume, the absence of the preheater lowered the sleeve temperature to 66"C. The outer surface was dimpled and faceted, as with earlier coatings.

(5) 160-2B HOR The base metal was moved closer to the nozzle exit plane to a 1.2 inch separation, which narrowed the deposit to .45 inch. The more concentrated heat flux raised the sleeve to 90"C and coating thickness to .009 inch. When the operating pressure was dropped from 23.9 to 22.9 psia, the coating appeared shiny from incipient melting induced by greater tin aspiration and higher deposited heat flux.

(6) 160-3A HOR The sleeve was moved even closer in an attempt to foster metallurgical bonding between the tin and carbon steel. The 1.0 inch separation yielded a .41 inch width. The greater spray concentration combined with a lower 22.0 psia nozzle pressure produced a molten pool that was pushed laterally by plume turbulence into two parallel ridges .012 inch high, with minor ridges between them. Temperature of the base metal was not recorded, because the TC was no longer in contact.

(7) 160-3B HOR The base metal was pulled back to a 1.5 inch distance to reduce molten ridge formation, which widened the coating to .50 inch. Operating pressure was lowered to 21.0 psia to ensure a mostly molten deposit. Molten ridges still formed under the incident nozzle plume, , but they were less pronounced, with maximum thicknesses of .010 inch.

(8) 178-1 VER Preparation of the vertical steel strip was confined to blasting with coarse No. 30 alundu grit, as for the two later 178 VER experiments. The base metal was positioned 1.5 inch from the nozzle exit to spray a .50 inch wide coating, as in 160-3B HOR. The heating element on the air gun was not used, so a cool gas flow was directed against the back side of the strip. This cooling and the 22.9 psia pressure prevented deposit.melting. Base metal speed was varied from 2.4 to 17.2 inches/minute, yielding .029 to .004 inch thicknesses. (9) 178-2 VER Nozzle-to-base metal distance was still 1.5 inch, so the coating width remained .50 inch. Operating pressure was also held at 22.9 psia for consistency with 178-1 VER. To guarantee a fully molten coating the air gun heating element was turned on at maximum power and the nozzle temperature was raised from 400^βC to 500^βC for the first time. The desired thin molten film was achieved once the base metal speed was increased from 2.4 to 16.1 inches/minute to eliminate ridging. The thin (.009 inch) layer formed between 200° and 225'C.

(10) 178-4 VER Base metal distance, coating width, nozzle temperature, and nozzle operating pressure were as in 178-2 VER. The air gun heating element was not engaged until half of the strip was coated to get baseline data on effect of the 500^βC nozzle without strip preheating. Base metal speed was from 17.3 to 10.0 inches/minute over the cool strip portion, producing dimpled deposit thicknesses from .010 to .016 inch. Ridging began promptly after the preheater was turned on. The ridges disappeared when the speed was increased back to 17.3 inches/minute, leaving a thin (.009 inch) molten film at base metal temperatures between 190 and 200^βC. Bend test results were performed and found to be best on the 153 RDL and the 160-0 HOR experiments. As well as being deposited 125' to 130"C onto preheated base metal, these coatings were relatively wide from the 3 inch nozzle to sleeve separation. These conditions also apply to the 150 RDL coatings, but they tended to fracture at somewhat lower deflections for indefinite reasons.

Thickness was not an important factor in coating ductility on the superior samples, because 153 RDL deposits were typically .025 inch in thickness and 160-0 HOR samples were .012 inch in thickness. The amount of surface roughening during base metal preparation also had no obvious influence on flexural strength; in both sets fine and coarse grit sizes yielded similar results. This likely reflects comparably good consolidation and adhesion.

Ten of seventeen samples had yield adhesion strengths that surpassed 1000 psi and samples 160-0 HOR- 2 and 160-0 HOR-4 surpassed 3000 psi. These strengths are especially impressive because they fall within the accepted strength range of commercial metal-on-metal bonds produced by plasma spraying. This is particularly remarkable because plasma sprayed metallic coatings often rely on metallurgical bonds from the high deposition temperatures of the plasma process whereas the bonds of the CAP process are mechanical adhesion bonds. The fact that three adhesion samples definitely exceed the 2100 psi tensile strength of wrought tin is equally significant. In these three cases, the tin layers would have failed earlier without the benefit of the rapid solidification in the CAP process because smaller grains improve tensile strength beyond that achieved in conventionally cast tin. Accordingly, spray forming can yield better performance in coating layers, as well as strong bonds.

Preliminary tin-spraying efforts establish that some roughening of the base metal surface was required for substantial adhesion. Varying the grit size of the particles used to roughen the surface had little effect on adhesion strength.

In general, the incident tin droplets were able to wet even the roughest steel surfaces, filling in all depressions very well. This explains why grit sizes had no obvious effect on adhesion strength among the 160-HOR samples.

Photomicrographs were taken of various prepared samples. The area averaged porosity concentrations ^ere generally between 0 and 1% in the overwhelming majority of cases, independent of the spraying conditions, base metal preparation, deposition temperatures and other process parameters. Excellent consolidation of the incident tin droplets typically was accomplished over the entire test matrix, and porosity was virtually never interconnected. Photomicrographs also confirmed that the isolated areas of relative high porosity generally occurred near external coating surfaces where complete consolidation was not expected as with formation of surface roughness. Even in these worse case locations, porosity was still very low near the steel interface, so corrosion resistance would not be jeopardized.

As regards surface roughness of the coating, it was found that the amount of heat directed toward the surface during the coating deposition affected the depth of the indentations. It was found that surface roughness can be controlled within reasonable limits according to heat deposition. For rapidly cooled coatings, at least .001 inch of material has to be ground away to produce a smooth surface. Little or no grinding is required for coatings deposited at higher temperatures. However, for certain applications, some roughness can be desirable, where lubricant retention or paint adherence are critical, for example. In such applications, the dimpled, faceted surface of the rapidly cooled coatings may be ideal.

The typically excellent mechanical interlinkage achieved strong adherence between the coating and the base metal without high temperature metallurgical bonding. Although the examples were performed with steel coupons and tin metal coatings, other metals such as lead, cadmium, zinc, aluminum, copper, iron, chromium, cobalt and nickel as well as various alloys thereof are applicable to the process. Since the droplets can be cooled rapidly in flight, such coatings can be placed onto heat-sensitive materials with little or no heat degradation, plus roughened plastics, cloth fibers and even paper with appropriate measures. Examination of the plumes produced in this process show that the mass flux at any point in the plume after the nozzle exit is substantially the same and it is this uniformity of mass flux in the plume which is an important feature of the invention and which helps to provide a coating of uniform thickness.

Measurements have also shown that the thermal flux at any point in the plume after exit from the converging/diverging nozzle exit is substantially the same and this is an important factor in depositing coatings of uniform microstructure and adhesion due, impart, to complete droplet consolidation, as previously described.

Thermal flux is determined by convective cooling requiring both different gas/metal temperatures and gas/metal velocities. There are four complementary cooling regimes in the low velocitv CAP process:

(1) In the nozzle throat and exit region, where warm gas forms the droplets and accelerates them to nearly the gas velocity. This acceleration, as well as the nebulization, takes energy from the gas, slowing it down somewhat. (2) Shortly after exiting the nozzle, where entrainment of cool stationary gas both cools the gas plume and slows it down.

(3) Closer to the substrate, where the metal droplets are now moving faster than the gas plume, due to their relatively large inertia.

(4) After impaction and deposition, where the gas plume cools the exterior deposit surface.

As hereinbefore stated, the process of the present invention produces a plume wherein the nebulized metal has a very narrow droplet size distribution. This distribution is in the range of from about 5 microns to about 15 microns, and it is. this uniform size distribution of droplets along with the uniform mass flux and thermal flux of the plume which results in these very dense, highly uniform thickness, strongly adherent, small grain size coatings. Another feature of the invention is that the rapidly solidified metal coatings produce a homogeneous dispersion of impurities throughout the coatings, thereby precluding the segregation of impurities into inclusion defects. Coatings of the type hereinbefore described have good adhesion strength attributable solely to mechanical adhesion, whereas adherent plasma deposited coatings typically rely on metallurgical bonding. The CAP deposited coatings rely purely on mechanical adhesion, and tin coatings have exceeded 2100 psi tensile strength with some coatings exceeding 3000 psi.

To be economically attractive, spray forming technology must be extended for covering large surface areas with uniformly thick coating. By using a rectangularly shaped nozzle with multiple liquid orifices, flat tin deposits having thicknesses in the range of .02 inch to about .08 inches have been formed, with thickness uniformity within 2% over 4 inch widths. In principle, this nozzle configuration may be as wide as desired.

Another important aspect of the spray coating economics is the metal-to-coating conversion efficiency. Careful measurements have verified efficiencies of 99.8% or better with the tin coatings tested. In actual commercial situations, any overspray (unconsolidated powder) will be collected within the steel chamber and recycled, so even for scale-up situations, the verified efficiency looks very encouraging. In testing, the argon gas was found to be remarkably clean after filtration, so it could be conveniently recycled if argon is used as the nebulizing/purging gas. Where applicable, nitrogen which is less costly than argon may also be used but for some coatings, nitrogen is unsuitable.

Accordingly, there has been provided a novel spray- coating system which permits production of a well defined plume of nebulized metal to be coated wherein the droplet size distribution is in the range of from about 5 microns to about 15 microns. The plume is directed toward the prepared substrate at a low velocity and by adjusting the upstream gas pressure, the nebulizer can be turned on and off and the metal spraying rate can be throttled in accordance with CAP or Control Aspiration Process.

Metal-to-coating converting efficiencies of 99.98% were obtained under normal conditions, and tin coatings were successfully deposited onto low carbon steel with or without base metal preheating. The coating widths ranged from 0.4 to 1.1 inch, base metal temperatures were varied from 50°C to 220"C. The various data collected show the basic versatility of the spray coating technology. Rapidly solidified coatings (by that we mean flight times in the range of from about 0.001 sec to about 0.2 sec, after which solidification is complete) from larger flight distances in the order of 12 inches and more mixing with ambient cool surrounding gas displayed a dimple, faceted appearance of incomplete consolidation of the last droplets. Thicknesses of such coatings range from .004 to .025 inch, depending on the plume expansion in flight and the base metals*s speed of translation.

The highest overall bend-test deflections were obtained from wide coatings deposited by a cool spray plume, where the base metal is preheated to 125*C. The samples showed good ductility and no influences were found from coating thickness which varied from about 3 mils to about 25 mils or the grit size used to roughen the steel surface for mechanical bonding. Superior overall adhesion strengths were found on wide, cool coatings without high temperature metallurgical bonding. Mechanical bonds on two of these samples exceeded 3000 psi, within the accepted range of metal layers coated by plasma spraying, a long established commercial process with high temperatures that often induce metallurgical bonds. Spray-coating thus deposits strong coatings at low temperatures that will not compromise heat sensitive base materials.

Varying the mass ratio of transport gas to metal in the range of from about 0.5:1 to about 4:1 helps control the temperature of the coating at the substrate surface. In fact, these coatings can be adhered to base materials which degrade at temperatures well below the melting point of the coating metal. Thermal degradation of substrates having melting points lower than the metal deposited thereon is possible because:

(1) The thermal flux seen by the substrate is the average of the gas and metal temperatures. With a typical gas to metal mass ratio of 2:1, the average temperature tends to be dominated by the gas, with allowance for different heat capacities.

(2) The 0.5:1 to 4:1 mass ratio is measured in the nozzle. This ratio increases after entrainment of surrounding gas (at ambient temperature) into the plume, which also lowers the plume temperature.

(3) Metal droplets are partially solidified or undercooled (solidification takes time and can be slower than cooling, especially for very small droplets) at impact, with a volume fraction of about 1/4 to 1/3 liquid, so the average droplet temperature is typically below the metal melting point.

For instance, the melting point of Sn is about 235"C but the temperature at the substrate surface is about 50^*C to about 125^*C; the melting point of Al is about 670"C, but the temperature at the substrate surface would be about 300^βC to about 400*C; the melting point of Cu is about 1085^βC, but the temperature at the substrate surface would be about 500^βC to about 600'C; the melting point of stainless steel is about 1400"C but the temperature at the substrate surface would be about 800'C to about 900^*C; the melting point of Co is about 1495°C but the temperature at the substrate surface would be about 900"C to about 1000"C; the melting point of Cr is about 1857^βC but the temperature at the substrate surface would be about 1200°C to about 1300^βC; the melting point of Ni is about 1455"C but the temperature at the substrate surface would be about 850*C to about 950'C.

Adhesion strengths on three samples substantially exceeded the tensile strength of the wrought tin parent material, illustrating how rapid solidification benefits can improve properties in the coating layer. Area- averaged porosity concentrations and tin layer micrographs were overwhelmingly between 0 and 1%, and the pores never provided an interconnected corrosion pathway to the base metal. This porosity level is superior to standard commercial plasma spraying and equal to plasma spraying conducted in a vacuum chamber. Spray coated porosity is particularly low near the base metal interface, as compared to atmospheric pressure plasma spraying where much gas is entrapped. The tin coatings produced had edges that tapered in thickness. The extent of the crowning was primarily determined by the flight distance which generally varied between about 1 inch and 4 inches and the associated amount of plume expansion. This thickness nonunifor ity is a fundamental byproduct of the circular geometry nozzle design used for small scale feasibility studies herein reported. Nozzles with rectangular cross sections containing a slot-throat and multiple liquid orifices will obviate nonuniformity found with circular nozzle designs.

Surface roughness on rapidly cooled, dimpled coatings decreased from .004 inch deep indentations to .002 inch when the preheating flow was removed from the front base metal surface and later directed against the opposite side from the spray plume impact zone. On shiny coatings consisting of partially or fully molten layers, the complete droplet consolidation decreased the roughness level to .001 inch. Surface roughness can therefore be controlled according to the deposition conditions, which is advantageous because some roughness is desirable for certain coating applications.

While there has been disclosed what is considered to be the preferred embodiment of the present invention, it is understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of applying a dense metal coating to a substrate comprising: providing a substrate to be coated having a clean and roughened surface, and directing a plume of nebulized metal droplets toward the substrate surface wherein the droplet size distribution is in the range of from about 5 microns to about 15 microns for a time sufficient to produce a coating having a thickness not less than about 3 mils and a density not less than about 98% of theoretical density.

2. The method of claim 1, wherein the coating has a uniform thickness.

3. The method of claim 1, wherein the droplet size is about 10 microns.

4. The method of claim 1, wherein the coating density is not less than about 99% of theoretical density.

5. The method of claim 1, wherein the coating thickness is in the range of from about 3 mils to about 50 mils.

6. The method of claim 1, wherein the metal coating is selected from the class consisting of Al, Cu, Fe, Sn, Co, Cr, Ni, Pb, Cd, Zn, and alloys thereof.

7. The method of claim 1, wherein the substrate is substantially free of grease and particulate matter.

8. The method of claim 1, wherein the substrate is heated to an elevated temperature greater than about 100"C before the plume is directed thereat.

9. The method of claim 1 , wherein the substrate thermally degrades at a temperature below the melting point of the coating metal.

10. A method of applying a dense metal coating to substrate comprising: providing a substrate to be coated having a clean and roughened surface, directing a plume of nebulized metal droplets toward the substrate from a converging/diverging nozzle having a throat at which the nebulized metal is introduced and an exit from which the metal droplets leave entrained in a carrier gas. wherein the mass flux at any point in the plume at and after the nozzle exit is substantially the same to provide a coating having a density not less than about 98% of theoretical density.

11. The method of claim 10, wherein the substrate is positioned up to about 12 inches from the nozzle exit.

12. The method of claim 11, wherein the mass ratio of transport gas to nebulized metal is in the range of from about 0.5:1 to about 4:1.

13. The method of claim 12, wherein the metal droplets cool sufficiently from the nozzle throat to the substrate that at least a portion of the droplets impact the substrate in a partially solidified or undercooled liquid state.

14. The method of claim 10, wherein the metal is selected from the group consisting of Cu, Al, Fe, Sn, Co, Cr, Ni, Pb, Cd, Zn and alloys thereof.

15. The method of claim 14, wherein the metal is Sn, the temperature at the nozzle throat is about 235°C and the temperature of the substrate surface during deposition is in the range of from about 50"C to about 125^*C.

16. The method of claim 14, wherein the metal is Al, the temperature at the nozzle throat is about 670"C and the average plume temperature at base metal contact is in the range of from about 300^βC to about 400^βC.

17. The method of claim 14, wherein the metal is Cu, the temperature at the nozzle throat is about 1085^βC and the average plume temperature at base metal contact is in the range of from about 500"C to about 600"C.

18. The method of claim 14, wherein the metal is stainless steel, the temperature at the nozzle throat is about 1400^βC and the average plume temperature at base metal contact is in the range of from about 800°C to about 900°C.

19. The method of clai.- 14, wherein the metal is Co, the temperature at the nozzle throat is about 1495*C and the average temperature at base metal contact is in the range of from about 900'C to about lOOO'C.

20. The method of claim 14, wherein the metal is Cr, the temperature at the nozzle throat is about 1857*C and the average temperature at base metal contact is in the range of from about 1200^βC to about 1300*C.

21. The method of claim 14, wherein the metal is Ni, the temperature at the nozzle throat is about 1455^βC and the average temperature at base metal contact is in the range of from about 850"C to about 950"C.

22. The method of claim 14, wherein the metal is Pb, the temperature at the nozzle throat is about 327'C and the average temperature at base metal contact is in the range of from about 140"C to about 200^βC.

23. The method of claim 14, wherein the metal is Cd, the temperature at the nozzle throat is about 321^βC and the average temperature at base metal contact is in the range of from about 140 " C to about 200^βC.

24. The method of claim 14, wherein the metal is Zn, the temperature at the nozzle throat is about 420"C and the average temperature at base metal contact is in the range of from about 250^βC to about 325"C.

25. The method of claim 10, wherein the nozzle exit is rectangularly shaped in a plane perpendicular to the flow of the nebulized metal leaving therefrom and

further providing relative movement between the nozzle and the substrate during the coating process.

26. A method of applying a dense metal coating of uniform thickness to substrate comprising: providing a substrate to be coated having a clean and roughened surface, directing a plume of nebulized metal droplets toward the substrate from a converging/diverging nozzle having a throat at which nebulized metal is introduced and an exit from which the metal droplets leave entrained in a carrier gas, wherein the thermal flux at any point in the plume after the nozzle exit is substantially the same to provide a coating of uniform microstructure and strong adhesion.

27. The method of claim 26, wherein the substrate is up to 12 inches away from the nozzle exit and the time for the nebulized metal to travel from the nozzle throat to the substrate is in the range of from about 0.001 second to about 0.2 second.

28. The method of claim 26, wherein the mass ratio of transport gas to nebulized metal is in the range of from about 0.5:1 to about 4:1.

29. The method of claim 26, wherein the melting point of the substrate is below the melting point of the metal coating.

30. The method of claim 29, wherein the substrate is up to about 12 inches from the nozzle exist and the mass ratio of transport gas to nebulized metal is in the range of from about 0.5:1 to about 4:1 and relative movement is provided between the substrate and the nozzle exit during the coating process.

31. The method of claim 30, wherein the nozzle exit is rectangularly shaped in a plane perpendicular to the flow of nebulized metal leaving therefrom.

32. A substrate having a rapidly solidified metal coating layer mechanically adhered thereto wherein said metal coating is uniform throughout in density, said coating having a thickness greater than about 3 mils and a density not less than about 98% of theoretical density.

33. The metal coated substrate of claim 32, wherein the substrate thermally degrades at a temperature lower than the melting point of the metal coating.

34. The metal coated substrate of claim 32, wherein the adhesion strength of the mechanical bond between the metal coating layer and the substrate is not less than about 2000 psi.

35. The metal coated substrate of claim 32, wherein the rapidly solidified metal coating has a homogeneous dispersion of any impurities therein due to the rapid solidification of said coating on said substrate.

36. The metal coated substrate of claim 35, wherein the metal coating solidifies from the nebulized liquid to a solid in the range of from about 0.001 to sec. to about 0.2 sec.

37. An article of manufacture produced by the process of providing a substrate having a clean and roughened surface, forming a plume of nebulized metal in a carrier gas wherein the metal droplet size distribution is in the range of from about 5 to about 15 microns and directing the plume toward the substrate for a time sufficient to provide a metal coating having a thickness of not less than about 3 mils on said substrate.

38. The article of claim 37, wherein the nebulized metal is selected from the class consisting of Cu, Al, Fe, Sn, Co, Cr, Ni, Pb Cd, Zn and alloys thereof.

39. The article of claim 38, wherein the coating density is not less than 98% of theoretical density.

40. The article of claim 37, wherein said substrate thermally degrades at a temperature less than the melting point of the metal coating.

41. The article of claim 37, wherein said coating surface has indentations in the range of from about .004 inch to about 0.001 inch.