WO1999052651A1

WO1999052651A1 - A method for modifying a workpiece surface using a high heat flux process

Info

Publication number: WO1999052651A1
Application number: PCT/US1999/008438
Authority: WO
Inventors: Vinod K. Sikka; Craig A. Blue; Edward C. Hatfield; Catherine J. Swindeman; Evan K. Ohriner
Original assignee: Lockheed Martin Energy Research Corporation
Priority date: 1998-04-16
Filing date: 1999-04-15
Publication date: 1999-10-21
Also published as: AU3568299A

Abstract

A method for forming a workpiece coating having desired coating characteristics is provided. The method employs infrared (IR) radiation to melt the coating while maintaining a lower workpiece core temperature. As a result, the method precludes undesirable microstructural modification of the underlying workpiece. The desired characteristics of the coating are produced by controlling the intensity of, and the time of exposure to, the IR radiation.

Description

A METHOD FOR MODIFYING A WORKPIECE SURFACE USING A HIGH HEAT FLUX PROCESS

Statement Regarding Federally Sponsored

Research or Development This Invention was made with government support under contract DE-AC05-96OR22464, awarded by the United States Department of Energy to Lockheed Martin Energy Research Corporation, and the United States Government has certain rights in this invention.

Field of the Invention This invention relates generally to depositing and treating material coatings, and more particularly to a method for depositing and modifying a workpiece coating using infrared (IR) radiation.

Background of the Invention The application of protective material coatings to tools and other components is well known. Coatings are commonly applied to provide wear, corrosion, thermal and radiation protection to an underlying workpiece. Various methods have been developed for applying such coatings. Although known deposition methods are generally useful for applying coatings, it is sometimes preferable to provide a post-deposition surface treatment to modify the coating microstructure or to enhance adhesion between the coating and underlying workpiece.

Thermal spraying is one example of a coating deposition process requiring post-coating treatment. Thermal spraying is becoming an increasingly popular deposition technique due to its efficiency, cost-effectiveness, and adaptability to a wide range of coating applications. However, it is not uncommon for a thermal spray coating to exhibit less-than-desirable cohesive strength and/or less-than desirable adhesion to the underlying substrate. In addition, thermally- sprayed coatings often exhibit high porosity. A variety of post-deposition

1 treatments have been developed to modify or enhance coating characteristics, including flame heating, microwave heating, induction and vacuum heating, inert atmosphere heating, controlled atmosphere heating, and laser glazing, to name just a few. While the aforementioned post-deposition techniques are generally useful for improving coating characteristics, they share the undesirable effect of heating, and thereby altering the microstructure of, the underlying workpiece. In some instances, the material structure of the underlying substrate is weakened as a result of heat transferred through the treated coating. Existing coating techniques also tend to be difficult to control and are not well-suited for the treatment of components having complex-shaped surfaces.

Clearly, a need exists for a method of treating a workpiece coating that does not result in undesirable microstructural changes in the underlying workpiece. It would be advantageous for such a method to also be easy to control and adaptable for use with complex-shaped workpiece geometries.

Summary of the Invention A method is provided for heat treating the surface coating of a workpiece without negatively affecting the structural characteristics of the underlying workpiece. Initially, a workpiece is supplied with a coating. The coating is then treated using an infrared (IR) radiation high heat flux process. Infrared radiation rapidly heats the coating material while the temperature of the body, or core, of the workpiece is maintained at a substantially lower temperature. The heating temperature is accurately controlled by varying the intensity of the IR radiation and the time of exposure to the IR radiation source. The intensity and time of exposure are varied, depending on characteristics of the workpiece core and coating materials, as well as the microstructural modification desired. Particular applications of this method may incorporate nonuniform and/or noncontinuous heating profiles. Regardless of the profile used, the IR intensity and exposure time may be controlled to prevent microstructural alteration of the workpiece core.

Infrared heating rapidly increases coating density, eliminating pores formed in the coating during deposition. Infrared heating also improves the cohesiveness of the coating material and/or the adhesion of the coating material to the workpiece surface. In some instances, coating adhesion to the workpiece is accomplished by partially melting the workpiece surface to enhance diffusion of the workpiece surface into the coating material. Other modifications or enhancements that are attainable with this method include sintering, alloying and precipitation. These coating modifications can, in turn, be used to perform fusing or hardening of a coating or deposit, enhance joining of a coating to a workpiece substrate, or modify composition or microstructural features to achieve specific mechanical, chemical, or electrical properties.

In an alternate embodiment of the present invention, a method is provided for depositing, and subsequently heat treating, a metal, ceramic,, or cermet material on the surface of a metal or ceramic workpiece. The coating material is provided as a powder. The coating powder is mixed with a liquid suspension medium which functions as a binder, facilitating application of the powder to the workpiece surface. For most applications, a low melting temperature metallic binder is added to the coating mixture. The powder and suspension medium are mixed to produce a homogeneous paint or slurry for deposition on the workpiece surface via brush- or spray-painting.

The coating is subjected to an IR radiation heating profile. Upon heating, the polymeric suspension medium is burned out. In some cases, a portion of the workpiece surface may diffuse into the coating. In some instances, the coating material facilitates the diffusion process. For instance, when carbon steel is coated with tungsten-carbide and then heated using IR radiation, carbon from the carbide dissolves into the steel and significantly lowers the melting point of the steel. For other applications, a low melting temperature metallic binder, such as a solder or braze alloy, is added to the coating mixture to facilitate bonding. The time of exposure to IR radiation is varied to control the extent of base metal dissolution into the coating, thereby controlling the thickness and final composition of the coating. Nonuniform heating profiles may be applied across a target surface area to produce coating thickness or coating composition gradient structures. It is preferred that IR heating be performed in an inert atmosphere to minimize oxidation. For example, we have found that an argon- hydrogen(4%) atmosphere works well.

Detailed Description of Preferred Embodiments

While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description.

A method is provided for heat treating the surface coating of a workpiece, without negatively affecting the microstructural characteristics of the underlying workpiece. The term "workpiece," as used herein, refers to a structure or body of material having a surface coating requiring heat treatment. For example, tools and equipment used for cutting and grinding often require surface coatings having particular characteristics such as good hardness. The workpiece may comprise a metal, ceramic, polymer, composite or some combination thereof.

The workpiece may be provided with or without a coated surface. In the latter instance, it is necessary to initially deposit a coating. A number of coating deposition techniques are available. An excellent example of a coating process is thermal spraying. Thermal spraying is adaptable to the deposition of ceramics, metals and metal alloys, polymers, composites, ceramic-metals and multi-component, graded, or multilayered combinations of these materials. Formation of a coating having desired characteristics is accomplished by heat- treating the coating using a high heat flux process. In this method, heat treatment is accomplished using infrared (IR) radiation. In contrast to commonly used coating treatments, IR radiation heating provides a means for rapidly heating the coating material while maintaining a substantially lower workpiece substrate temperature. Infrared radiation heating is preferably performed in an IR heating furnace. A variety of IR sources are available. For instance, Infrared Technologies, LLC, located in Oak Ridge, Tennessee, manufactures specialized IR furnaces which incorporate tungsten- halogen based IR sources. A more powerful IR furnace, incorporating a plasma- based IR source, is manufactured by Vortek, Inc. of Vancouver, Canada. This particular plasma-based furnace operates as a line-focus type system, whereby the coating is treated by scanning across the coating surface.

By maintaining the workpiece temperature below a critical value, the coating is modified while controlling the microstructure of the underlying workpiece material. The temperature to which the coating is heated is accurately controlled by varying the intensity of IR radiation and the time of exposure to the IR radiation source. The intensity of IR radiation and time of exposure to IR radiation will vary, depending on characteristics of the workpiece and coating materials, and the coating modification or enhancement desired. For most applications, the IR exposure time ranges from 5 to 300 seconds, with an exposure time of 30 to 60 seconds preferred. The preferred IR intensity, or heat flux density, will generally range up to a maximum value of about 3,500 Watts/cm². However, these variables are application specific and may be deviated from. For instance, particular applications may incorporate nonuniform and/or noncontinuous heating profiles.

Infrared heating rapidly increases coating density by eliminating pores formed in the coating during deposition. IR heating is also used to improve the cohesiveness of the coating material and/or the adhesion of the coating material to the workpiece surface. It may be desirable to heat a portion of the workpiece surface, in addition to heating the coating, such that the microstructure of the heated portion of the workpiece surface is altered. The degree to which the workpiece surface microstructure is altered depends on a number of factors, including the respective workpiece and coating materials used, and the microstructural properties desired. The step of IR heating may be controlled to initiate various material microstructure modifying mechanisms, including sintering, alloying and precipitation. In the present method, "sintering" refers to densification and chemical bonding of adjacent particles which is effected by heating to a temperature below the melting point of both the workpiece and coating materials. Sintering may occur at the interface between the coating and the underlying workpiece surface to improve interfacial adhesion. In addition, sintering may occur within the coating material itself, to improve densification and mechanical strength of the coating material. The term "alloying" refers to heating the workpiece and coating materials above their respective melting points to produce an interface comprising a mixture of the workpiece and coating materials. Alloying is a desirable mechanism for producing improved adhesion between the coating and underlying workpiece surface. The term "precipitation" describes a material modification process whereby the material being modified, i.e., the coating and/or the workpiece surface, is heated to produce a new solid phase which gradually precipitates within the particular solid alloy material as a result of slow, inner chemical reaction. This type of reaction is generally effected to harden the particular material.

The present method can be performed in vacuum, air, or controlled and inert environments. Infrared heating is unique in that it can be applied to complex surface geometries with nominal effect on heating system geometry. Commonly used high heat flux methods require physical coupling to the coated surface, for example, with an induction coil. However, where the workpiece surface comprises an obscure geometry, a typical induction coil will not couple uniformly to the entire surface. Therefore, avoiding nonuniform heating of the coating surface requires specially designing a coil which follows the contours of the particular workpiece. Using the instant IR heating method, the specific intensity of the thermal energy may decrease as a function of distance between the IR source and the coating surface due to dispersion of the radiation. However, in contrast to known methods, this decrease in energy is nominal.

Therefore, regardless of surface geometry, the workpiece coating can be uniformly heated. The instant method provides the further advantage of enabling the flexibility to heat, and thereby treat, a specified portions of a surface. This is possible since the IR radiation may be directed or focused toward a particular area. The method described herein has been successfully applied to a variety of coatings which, historically, have proven difficult to modify. For example, the method has been used to uniformly flux and sinter powder coatings over entire surface areas at a time, effectively eliminating residual coating porosity without heating the underlying substrate to the sintering temperature. Although other methods have been used to sinter an entire coating surface at the same time, without heating the underlying substrate, they typically produce inconsistent results over the treated area. Nonuniform sintering is further exacerbated when irregular surface geometries are being treated. In contrast, this method is useful for effectively sintering powder coatings across workpiece surfaces having complex geometries.

The present method has also been applied to non self-fluxing alloys. The term "self-fluxing" refers to coatings containing elements for dissolving oxides, facilitating wetting of the coating to the underlying workpiece substrate. Coatings which are not self-fluxing typically must be treated in a special atmosphere to prevent oxidation. Furthermore, the absence of a fluxing element hinders wetting to a workpiece surface. Aluminum alloy substrates (2.54cm x 2.54cm x 0.635cm) were thermal spray coated with aluminum. The samples were unidirectionally heated in an IR furnace to heat the surface coating and fuse pores formed in the coating. The workpiece coating was exposed to IR radiation, heating the coating to a temperature of 950°C for 60 seconds. This was accomplished without melting the underlying aluminum substrate, using a water-cooled backing plate, despite a substantially lower substrate melting point temperature of only 660°C.

In an alternate embodiment of the instant invention, a method is provided for depositing, and subsequently heat treating, a metal, ceramic or combination thereof, on the surface of a metal or ceramic workpiece. The coating material is provided in powder form. The coating powder may be mixed with a liquid suspension medium, or carrier, to form a slurry. The term" slurry" is used to describe a mixture having a watery consistency and comprising insoluble matter in a liquid. The carrier facilitates application of the powder to the workpiece surface. For example, the liquid carrier may consist of alcohol, a water-alcohol mixture, an alcohol-ethylacetoacetate mixture, or an alcohol-acetone mixture, to name just a few. The carrier acts as a medium for carrying or transporting the coating materials to the workpiece surface. The carrier is typically evaporated during the coating curing process. There are a number of commercially-available suspension media which can be used. For example, experiments were performed using HPC, the commercial designation of a carrier medium manufactured by ZYP^® Coatings, Inc. of Oak Ridge, Tennessee. This particular suspension medium consists of 98% water and 2% Mg-AI-silicate.

For some applications, a low melting temperature binder is added to the coating mixture. The binder acts as a glue to hold the coating materials together. In some instances, the binder material, like the carrier, is lost during the curing process. In other instances, the binder may remain in the cured coating, acting as a matrix material.

The slurry may comprise additional components for controlling physical characteristics of the slurry. For example, surface active agents, or surfactants, such as sodium lauryl sulfate, polyvinyl alcohol and carbowax, may be added to maintain suspension of the solid phase. Lubricants, such as stearic acid, may be added to assist in consolidation of the slurry components.

The slurry is deposited on a workpiece surface, preferably by brush- or spray-painting. However, it will occur to one skilled in the art that alternate deposition methods are available. For example, the workpiece could be immersed in the mixture or the slurry could be spray-dried upon the workpiece. The time of exposure to IR radiation determines the extent of base metal dissolution into the coating. Therefore, time of exposure can be used to control both the thickness and final composition of the coating. Nonuniform heating profiles may be applied across a target surface area to produce coating thickness

8 or coating composition gradient structures. It is preferred that IR heating be performed in an inert atmosphere to minimize oxidation. For example, we have found that an argon-hydrogen(4%) atmosphere works well.

For applications in which the aforementioned diffusion mechanism is not as effective, a low melting temperature metallic binder, such as a solder or braze alloy, can be added to the coating mixture. For example, a metallic matrix may be incorporated when a ceramic coating is being applied to a metal workpiece surface. The term "low melting temperature" refers to the fact that the metallic binder has a melting point below the melting point of the coating powder and the workpiece material. Upon melting, the metallic matrix wets to the workpiece surface and wets/embodies the coating powder particles. Thus, the binder forms a metallic matrix having a hard reinforcement material formed therein.

This method has been successfully implemented to deposit a variety of coatings on both metal and ceramic workpiece substrates. Some examples are provided below. Corresponding cross-sectional micrographs are provided to further illustrate the successful implementation of the present method.

Example 1 Self-fluxing, hard-facing material coatings were thermally-sprayed on

4140 steel and then heat treated with IR radiation. The coatings were heated to 1 100-1 1 50°C for approximately 2 minutes. Cross-sections of the pre- and post- treatment interface structure are depicted in Micrographs 1 a and 1 b, respectively.

Example 2

A tungsten-carbide powder (containing 10% cobalt binder) was blended in 50 wt% HPC media (98% water, 2% Mg-AI-silicate) . The resulting blend was painted on a clean carbon steel surface. Specimens were air dried and heated in an IR furnace. The coatings were exposed to a heating cycle of 1 350°C for 30 seconds + 1 1 50°C for 1 5 minutes. The resulting coating had an average thickness of 65 micrometers. The treated coating exhibited no porosity and excellent bonding to the base metal. A cross-section of the resulting structure is depicted in Micrographs 2a (unetched) and 2b (etched with 2% nital) .

Example 3

A tungsten-carbide powder (containing 1 3.8 wt% nickel-aluminum alloy binder) was blended in 50 wt% HPC media (98% water, 2% Mg-AI-silicate). The blend was painted on carbon steel substrates. The coated samples were air dried and heated in an IR furnace. A heating profile of 1 350°C for 1 0 seconds resulted in a fully dense coating. A cross-section of the resulting structure is depicted in Micrographs 3a (unetched) and 3b (etched with 2% nital) .

Example 4 A blend comprising 50% boron in HPC media (98% water, 2% Mg-AI- silicate) was blended and painted onto clean samples of carbon steel. The painted samples were air dried and exposed in an IR furnace to 1 350°C for 30 seconds. The resulting coating comprised an iron layer enriched with boron. Enrichment of boron is defined by the depth of diffusion of boron into the iron layer and is dependent upon heating time. A cross-section of the resulting structure is depicted in Micrographs 4a (unetched) and 4b (etched with 2% nital) .

Example 5

A tungsten-iron blend (25 wt% iron) was blended with 50 wt% HPC media (98% water, 2% Mg-AI-silicate) . The blend was painted on a ceramic substrate and exposed to an IR profile of 1 350°C for 20 seconds. The final coating was 30 micrometers thick and exhibited excellent adhesion to the ceramic substrate. A cross-section of the resulting structure is depicted in

Micrographs 5a (magnified 200x) and 5b (magnified 1 000x) .

1 0 Example 6

The present method was used to treat a component known as a "bobbin traveler. " A bobbin traveler is C-shaped steel component used in the textile industry to manufacture thread. Materials fed through the bobbin traveler tend to wear down, and thereby reduce the life of, the component. In an effort to provide a component with increased wear resistance, samples were painted with WC/Co and IR treated . The IR treatment comprised heating the coated component to approximately 1 350°C for a few seconds in an argon- 4%hydrogen atmosphere. The component was subsequently cooled. The method resulted in a well-coated component having excellent microhardness. A cross-section of the resulting structure is depicted in Micrographs 6(a)-(e) .

Numerous applications for the methods embodied in the present invention have been found. Some sample applications include:

(1 ) Deposition and treatment of aluminum block engine cylinder liners. The implementation of aluminum block engines requires liners for the cylinder walls. Typically, steel sleeve inserts are used. However, recent efforts by the Department of Energy and automobile manufactures, such as General Motors and Ford, have focused on thermally spraying the aluminum block cylinder liners, in lieu of using inserts. There are a number of issues associated with the application of steel coatings on aluminum substrates. For example, the inefficiency of the operation, negative effect on material microstructure, and inadequate mechanical adhesion between the steel and aluminum. The instant invention can be used to efficiently deposit and treat such a coating structure without influencing the metallurgical properties of the substrate. The system could be configured to match the geometry of the cylinder. Optimal performance, microstructure and adhesive properties could be achieved by increasing the density of the coating or liner insert, and fusing the interface between the steel coating/liner and the aluminum block substrate.

(2) Gun Barrel Inserts or Liners. Materials required to provide enhanced, improved or optimum performance are difficult to manufacture, especially as gun barrel liners, liner inserts, or liner coatings. The present coating deposition and

1 1 treatment methods are amenable for use with these particular materials (e.g., Ta, Mo, Re, etc.) . Samples of thermally-sprayed coatings incorporating these materials were produced and treated to produce virtually 1 00% densification and enhanced mechanical strength. (3) Coatings for Molten Glass Tooling. Deposition and/or post treatment of thermally-sprayed tungsten coatings using the instant method may be used to produce mechanically enhanced cost-effective coatings.

(4) Formation & Treatment of Abrasive Grinding Tools. This method may be used to manufacture an enhanced abrasive surface on a tool. A variety of abrasives can be used, including: WC, AI₂O₃, diamond, glassy carbon, B₄C, and

Cubic BN in metal-bonded, resin or organic bonded, and vitrified matrices.

(5) Improved Thermal Barrier Coatings. Hollow ceramic structures such as hollow ceramic spheres and cylinders can be incorporated into the coating structure of thermal barrier coatings using the present coating method. Thermal barrier coating ceramic microstructures provide thermal insulation of the underlying substrate. However, the high porosity associated with thermal barrier coatings produced using existing coating methods results in low mechanical coating strength. The present invention provides a better, enhanced, controlled, reproducible method of accomplishing a specific engineered porosity level. As a result, thermal conductivity of the thermal barrier coating can be controlled without seriously impacting the mechanical strength of the coating.

(6) Beryllium Consolidation, Densification and Performance Enhancement. The present method can be applied to a variety of applications where it is desirable to consolidate or densify beryllium coatings and/or components. One such application is the treatment of target chamber walls of thermonuclear reactors. Our experiments have shows that densified Beryllium manifests improved thermal conductivity and mechanical strength.

(7) Diesel Engine Enhancement & Repair. Applications include cylinder liners, piston rings, piston crowns, fuel injection components, bushings, water pumps, turbo pumps and cam shafts, to name a few.

1 2 (8) Steel Arresting Hooks of Aircraft Carriers. Investigations suggest that the hard facing and manufacture of F/A-1 8 hookpoints can be significantly improved using the present coating treatments. Known methods for fusing hardfacing materials on hookpoints are more of an art than a reproducible manufacturing process. In addition, several post-deposition heat treatments are typically required using known methods. As a result, it has been difficult for manufacturers to achieve a high-yield cost-effective manufacturing process. The present invention provides a means of fusing these thermal sprayed coatings in a reproducible, cost-effective manner without detrimentally affecting the metallurgical state of the underlying substrate.

While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention.

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Claims

What is claimed is:

1 . A method for treating a workpiece coating, the workpiece having a core and at least one surface portion, said coating disposed upon said at least one surface portion, the method comprising the steps of: providing a source of infrared (IR) radiation; and exposing said workpiece coating to said IR radiation, said exposure effecting a microstructural modification of said workpiece coating.

2. A method as recited in claim 1 , further comprising the step of controlling said IR radiation exposure to prevent microstructural modification of the workpiece core.

3. A method as recited in claim 1 , wherein the step of exposing further comprises directing said IR radiation toward a specific portion of said coating.

4. A method as recited in claim 1 , further comprising the step of melting said workpiece coating.

5. A method as recited in claim 4, wherein the surface portion of said workpiece at least partially diffuses into said coating.

6. A method as recited in claim 1 , wherein said workpiece coating comprises a self-fluxing alloy.

7. A method as recited in claim 1 , wherein said workpiece coating comprises a non-self-fluxing alloy.

8. A method as recited in claim 1 , wherein said workpiece coating is exposed to IR radiation for about 5 to 300 seconds.

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9. A method as recited in claim 8, wherein the preferred exposure time is about 30 to 60 seconds.

1 0. A method as recited in claim 1 , wherein said IR radiation has a heat flux density, said heat flux density having a maximum value of about 3500 Watts/cm².

1 1 . A method as recited in claim 8, wherein said IR radiation has a heat flux density, said heat flux density having a maximum value of about 3500 Watts/cm².

1 2. A method for forming a coating on a workpiece, comprising the steps of: providing a workpiece having a core and at least one surface portion; depositing a coating upon said at least one surface portion; and exposing said coating to infrared (IR) radiation.

1 3. A method as recited in claim 1 2, further comprising the step of drying said coating.

1 4. A method as recited in claim 1 2, further comprising the step of controlling the intensity and time of said exposure.

1 5. A method as recited in claim 1 4, wherein said intensity and time of exposure are controlled to prevent microstructural modification of the workpiece core.

1 6. A method as recited in claim 1 4, wherein said intensity and time of exposure are controlled to form a coating having a gradient structure.

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1 7. A method as recited in claim 1 6, wherein said gradient comprises a coating thickness gradient.

1 8. A method as recited in claim 1 6, wherein said gradient comprises a coating composition gradient.

1 9. A method as recited in claim 1 2, wherein said step of depositing comprises thermal spraying.

20. A method as recited in claim 1 2, wherein said workpiece coating is exposed to IR radiation for about 5 to 300 seconds.

21 . A method as recited in claim 20, wherein the preferred exposure time is about 30 to 60 seconds.

22. A method as recited in claim 1 2, wherein said IR radiation has a heat flux density, said heat flux density having a maximum value of about 3500 Watts/cm².

23. A method as recited in claim 20, wherein said IR radiation has a heat flux density, said heat flux density having a maximum value of about 3500 Watts/cm².

24. A method for forming a coating on a workpiece, comprising the steps of: providing a workpiece having a core and at least one surface portion; providing a coating mixture; applying said mixture to said at least one surface portion; and exposing said coating mixture to infrared (IR) radiation.

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25. A method as recited in claim 24, further comprising the step of drying said coating.

26. A method as recited in claim 24, further comprising the step of controlling said IR radiation exposure to effect a microstructural modification of at least the surface portion of said workpiece.

27. A method as recited in claim 26, wherein said exposure is controlled to effect at least a partial diffusion of said at least one surface portion into said coating.

28. A method as recited in claim 26, wherein said exposure is controlled to effect sintering of said coating to said at least one surface portion.

29. A method as recited in claim 24, wherein said coating mixture includes a powder.

30. A method as recited in claim 24, wherein said coating mixture includes a binder.

31 . A method as recited in claim 24, wherein said step of exposing is performed in an inert atmosphere.

32. A method as recited in claim 31 , wherein said inert atmosphere comprises an Argon-Hydrogen atmosphere.

33. A method as recited in claim 24, wherein the workpiece is a metal or a ceramic.

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34. A method as recited in claim 24, wherein the coating mixture includes a metal or metal mixture.

35. A method as recited in claim 24, wherein the coating mixture includes a ceramic material.

36. A method as recited in claim 35, wherein the ceramic material is an oxide, a carbide, a boride or a nitride.

37. A method as recited in claim 24, wherein the coating mixture includes a ceramic-metal material.

38. A method as recited in claim 24, wherein said step of applying comprises brushing the mixture onto said at least one surface portion.

39. A method as recited in claim 24, wherein said step of applying comprises spraying the mixture onto said at least one surface portion.

40. A method as recited in claim 24, wherein said step of applying comprises immersing said at least one portion in said coating mixture.

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