MXPA00000925A - Thermal spray coating for gates and seats - Google Patents

Abstract

A thermal spray powder composition, a coating made using a powder of this composition, and a process for applying the coating. The chemical composition of the powders of the invention comprise a blend of a tungsten carbide-cobalt-chromium material and a metallic cobalt alloy.

Description

COATING OF THERMAL SPRAY FOR GATES AND SEATS Field of the invention The invention relates to a powder composition for thermal spray, a coating made using a powder of this composition, and a process for applying the coating. The invention also relates to the application of the coating to the wear surfaces of gate valves or balloon and landing gear of aircraft and to the surfaces of other components requiring wear resistance.

BACKGROUND OF THE INVENTION This invention relates to the problem of providing low friction surfaces, resistant to wear in components operating under high stresses and frequently in corrosive conditions. A variety of means have been used in attempts to meet these requirements including: hardening of steel surfaces by heat treatment, carburization, nitriding, or implanting; the use of solid ceramic or cermet components; the application of coatings produced by thermal spraying, chemical vapor deposition, physical deposition, electroplating (particularly with chromium); and other techniques. Depending on the application, all of these approaches have limitations. A particularly difficult application is that of high pressure gate valves that open or close at high speed in the oil and gas production industry. Another application that is difficult to satisfy is the coating of aircraft landing gear components where, in addition to wear and friction problems, the fatigue characteristics of the substrate are of particular concern. It is the intention of this invention to provide thermal spray coatings that can satisfy these and a wide variety of other problems. The gate valves consist of a valve body which is axially positioned in pipe or ducts through which the fluid to be controlled flows. Within the valve body is a "gate" which is a solid, usually metallic, rectilinear plate component with a circular hole through it. The gate slides between two "seats" which are circular circular, ceramic, or cermet components with an inner diameter approximately equal to the diameter of the hole in the gate. The seats are aligned coaxially with and directly or indirectly attached to the ends of the tube or pipe within which the valve is located. When the hole in the gate is aligned with the holes in the seats, the fluid flows freely through the valve. When the hole in the gate is partially or completely misaligned with the seats the flow of fluid is impeded or interrupted; that is, the valve is partially or completely closed. To avoid leakage of fluid it is essential that the surfaces in contact between the gate and the seats are very smooth and are held together hermetically. The valves may have springs or other devices within them to hold the seats firmly against the gate. When the valve is closed, the fluid pressure on the upstream side of the valve also presses the gate against the seat on the downstream side. The gate valves are usually operated by sliding the gate between the seats using an actuator attached to the gate with a rod or arrow called a "rod". Using a manual actuator results in a relatively slow gate movement, a hydraulic actuator results in a faster gate movement, and a pneumatic actuator usually results in a very fast gate movement. The actuator must be able to exert sufficient force to overcome static and dynamic friction forces between the seats and the gate. The friction force is a function of the design of the valve and the force of the fluid in the tube when the valve is closed. This friction force can become extremely high when the fluid pressure becomes too high. The adhesion wear of the seats and / or the gate that can occur when the valve is opened and closed can also be a problem and become excessive under high pressure conditions. An additional potential problem is that of corrosion. Oil and gas from many wells can contain very corrosive constituents. A) Yes, for many wells, the valves should be made of corrosion resistant materials, particularly the seats and the gate where corrosion of the surfaces exacerbates wear and friction problems.

For valves operated manually at low pressure, hardened steel seats and gates may be sufficient to combat wear and friction problems. For pneumatic and hydraulic valves at higher pressures, thermally sprayed coatings, such as coatings based on tungsten carbide or chromium carbide, may be sufficient on both gate and seat surfaces. Three of the best coatings of that type are UCAR LW-15 detonation gun coatings, a tungsten-cobalt-chrome carbide coating, UCAR LW-5, a tungsten-nickel-chrome carbide coating, and UCAR LC -1 C, a coating of chromium carbide + nickel-chromium. For some applications it may be appropriate to use a solid cobalt base alloy, Stellite 3 or 6, for the seats with a hardened steel gate. Other approaches have included superimposed arc layers by laser or plasma transfer of Stellite 6 and dew and melted alloys. As the wells become deeper, the pressures increase and the methods described above become inadequate. Two new coatings were developed that have become the benchmark of the industry. One is UCAR LW-26, a coating based on tungsten carbide, described more fully in U.S. Patent No. 4, 173,685. This coating is usually applied by plasma spray followed by a heat treatment. It has outstanding performance characteristics, but it is relatively expensive to produce. The other is UCAR LW-45, a detonation gun coating of tungsten-cobalt-chromium carbide with a unique microstructure that is able to perform well in most of the harsh conditions of today's oil and gas wells. . However, as the wells are drilled even deeper and the pressures become even greater, even these reference point coatings can not meet the requirements for these extreme conditions, and no other solution is currently available. Frequently coatings should be used for wear resistance on components that are very sensitive to fatigue. An example is the cylinder in an aircraft landing gear cylinder. Any coating that would break under stresses imposed on the cylinder due to bending moment during the operation could propagate to the cylinder and cause a fatigue failure of the cylinder with disastrous results. The present coating on the cylinder is electrodeposited hard chrome, which has a negative effect on fatigue which must be compensated with an excessively thick cylinder wall. The chrome plating runs against an aluminum-nickel-bronze bushing or bearing, such that any replacement for the chrome plating should have good equalization characteristics (adhesive wear) with this material as well. In addition, any coating must have good resistance to abrasion in the event that sand or other hard particles are trapped in the bearing. The chromium electrodeposition currently used is only marginally suitable. It should also be noted that the electrodeposition of chromium has very undesirable environmental characteristics, and it would be advantageous to replace it in this and other applications. An alternative to the present system of a hard coating on the cylinder running against a relatively smooth bearing or bearing surface would be to have both surfaces coated with a hard coating. This system would resist abrasion but the coated surfaces should also have a low friction and be resistant to adhesive wear when running against each other. The fatigue effects of a coating have frequently been related to the deformation to fracture (STF) of the coating; that is, the degree to which a coating can be stretched without cracking. The STF has been related, in part, with the residual stress in a coating. The residual tensile stresses reduce the external tensile stresses that must be imposed on the coating to crack it, while the residual compressive stresses increase the added tensile stress that must be imposed on the coating to break it. Typically, the higher the STF of the coating, the lower is a negative effect of the coating that will have on the fatigue characteristics of the substrate. This is true because a crack in a well-bonded coating can propagate to the substrate, initiating a fatigue break and finally a fatigue failure. Unfortunately, most thermal spray coatings have very limited STF, even if they are made of pure metals which would normally be expected to be very ductile and to plastically deform easily instead of breaking.

Thermal spray coatings produced at low or moderate particle velocities during deposition typically have a residual stress stress that can lead to cracking or chipping of the coating if it becomes excessive. The residual stresses usually also lead to a reduction in the fatigue properties of the coated component by reducing the STF of the coating. Some coatings made at high particle speeds, particularly detonation gun and Super D-Gun coatings with very high particle velocities during deposition may have moderate to very high residual compression stresses. This is especially true in coatings based on tungsten carbide. High compression efforts can beneficially affect the fatigue characteristics of the coated component. The high compression efforts can, however, lead to chipping of the coating when it comes to coating sharp edges or similar geometric shapes. Thus, it can be difficult to take advantage of superior physical properties such as hardness, density, and wear resistance of detonation gun and Super D-Gun coatings when such configurations are coated.

BRIEF DESCRIPTION OF THE INVENTION Now, according to the present invention, coatings are provided that meet the requirements of wear resistance and corrosion for many applications including, but not limited to, the examples that only describe components of gate valves and balloon and components of aircraft landing gear.

In addition to wear and corrosion resistance, these coatings must also have low residual stress and high STF to have little or no effect on the fatigue properties of the coated components and make it possible to produce thick coatings and coat complex shapes. The present invention is based on the discovery that a thermally sprayed coating of a mixture of a tungsten-cobalt-chromium carbide material and a cobalt metal alloy provides the low friction and superior resistance to wear and corrosion required for valves of gate that operate at very high pressure with pneumatic actuators, for aircraft landing gear cylinders, and many other applications. The deposited coatings must have not only excellent friction, wear and corrosion characteristics, they must have a very high bond strength on a variety of metal substrates and must have a relatively low residual stress. Any thermal spray deposition process that generates adequate particle velocities can be used to give a dense, well-bonded coating. The coatings of this invention are produced by thermal spray deposition. It is well known that when materials are sprayed thermally they cool quickly on the substrate. This can result in the formation of metastable crystallographic phases or even amorphous materials in some cases. For example, an alpha alumina powder is usually completely fused during the spraying process and then deposited as a mixture of gamma, alpha and other phases. During the thermal spray process minor composition changes may also occur as a result of the reaction with gases in the environment or the gases of the thermal spray or as a result of differential evaporation of one of the constituents of the material being sprayed. Most often the reaction is one of oxidation from exposure to air or carburization if a combustible gas is used as in deposition by detonation gun or high-speed oxy-fuel deposition. A more complete discussion of thermal spray deposition can be found in the following publications: Thermal Spray Coatings, RC Tucker, Jr., in Manual of Deposition Technologies for Films and Coatings, Second Edition, RF Bunshah, ed., Noyes Publicatíons, 1994 , pp. 591 to 639; Thermal Spray Coatings, R. C. Tucker, Jr., in Surface Engineering ASM Handbook, Volume 5, 1994, ASM International, pp. 497 to 509; M. L. Thorpe, Journal of Thermal Spray Technology, Volume 1, 1992, pp. 161 to 171. One of the main constituents of the coatings of this invention is tungsten carbide. Most tungsten carbide powders used in thermal spray are either WC or a combination of WC and W2C. Other phases may be present. Tungsten carbides are most often combined in the powder with some amount of cobalt to facilitate melting and to add cohesive strength to the coatings. Occasionally, chromium is added for corrosion resistance or other purposes. As examples, the cobalt or cobalt plus chromium can be simply combined with the carbide in a dried and sintered dew powder with most of the cobalt or cobalt plus chromium still present as metallic. They can also be combined with the carbide in a mold and ground powder with some of the cobalt or cobalt plus chromium reacted with the carbide. When sprayed thermally, these materials can be deposited in a variety of compositions and crystallographic forms. As used herein, the terms "tungsten carbide" or "WC" will mean any of the crystallographic or compositional forms of tungsten carbide. The terms tungsten-cobalt carbide, tungsten-cobalt-chromium carbide, WC-Co or WC-Co-Cr will mean any of the crystallographic or compositional forms of the combinations of tungsten carbide with cobalt or cobalt plus chromium. Another constituent of the coatings of this invention is a cobalt alloy. As used herein, the term "cobalt alloy" will include any of the crystallographic forms of any cobalt alloy.

DESCRIPTION OF PREFERRED MODALI DADES The chemical composition of the powders of the invention comprises a mixture of tungsten-cobalt-chromium carbide material and a cobalt metal alloy. Note that all compositions herein are in percent by weight not including unavoidable trace contaminants. Preferably the tungsten-cobalt-chromium carbide material comprises tungsten carbide - 5 to 20 cobalt and 0 to 12 chromium, most preferably about 8 to 13 cobalt and 0 or 4 to 10 chromium. The metal alloy is preferably a cobalt alloy with a composition comprising in weight percent 27 to 29 chromium, 7 to 9 tungsten, 0.8 to 1.2 carbon, and the remainder cobalt - particularly preferred is a cobalt alloy having the nominal composition comprising cobalt-28 chromium-8 tungsten-1 carbon (nominally Stellite 6); or, a composition comprising in weight percent 25 to 31 molybdenum, 14 to 20 chromium, 1 to 5 silicon, less than 0.08 carbon, and the rest cobalt - particularly preferred is an aluminum alloy. cobalt having the nominal composition of cobalt-28 molybdenum-17 chromium-3 silicon-less than 0.08 carbon (nominally Tribaballoy 800). Preferably the mixture comprises from 5 to 35 cobalt metal alloy, most preferably from 10 to 30 cobalt metal alloy. The tungsten-cobalt-chromium carbide material is preferably made by the technique of manufacturing mold and ground powder when the chromium content is approximately zero and by a sintering process when the chromium content is from two to twelve. The metallic cobalt alloy is preferably produced by vacuum melting and inert gas atomization. If a detonation gun deposition process is to be used to produce the coating, the tungsten-cobalt carbide powder should preferably be of a size less than 325 standard E. U screening screen. (44 micrometers) and the metallic cobalt alloy of a size less than 270 meshes (60 micrometers), but greater than 325 meshes (44 micrometers) by screening. If other thermal spray deposition techniques are to be used, the powders should have appropriate sizes. The invention is furthermore a process for producing a low friction wear and corrosion resistant coating, comprising the steps: a) forming a powder feed composition comprising a mixture of tungsten-cobalt carbide material and an alloy cobalt metal; and b) thermally depositing, preferably with a particle velocity greater than 500 m / sec., said powder feed of step a) onto a component forming a coating comprising a tungsten-cobalt carbide blended with a cobalt metal alloy. The mixing of the WC-Co-Cr material and the cobalt alloy is usually done in the form of powders before being charged to the powder jet of the thermal spray deposition system. However, it can be done by using a separate powder jet for each of the constituents and feeding each one in an appropriate ratio to achieve the desired composition in the coating. If this method is used, the powders can be injected to the thermal spray device upstream of the nozzle, through the nozzle or to the effluent current under the nozzle. Any thermal spray deposition process that generates a sufficient powder speed (generally greater than about 500 meters / second) to achieve a dense, tightly bonded microstructure with a cohesive force can be used to produce the coatings of this invention. The preferred thermal spray technique is the detonation gun process (e.g., as described in U.S. Patent Nos. 2,714,563 and 2,972,550) with a particle velocity greater than about 750 m / s., and most preferably the Super D-Gun process (e.g., as described in U.S. Patent No. 4,902,539), with a particle velocity greater than about 1000 m / s. The latter process produces a better bonded coating, in some way more dense with high cohesive force that is smoother as it is deposited than the first. Both produce coatings with very high bond strengths and density greater than 98%, measured metallographically. Alternative methods of thermal spray deposition may include plasma spray, high-speed oxy-fuel, and high-speed air-fuel deposition processes. The invention also comprises components having a wear-resistant coating of this invention including, but not limited to, gate or globe valves in which the sealing surfaces of the seats and / or the balloon or gate are coated and train components. of landing aircraft in which the cylinders or their corresponding surfaces (bushings or bearings), are at least partially coated, said coating being a wear and corrosion, low friction coating comprising a mixture of a carbide material of tungsten-cobalt-chromium and a metallic alloy of cobalt. The following examples are provided to further describe the invention. The examples are intended to be illustrative in nature and are not constructed as limiting the scope of the invention.

Example 1 A laboratory wear test has been developed to evaluate materials for use in gate valves as seat or gate materials or coatings. A plate that is approximately 152 mm long, 76 mm wide, and 13 mm thick represents the gate. Three cotter pins that are approximately 6.35 mm in diameter represent the seats. Either the plate or the keys may be made of the same solid material from which the seats and hatches would be made or they may be coated on their corresponding surfaces (one side of 76 x 152 mm of the plate or the flat ends of the keys). The keys are secured in an element which ensures that one end of each key is held against the plate in an annular arrangement with a diameter of approximately 75 mm at a pressure of 1 12.47 MPa on each key. The fixing element is then oscillated through an arc of approximately 100 degrees. Sensors allow the calculation of the key velocity and the coefficient of dynamic friction. Each oscillation is considered a cycle. The keys and the plate are evaluated periodically during the test. The duration of the test is typically 25 cycles. The evaluation of the wear resistance is usually done qualitatively in this test based on the overall appearance of the wear marks on both the cotter pins and the plate. A numerical value is obtained for the dynamic coefficient of friction, but it is considered a relative value, specific for this test. The speed of the keys in relation to the plate that is reached in the test is an indication of the frictional force and general roughness due to wear. Thus, the higher the speed reached, the lower the friction force and the smoother the surfaces remain. A correlation between laboratory test results and operation in field use or actual production is necessary when using such a test to filter materials for use in the field. The operation of Stellite 3 mold seats running against the UCAR LW-45 coated floodgates is well established in the field. This coupling has been used, therefore, as a reference point in the laboratory test. An additional reference point is that of UCAR LW-45 coatings on both the keys and the plate, since this coupling is considered to be the normal reference point of the industry in service. A number of steel plates were coated with the coating UCAR LW-45 by detonation gun, then cemented and coated to a thickness of 100 to 200 micrometers and a surface roughness of less than 8 micrometers Ra. A number of steel keys were coated with UCAR LW-45, UCAR LC-1 C, a Super D-Gun coating of Stellite 6 alloy (SDG Stellite 6), and a Super D-Gun coating of this invention designated herein SDG A. The specific compositions of these materials were as follows: Stellite 3 molding Co- 30.5_ Cr- 12.5_ W UCAR LW-45 WC-10Co-5Cr UCAR LC-1 C Chromium-20 Carbide (Ni-20Cr) SDG Stellite 6 Co-28Cr-8W-1 C SDG A WC-9Co + 25 (Co-28Cr-8W-1 C) The coatings on the keyways and the Stellite 3 mold keys were also cemented and coated to a coating thickness of 100 to 200 micrometers and a surface roughness of less than 8 micrometers Ra. The laboratory test was run using these key materials against the plates coated with UCAR LW-45 with the results shown in the following table. Friction Material of Key Speed Value Wear Stellite 3 mold 30.5 2.3 Baseline-Moderate 30.5 2.1 Baseline-Moderate UCAR LW-45 54.9 1 .8 Baseline 48.8 1 .9 Baseline SDG Stell 6 45.75 2. 1 Similar a Baseline UCAR LC-1 C 51 .85 2.1 Baseline SDG A 48.8 1 .3 < < Baseline - light 61 * 0 0..55 * < < Baseline - lightweight the plate was somewhat smoother in this test.

The speed measurement is in m / sec. Both, the speed measurement and the relative friction dynamic coefficient value shown in the table are approximate average values for the cycles from 12 to 25, which represent the stabilized behavior of the wear coupling. It is evident that the Super D-Gun Stellite 6 coating worked better than the baseline coating in this test. However, the new coating of this invention, SDG A, performed much better than both coatings the baseline and the Stellite 6.

Example 2 A common test for corrosion resistance of materials is a salt spray test defined by a standard of the American Society for Testing and Materials, ASTM B 1 17. In this test the samples are exposed to a spray mist saline for a period of 30 days at a temperature of 33.3 to 36.7 ° C. The operation of a coating of this invention, SDG A, described in Example 1, was evaluated by coating a sample of AISI 4140 steel that was 76 mm thick. width, 127 mm long, and 12.5 mm thick over most of a face of 76 x 127 mm. A portion of the face was left uncoated to simulate the cutting or masking line present in many valve gates. Two coating thicknesses were applied. The coatings were then sealed using an epoxy-based sealer. Finally, the coatings were cemented to a thickness of either 100 to 130 micrometers, which represent the typical thickness of a new part, or to a thickness of 250 to 280 micrometers, which represent the thickness in a reworked part. The samples were then subjected to the test. After 30 days of exposure, the samples were cleaned and examined. There was no evidence of general corrosion, pitting, or crack of the coating. In contrast, the uncoated areas of the steel were heavily corroded, as expected.

Although the preceding salt spray test is very useful in filtering materials for many corrosive applications, it does not adequately represent those situations where a significant amount of hydrochloric acid is present. In these situations, the cobalt-based alloy used in SDG A can be attacked. A better selection in these situations may be a coating similar to SDG A, but with WC-Co material modified to include 4 to 12 Cr or a coating comprising WC-Co-Cr + 25 (Co-28Mo-17Cr- 3Si- <0.08C).

Example 3 The abrasive wear resistance of materials is often characterized using a dry sand wheel "rubber" test ASTM G 65-94. This test is useful in materials rated relatively for abrasive wear resistance in applications such as seals or bearings where abrasive particles may be embedded in the seal or bearing surface. Thus, test results can be useful when selecting materials for aircraft landing gear cylinders where particles of sand or other hard particles can get trapped on the surface of the brass bearing. 6 coatings per detonation gun of this invention were applied to test samples of AISI 1018 steel using a single powder with a composition of WC-9Co + 25 (Co-28Cr-8W-1 C). The microstructures and mechanical properties of the coatings were varied somewhat by varying the deposition parameters. The coatings were designated SDG B, C, D, E, F, and G. The wear tests were run at a speed of 144 m / min. under a load of 130 N (13.62 kg) during 3000 revolutions for the wheel which had an outer layer of polyurethane in contact with the coated test specimen. The bit was fed between the wheel and the Ottawa silica sand test sample with a nominal size of 212 micrometers. The wear marks were measured by weight loss of the coated sample converted to loss of volume and reported as an average loss per 1000 revolutions.

Brand Vol. Coating, mm / 1000 rev. SDG B 3.61 SDG C 3.69 SDG D 4.83 SDG E 4.85 SDG F 4.96 SDG G 4.69 UCAR LW-45 1.5 UCAR LC-1C 3.88 WC-Co Spray with Plasma 5.6 Cr Electrodeposited 8 to 10 It is clear that the coatings of this invention have an abrasive wear resistance that is substantially greater than that of electrodeposited hard chromium. Thus, they should be excellent replacements, on this basis, for electrodeposited hard chrome in applications such as coatings on aircraft landing gear cylinders if other restrictions are met. In this test the coatings of this invention have less wear resistance than that of the UCAR LW-45 coating by detonation gun, but that is to be expected due to the larger volume fraction of tungsten carbide in the UCAR LW-45. Surprisingly, they have greater resistance substantially than the plasma analog sprayed from UCAR LW-45. They are comparable in wear resistance to the UCAR LC-1 C coating of chromium carbide by detonation gun.

Example 4 The residual stress characteristics of the coatings of this invention described in Example 3 were evaluated and compared with the other coatings by coating Almen strips and measuring their deviations. The test is a modification of the one described in the US Military Specification for hammering shot Mil F-13165B. A positive deviation indicates a residual tension stress in the coating, while a negative value indicates a compressive stress. Samples of the Almen test were made of AISI 1070 steel heat treated to a hardness of HRA 72.5 to 76. They were 76.2 x 19.05 x 0.79 mm coated on one side of 76.2 x 19.05 mm with a coating of approximately 300 mm thick. The stress for fracture (STF) of the coatings was calculated by coating bars of AISI 4140 steel of 25.4 x 1.27 x 0.635 cm heat treated for HRC of 40 on one side of 25.4 x 1.27 cm for a thickness of 300 micrometers and then bending the bars in a 4-point bend test element. The initiation of the fracture was detected with a sonic sensor attached to the bar. The STF is a value without units reported in micrometers / centimeter or tenths of a percent.

Almen coating, mm STF, mm / cm SDG B +0.0254 0.0238 SDG C -0.1778 0.0348 SDG D -0.1778 0.0367 SDG E -0.0635 0.0296 SDG F -0.2413 0.0380 SDG G -0.2286 0.0374 SDG WC-15CO -0.6223 0.0387 SDG WC-10Co -0.1651 0.0180 D-Gun WC-15Co -0.04064 0.0180 First consider the Almen deviation information as an indication of residual stress. It is apparent that the residual stresses in the coatings of this invention are very low and can be changed from very slight tension to some compression by changing the deposition parameters, at least when using Super D-Gun deposition. This implies that coating complex shapes such as sharp edges should not be a problem and that thick coatings can be deposited without cracking or chipping. Then consider the STF information which is the unifying of the coating defect in the fatigue properties of the substrate; that is, a high STF is generally an indication that the coating will have little effect on the fatigue properties of the substrate. Note that the D-Gun WC-15Co coating has a low STF (even though it has a very low residual compression stress) and is known to have a significant detrimental effect on the fatigue properties of steel, aluminum and titanium substrates. The coating Super D-Gun WC-10Co has a somewhat higher compression residual effort, but no better STF. The Super D-Gun WC-15Co coating has a significantly higher STF and is known to have little or no effect on the fatigue properties of steel, aluminum or titanium substrates. However, this is achieved only with a very high residual compression stress, which makes it difficult to coat complex shapes or thick coatings. In contrast, the coatings of this invention can be deposited under conditions that give coatings with a high STF and relatively low residual compression stress. This suggests that the coatings will have little effect on the fatigue properties of the substrate and still be able to be applied to complex and very thick shapes without difficulty. These attributes should make them very useful in fatigue-sensitive components such as components of aircraft landing gear. Various other modifications of the embodiments described, as well as other embodiments of the invention, will be apparent to those skilled in the art by reference to this disclosure, or may be made without departing from the spirit and scope of the invention defined in the appended claims.

Claims (7)

REIVI N DICACIONES

1 . A powder composition for thermal spray comprising a mixture of a tungsten-cobalt-chromium carbide material and 5 to 35 weight percent of a cobalt alloy.

2. The powder composition of claim 1 wherein the mixture comprises tungsten-cobalt-chromium carbide and 10 to 30 weight percent of the cobalt alloy.

3. The powder composition of claim 1 wherein the tungsten-cobalt-chromium carbide material comprises tungsten carbide, 8 to 13 weight percent cobalt and 4 to 10 weight percent chromium.

4. The powder composition of claim 1 wherein the cobalt alloy comprises in weight percent 25 to 31 chromium and 0.5 to 1.5 carbon. The powder composition of claim 1 wherein the cobalt alloy comprises in weight percent 25 to 31 chromium, 5 to 1 1 tungsten, 0.5 to 1.5 carbon and the remainder cobalt . 6. The powder composition of claim 1 wherein the cobalt alloy comprises in weight percent 25 to 31 molybdenum, 14 to 20 chromium, 1 to 5 silicon, less than 0.08 carbon, and the rest of cobalt. 7. A process for producing coatings comprising the steps of: