NICKEL-CHROMIUM CARBIDE POWDER AND SINTERING METHOD

Abstract: A powder which consists essentially of chromium and nickel with the balance being from 50 to 95 percent by weight chromium carbide, with the chromium and nickel being present in a weight ratio of about 1 part by weight chromium to 4 parts by weight nickel. Also, producing the powder by the process, of agglomerating a blend of the constituents and sintering the agglomerates in a high temperature reactor.

Description: MCKEL-€HROMIUMCARBIDEPOWDERANDSINTERINGMETHOD BACKGROUND OF INVENTION This invention relates to a powder for thermal spray applications and a process to produce it. 5 These powders require various agglomeration methods to make free flowing powders from normally non-flowing small particles. One such agglomeration method is spray drying. Agglomerates are formed in spray drying by atom¬ izing a slurry of powder, binder and liquid into a dry- 10 ing chamber where the liquid is evaporated. The result is a generally spherical agglomerate held together by. the binder. O.S patent 3,617,358 describes an agglomer¬ ation process using an organic binder. Other agglomeration processes have been developed 15 to overcome what may be undesirable effects caused by the presence of organic binders. In some cases, the organic binder may cause fouling of the plasma gun due to vaporization of the organic. The presence of organ- • 3 ics may even decrease the apparent density of the powder m .20 or affect the flame spray coating. In U.S. patent 3,881,911 to Cheney et al., the agglomerates are presin- tered to remove the binder. U.S. Patent 3,973,948 to Laferty et al. uses a water soluble ammonia complex as a binder and U.S patent 4,025,334 to Cheney et al. uses an 25 aqueous nitrate solution. SUMMARY OF INVENTION Chromium carbide (Cr3C2) in combination with nickel-20% chromium powders are used to produce plasma- spray coatings for jet turbine engine applications. 30 These powders, as presently produced, are mechanical blends of the two components. As a result they have a tendency to become segregated both during shipment and durinq thermal spraying, yielding coatings which are not completely homogeneous. It is therefore desirable to have a powder which is comprised of particles each of which contains like amounts of both constituents. In accordance with the present invention, there is provided an agglomerated powder which can be plasma densified to a thermal spray powder of a substantially uniform composition consisting essentially of nickel and chromium with the balance being about 50 to about 95 percent chromium carbide, said nickel and chromium being present in a weight ratio of about 1 part chromium to about 4 parts nickel. The chromium may be partially or completely combined with the nickel to form a nickel- chromium alloy. DETAILED DESCRIPTION For the thermal spray powder ' s of the present inven¬ tion, a powder blend is prepared consisting of 1 part chromium to 4 parts nickel with the balance chromium carbide. The amount of chromium carbide present may be varied from 50 to 95 percent. The nickel and chromium in the blend may be present as a mixture of elemental nickel powder and elemental chromium powder or as an alloy of nickel and chromium provided the ratio of chro¬ mium to nickel is about 1:4. Preferably the overall powder blend has an average particle size less than about 10 microns. The ratio of chromium to nickel is described in terms of the ratio of chromium to nickel excluding the chromium in the chromium carbide. The powders are mixed by methods known in the art, such as by V-blending, tumbling or even by milling to obtain suitable particle sizes if size reduction is desired. The uniform powder blend is next agglomerated by methods known in the art. Such agglomeration tech- niques include forming powder compacts and subsequently crushing and screening them. However, agglomeration by spray drying is in general preferred for its flexibility and economy of operation on a production scale as well as its close control over the size of the agglomerated particles produced. Conditions under which slurries are formed and spray dried are well known. For example, U.S. patent • 3,617,358, issued November 2, 1971 describes formation of slurries. Other suitable methods for agglomerating are described in U.S. patents 3,881,911; 3,973,948 and 4,025,734, hereinafter discussed. The agglomerates may be conveniently classified to obtain a desired particle size distribution. It is generally desired to have at least 80% of the particles within a 50 micron average particle size range. The classified agglomerates are passed through a furnace at low temperatures to decompose the binders used for agglomeration and further treated at high tem- peratures to strengthen them for subsequent handling. An alternative method for the incorporation of the nickel and chromium into the powder is through the binder used for the agglomeration. Conventional binders include such materials as waxes and polyvinyl alcohols. As previously mentioned, these materials decompose during heat treatment, and thus contribute nothing to the constitution of the powder. Alternative binders include soluble salts of nickel and chromium. These can be introduced into the slurry for spray drying. Upon drying, these salts serve to bind the fine powders - together to form agglomerates. When the agglomerates are passed through a high temperature furnace under a reducing atmosphere the binder decc-poses to yield the desired quantity of nickel and chro-ium. - -ft - The sintered agglomerates can be subsequently • screened to yield a particle size distribution suitable for creating thermal sprayed coatings. Typically these distributions fall within two ranges, -200 +325 mesh or 5 -270 mesh. The coarser distribution powder typically contains 10% +200 and 10% -325 material. The finer distribution powder generally has a restriction .on the percentage of ultra fine particles allowable, e.g. a • maximum of 20% -20um. 10 Alternatively, the agglomerated and sintered par¬ ticles can also be subsequently plasma densified so as to produce fine, spherical, densified particles. The densification process comprises entraining agglomerated powders in a carrier gas and feeding the entrained par- 15 tides through a high-temperature reactor. The parti¬ cles pass through the reactor at such a flow rate that interparticle contact and coalescence are avoided but that at least the outer surfaces of the particles are • melted. After melting, the particles fall through a 20 distance sufficient to permit solidification and cooling prior to contact with a solid surface or each other. Because the particles are melted while entrained in a carrier gas, the solidified particles are substantially spherical, have smooth surfaces and thus excellent flow- 25 ability. In addition, the solidified particles have the same general size range as the starting material. How¬ ever, depending on the porosity of the starting mate¬ rial, they may have a smaller mean particle size, due to densification during melting. Preferably the melting 30 during densification is to such an extent that each par¬ ticle becomes prealloyed, i.e., the nickel and chromium alloy together and achieve intimate contact with the Ss*..*'.' densified chromium carbide. Some solution of the constituents in one another may also take place. A major portion and preferably substantially all of the densified powder consists essentially of particles 5 wherein each particle has a substantially uniform composition. The powder particles preferably have essentially the same composition so that the powder is uniform particle to particle. The plasma densification is preferably carried out 10 in a plasma flame reactor. Details of the principles and operation of such plasma flame reactors are well known. The temperature within the plasma flame can be adjusted between 10,000 P and 30,000 F. The temperature which the particles experience is a function of the rate 1 . 5 at which they are fed through the reactor. Commercially available feeding devices allow rates between approximately 1/2 and 30 pounds per hour, depending on the bulk density of the material being fed. Conditions for plasma densification are established such that the 0 particles reach a temperature at least above the melting point of the highest melting component and preferably below the vaporization point of the lowest vaporizing component. The melted particles must be cooled at a rate suf- 5 ficient to solidify at least an outer layer of the par¬ ticles prior to their contact with a solid surface or with each other in order to maintain their sphericity and particle integrity. While any of several methods may be used to achieve this result, it has been found 0 convenient to feed the melted particles into a liquid cooled chamber containing a gaseous atmosphere. The chamber may conveniently serve as a collection vessel. After the powders have been plasma densified they can be classified to achieve the desired particle size 5 distribution for use in thermal spray applications. Particle size distributions similar to those for the agglomerated and sintered particles are desired. Alternatively, the plasma densified powders can be crushed and classified to yield a powder with a finer particle size distribution, preferably one for which all the particles pass through a 270-mesh U.S. screen and at least 60 percent of the particles are less than 20 microns in average diameter. A typical particle size distribution has less than 10 percent of the particles below about 5 microns. The bulk density is from about 1.5 to about 3.0 grams/cc. EXAMPLE 1 A sintered agglomerated powder is prepared by blending 80/20 nickel-chromium alloy powder, with a particle size less than approximately 10 micron with chromium carbide powder of the same particle size in amounts sufficient to result in a blend comprising 25% of the nickel-chromium alloy and 75% chromium carbide. A slurry is prepared by combining the resulting powder- blend with polyvinyl alcohol in the " ratio of 98:2 respectively, with enough water to make a 50-80% solids concentration. Spray drying is carried out by pumping the slurry at low pressure through a two fluid nozzle located at the top of a commercially available spray dryer. The slurry is continually agitated throughout the spray drying run. The atomization air pressure to the nozzle is 40-60 psi. The inlet air temperature is 370 C to 430 C with an outlet temperature of 140 to 150 C. The spray dried powder is slowly passed through a hydrogen furnace at 450 C to remove the organic - binder. It is then fired for approximately " 7 hours at 1000 C to strengthen the agglomerated particles. The resulting particles are screened to yield powders with a -200+325 or a -.270+20 urn particle size distribution. These particles can then be used as thermal spray powders. EXAMPLE 2 The agglomerated spray dried and sintered particles of Example 1 are fed through a commercially available plasma torch into a jacketed water cooled collection tank. A mixture of 126 cubic feet per hour of argon and 70 cubic feet per hour of hydrogen is fed to the plasma torch. The torch power is about 28KVA. Nitrogen gas is fed to a powder feeder at the rate of 7 cubic feet per hour to entrain the powder which is fed through the torch. The powder produced is then screened as in Example 1. Analysis of the -270 powder indicated 15%-15 u particles. These prealloyed powder particles can then be used as a thermal spray powder. EXAMPLE 3 A plasma densified spray powder as produced in Example 2 is comminuted and air classified to produce a powder having the following distribution: 60-90% less ' than 20 um and no more than 15% less than 5 microns. EXAMPLE 4 A sintered agglomerate is prepared according to the process described in Example 1 by substituting nickel powder and chromium powder in the ratio of 4 to 1 for the 80/20 nickel-chromium alloy. Similar results are obtained. EXAMPLE 5 The sintered agglomerate powder of Example 4 is plasma densified according to the process as set forth in Example 2. The results were similar. EXAMPLE 6 The densified plasma spray powder of Example 5 is comminuted and classified as in Example 3 with similar results. EXAMPLE 7 A sintered agglomerate is prepared according to the process described in Example 1 by substituting nickel acetate and chromic acetate or nickel nitrate and chro- mic nitrate for the nickel-chromium alloy. The quantity of these salts is chosen such that upon decomposition they yield the proper ratios of nickel to chromium and chromium carbide. Polyvinyl alcohol is not required. EXAMPLE 8 The agglomerated spray dried and sintered particles of Example 7 are fed through a commercially available plasma torch into a jacketed water cooled collection tank. A mixture of 126 cubic feet per hour of argon and 70 cubic feet per hour of hydrogen is fed to the plasma torch. The torch power is about 28 KVA. Nitrogen gas is fed to the powder feeder at the rate of 7 cubic feet per hour to entrain the powder which is fed through the torch. The powder produced is then screened as in Example 7. Analysis of the -270 powder indicates 15%-15um particles. These prealloyed powder particles can then be used as a thermal spray powder. EXAMPLE 9 A plasma densified spray powder as produced in Example 7 is comminuted and air classified to produce a powder having the following distribution: 60-90% less then minus 20 microns, less than 15% less than 5 microns.

Claims: CLAI S 1. A thermal spray powder consisting essentially of nickel and chromium with the balance being from abou 50 to about 95 percent by weight chromium carbide, said chromium and nickel being present in a weight ratio of about 1 part by weight chromium to 4 parts by weight nickel wherein said powder being a substantially unifor composition. 2. A thermal spray powder according to claim 1 wherein at least a major portion of said densified powder consist essentially of particles wherein each particle has a substantially uniform composition. 3. A thermal spray powder according to claim 2 having a particle size distribution of about 60 to 90 percent minus 20 microns, and less than about 15 percent ' minus 5 microns. 4. . A thermal spray powder according to claim 2 having a particle size distribution of about 100 percent minus 270 mesh and less than 15 percent -15 microns. 5. A thermal spray powder according to claim 2 having a particle size distribution of about minus 200 plus 325. 6. A process for producing a thermal spray powder comprising preparing a uniform powder blend consisting essentially of chromium and nickel being present in a weight ratio of about 1 part by chromium to 4 parts by weight nickel with the balance being from about 50 to about 95 percent by weight chromium carbide, said powder blend having an average particle size less than about 10 microns, agglomerating the powder to produce agglome¬ rated particles, sintering the agglomerated particles, entraining the sintered agglomerated powder in a carrier gas, feeding the entrained agglomerated powder through a high temperature reactor having a temperature above the melting point of the highest melting component of the powder material to densify said particles. 7. A process for producing a thermal spray powder according to claim 5 wherein said densified particles consist essentially of particles wherein each particle has a substantially uniform composition. 8. A process for producing a thermal spray powder according to claim 6 wherein said densified particles have a particle size distribution of about 60 to 90 per- cent minus 20 microns, and less than about 15 percent minus 5 microns. 9. A process for producing a thermal spray powder according to claim 6 wherein said densified particles have a particle size distribution of about 100 percent minus 270 mesh and less than 15 percent minus 15 microns. 10. A process for producing a thermal spray powder - according to claim 6 wherein said uniform powder blend is spray dried to form said agglomerates. 11. A process for producing a thermal spray powder according to claim 10 wherein said chromium comprises a water soluble chromium salt and said nickel comprises a water soluble ' nickel salt. 12. A powder for thermal spraying consisting essentially of an agglomerated powder consisting essen¬ tially of nickel and chromium with the balance being from about 50 to about 95 percent by weight chromium carbide, said chromium and nickel being present in a weight ratio of about 1 part by weight chromium to 4 parts by weight nickel. 13. A powder for thermal spraying according to claim 12 having a particle size distribution of about 100 percent minus 270 mesh and less than 15 percent minus 15 microns. O * -11- 14. A powder for thermal spraying according to claim 13 having a particle size distribution of about minus 200 plus 325.

Inventors: CHENEY RICHARD F, HOUCK DAVID L, CHENEY, RICHARD, F., HOUCK, DAVID, L.
Applicants: GTE PROD CORP, GTE PRODUCTS CORPORATION
Application year: 1982
Main classification: B22F1
IPC codes: B22F1, C22C29, C23C4