CA1085239A

CA1085239A - Process for producing composite powder particles

Info

Publication number: CA1085239A
Application number: CA277,041A
Authority: CA
Inventors: Vilnis Silins; Ronald L. Prowse
Original assignee: Sherritt Gordon Mines Ltd
Current assignee: Viridian Inc Canada
Priority date: 1977-04-26
Filing date: 1977-04-26
Publication date: 1980-09-09
Also published as: DE2815876A1; IT1095304B; IT7822651A0; FR2388776A1; AU3490678A; ZA782341B; JPS53133582A; GB1597684A

Abstract

ABSTRACT OF THE DISCLOSURE

A process for producing composite powder particles having cores within a predetermined size range coated by a metal-lic outer layer, the predetermined core size range being within the overall size range of from about 5 to about 250 µm. Core particles of a size smaller than the lower limit of the predeter-mined size range are mixed with a polymeric bonding substance, and the resulting mixture is formed into solid agglomerates of a size within the predetermined range to form the cores, with each core comprising a plurality of core particles bound to-gether by the polymeric bonding substance. The cores are then coated with a metallic outer layer to form composite powder particles.

Description

1~5~3~

This invention relates to a process for producing metal-coated composite powder particles, that is to say par-ticles which each comprises a metal-coated core.
Such composite powders are in commercial use in a number of fields. For example, nic]cel-coated graphite powder is used in the formation of abradable seals for gas turbine engines.
Cobalt-coated tungsten carbide powder is thermal-sprayed onto knife blades to form hard, wear-res:istant cutting edges. Nickel coated aluminum powder is thermal-sprayed onto various substrates to provide a strongly adherent bond coat in preparation for further coating.
Metal coated composite powder particles may be formed in a number of different ways. One process in commercial use in-volves suspending core particles in a solution containing dissolved `
values of the coating metal or metals, and reacting the result-ing suspension at elevated temperature and pressure with a re-ducing gas to cause the coating metal to precipitate from solu-tion onto the core particles.
In general, according to the foregoing process, each dis-crete particle of core material receives a metal coating and, al-though some agglomeration of particles may occur during the coat-ing process, the particle size of the final composite powder is largely determined by the particle size of the core material. This does not present any problem where the particle size of the final composite powder is of no particular importance, or where core material particles of the required size can readily be obtained.
However, it is frequently desirable, and in some cases even essen-tial that the particle size of the composite powder be closely ' controlled, for example when the powder is to be employed in a thermal-spraying process. Also, it is not always possible to readily obtain core material particles of the required size.

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For example, minerals having a phyllosilicate structure, such as pyrophyllite, talc, kaolin, halloysite, chlorite, mica, montmoril-lonite and bensonite, are usually obtainable only with particle sizes less than 5 ~um (microns~, which is too fine for production of useful composite powder by the process described above. Wonder-stone is another mineral which is usually only obtainable with particle sizes which are too small for this purpose. Compounds such as graphite and boron nitride, as well as refractory compounds such as tungsten carbide and titanium carbide also fre~uently fall into this category.
An object of this invention is therefore to provide a process for producing metal-coated composite powder utilizing core material having a particle size smaller than the required core size.
According to the present invention, a process for producing composite powder particles having cores within a pre- -determined 5iZe rar.ge coated by a metallic outer layer, said predetermined core size range being within the overall size range of from about 5 to about 250 ,um, includes providing ~ .
core particles of a size smaller than the lower limit of said predetermined size range, mixing said core particles with a polymeric bonding substance, forming the resulting mixture into solid agglomerates of a size within said predetermined range to form said cores, with each core comprising a plurality of core particles bound together by said polymeric bonding sub-stance, and coating said cores with a metallic outer layer to form composite powder particles.
The product of the invention is a composite powder comprising particles having cores within a predetermined size range, said predetermined size range being within the overall size range of from about 5 to about 250 ~um, each core compris-ing a plurality of core particles, and each core being coated .. . . . .

with a metallic outer layer. The composite powder particles will accordingly be of a size in the range of from about 6 to to about 500 um.
Core materials which may be utilized with the present invention include metals and non-metals which can be coated with a layer of metal by the known hydrometallurgical method des-cribed above. Possible materials are compounds such as graphite, calcium fluoride and boron nitride, and refractory materials such as aluminum oxide, tungsten carbide, titanium carbide, tungsten-titanium carbide, chromium carbide, chromium oxide, zirconiumdioxide, titanium dioxide and molybdenum disulphide. Also, wonderstond and minerals having a phyllosilicate structure such as those mentioned above are suitable as core material. Wonder-stone appears to be pattern A.S.T.M. 2-613 for aluminum silicate hydrite.
As mentioned previously, the present invention is principall~ concerned with the coating of finely divided core materials which are not readily available in a predetermined required size prior to coating with a layer of a metal by the ~0 method described above. As also mentioned previously, it is frequently necessary that the particle size of the composite powder be within a predetermined size range. For example, pow-ders which are to be applied by thermal-spraying methods prefer-ably should be free flowing and with a particle size between about 6 and about 150 um. According to the known hydro-metallurgical coating method, control of particle size of the composite particle is achieved primarily through control of the particle size of the core material. The known method, there-fore, does not lend itself to coating of core particles smaller than the lower limit of the range required for the core size, and it is to the production of composite powders of controlled 5235~

size using such core particles that the present invention is directed.
The first step in the process of the invention in-volves mixing the particles of core material with a polymeric bonding substance which adheres to the core particles and re-tains its bonding properties under the conditions employed dur-ing the latter stages of the process to coat the core material with a layer of metal. For example, it may be necessary that the polymeric bonding substance retai'ns its bonding properties when the formed cores are immersed in water and/or sub~ected to high temperatures. Good results have been achieved with polyurethane and with vinyl ester resins, as will be described later. The mixture of core particles and polymeric bonding substance is then formed into solid agglomerates of a size within the predetermined core size range to form cores.
The core particles and polymeric bonding substance may be mixed in a mixing apparatus such as an attritor. Alter-natively, the core particles and polymeric bonding substance may be mixed by first dissolving the polymeric bonding substance in a volatile solvent, and combining the resulting solution with core particles in a pelletizing apparatus such as a disc pelleti-zer or drum pelletizer. This type of apparatus is well known to those skil]ed in tha ar~-and,insofar as the present invention is concerned, is operated in accordance with conventional proce-dures to produce solid, polymer-bonded agglomerates of core par-ticles within the predetermined core size range. As a specific example, polyurethane may be dissolved in acetone in a 1:1 ratio and combined with core particles in a rotary granulating pan.
The acetone evaporates rapidly and, as the pan rotates, the polyurethane deposits on and binds the core particles into agglomerates. The size of the agglomerates can be controlled ., , . .,. ,.. , ," . . ..

~0~35239 ~y controlling the operation of the pan and subsequent screening of the agglomerates.
The quantity of polymeric bonding substance mixed with the core particles can vary between 1 and 10~ based on the combined weight of the polymeric bonding substance and core particles.
Quantities of polymeric bonding substance larger than 10% are usually not required to bind the core particles in a form suitable for coating but, where the particular polymeric bonding substance used does not have strong binding properties r more polymeric bond-ing substance may be required. Similarly, less than 1% polymericbonding substa~ce is usually not sufficient to maintain the core particles in the form of agglomexates unless the polymeric bond-ing substance has unusually good binding properties~
In some cases, it may be necessary to heat treat the cores to prepare them for coating with metal. The polymeric bonding substance is chosen so that, although the heat may cause the polymeric bonding substance to decompose, with volatile de-composition products being lost, the core particles remain bound together as a core by the residue of the polymeric bonding sub~
stance. This procedure stabilizes the cores against weight and volume changes and reduces their carbon content~
Following agglomeration, the resultant cores are coated with a layer of at least one metal such as cobalt, nickel, copper, ruthenium, rhodium, osmium, iridium, gold, silver, platinum, arsenic, tin, cadmium or molybdenum, for example, by the known hydrometallurgical method referred to previously and described in a number of publications such as United States Patents Nos. 2,853,398, 2,853,401, 2,853,403, 3,062,680, 3,218,192 and 3,241,949 or by other known coating techniques.
The coating of the cores with metal may be improved by the use of a catalyst or a nucleation promoter, for example, a small amount of anthraquinone or a substituted anthraquinone.

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lO~X239 The resulting metal coated composite particles may be used as such in the flame or plasma spraying field, in the powder metallurgical ~ield and in other ways. Further treatment may, however, be necessary or desirable to render the composite powder particles suitable for use in particular applications. It may, for example, be desirable to protect the metal coating from high temperature oxidation. For instance, composite powder par-ticles having malleable or ductile coatings and cores composed of wonderstone and previously mentioned minerals having a phyllo-10 ~ silicate structure are particularly suitable for use as abradableseals. The particles may be flame or plasma sprayed onto gas turbine engine parts such as compressor counterfaces, casings, stator vanes or rotor vanes to provide abradable seals. Where the metal coating is nickel, the temperature within the turbine ;
for example sannot be allowed to exceed about 480C. since the nickel coating will oxidize above this temperature and the abradable seal may deteriorate. For higher service tempera-tures, the nickel coating must be protected from oxidation. Such protection can be afforded by alloying the coating with a metal such as chromium, aluminum or silicone prior to use. One method ~or effecting such alloying is described in Canadian patent No.
901.892.
Cobalt-coated composite particles may be similarly protected against oxidation by the method described in the Canadian patent mentioned immediately above. It should be noted that the temperature prevailing during the alloying operation described in this patent may be such as to cause the polymeric binding substance to decompose and/or volatilize. The composite particles following alloying will accordingly have a core con-sisting essentially of a plurality of core particles with analloy coating or the core particles will be bound together by ~135;~3~
the residue from the decomposition of the polymeric binding substance. Specific examples of the invention will now be described.

This example describes the production of nickel-coated boron nitride powder.
A slurry was prepared by combining 44 parts by weight of boron nitride particles, essentially of 1 ,um in size, 36 parts by weight acetone and 20 parts by weight urethane parti~cles.
The slurry was placed in a rotary pelletizing pan which was rotated at 24 rpm for 30-45 minutes. As the pan rotated, the acetone evaporated and the urethane polymerized. The result-ing agglomerates were cured for 16 hours in air maintained at 30C.
The cured agglomerates were separated into coarse and fine fractions by means of a 150 mesh standard Tyler screen (105 ~um) and the coarse fraction was pulverized by passing it through a hammer mill. The pulverized fraction was separated into coarse and fine fractions using the 150 mesh screen. The total fine fraction (minus 150 mesh) amounted to 62% of the overall weight of the agglomerates, and was utilized as cores.
The cores were then activated by immersion in a solu-tion containing palladium chloride. The activated cores were removed from solution by filtering, and were dispersed in an ammoniacal ammonium salt solution containing dissolved nickel values. The resulting slurry was reacted at 180C. with hydrogen gas under a partial pressure of 350 p.s.i. The re-action resulted in precipitation of the nickel from solution and deposition thereof as a continuous coating onto the cores.

This example also describes the production of nickel-coated boron nitride powder.

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A slurry was prepared by combining 750 g of commercial hexagonal boron nitride powder, essentially of 1 _um in size, 1800 ml of acetone and 450 ml of polyurethane. The slurry was placed in a 46 in. diameter disc peLletizer equipped with a stationary arm to provide mixing and aeration of the ingredients, and the pelletizing disc was rotated at 24 rpm.
The slurry changed first to a dough-like mix, and then to agglomerates, which were reduced in size by mechanical interaction against the stationary arm. The disc pelletizer was operated for 60 min.
The agglomerates were then air dried for 16 hrs. at 70C. and screened on an 80 mesh (177 um) screen. The minus 80 mesh fraction was utilized as cores, which were then coated -with nickel as in the previous example. The final nickel coated boron nitride powder had particles with 85% by weight nickel coating and 15~ boron nitride core particles, and with the following ~ize range:
Mesh (Tyler) um %
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+150 ~-105 7.6 -~50 / +200 74 - 105 9.8 -200 / +250 63 - 74 7.0 -250 / +325 44 ~ 63 50.8 -325 ~ ~4 24.8 The powder was of a size generally suitable for flame spraying or further processing into an alloy composite powder of the method described in previously mentioned Canadian patent No. 901,892.

This example describes the produstion of nickel-coated bentonite powder.

~L085239 4000 g of bentonite, with a particle size range of 0.8 to 6.5 ,um and an average powder particle size of about 4 ,um, were dried in air for 16 hours at 200C. to remove adsorbed mois-ture. A polymeric binder was prepared by mixing 840 g of Derakane resin 470-45P with 25 g Mia-Cat 60. (Derakane is a trade mark of Dow Chemical Company, Michigan, U.S.A. and Derakane resin 470-45P is a vinyl ester resin. Mia-cat is a trade mark of Mia Chemicals Limited, Ontario, Canada and Mia-cat 60 is a methyl ethyl ketone peroxide catalyst). The polymeric binder was added ; 10 to 3150 g of dried bentoni~e in an attritor at the rate of 300 g/min with the attritor agitator rotating at 180 rpm. Agita-tion was continued for -10 minutes after all the polymeric binder had been added, during which time the bentonite particles agglo- ' merated.
The agglomerates were then discharged from the attritor and cured in air for 2 hours at 200C. with periodic raking. The cured agglomerates were screened to remove the +150 mesh (~ 105 ,~m) fractions, and 1700 g of cured agglo-merates with an apparent density of 0.77 g/cm3 and having the following size characteristics were recovered:
MeS.h` (TYler) _~ %
+150 ~ 105 0.1 -150 / ~17088 - 105 ~.5 -170 / ~2007~ - 88 9.5 -200 / +25062 - 74 4.5 -250 / +27053 - 62 17.4 -270 / +32544 - 53 23.0 -325 > 44 43.0 The recovered cured agglomerates were used as cores and prepared for coating with nickel by heat treatment in a hydrogen atmosphere for 0.5 hr. at 950C. This heat treatment _ 9 _ ~ 1al85~9 -~ resulted in a partial loss of polymeric binder, lowering the carbon content of the cores from about 9~ to about 4% by weight, and stabilized the cores against further weight and volume changes. Following the heat treatment, 1240 g of stabilized cores with an apparent density of 0.96 g/cm3 and the following size characteristics were recovered:
Mesh (Tyler) ,,um +150 > 10~ 0.1 -150 / ~170 88 - 105 1.0 10 -170 / +200 74 - 88 6.5 -200 / +250 62 - 74 5.0 -250 / +270 53 - 62 19.4 -270 / +325 44 - 53 26.0 -325 ~ 44 42.0 The stabilized cores did not disintegrate when stirred in boiling water, and were subsequently coated with nickel in the manner described in the previous examples.

This example describes the production of NiCrAl/Ben-'tonite composite powder, and also describes the flame spraying of NiCrAl/Bentonite composite powder to form coatings which function as abradable seal structu~es for turbine engines.
Thirty-four kilograms of bentonite were dried at 200C. for 8 h. The resulting moisture loss left 31.5 kg of dried bentonite. A polymeric binder was prepared by mixing 7.3 kg of Derakane resin 470-45P with 25 g Mia-cat 60. The polymeric binder was added to the dried bentonite at the rate of 2.5 kg/min beginning at the start of pelletizing. Pelleti-zing was effected in a 15 gallon attritor (Szgvari type) for 5 minutes with its agitator rotating at 190 rpm. The pelletized ~A ,.
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1~ ;i239 bentonite was heat treated for 1 h. at 950C. in hydrogen to stabilize it against further weight and volume changes. During this step, the volatile constituents of the polymeric binder were driven off, leaving a carbonaceous char. The pelletized bentonite was then screened successively on 400, 150 and 400 mesh screens. Three more batches of the material were pre-pared, and the recoveries are given below:

Heat Attritor Treated Size Distribution of Charge Core Heat Treated Core, %
kg __ kg +150 mesh -1507+400 -400 . _ .
Batch 1 39 30 21 28 51 Batch 2 39 29.3 18 28 54 Batch 3 39 28.1 18 34 47 Batch 4 39 26.2 23 36 41 The middle fractions of the four batches were blended to give 35.5 kg core product of the following size distribution: -Mesh (Tyler) ium wt +150 ~ 105 8.0 -150 / +170 88 - 105 5.2 -170 / +200 74 - 88 7.2 -200 / +250 62 - 74 4.8 -250 / +270 53 - 62 15.2 --270 / +325 44 - 53 22.0 -325 ~ 44 37.6 Twenty kilograms of this core product were coated with nickel as in the previous examples to obtain 87 kg of com-posite powder analyzing 78.5% nickel and 21.5% Bentonite. This product had the following size distribution:

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Mesh ~Tyler ~m wt %
~150 ~ 105 4.0 -150 / +170 88 - 105 7.8 -170 / +200 74 - ~8 12.8 -200 / +250 62 - 74 8.8 -250 / ~270 53 - ~2 26.0 -270 / +325 44 - 53 22.0 -325 > 44 14.0 Five thousand grams of 78.5 Ni/Bentonite 21.5 compo-site powder was alloyed using the following procedure, which is in accordance with the process described in Canadian patent No.
901,892:
1. Mix with 156 g -325 mesh chromium powder (Union Carbide El-Chrome grade) and 50 g granulated ammonium chloride;
heat for 4 h. at 950C. Cool and crush the resulting cake into powder in a pulverizer.

2. Mix with 117 g above chromium powder and 50 g granulated ammonium chloride; heat for 4 h. at 950C. in hydrogen atmos-phere. Cool and pulverize. '

3. Mix with 312 g aluminum flake powder (Leafing Grade950, Cambro Division of International Bronze Powders) and treat for 4 h. at 950C. and hydrogen atmosphere. Cool and pulverize.
The resulting composite consisting of bentonite core coated with NiCrAl alloy was further screened through 150 and 400 mesh screens to give the product of the following chemical and physical characteristics:

Total Ni Cr Al Si Fe Metallic Core Chemical 30 Analysis, % 71.6 3.22.1 1.3 1.279.2 21.0 ~A~

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~etallic/Bentonite, ~ 79/21 Size Analysis:
Mesh (Tyler) _~ wt %
+150 ~ 105 0.4 -150 / +17~ 88 - 105 lOoO
-170 / +2~0 74 - 88 1~.2 -200 / ~250 62 - 74 10.8 -250 / ~270 53 - 62 29.2 -270 / +325 44 - 53 22.2 . -325 ~44 9.2 This powder was deposited by thermal spraying techniques using a Metco 6P flame spraying system to form a coating suitable for use as an abradable seal in a turbine engine. During spraying, substrate pieces were attached on the inside of a 22 in. ring - rotating at 60 rpm with the spraying gun located inside the ring. I~
- The spray parameters used as well as the coating properties ob- ~ :
tained are given below:
Spray Parameters Nozzle P7A-M
Carrier Gas (Nitrogen) Flow, % 37 Pressure, psig (kPa) 55 ~379) ;~
Oxygen Flow, % 42.5 Pressure, psig (kPa) 25 ~172) Acetylene Flow, % 42.5 Pressure, psig (kPa) 18(124) Cooling Air Pressure (6P-3), psig (kPa) 25 (172) Powder Feed Wheel 5 Powder Feed Rate, g/min 55 Gun-to-Substrate Distance, in. (cm). 8.5 (21.6) Gun Traverse, in/min (cm/min) 12 (30.5) ~.A

Physical Propertles Coating Thickness, in (mm) 0.060 (1.5) Hoffman Scratch Hardness 12-15 Rockwell 15Y Hardness 55 Erosion Weight Loss, g/min 0.52*
Tensile Strength, psi (MPa) 700 (4.8) * Erosion test involves impingement of the coating surface with 240 grit silica flowing at 71 m/s and 32 g/min; ~ozzle to substrate distance is 10 cm.

This example illustrates the preparation and flame spraying of a NiCrAl/Bentonite composite powder similar to that in Example 4 but with a higher Eatio of metallic phase to bentonite. Flame sprayed coatings using this powder have a very low erosion weight loss. This is achieved without undesirable hardening of the coating which would compromise its abradability.
The preparation of this product proceeded in exactly the same manner as in Example 3 up to the stage where bentonite core -150/+400 had been obtained. In this example, the core was further screened on a 200 mesh screen to obtain two new fractions: -150/+200 and -200/~400, which, for the purpose of preparation of coarse 85 NiCrAl/Bentonite 15, were blended in the proportion of 4:1 respectively. The core blend was nickel coated as in the previous examples to obtain a composite powder of 85.3 Ni/Bentonite 14.7. The alloying of the nickel coating was performed in essentially the same manner as in Example 4, but the amount of chromium and aluminum was increased at each step by 85.3/78.5 (= 1.09) to obtain the same composition of NiCrAl coating but different metallic/bentonite ratio, i.e.
86.2/13.8. The size analysis of this product was:

l~S23g Mesh (Tyler _ wt ~150 ~ 105 15.0 ;
-150 / +170 88 ~ ]05 17.4 -170 / +200 74 - 88 17.4 -200 / +250 62 - 74 10.2 250 / +270 53 - 62 23.4 -270 / ~325 44 - 53 11.0 -325 ` 44 3.6 This powder was deposited by thermal spraying tech-nique using the Metco 6P spray system to form an abradable seal coating. During spraying, substrate pieces were attached on the inside of a 22 in. ring rotating at 60 rpm with the spraying gun ¦-~
located inside the ring. The spray parameters used and the pro-perties of the resulting coating are given below:
Spray Parameters :
Nozzle P7A-M 1-Carrier Gas (Nitrogen) Flow, % 37 Pressure, psig (kPa) 55 (379) Oxygen ;~
Flow, % 42.5 ~-Pressure, psig (kPa) 21 (145) - Acetylene Flow, ~ 42.5 Pressure, psig (kPa) 15 (103) Cooling Air Pressure (6P-3), psig (kPa) 20 (138) Powder Feed Wheel 5 Powder Feed Rate, g/min. 55 Gun-to-Substrate Distance, in. (cm) 8.5 (21.6) Gun Traverse, in/min (cm/min) 12 (21.6) Physical Pro~erties Coating Thickness, in (mm) 0.060 (1.5) Rockwell 15Y Hardness 56 Erosion Weight Loss, g/min 0.11 -Tensile Strength, psi (MPa) 800 (5.5) ~ 15 3~
Other embodiments and examples within the scope of the invention will be readily apparent to one skilled in the art, the scope of the invention being defined in the appended claims.

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Claims

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

1. A process for producing composite powder particles having cores within a predetermined size range coated by a metal-lic outer layer, said predetermined core size range being within the overall size range of from about 5 to about 250 µm, including providing core particles of a size smaller than the lower limit of said predetermined size range, mixing said core particles with a polymeric bonding substance, forming the resulting mixture into solid agglomerates of a size within said predetermined range to form said cores, with each core comprising a plurality of core particles bound together by said polymeric bonding sub-stance, and coating said cores with a metallic outer layer to form composite powder particles.

2. A process according to claim 1 including alloying the metallic outer layer of said powder particles with a metal selected from the group consisting of chromium, aluminum and silicon.

3. A process according to claim 1 wherein said core par-ticles are of a phyllosilicate mineral selected from the group consisting of pyrophyllite, talc, kaolin, halloysite, chlorite, mica, montmorillonite and bentonite.

4. A process according to claim 1 wherein said core pre-ticles are of a material selected from the group consisting of boron nitride and wonderstone.

5. A process according to claim 1 wherein the polymeric bonding substance is a polyurethane.

6. A process according to claim 1 wherein the polymeric bonding substance is a vinyl ester resin.

7. A process according to claim 1 wherein the core particles comprise bentonite particles, and the metallic outer layer comprises nickel.

8. A process according to claim 7 wherein the polymeric bonding substance is a vinyl ester resin.

9. A process according to claim 1 wherein the core par-ticles comprise boron nitride particles and the metallic outer layer comprises nickel.

10. A process according to claim 9 wherein the polymeric bonding substance is a polyurethane.

11. A process according to claim 1 wherein the core par-ticles comprise bentonite particles, and the cores are coated with an outer layer of nickel by slurrying the cores in a solu-tion containing dissolved nickel values and reacting the slurry with a reducing gas at elevated temperature and pressure to precipitate elemental nickel on said cores.

12. A process according to claim 1 wherein the polymeric bonding substance is a vinyl ester resin.

13. A process according to claim 1 wherein the core particles comprise boron nitride particles, and the cores are coated with an outer layer of nickel by slurrying the cores in a solution containing dissolved nickel values and reacting the slurry with a reducing gas at elevated temperature and pressure to precipitate elemental nickel on said cores.

14. A process according to claim 13 wherein the polymeric bonding substance is a polyurethane.

15. A process according to claim 1 wherein, before the cores are coated with said metal, the cores are heated to stabilize the cores against weight and volume changes and to reduce their carbon content.

16. A process according to claim 15 wherein the core particles comprise bentonite particles.

17. A process according to claim 16 wherein the polymeric bonding substance is a vinyl ester resin.

18. A process according to claim 17 wherein the metallic outer layer comprises nickel.

19. A composite powder comprising particles having cores within a predetermined size range, said predetermined size range being within the overall size range of from about 5 to about 250 µm, each core comprising a plurality of core particles, and each core being coated with a metallic outer layer.

20. A composite powder according to claim 19 wherein each core comprises an agglomeration of core particles bound together by a polymeric bonding substance.

21. A composite powder according to claim 19 wherein the metallic outer layer is an alloy of a metal and at least one other metal selected from the group consisting of chromium, aluminum and silicon.

22. A composite powder according to claim 19 wherein the core particles are of a phyllosilicate mineral selected from the group consisting of pyrophyllite, talc, kaolin, halloysite, chlorite, mica, montmorillonite and bentonite.

23. A composite powder according to claim 19 wherein the core particles are of a material selected from the group consist-ing of boron nitride and wonderstone.

24. A composite powder according to claim 20 wherein the polymeric bonding substance is a polyurethane.

25. A composite powder according to claim 20 wherein the polymeric bonding substance is a vinyl ester resin.

26. A composite powder according to claim 19 wherein the core particles comprise bentonite particles, and the metallic outer layer comprises nickel.

27. A composite powder according to claim 26 wherein the bentonite particles are hound together with a vinyl ester resin.

28. A composite powder according to claim 19 wherein the core particles comprise boron nitride particles and the metallic outer layer comprises nickel.

29. A composite powder according to claim 28 wherein the boron nitride core particles are bound together by a polyure-thane.

30. A process for forming an abradable seal on a metallic substrate including spraying a composite powder according to claim 19 onto the metallic substrate to form an abradable sealing coating on the substrate.

31. A process according to claim 30 wherein the cores of the composite powder particles comprise bentonite particles, and the metallic outer layer coating each core comprises nickel.

32. A process according to claim 31 wherein the metallic outer layer of each composite core particle comprises nickel alloyed with at least one other metal selected from the group consisting of chromium, aluminum and silicon.

33. An abradable seal assembly including a metallic sub-strate coated with an abradable seal, said abradable seal being formed by spraying thereon composite powder particles according to claim 19.

34. An abradable seal assembly according to claim 33 wherein the cores of the composite powder particles comprise bentonite particles, and the metallic outer layer coating each core comprises nickel.

35. An abradable seal assembly according to claim 34 wherein the metallic outer layer of each composite powder par-ticle comprises nickel allowed with at least one other metal selected from the group consisting of chromium, aluminum and silicon.