CA1128761A

CA1128761A - Alumina coated silicon carbide abrasive particulate useful as a dispersoid in metal matrices

Info

Publication number: CA1128761A
Application number: CA310,982A
Authority: CA
Inventors: Gerard F. Johnson; Peter W. Schilke
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1977-09-09
Filing date: 1978-09-08
Publication date: 1982-08-03
Also published as: FR2402688A1; IL55480A; SE7809198L; AU3969878A; FR2402688B1; GB2004906B; JPS6333987B2; DE2838250A1; DE2838250C2; JPS5451089A; BE870324A; AU520591B2; GB2004906A; IL55480A0

Abstract

ALUMINA COATED SILICON CARBIDE
PARTICULATE USEFUL AS AN ABRASIVE
DISPERSOID IN METAL MATRICES

ABSTRACT OF THE DISCLOSURE

Disclosed is an improved abrasive material particularly useful as a dispersoid in metal matrices to provide articles of use having superior abrasive properties, especially at elevated temperatures. The improved abrasive material typically comprises silicon carbide particulate, the individual particles of which have a predominantly alumina coating thereon to inhibit dis-solution of the particles in the metal matrix at high temperatures.
The invention finds special application in abrasive blade tips for service in gas turbine engines, the tips comprising a nickel base or cobalt base superalloy having the alumina coated silicon carbide particles dispersed therein.
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Description

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BACKGROUND OF THE INVENTION
Field of the Invention The present invention rela~e~ ~o abrasive materials and their manuacture and, more particularly, to abrasive particulate and its dispersal in a metal matrix to provide an article~of use having superior abrasive properties.
Description of the Prior Art - Improved efficiency is an increasingly important factor in the development of gas turbine engines. Such engines have rows of rotating blades fixtured within a generally cylindrical case and leakage of gas between the ends of the blades and case contributes toward inefficiency.
In current engine design, this leakàge is minimized by blade and sealing systems in which the blade tips rub against a seal which is attached to the interior of the engine case. Generally, it has been found most desirable to construct the system such that the blade tips are harder and more abrasive than the seal and thus cut into the seal during certain sta~es Qf engine operation to establish essentially a zero cLearance therebetween.
Previous abrasive blade tips have been constructed of various materials including certain nickel base supexalloys, U.S.

2,994,605, and nickel matrices having tungsten carbide partîcles randomly dispersed therein, U.S. 3,199,836. However, as a result of the increasing temperatures enco~ntered in gas turbine engines, especially in the turbine section, and of the presence of corrosive gases, ~he abrading and cutting effectiveness of these blades tips has not been entirely satisfactory. The prior art blade tips have proved especial~y deficient when used in con-junction with ceramic seal materials which have been developed for ~2S turbine engine use, for example, graded ceramic seals based on zirconia.
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SUMMAR~ OF THE INVE~TION
The present invention contemplates an improved abrasive material particularly useful as a dispersoid in a metal matrix to provide an article of use having superior abxasive properties, especially at elevated temperatures. Typically, the invention utilizes abrasive silicon carbide particulate, the individual particles of which have a coating substantially of alumina (A12O3) thereon. m e alumina coating functions to inhibit diffusion or dissolution of the dispersed silicon carbide particles into the metal matrix during service at elevated temperature and thereby increases the useful life of the part-iculate as an abrasive dispersoid. The invention also envisions providing an intermediate coating of silica (SiO2) between the particle and outer alumina coating to improve high temperature bonding by formation of a mullite phase (3~12O3 . 2SiO2) there-between. According to the invention, the alumina and silica coatings are preferably applied by sputtering or vapor deposit-ion processes.
m e alumina coated silicon carbide particulate can be dispersed in a wide variety of metal matrices to suit particular needs. Dispersal of the par~iculate can be affected by powder metallurgical and casting techniques as well as others.
~ he invention finds special application in abrasive blade tips for service in gas turbine engines. Such blade tips typically comprise the alumina coated silicon carbide particles dispersed in nickel base or cobalt base superalloy matrices.
Abrasive blade tips produced in accordance with the present invention have been found highly effective in abrading and cut-ting ceramic seal materials at elevated temperatures.

In accordance wlth a broad aspect of the invention, there is provided a high temperature particulate, useful as an improved dispersoid in a metal matrix, comprising silicon

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carbide particulates having a predominantly alumina coating, the coating being resistive to diffusion or dissolution in superalloys at temperatures of 2000 F. or more.
In accordance with another broad aspect of the invention, there is provided a method for making a sili~on carbide and metal matrix abrasive article useful at elevated temperatures comprising depositing an alumina coating on in-dividual silicon carbide particles and dispersing the coated particles in a metal matrix, the alumina coating inhibiting diffusion of the particles into the matrix at elevated temper- ;
atures and increasing the useful life of the article.
These and other objects, advantages and uses of the present invention will become more readily apparent from the ; following detailed description of the preferred embodiments.

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DESCRIPTIOI~ 0~ THE PREFEP~ED EMBODIMENT
As previously discussed, the improved abraslve Ma-te~ia the present invention finds particular usefulness as a dispersoid in ~etal matrices to provîde articles of use havlng excellen~
high temperature abrasive properties. One such article of use is the abrasive blade tip used in the blade and seal systems o modern gas turbine engines. For the sake of illustration, the present invention will be described in detail hereinbelow with re~erence to such blade tips.
In modern gas turbine engines, the abrasive blade tips must be capable of providing effective abrading and cutting action into the seal material at very high temperatures. In the turbine section of the engine, for example, blade tip temperature may approach or exceed 2000F during normal engine operation. Under - these severe conditions, prior art blade tips have not been entirely satisfactory, especially- when used in conjunction with ceramic seal materials. The present invention provides an im-proved abrasive material which can be dispersed in a nickel base or cobalt base superalloy matrix, or other appropriate matrix, to provide an abrasive blade tip characterized by superior ability to abrade and cut seal materials, including ceramîc seal materials, at the very high temperatures encountered in engine operation. More specifically, the invention involves the discovery that silicon carbide particulate, when dispersed in nickel base and cobalt base superalloy matrices, exhibits superior abrasive ability and that by coating the individual particles of silicon carbide with alumina, such superior abraslve characteristics can be maintained at turbine operating temperatures.
Commercially available silicon carbide particulate appears satisfactory in sizes from 8 mils to 30 mils average diameter, ~ z~761 although the particular size utilized can of course be varied as desired depending on the particular c~brasive application In order to inhibit the di~usion or dissolutlon o~ the par~icl~s in the metal matrix at high temperatures, thereby maintaining abrasive ability, the individual particles are coated with commercially available alumina (A1203) or, for better bonding, with a first coating of silica (Si02) followed by a second coating of alumina. The use of the intermediate silica coating fosters improved high temperature bonding of the alumina coating to the particle by the formation of a mullite phase (3A1~03 2Si02) therebetween during high temperature service. Or, if desired, the duplex coated particles can be suitably heat treated prior to dispersal in the matrix to effect formation of the mullite phase. The alumina and silica coatings can be conveniently deposited on the individual silicon carbide particles by sputtering, physical vapor deposition and chemical vapor depo-sition techniques wherein the particles are rotated slowly during deposition. Of course, the thickness of the coatings can be varied as desired. For abrasive blade tip applications> an alumina coating of about 0.1 mil in thickness has been found satisfactory.
As those skilled in the art will recognize, dispersal o the alumina coated silicon carbide particulate in the nickel base or cobalt base superalloy matrices can be effectPd by a variety o~ techniques including, but not limited to, powder metallurgy, casting and others. Powder ~etallurgical techniques are preferred since random dispersal of the particles is optimized.
For example, a preferred tec.hnique is to mix the coated silicon carbide particles and superalloy powder in desired amounts and then hot press the mixture to a final or intermediate consolidat~d shape. However, in some cases, it may be desirable to provide ~Z876~;

for a nonrandom dispersion~ that is, the particulate being segregated in certain regions of the tip, such as near the end contacting the seal material. A significant advan-~age af the present invention is t'hat the alumina coated silicon carbide particulate can be dispersed in a wide variety o~ matrix alloys with no signi~icant deleterious interaction therebetween. This feature allows the matrix alloy to be selected and tailored to the specific enviro~ment and conditions of service, resulting in an overall-improved abrasive blade tip or other article of use.
For abrasive blade tips, various nickel base and cobalt base alloys can be utilized', examples o~ w~ich are a nickel base alloy of nominal composition, by weight, 21-25% Cr, 4.5-7% Al,

4-10% W, ~.5-7% Ta, .02-.15V/o Y, .1-.3% C, balance nickel and a cobalt base alloy of nominal composition, by weight, 18-30% Cr, 10-30% Ni+Fe, 5-15% ~+Mo, 1-5% Ta+Cb, .05-.6% C, 3.5-8.0% Al, .5-2.0% Hf, .02-.1% Y, balance cobalt. Blade tips having the alumina coated silicon carbide particulate dispersed in these alloys can be attached to the end of a nickel basQ- or cobalt base superalloy turbine blade by solid state diffusion bonding, TLP~
bonding, brazing and other processes.
l'hose skilled in the art will recognize that the amount of alumina coated silicon carbide particulate dispersed in the alloy matrix can be varied as desired. In blade tip applications, the amount has ranged from 45 volume percent for 8 mil diameter particles to 30 volume percQnt ~or 15 mil diameter particles. In addition, other abrasive particulate may be simultaneously dis-persed in the matrix to provide useful properties. For example, hot pressed silicon nitride (Si3N4) can be dispersed along with alumina coated silicon carbide. Abrasive blade tips of the in-3~ vention have been wear tested and found to possess acceptable abrasive'properties to tip temperatures of about 2100F, the matrix alXoy being a cobalt base alloy.

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Although the following invention has been shown and described with respect to a pre~erxed embodiment thereo~ should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without dep-rting f-om he ~pirit and the scope of the inve~tion.

Claims

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

lo A high temperature particulate, useful as an im-proved dispersoid in a metal matrix, comprising silicon carbide particulates having a predominantly alumina coating, the coating being resistive to diffusion or dissolution in superalloys at temperatures of 2000° F. or more.
2. A material for high temperature wear and rubbing resistance, comprising, a high temperature superalloy matrix having silicon carbide particles dispersed therein, the particles having a predominantly alumina coating, wherein the alumina coating is resistive to diffusion or dissolution in superalloys at temperatures of about 2000° F.
3. A material for high temperature wear and rubbing resistance comprising a high temperature superalloy matrix having silicon carbide particles dispersed therein, the particles having a first coating to improve bonding and a second coating of pre-dominantly alumina to resist diffusion or dissolution in super-alloys at temperatures of about 2000° F., wherein the first coating is selected from the group consisting of silica and mullite.
4. A high temperature particulate material useful as an improved dispersoid in a metal matrix, comprising silicon car-bide particulates having a first coating to improve bonding and a second coating of alumina to resist diffusion or dissolution in superalloys at temperatures of 2000° F. or more, wherein the first coating material is selected from the group consisting of silica and mullite.
5. The abrasive material of claim 1 wherein the thickness of the alumina coating is about 0.1 mil.
6. In the method for making a silicon carbide and metal matrix abrasive article useful at elevated temperatures, the improvement comprising, depositing an alumina coating on in-dividual silicon carbide particles and dispersing the coated particles in a metal matrix, the alumina coating inhibiting diffusion of the particles into the matrix at elevated temper-atures and increasing the useful life of the article.
7. The method of claim 6 wherein the coated particles are dispersed in the metal matrix by powder metallurgical techniques including mixing the coated particles with metal powder and then pressing the mixture into a consolidated body.
8. The method of improving the durability of high temper-ature silicon carbide particulate-containing metal which comprises coating the silicon carbide particulates, prior to inclusion within the metal, with at least one material selected from the group consisting of alumina, silica, mullite.
9. An abrasive blade tip for a gas turbine engine blade comprised of a superalloy having silicon carbide particulate of an average particle dimension of from 8 to 30 mils, the particulate having an alumina coating, the coating being re-sistive to diffusion into the superalloy at temperatures of about 2000° F.
10. A composite gas turbine blade made of a superalloy, having a tip according to claim 9.