MULTIPHASE CERAMIC COMPONENTS PRODUCED BY REACTION INJECTION MOLDING

Abstract: A reaction injection molding process for preparing ceramics having at least two compositionally distinct ceramic phases is disclosed. For example, block copolymers prepared from an aluminum-nitrogen polymer and a polysilazane are filled with a ceramic powder, a metal powder or mixtures thereof and cured in the mold. An AlN/SiC-containing ceramic is formed by pyrolysis of the molded articles in a nonoxidizing atmosphere.

Description: DESCRIPTION MULTIPHASE CERAMIC COMPONENTS PRODUCED BY REACTION INJECTION MOLDING Technical Field This invention relates to reaction injection molding of ceramic and/or metal powders. Background Art Reaction injection molding (RIM) is a relatively new forming process that has recently been adapted for forming shaped ceramic green bodies. U.S. Patent 4,906,424 (Hughes) discloses a RIM process for molding a mix of ceramic powder and a polymerizable, low viscosity, multifunctional organic acrylate monomer or mixtures of monomers. The ceramic-monomer mixtures are formulated to be highly filled, i.e., greater than 50 vol. %, with ceramic powder, yet have adequate fluidity to be processed at ambient temperature and readily injected into a hot mold. The part is then ejected from the mold and subjected to subsequent post-curing, binder removal, sintering and, if needed, machining to produce a dense ceramic part. However, organic binders such as polyacrylates must be burned out of the molded part in the process of converting the part to a dense, sintered ceramic article. The carbon-containing char that would otherwise remain in the sintered body would have a deleterious effect on the structural integrity and high temperature performance of the sintered part. Often, the carbon in such binders cannot be completely eliminated in the firing step. In addition, removal of an organic binder can cause structural defects in a sintered part due to voids formed from the rapid generation of volatile materials in the binder burnout step. A further complication arises in fabricating sintered parts of well-defined dimensions. Excessive shrinkage occurs when a high fraction of a ceramic green body must be removed in a binder burnout step. When the part finally densifies at high temperatures, dimensional distortion can be extreme, requiring a complex mold design. Curable, liquid binder systems that contribute to the ceramic body ("nonfugitive" binders) have been used in traditional molding methods. For example, U.S. Patents 4,689,252; 4,722,988 and 4,772,494 disclose a crosslinkable silazane polymer that can be cured and subsequently pyrolyzed to convert the polysilazane to a ceramic material. The silazane polymer can be used for coating or impregnating a substrate, making ceramic fibers or as a sinterable binder for ceramic powders. U.S. Serial No. 07/905,484 filed June 26, 1992, describes a reaction injection molding (RIM) process for preparing sintered ceramic articles using poly(thio)ureasilazane binders. A high solids, nondilatant dispersion of ceramic powder in a curable poly(thio)ureasilazane ceramic precursor binder is injection molded at a temperature less than 120°C, the binder is subsequently cured in the heated mold, and the molded article is sintered to convert the binder to a single phase ceramic. None of the molding techniques described above teach a reaction injection molding method for preparing a sintered composite ceramic article from a fluid, solvent-free, nondilatant mixture of a ceramic powder, a metal powder or mixtures thereof, and a liquid ceramic precursor polymer binder phase that forms at least two compositionally distinct ceramics, e.g., SiC and A1N, upon pyrolysis under a suitable atmosphere. Summary of the Invention In a reaction injection molding process for preparing a sintered ceramic article comprising (a) injecting into a heated mold a fluid, solvent-free, nondilatant mixture comprising a ceramic powder, a metal powder, or mixtures thereof, and a curable ceramic precursor binder phase that is a liquid below its curing temperature, said powder being present in an amount of at least 30% by volume, to cure the binder and produce a hardened molded article, (b) heating the hardened molded article to a temperature sufficient to convert the cured binder phase to a ceramic and (c) sintering the article to the desired density, the improvement according to this invention comprises using as the curable, liquid ceramic precursor binder phase a polymer or mixture of polymers that forms at least two compositionally distinct ceramics upon pyrolysis under a suitable atmosphere. In one embodiment of the process of this invention a single ceramic precursor binder that contains both silicon and aluminum atoms in its polymeric structure is pyrolyzed in a nonoxidizing atmosphere to form both SiC and A1N ceramic phases. The binder is a block copolymer prepared by reacting an aluminum-nitrogen polymer with a silazane or (thio)ureasilazane polymer. A wide range of ceramic compositions containing SiC and A1N can be prepared from such polymeric ceramic precursors by adjusting the ratio of silazane or (thio)ureasilazane polymer to aluminum-nitrogen polymer during preparation of the block copolymer. The stoichio etry of Si to C to Al to N desired in the ceramic product is determined during the synthesis of the polymeric ceramic precursor. Therefore, such compositions do not require extended treatment at high temperatures after pyrolysis to promote the solid state diffusion often required to form solid solution SiC/AIN ceramics from powder mixtures. Detailed Description of the Invention and Preferred Embodiments The reaction injection molding process of this invention is not limited to the use of any particular polymeric ceramic precursor. Ceramic precursor polymers suitable for the process of this invention contain at least one metallic element, e.g., silicon, aluminum, titanium, or zirconium, that can form at least two compositionally distinct ceramics upon pyrolysis. A polyureasilazane or a polythioureasilazane containing organic substituents, for example, is a suitable polymer for the practice of this invention if a suitable atmosphere is used in the pyrolysis step. A curable, liquid mixture of two or more polymers, each containing at least one metallic element can also be used, e.g., a solution of a solid polycarbosilane or polysilane in a liquid poly(thio)ureasilazane. It is preferable, however, to use a single polymer containing one or more metallic elements, since the resulting multiphase ceramic prepared from a single polymer will be more homogeneous. When SiC and A1N are the desired ceramic phases, the polymer is preferably a single block copolymer containing both silicon and aluminum atoms prepared by heating a mixture of an aluminum-nitrogen polymer containing Al-C or Al-H bonds and a silazane or (thio)ureasilazane polymer containing N-H bonds at a temperature not greater than 400 β C, preferably from about 90 β C to about 220 β C. At least some of the organic groups attached to the Al and N of the Al-N polymer or to the Si atom of the silazane or (thio)ureasilazane are unsaturated, although not all of them need be unsaturated. The notation "(thio)ureasilazane" indicates that both ureasilazanes and thioureasilazanes can be used. Pol silazanes are well known in the art, for example, as described in U.S. Patents 3,853,567; 4,482,669; 4,612,383; 4,675,424; 4,689,252 and 4,722,988. Addition polymers of polysilazanes that are prepared by treating a polysilazane with an isocyanate, isothiocyanate, ketene, thioketene, carbodiimide or carbon disulfide can also be used. Preparation of these addition polymers is described in U.S. Patent 4,929,704, which is incorporated by reference in its entirety. When silazanes are used to prepare the block copolymers, the ceramic precursor binder preferably comprises a multiplicity of blocks of units having the formula and a multiplicity of blocks of units having the formula wherein x > 1 and y > 1, R and R 2 are the same or different and are selected from the group consisting of hydrogen, substituted or unsubstituted 1-12 carbon alkyl, 3-12 carbon cycloalkyl, 2-12 carbon alkenyl, 3-12 carbon cycloalkenyl, and aryl groups; R 3 and R^ are the same or different and are selected from the group consisting of hydrogen, substituted or unsubstituted 1-6 carbon alkyl, 3-6 carbon cycloalkyl, 3-6 carbon cycloalkenyl, aryl, 2-6 carbon alkenyl and 2-6 carbon alkynyl groups, provided that R 1 , R 2 , R 3 and R 4 are not all hydrogen and at least one of R , R , R 3 and R 4 is an unsaturated organic group. In a preferred embodiment, the polysilazane component of the block copolymer is obtained by treating a polysilazane containing sites of organounsaturation and N-H bonds with an iso(thio)cyanate to form a poly(thio)ureasilazane. When the resulting (thio)ureasilazane polymer is reacted with an aluminum-nitrogen polymer containing Al-C or Al-H bonds, the resulting block copolymer comprises a multiplicity of blocks of units having the formula and a multiplicity of blocks of units having the formula wherein x > 1 and y > 1, R and R 2 are the same or different and are selected from the group consisting of hydrogen, substituted or unsubstituted 1-12 carbon alkyl, 3-12 carbon cycloalkyl, 2-12 carbon alkenyl, 3-12 carbon cycloalkenyl, and aryl groups; R , R 4 and R 5 are the same or different and are selected from the group consisting of hydrogen, substituted or unsubstituted 1-6 carbon alkyl, 3-6 carbon cycloalkyl, 3-6 carbon cycloalkenyl, aryl, 2-6 carbon alkenyl, and 2-6 carbon alkynyl groups, and A is 0 or S, provided that R , R , R , R 4 and R 5 are not all hydrogen, and at least one of R , R , R , R and R 5 is an organounsaturated group. Most preferred is a polyureasilazane prepared by reacting a vinyl- substituted polysilazane containing N-H bonds and phenylisocyanate as described in Example E. The aluminum-nitrogen polymers employed in the practice of this invention can be soluble or insoluble solids, or liquids of various viscosities, and have a backbone comprising alternating aluminum- and nitrogen-containing groups. Suitable polymers include aluminum amide polymers, aluminum imide polymers, aluminum imine polymers and polyaminoalanes. Polymers containing an aluminum-nitrogen bond suitable for purposes of the present invention include, for example, (1) aluminum amide polymers comprising structural units of the general formula where n > 2 and R, R', R" and R'" are selected from the group consisting of alkyl, cycloalkyl, alkenyl, aryl, and hydrogen; (2) aluminum imine polymers comprising structural units of the general formula R' R" where n > 2 and R" is an imine group, and R and R' are selected from the group consisting of alkyl, cycloalkyl, alkenyl, aryl and hydrogen; and (3) polyaminoalanes comprising structural units of the general formula R R" where n > 2 and R and R' are selected from the group consisting of alkyl, cycloalkyl, alkenyl, aryl and hydrogen. Preferred aluminum-nitrogen polymers are prepared by heating the reaction product of a dialkylaluminum hydride and an organic nitrile. Most preferred is an aluminum-nitrogen polymer prepared from acetonitrile and diisobutylaluminum hydride as described in Example A. The block copolymers of this invention can be crosslinked, i.e., cured, by supplying energy to generate free radicals. Suitable energy sources include heat, UV light or electron beam radiation. UV curing agents such as alpha, alpha-dimethoxy-alpha-acetophenone (DMPAP) enhance UV curing. In a preferred embodiment, block copolymers containing alkenyl or alkynyl groups on silicon can be crosslinked by heating in the presence of a free radical generator such as a peroxide or an azo compound. The cured block copolymers are infusible solids that retain their shape upon pyrolysis and are insoluble in common organic solvents. Suitable peroxides include for example, diaroyl peroxides such as dibenzoyl peroxide, di-p-chlorobenzoyl peroxide and bis-2,4-dichlorobenzoyl peroxide; dialkyl peroxides such as 2,5-dimethyl-2,5-di (t- butylperoxy)hexane and di-t-butyl peroxide; diaralkyl peroxides such as dicumyl peroxide; alkyl aralkyl peroxides such as t-butyl cumyl peroxide and l,4-bis(t-butyl-peroxyisopropyl)benzene; alkylaroyl peroxides; and alkylacyl peroxides such as t-butyl perbenzoate, t-butyl peracetate and t- butyl peroctoate. Dicumyl peroxide is preferred. Suitable azo compounds include, for example, symmetrical or unsymmetrical azo compounds such as, for example, 2,2'-azobis(2- methylpropionitrile); 2,2'-azodimethyl-4-methoxyvaleronitrile); 1-cyano-l- (t-butylazo)cyclohexane; and 2-(t-butylazo)-isobutyronitrile. These compounds are well known and are described, for example, in U.S. Patents 2,492,763 and 2,515,628. The curable, liquid ceramic precursor binder phase used in the process of this invention must be a liquid at temperatures below its curing temperature and have the properties discussed below in order to be useful in the reaction injection molding process of this invention. The curable, liquid precursor binder phase should preferably cure rapidly by thermal, radical or ionic means. The term "cure" is defined as a chemical polymerization or crosslinking process that leads to thermally irreversible binder solidification to the extent necessary to remove a powder-filled green part from a mold without dimensional distortion. There is an increase in binder molecular weight during curing, with formation of covalent bonds and rapid propagation of bond formation such that the cure is accomplished in less than 60 minutes and preferably less than 10 minutes. Rapid cure mechanisms such as those involving radical propagation are thus preferred. The curable, liquid ceramic precursor binder phase preferably has a viscosity of less than 5000 poise (p), more preferably less than 500 poise, and most preferably between 500 poise and 1 centipoise at 25 β C. The viscosity of the precursor binder should not increase appreciably over the range of temperatures at which the injection molding is conducted. This is usually not a problem, since molding mixes are injected at relatively low temperatures in a RIM process, i.e., generally less than 80 β C and certainly less than 120 β C. This requirement limits suitable precursors to those that do not exhibit appreciable molecular weight increase at temperatures between 25 β C and about 120 β C. However, molecular weight buildup during injection molding is seldom a problem if room temperature viscosities fall within the ranges defined above. Indeed, the viscosity of the SiC/AIN ceramic precursors described in this specification decrease appreciably as they are heated to temperatures of between 25 β C and about 120°C so that fluidity of the powder/polymer mixtures does not present a problem in the molding process described. The ceramic precursor binders preferably have a polydispersity less than or equal to three, more preferably less than or equal to two. Polydispersity is defined as the ratio of the polymer weight average molecular weight to the polymer number average molecular weight. Polymers or oligomers having a higher polydispersity often exhibit complex rheological behavior and often show shear thickening (dilatant) behavior when highly filled (greater than 30% by weight) with a ceramic or metal powder. Such polymers or polymer mixtures, when filled, are therefore unsuitable for injection molding because the mixtures will not flow easily when sheared. In contrast, the highly filled polymers or oligomers of this invention exhibit non-dilatant behavior, even without heating. After curing, the ceramic precursor should preferably contain no more than 10 wt. %, more preferably no more than 5 wt. % of species that volatilize below the decomposition temperature of the cured binder. Extensive voids are created if a higher percentage of volatile species are present, leading to unacceptable porosity and increased shrinkage in the fired article. The ceramic precursor should preferably form a coherent char upon decomposition and at temperatures less than the sintering temperature of the filler. While monomeric ceramic precursors can satisfy all of the requirements mentioned above, monomers that polymerize to form binder polymers of appreciable ceramic yield (greater than 60 wt. %) often have so low a molecular weight that volatilization at modest molding temperatures becomes a problem. Because monomers are generally too volatile to be used in this RIM process, the preferred ceramic precursors of this invention are either oligomeric or polymeric. An oligomer is defined as a polymer molecule consisting of only a few monomer repeat units, i.e., greater than two and generally less than 30. When the precursor used in the practice of this invention is an oligomer or a polymer, the synthesis of the precursor is controlled in order to produce a low molecular weight product that exhibits the requisite rheological characteristics. In general, polymers suitable for the practice of this invention have numbers of repeat units of less than about 200. The ceramic precursor binder phase used in the practice of this invention is mixed with a ceramic powder, a metal powder or mixtures thereof as a filler. Suitable fillers include, for example, SiC, A1N, Si 3 N , Si0 2 , BN, A1 2 0 3 , TiN, TiC, Si, Ti , Zr, B, Al , ZrC, Zr0 2 , B 4 C, TiB 2 , HfC and Y^. A powder, whisker or platelet that comprises a solid solution of SiC and A1N can also be used as a filler. SiC and A1N are the preferred fillers. SiC is most preferred. Alpha-SiC, beta-SiC and mixtures thereof can be used. The ceramic and/or metal powder filler comprises at least 30% by volume of the mixture. The percentage by weight will vary, depending on the density of the filler. Although the physical state of the metal and ceramic is referred to as a "powder" throughout this specification, it should be understood that the ceramic or metal can also be present in various other forms such as fibers, whiskers or platelets. The curable, liquid ceramic precursor binder phase and the ceramic and/or metal powder filler can be mixed by milling, or they can be mixed without milling. Processing aids such as dispersants, rheology modifiers, sintering aids and lubricants can also be added to the mixture. When curing is to be accomplished by heating, the mixture of ceramic precursor and ceramic and/or metal powder can also include a free radical source, a curing agent or a catalyst, depending upon the precursor used. With regard to the injection molder used in the practice of this invention, a ram extruder is preferred over a reciprocating screw extruder due to the rheological behavior of the mixtures used. The mixture of powder and binder used in the practice of this invention has a sufficiently low viscosity at low temperatures to be extruded through an injection port into a mold at low pressures. In contrast, when reciprocating screw type injectors typically employed for conventional thermoplastic injection molding are used, the material flows up the screw flights rather than out of the nozzle into the mold. The pressure applied to the mix during injection is at least 50 psi and preferably between 100 and 2000 psi. The velocity of the ram is at least 1 inch per second (ips) and preferably between 3 and 10 ips. Excessively fast ram velocities are undesirable due to the jetting of the material into the mold cavity with subsequent formation of knit lines in the green body and degradation of the mechanical integrity of the sintered parts. Once filled, the mold pressure is held until the precursor cures. This holding pressure is at least 500 psi and preferably 1000 and 4000 psi. Higher pressures are desired to minimize part shrinkage and cracking upon removal from the mold. The mold is held at a temperature high enough to initiate polymerization/crosslinking of the precursor binder. For example, when a dicumyl peroxide initiator is used to crosslink a vinylsilyl group in a precursor binder, the mold temperature is generally set at 150°C. Other initiators require different temperatures. When a free radical initiator is used, a temperature is generally selected so that the hold time in the mold is greater than or equal to one or preferably two half lives of the initiator at that temperature. It is important for the part to cure sufficiently while in the mold so that removal stresses can be sustained without cracking of the molded part. The mold should be fabricated in such a manner that the facile flow of the highly filled precursor mixtures can be accommodated without leaking, since the mixtures are generally highly fluid at temperatures just below their cure temperature. The material used to fabricate the mold should be selected so that there is low adhesion of the cured part to the surface of the mold. This facilitates part removal. The exact nature of the material used to fabricate the mold depends on the composition of the mix to be injection molded and is readily apparent to one skilled in the art. After curing of the ceramic precursor, the shaped article is heated under a suitable atmosphere to convert the cured ceramic precursor binder phase to a ceramic comprising at least two compositionally distinct ceramic phases, and then heated under a suitable atmosphere to a temperature sufficient to densify the article. When high density sintered parts are desired, one or more sintering aids are preferably included in the molding formulation. Such sintering aids are well known in the art and are specific to the material being molded. For example, typical sintering aids for silicon carbide/aluminum nitride ceramics include Y 0 3 , CaO, A1 0 3 , and boron. The densified articles retain their net shape after firing. The term densify is meant to include solid phase sintering, liquid phase sintering and reaction bonding. The atmosphere selected for each of these steps can be the same or different, and depends upon the type of ceramic that is desired in the final product. For example, if the preferred block copolymers of this invention are used as the binder in the molding process and a SiC/AIN composite ceramic is desired, the molded object can be pyrolyzed under a nonoxidizing atmosphere, e.g., an argon atmosphere, a reducing atmosphere or a nitrogen atmosphere. As a further example, a ceramic comprising both silicon dioxide and aluminum oxide can be obtained by treating the same block copolymer composition under an atmosphere of air or oxygen in both steps of the heating process. In contrast, the ability to obtain a multiphase ceramic from the poly(thio)ureasilazanes mentioned as suitable ceramic precursor binders is very dependent upon the atmosphere used. These polymers convert to a single ceramic phase, i.e., silicon nitride, upon pyrolysis in a reducing atmosphere containing nitrogen atoms, such as ammonia or a mixture of N and H . If an argon atmosphere is used, only SiC is obtained. However, upon pyrolysis in a nitrogen atmosphere, mixed SiC/Si 3 N 4 ceramics are obtained. In the sintered multiphase ceramic article of this invention, each of the ceramic phases comprises at least 1 wt. % of the total mass of the sintered article, preferably at least 5 wt. % and most preferably at least 10 wt. %. Ceramics having two or more phases can be prepared by the process of this invention. For example, a mixture of A1N, SiC and Si 3 N 4 ceramic phases can be obtained from a mixture of A1N powder, a polycarbosilane and a polyureasilazane. The properties of the ceramic product are enhanced by the presence of multiple phases, since the advantageous properties of each are incorporated. For example, SiC has excellent high temperature properties, Si 3 N 4 is very strong and A1N contributes oxidation or corrosion resistance. In this specification all parts and percentages are by weight unless otherwise noted. Example A An aluminum-nitrogen polymer was prepared as follows. A 250 ml Schlenk round bottom flask was fitted with a pressure-equalized dropping addition funnel and purged. Acetonitrile (50 mol , 946 mmol) was added to the flask. The funnel was charged with diisobutylaluminum hydride (100 ml, 1.0 M in toluene, 100 mmol) and the flask was cooled to 0 β C. The diisobutylaluminum hydride was added dropwise over 30 minutes and stirred at 0 β C for an additional hour. The flask was warmed to room temperature and the colorless solution was stirred overnight. The solvent was removed under vacuum leaving 18 g of a yellow liquid. This liquid was heated under nitrogen to 300 β C over a period of 2 hours and held at 300°C for 6 hours to form -[(CH 3 CH 2 )NAl(C 4 H 9 )]- n -[(CH 3 CH 2 )NAl]- m . Upon cooling, 10.6 g of a solid polymer were obtained. Example B A polysilazane was prepared as follows. A 5 liter, three-necked flask was equipped with an overhead mechanical stirrer, a dry ice/acetone condenser (-78°C), an ammonia/nitrogen inlet tube and a thermometer. The apparatus was sparged with nitrogen and then charged with hexane (1760 ml, dried over 4A molecular sieves), methyldichlorosilane (209 ml, 230.9 g, 2.0 mol) and vinylmethyldichlorosilane (64 ml, 69.6 g, 0.5 mol). The ammonia was added at a rate of 3.51/min. (9.37 mol) for one hour. During the addition, the temperature of the reaction rose from 25 β C to 69°C. After one hour, the ammonia flow was stopped and the reaction mixture cooled to room temperature. The reaction mixture was filtered on a glass-fritted funnel to remove the precipitated ammonium chloride. The hexane was removed from the filtrate under reduced pressure (28 mm Hg, 60 β C) to give -[(CH 3 SiHNH) 0#8 (CH 3 SiCH=CH 2 NH) 0#2 ]- χ as a clear oil (150.76 g, 2.34 mol, 95% yield). The oil had a viscosity of 43 cp at 25T. Example C A polyureasilazane was prepared as described in U.S. Patent 4,929,704 by treating 1451.8 g of the polysilazane, -[(CH 3 SiHNH) 0# g(CH 3 SiCH=CH 2 NH) 0- 2J"χ> P re P ared as described in Example B, with 6.62 ml phenylisocyanate and heating the mixture to 70°C for 1 hour. Example D A block copolymer was prepared by combining 10.0 g of the polysilazane prepared as described in Example B, and 7.2 g of the aluminum- nitrogen polymer prepared as described in Example A, and heating under nitrogen to 110 β C for 5 hours. Isobutane was formed as a by-product of the reaction. The resulting block copolymer was an orange liquid having a viscosity of 22 poise. Example E A block copolymer was prepared by combining 10.0 g of the isocyanate- modified polysilazane prepared as in Example C, and 7.2 g of the aluminum- nitrogen polymer prepared as described in Example A, and heating under nitrogen to 110°C for 4 hours. Isobutane was formed as a by-product of the reaction. The resulting block copolymer was an orange liquid having a viscosity of 357 poise. Example 1 This example describes the formation of a multiphase ceramic from a mixture of polymeric ceramic precursor binders. A curable, liquid mixture of a 25 wt. % solution of polycarbosilane in polyureasilazane was prepared by dissolving 3.14 grams of solid, Nicalon ® X9-6348 polycarbosilane supplied by Dow Corning Corp., Midland, MI, in 9.40 grams of the liquid polyureasilazane prepared as described in Example C by heating at HOT. Dicumyl peroxide (1.5 wt. % based on the total weight of the polymer) was added to a sample of the liquid mixture of polycarbosilane and polyureasilazane and the polymer was cured by heating to a temperature of 155°C for 5 minutes. The cured polymer mixture was pyrolyzed to a mixed SiC/Si N 4 ceramic by heating to 1600 β C at 10 β C/minute in a nitrogen atmosphere. The ceramic content was confirmed by X-ray diffraction analysis. Example 2 This example describes the formation of a multiphase ceramic from a polymeric ceramic precursor binder that contains only one metallic element. Dicumyl peroxide (1.5 wt. % based on the total weight of the polymer) was added to a sample of polyureasilazane prepared as described in Example C and the polymer was cured by heating to a temperature of 155 β C for 5 minutes. The cured polymer was pyrolyzed to a mixed SiC/Si 3 N 4 ceramic by heating to 1600°C at 10 β C/minutes in a nitrogen atmosphere. The ceramic content was confirmed by X-ray diffraction analysis. Example 3 This example describes the formation of a multiphase ceramic from a polymeric ceramic precursor binder that contains both silicon and aluminum. Dicumyl peroxide (1.5 wt. % based on the total weight of polymer) was added to a sample of the block copolymer prepared as described in Example D and the polymer was cured by heating to a temperature of 155°C for 5 minutes. The cured polymer was pyrolyzed to a mixed SiC/AIN ceramic by heating to 1600 β C at 10 β C/minute in an argon atmosphere. The ceramic content was confirmed by X-ray diffraction analysis. Example 4 A blend of 30% by volume (57 wt. %) HC Starke beta-silicon carbide powder and 70% by volume (43 wt. %) of the block copolymer prepared as in Example D containing 1.5% by weight of dicumyl peroxide based on the total weight of the polymer was prepared by hand mixing until a homogeneous mixture was obtained. The mixture was then poured into the barrel of Frohring Mini-Jector (Model 60 PC 100) and injected at an injection pressure of 800 psi and an injection temperature of 25 β C into a mold which was heated to 155 β C. After 5 minutes the molded part was removed from the mold. The molded part replicated the shape of the mold and was quite strong. The molded part was then fired to 1600°C at 10°C/minute in a continuous ramp under an argon atmosphere. The finished part consisted of approximately 92 wt. % SiC and 8 wt. % AIN based on the total weight of the fired ceramic. Example 5 A blend of 40% by volume (68.5 wt. %) aluminum nitride powder and 60% by volume (31.5 wt. %) of the block copolymer prepared as described in Example D containing 2.0% by weight of dicumyl peroxide, based on the total weight of the polymer, was mixed by hand until a homogeneous mixture was obtained. The mixture was then injected by syringe into a mold at 25°C. The mold composition was cured by heating to 155°C. After 5 minutes the molded part was removed from the mold. The molded part replicated the shape of the mold and was quite strong. The molded part was then fired to 1600°C at lO'C/minute in a continuous ramp under an argon atmosphere. The finished part consisted of approximately 87 wt. % AIN and 13 wt. % SiC based on the total weight of the fired ceramic. Example 6 An injection molding mix is made by mixing 766.4 g of beta-silicon carbide, 276.6 g of the polyureasilazane prepared as described in Example C, 1.04 g of glycerol monooleate dispersant, and 1.11 g of dicumyl peroxide in a Ross Model A6C17XC20C planetary mixer for two hours. The mix was injection molded at 50 β C on a Hull Model 120-25 injection molder at a ram speed of 4.0 inches per second under 500 psi pressure. The part was cured in the die at a temperature of 150 β C for a period of 30 minutes. The cured part was strong, and had a good surface finish. The cured part was then converted to a ceramic part containing approximately 97 wt. % SiC and 3 wt. % Si 3 N 4 , based on the total weight of the fired ceramic, by heating to 1600"C in a nitrogen atmosphere to form a mixed silicon carbide/silicon nitride ceramic. Example 7 A blend of 30% by volume silicon carbide powder and 70% by volume of the liquid polycarbosilane/polyureasilazane mixture prepared as described in Example 1 containing 2% by weight of dicumyl peroxide was mixed by hand until a homogeneous mixture was obtained. The mixture was then injected by syringe into a mold at 25°C. The molded composition was cured by heating to 155 β C. After 5 minutes the molded part was removed from the mold. The molded part replicated the shape of the mold and was quite strong. The molded part was then fired to 1600 β C at lO'C/minute in nitrogen in a continuous ramp. The fired part contained 97 wt. % SiC and 3 wt. % Si 3 N 4 , based on the total weight of the fired ceramic. Example 8 A blend of 30% by volume aluminum nitride powder and 70% by volume of the liquid polycarbosilane/polyureasilazane mixture prepared as described in Example 1 containing 2% by weight of dicumyl peroxide was mixed by hand until a homogeneous mixture was obtained. The mixture was then injected by syringe into a mold at 25 β C. The molded composition was cured by heating to 155 β C. After 5 minutes the molded part was removed from the mold. The molded part replicated the shape of the mold and was quite strong. The molded part was then fired to 1600"C at 10 β C/minute in nitrogen in a continuous ramp. The fired part contained a mixture of AIN, SiC, and Si 3 N 4 ceramic phases as confirmed by X-ray diffraction analysis.

Claims: CLAIMS 1. In a reaction injection molding process for preparing a sintered ceramic article comprising: (a) injecting into a heated mold a fluid, solvent-free, non¬ dilatant mixture comprising (1) at least one of a ceramic powder, a metal powder, and mixtures thereof; and (2) at least one curable ceramic precursor binder phase which is a liquid below its curing temperature, said powder being present in an amount of at least 30% by volume, to cure the binder and produce a hardened molded article, (b) heating the hardened molded article to a temperature sufficient to convert the cured binder phase to a ceramic, and (c) sintering the article to the desired density, the improvement comprising using as said at least one curable, liquid ceramic precursor phase a polymer or mixture of polymers that contains at least one metallic element and forms at least two compositionally distinct ceramics upon pyrolysis under a suitable atmosphere. 2. The process of claim 1 wherein the mixture in (a) also comprises a free radical source and the ceramic precursor is cured by heating. 3. The process of claim 2 wherein the free radical source comprises an organic peroxide. 4. The process of claim 1 wherein the mixture in (a) also comprises at least one sintering aid. 5. The process of claim 1 wherein the curable, liquid precursor contains the metallic elements silicon and aluminum. 6. The process of claim 5 wherein the curable, liquid precursor comprises a block copolymer comprising a multiplicity of units having the formula and a multiplicity of blocks of units having the formula R- -Er - k wherein x > 1 and y > 1, R and R 2 are the same or different and are selected from the group consisting of hydrogen, substituted or unsubstituted 1-12 carbon alkyl, 3-12 carbon cycloalkyl, 2-12 carbon alkenyl, 3-12 carbon cycloalkenyl, and aryl groups; R 3 and R 4 are the same or different and are selected from the group consisting of hydrogen, substituted or unsubstituted 1-6 carbon alkyl, 3-6 carbon cycloalkyl, 3-6 carbon cycloalkenyl, 2-6 carbon alkenyl, 2-6 carbon alkynyl, and aryl groups, provided that R 1 , R 2 , R 3 , and R 4 are not all hydrogen and at least one of R , R 2 , R , and R 4 is an organounsaturated group. 7. The process of claim 6 wherein the organounsaturated group comprises a vinyl group. 8. The process of claim 5 wherein the curable liquid precursor comprises a block copolymer comprising a multiplicity of blocks of units having the recurring formula -E •Al and a multiplicity of blocks of units having the formula wherein x > 1 and y > 1, R and R 2 are the same or different and are selected from the group consisting of hydrogen, substituted or unsubstituted 1-12 carbon alkyl, 3-12 carbon cycloalkyl, 2-12 carbon alkenyl, 3-12 carbon cycloalkenyl, and aryl groups; R , R 4 and R 5 are the same or different and are selected from the group consisting of hydrogen, substituted or unsubstituted 1-6 carbon alkyl, 3-6 carbon cycloalkyl, 3-6 carbon cycloalkenyl, aryl, 2-6 carbon alkenyl, and 2-6 carbon alkynyl groups, and A comprises 0 or S, provided that R 1 , R 2 ., R , R 4 and R 5 are not all hydrogen, and at least one of R 1 , R 2 , R 3 , R 4 and R 5 comprises an organounsaturated group. 9. The process of claim 8 wherein the organounsaturated group comprises a vinyl group. 10. The process of claim 8 wherein R 4 is vinyl, R 5 is phenyl and A comprises 0. 11. A sintered multiphase ceramic article prepared by the process of claim 1. 12. The sintered multiphase ceramic article of claim 11 wherein each ceramic phase comprises at least 1 wt. % of the total mass of the sintered article. 13. The sintered multiphase ceramic article of claim 12 wherein each ceramic phase comprises at least 10 wt. % of the total mass of the sintered article. 14. The sintered multiphase ceramic article of claim 11 wherein said phases comprise at least two materials selected from the group consisting of SiC, AIN and Si 3 N 4 . 15. The sintered multiphase ceramic article of claim 11 comprising SiC and AIN. 16. The sintered multiphase ceramic article of claim 11 comprising Si 3 N and SiC. 17. The sintered multiphase ceramic of claim 11 comprising at least three compositionally distinct ceramic phases. 18. The sintered multiphase ceramic article of claim 17 comprising AIN, Si 3 N 4 and SiC. 19. In a reaction injection molding process for preparing a sintered ceramic article comprising: (a) injecting into a heated mold a fluid, solvent-free, non- dilatant mixture comprising at least one curable ceramic precursor binder phase which is a liquid below its curing temperature to cure the binder and produce a hardened molded article, (b) heating the hardened molded article to a temperature sufficient to convert the cured binder phase to a ceramic, and (c) sintering the article to the desired density, the improvement comprising using as said at least one curable, liquid ceramic precursor phase a polymer or mixture of polymers that contains at least one metallic element and forms at least two compositionally distinct ceramics upon pyrolysis under a suitable atmosphere. 20. A sintered multiphase ceramic article prepared by the process of claim 19.

Inventors: JENSEN JAMES ALLEN, LUCKACS ALEXANDER III, JENSEN, JAMES, ALLEN, LUCKACS, ALEXANDER, III
Applicants: JENSEN JAMES ALLEN, LANXIDE TECHNOLOGY CO LTD, LUCKACS ALEXANDER III, JENSEN, JAMES, ALLEN, LANXIDE TECHNOLOGY COMPANY, LP, LUCKACS, ALEXANDER, III
Application year: 1993
Main classification: C04B35
IPC codes: C04B35