Ceramic materials are of critical importance for a number of high temperature, high performance applications. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness.
Silicon carbide is an extremely hard ceramic that exhibits short-term high strength at elevated temperatures. It oxidizes slowly in air and is serviceable to about 1500.degree. C. Silicon carbide possesses high corrosion resistance, low bulk density, and excellent creep and wear resistance. Nevertheless, microstructural instability during heating often limits the use of SiC in many such applications for extended periods of time. The morphology of SiC formed at low temperatures is cubic (beta-SiC, 3C). Alpha-SiC, which can have either hexagonal (alpha-SiC, 2H, 4H, 6H) or rhombohedral (alpha-SiC, 15R, 21R) morphologies, forms at higher temperatures. Also, beta-SiC transforms to alpha-SiC at higher temperatures. These various polytypes can undergo one or more phase transformations between 1400.degree. C. and 2200.degree. C., often resulting in exaggerated grain growth. Such grain growth can result in brittle failure of the ceramic under structural loading.
It is known that the presence of &gt;1% AlN in hot-pressed SiC samples results in sintered parts having reduced grain size and improved microstructural uniformity. These phenomena have been attributed to the formation of solid solutions of the AlN in the SiC ceramic. The formation of dense bodies of SiC/AlN solid solutions from a mixture of SiC and AlN powder requires not only powder consolidation (sintering), but also thorough solid-state diffusion of the AlN into the SiC microstructure. Therefore, the high melting points and low solid state diffusivities of both AlN and SiC have limited the use of solid solution SiC/AlN ceramics. As a result, consolidated samples having representative properties have, for the most part, been prepared by pressure-assisted densification methods (e.g., hot-pressing) at relatively high temperatures (2100.degree. C.). Such techniques are energy-inefficient and severely limit the shape complexity of the part that can be fabricated.
Polymer precursors have also been used to prepare SiC/AlN ceramics, including solid solutions. For example, J. F. Janik etal., Inorg. Chem., 1987, 26, 4341-4345, reported the synthesis of the dimer {[(CH.sub.3).sub.3 Si].sub.2 AlNH.sub.2 }.sub.2 by combining [(CH.sub.3).sub.3 Si].sub.3 A1-O(C.sub.2 H.sub.5).sub.2 and ammonia in a 1:1 ratio. Upon pyrolysis in ammonia at 900.degree. C., a solid mixture of AlN/SiC forms. Interrante et al. in J. Am. Ceram. Soc., 1990, 73, 352-357, report the formation of solid solutions of 2H-SiC/AlN by pyrolysis of mixtures of the carbosilanes [((CH.sub.3).sub.3 Si).sub.0.80 ((CH.sub.2 .dbd.CH)CH.sub.3 Si).sub.1.0 (CH.sub.3 HSi).sub.0.35 ].sub.n, or [CH.sub.3 HSiCH.sub.2 ].sub.n with [R.sub.2 AlNH.sub.2 ].sub.3, where R.dbd.C.sub.2 H.sub.5 or i-C.sub.4 H.sub.9.
U.S. Patent 4,687,657 discloses a ceramic comprising a solid solution of silicon carbide and aluminum nitride formed by mixing a preceramic organosilicon polymer such as poly(diorganosilanes), poly(haloorganosilanes) and poly(carbosilanes) with a poly-N-alkyliminoalane and pyrolyzing the mixed polymers at a temperature above 1000.degree. C. in an inert atmosphere. Seyferth and Brodt in Technical Report #32, Office of Naval Research, Contract N00014-82-K0322, May 16, 1990, report that reaction of trimethyl-aluminum with (CH.sub.3 SiHNH).sub.n cyclic oligomers results in the formation of soluble, crosslinked aluminasilazanes. Pyrolysis in argon gives a good yield of aluminosilicon carbonitride. U.S. Pat. No. 4,730,026 discloses crosslinked polysilazanes in which the silazane units are linked together by at least one bridge of the formula -MR'.sub.n - attached to the nitrogen atoms of silazane repeat units, where M is a metal selected from Groups IIIA, IIB, IVB and IIA of the Periodic Table. W. R. Schmidt et al. disclose preparing a single component precursor to SiC/AlN by treating 1,3,5-trivinylcyclotrisilazane with triethylaluminum. The silazane units are bridged by one -Al(C.sub.2 H.sub.5) unit (The Second International Ceramic Science and Technology Congress, Nov. 12-15, 1990, Orlando, Fla.).
None of these polymeric AlN/SiC ceramic precursors are block copolymers in which blocks of ##STR1## repeat units alternate with or are bridged by ##STR2## repeat units where &gt;1 and y &gt;1, and none offer the ability to cure from a liquid to an infusible solid in a controlled fashion at low temperatures.
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