CERAMIC MATRIX COMPOSITES Ceramic Matrix Composites

Reviewed here are 17 of the latest reports of research on the fabrication and properties of ceramic matrix composites, special materials developed over the past quarter century. What is revealed is a body of university and industrial efforts at instrumentation-intensive research on man-made materials that still have limited and problematical uses, though illusive promises of magical performance, innumerable applications, and lucrative sales entice approaching researchers, anxious for reward, like Sirens beckoning to mariners. Dangers lurk.

Ceramic matrix composites (CMCs) are space-age, three-decker sandwich materials useful in applications requiring resistance to heat and certain (moderate) structural stresses. Aerospace vehicle skins, oven walls, automotive brake shoes, and certain flues and ducts for hot gases are obvious applications. Composites are a base matrix material, usually the ceramic part; an organic or inorganic-organic fiber attached to the outside of, or impregnated within, the matrix; and--in the latest versions--an interphase, third material to protect one or both of the other two and to bind them together. The best overview of the art of fabricating these materials and researching ever-improved versions is that by Naslain (13). The brochure by AlliedSignal, Inc. (ASI, hereafter) exposes the leading edge of fabrication and achievement of superior strength, heat resistance, oxidation resistance, a

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mina, 253 MPa; Zr(8Y)O2, 413 MPa; Zr(4Y)O2, 582 MPa; and mullite, 588 MPa) (8:1813). The grain sizes of the zirconium and the mullite were made roughly one-sixth to one-fifth as large as the raw alumina matrix material by the infiltration process--a clear fabrication advantage. The process used by Honeyman-Colvin and Lange (8) to make their test matrices, however, was elaborate. a-Aluminum oxide powder (0.2 �m diameter) was first made into an aqueous slurry and then pressure filtered (at 17 MPa); it was then dried at room temperature for 24 hours, following which it was heated to 100(C for another 12 hours; it was then 'strengthened' by heating to 1,000(C, raised at 2(C per minute (more than 8 additional hours). The disks (first formed in the pressure-filtering step) were then cut into bars of various lengths, but all having 0.5 x 0.3 cm cross-sections. All of this initial fabrication took place before any infiltration steps (8:1810-11). Again, the process is elaborate, which has to be a disadvantage. Strangely, a year earlier Kim et al. were claiming to have overcome some of these same fabrication disadvantages (11). Like AlliedSignal, Inc. (1), these workers were particularly impressed with BN matrix composites. At

Category: Science - C