Effects of amount, location, and character of porosity on stiffness and strength of ceramic fiber composites via different processing

The porosity dependence of ceramic fiber composite Young's modulus, and esp ecially tensile strengths, is reviewed. Though limited, data shows markedly different porosity dependencies for composite matrices derived from: (1) c hemical vapor infiltration (CVI), (2) preceramic polymer pyrolysis, or (3) hot pressing of powders. CVI results in initially limited, then acceleratin g, rates of property decreases as porosity increases, as for typical monoli thic ceramics. In contrast, hot pressing and polymer pyrolysis result in th e opposite behavior, i.e., high initial then diminishing rates of property decreases. This markedly differing behavior is attributed to pores being ro unded and especially away from the fiber-matrix interfaces in CVI while in hot pressing and polymer pyrolysis fiber-matrix interface, cusp/lenticulars haped pores (more difficult to remove and dominant at lower porosity levels ) are more detrimental to properties, similar to grain boundary pores in mo nolithic ceramics. Competition between such interfacial pores and those tot ally in the matrix in both their elimination and the effects of those pores remaining in the processed composite is consistent with data differences a nd scatter. Implications for properties achievable by the above 3 types of processing, as well as for sintering of composites and possible use of poro us layers at the fiber-matrix interface to limit oxidative embrittlement ar e noted. (C) 1999 Kluwer Academic Publishers.