C(B) MATERIALS AS INTERPHASES IN SIC SIC MODEL MICROCOMPOSITES/

A specific test procedure has been developed to compare the high tempe rature lifetimes of SiC/SiC microcomposites with various interphases i n air and under mechanical loading. The interphases, namely pure pyroc arbon (PyC) or C(B) materials with uniform or variable boron contents in the thickness, were prepared by chemical vapour deposition (CVD). U niform addition of boron in PyC interphases improved their oxidation r esistance and consequently the lifetimes of the microcomposites. Howev er, room temperature tensile tests have shown that this improvement oc curs to the detriment of the mechanical properties even when a non-bri ttle behaviour is maintained. In the case of variable boron contents, compositional gradient interphases (CGI) in which boron content increa ses from the fibre interface to the matrix interface allow the mechani cal fuse properties of PyC to be combined with the oxidation resistanc e of a C(B) material.