CHARACTERIZATION OF SIC C (B)/SIC MICROCOMPOSITES BY TRANSMISSION ELECTRON-MICROSCOPY/

Uniform or composition-graded C(B) (i.e., boron-containing carbon) int erphases in SiC/SiC model microcomposites were characterized by transm ission electron microscopy after tensile tests and thermal ageing in a ir. A specific method was developed to prepare thin longitudinal secti ons of the tested specimens. Deflection of matrix cracks occurs within the uniform C(B) interphase, as long as its anisotropy remains high e nough (i.e., when the boron content is not too high). It takes place c lose to the most anisotropic layer (i.e. that containing 8 at% of boro n) in composition-graded interphases. In both cases, the crack deflect ion path does not reach the fibre, a feature which is consistent with the good mechanical properties. After ageing in air under tensile load ing beyond the proportional limit (600 degrees C; sigma = 800 MPa), th e composition-graded interphase (made of five sublayers in which the b oron content increases from 0 at% near the fibre to 33 at% near the ma trix) was observed to act as a glass-forming protection, the pyrocarbo n sublayer (at the fibre surface) remaining unoxidized.