Microstructural instability and the resultant strength of Si-C-O (Nicalon) and Si-N-C-O (HPZ) fibres

Microstructural changes and the related strength degradation were investigated for Si-C-O Nicalon and Si-N-C-O HPZ fibres during heat treatments in air and argon at 1000-1400 degrees C for 0.5 to 90 h. While the as-received Nicalon fibre contains beta SiC and carbon nanocrystals in an amorphous SiO1.12 C-0.44 phase, the HPZ fibre is completely amorphous. It is also inhomogeneous in surface composition compared with that of the bulk. The time-dependent strength degradation of the Nicalon fibre in argon is related to the gradual decomposition of the SiO1.12 C-0.44 phase from the surface which produces surface defects, beta-SiC grain growth and intergranular porosity. The strength degradation of the HPZ fibre results from surface crystallization into alpha-SiO2, Si2N2O and beta-SiC. On the other hand the HPZ fibre cove -- which has composition close to SiN1.02C0.23 -- shows structural stability for all heat treatment conditions.