The degradation behaviour of Nippon Carbon Co. SiC fibres (Nicalon®) after heat treatment in various environments was studied. Regardless of the heat-treatment conditions, the Nicalon® fibre strength degraded when the fibres were subjected to temperatures higher than 1200° C (temperatures below 1200° C were not investigated). This degradation is associated with the evaporation of CO from the fibres as well as with
-SiC grain growth in the fibres.