The effects of thermal fatigue on a SiC fibre/aluminosilicate glass composite

The thermal fatigue (TF) of ceramic matrix composites introduces stresses within the composite due to the thermal expansion mismatch of fibre and matrix; this will affect the lifetime and dimensional stability of the composite. A review of various laboratory TF methods is given, and the controlled, photon heating method used in this research is explained. A Nicalon fibre/glass matrix composite was subjected to rapid, controlled TF from 250 to 700 °C and 250 to 800 °C under no load and dead load conditions in order to illustrate a variety of elastic and inelastic cyclic strain conditions. To characterize simple environmental exposure at elevated temperature, ageing experiments were also run. After TF, the surfaces of the composites were characterized using SEM for evidence of thermal damage and microcracking. The composites were then tested for flexural strength. Results show that the tensile modulus after TF testing remains constant, and that dimensional changes are slight, except near any local hot spots. The 700 °C maximum TF specimens showed appreciably greater embrittlement and lower strength than the 800 °C maximum TF specimens. Observations in the SEM of the surfaces of the 700 °C specimens showed little matrix flow of the type which could decrease oxygen infiltration. Greater matrix flow was observed for the 800 °C specimens. Thermally aged specimens gave results similar to those for the TF experiments.