Damage mechanisms of C/SiC composites subjected to constant load and thermal cycling in oxidizing atmosphere

Properties of a carbon fiber reinforced silicon carbide matrix composite were investigated in controlled environments including constant load, thermal cycling and wet oxygen atmosphere. Damage was assessed by residual mechanical properties and scanning electron microscopy characterization. Thermal strain was shown to change with cyclic temperatures over the same period (120 s). Strain varies approximately from the initial linear elastic strain of 0.63% to the final nonreversible damage strain of 1.6% during the short time of the test. The experimental strain difference between two selected temperatures is about 0.16% and the theoretical calculation value is 0.1566%. After 50 thermal cycles, the Young's modulus of the composites is reduced by a factor of 0.5 while the residual strength still retains 82% of the initial strength. It is observed that matrix cracks transversely and wave-shaped cracks are arranged on the coating surface at relatively regular spacing. A typical superficial oxidation can be found along the opening and propagating cracks beneath the coating.