Fatigue and stress-rupture behaviors of SiC/SiC composite under humid environment at elevated temperature

The effects of moisture on fatigue and stress-rupture behaviors of a ceramic matrix composite (CMC) at an elevated temperature were investigated. The CMC consisted of silicon carbide fiber with an in situ BN layer (iBN) reinforced in silicon carbide matrix with boron nitride (BN) interphase. Two series of tests were conducted; tension–tension fatigue tests at a frequency of 10 Hz with a stress ratio of 0.1 and stress-rupture tests. Both type of tests were conducted at 750 °C: (1) under dry environment test condition (with 0% moisture content) and (2) under humid environment test condition (with 60% moisture content). Stress versus time to failure relationships were established under these test conditions. Moisture (humid environment) decreased the time to failure at a prescribed stress level in comparison to that without moisture (dry condition) in both series of tests. For a given test environment, time to failure was shorter under the cycling loading condition of fatigue test than that under the constant load of stress-rupture test. Damage analysis showed that the exposure to the elevated temperature caused the embrittlement of BN interphase between fiber and matrix. The presence of moisture accelerated this embrittlement process, which in turn caused the more degradation of fatigue or stress-rupture performance of the tested CMC system in humid test environment in comparison to that in dry test environment.