Fatigue Characterization of SiC/SiC Ceramic Matrix Composites in Combustion Environment

Abstract

Fatigue behavior of woven melt infiltrated (MI) SiC/SiC ceramic matrix composites (CMCs) was investigated under a tension–tension fatigue condition in a combustion environment. A special experimental facility is designed to subject the CMCs under simultaneous mechanical and combustion conditions which is more representative of some conditions experienced by the hot section components of a jet engine. The MI SiC/SiC CMCs considered in this study consists of a SiC matrix densified with liquid Si infiltration, BN interphase, and reinforced with two different fibers, namely, Hi–Nicalon type S and Tyranno SA fibers. A high velocity oxygen fuel (HVOF) gun is used to create the representative combustion condition and a horizontal hydraulic MTS machine to apply the mechanical loading. Several fatigue tests were conducted at different stress levels with a stress ratio of 0.1, frequency of 1 Hz, and the specimen surface temperature at 1200 °C. Similar tests were conducted in an isothermal furnace condition at 1200 °C for comparison. Electrical resistance (ER) was used to monitor the tests. A reduction in the fatigue life was observed for the two MI systems under combustion conditions in comparison to the isothermal furnace condition at the same applied stress level. This is attributed to the presence of harsh combustion environment present in the burner rig. ER showed some promising results in monitoring the temperature and detecting damage in the specimen. Runout condition was set as 24 H (86400 cycles) in burner rig and 100 H (360000 cycles) in furnace environment. Specimens that achieved the runout condition were subsequently tested under monotonic tension testing at room temperature after cooldown to evaluate the residual properties. Residual strength results showed a significant strength reduction in both the furnace and burner rig environments. Post-test microscopy was conducted on the fracture surfaces of the failed specimens to understand the oxidation behavior and damage mechanisms.