Effect of Oxidizing Environment on High-Temperature Fatigue of an Advanced SiC/SiC Ceramic Matrix Composite Component

SiC fiber-reinforced SiC ceramic matrix composites (SiC/SiC CMCs) are known for their low weight, good toughness, and excellent durability at elevated temperatures, making them suitable for use in various parts of advanced aircraft engines. When hot air flow of ~2000°C passes through the SiC/SiC CMC liner of a combustion chamber, development of a high gradient of temperature occurs, and leads to the generation of high bending stresses. The SiC/SiC CMC component (liner) has Y-shape discontinuity where high stresses are concentrated locally and hence more susceptible to damage during fatigue. Concerning this issue, the present study aims to understand the fatigue degradation with oxidization behavior at 1000°C on the high-stress concentrated part of the component made of 2.5 D Tyranno SA3 SiC fiber/BN interface/CVI-PIP SiC matrix composite. The unique point-load-bending-fatigue tests followed by oxidizing treatment were adapted to simulate the SiC/SiC CMC component’s degradation behavior at 1000°C. The fatigue run-out samples were exposed to 1000°C for various time, and retained strengths were measured. The component withstood 1000 fatigue run-out cycles at 25 N (~170 MPa) and 27.5 N (~190 MPa) but failed at less than 50 cycles at 30 N (~215 MPa). SEM microstructures of 27.5 N-loaded fatigue run-out sample revealed significant matrix-cracks, and when exposed to 1000°C for 50 h, showed significantly lower retained strength of ~235 MPa as compared to the non-fatigue sample, which exhibited the strength of ~335 MPa. This degradation was attributed to the oxygen ingress through matrix-cracks, which degraded the BN interphase and led to the formation of boria-rich glassy phases. The final failure of the component showed the delamination of the surface plies due to the generation of shear stress between longitudinal and transverse plies.