Investigation of Residual Stresses in Melt Infiltrated SiC/SiC Ceramic Matrix Composites using Raman Spectroscopy

Ceramic matric composites (CMCs) are being developed for use in extreme operating conditions. Specifically, there is interest to replace superalloys with Silicon Carbide/Silicon Carbide (SiC/SiC) CMCs in the hot section of gas turbine engines because of their lower densities, high temperature performance, and oxidation resistance. Residual stresses in SiC/SiC CMCs are a direct result of the high temperature processing conditions, a mismatch in the coefficients of thermal expansion between composite constituents, and silicon crystallization expansion upon cooldown. Understanding the residual stress state and magnitudes of these stresses will enable better prediction of behavior and life performance in application environments. This study focused on using micro-Raman spectroscopy, indentation cracking, and mechanical testing on as-received and annealed SiC/SiC CMCs to measure and investigate the residual stresses within the composite. Following the silicon Raman active mode at 520 cm-1 and the SiC Raman active mode at 796 cm-1, residual stresses within the matrix and reinforcing fibers were investigated with a spatial resolution of 1 micron. Indentation cracking allowed for an estimate of the residual stresses solely in the matrix material. Mechanical testing, paired with acoustic emission, enabled an understanding of the macro-mechanical behavior of the composite. Results from this study will aid in behavior and damage modeling of SiC/SiC CMCs.