Residual Stresses and Oxidation of Silicon Carbide Fiber Reinforced Silicon Carbide Composites.

Abstract

The focus of this study was to determine the effects of environmental exposure on mechanical properties of a continuous fiber, melt-infiltrated (MI), reaction bonded silicon carbide/silicon carbide (RBSiC/SiC) ceramic matrix composite (CMC). We have determined that crystallization expansion of silicon causes large residual stresses to accumulate at multiple length scales within the composite. Large residual microstresses, stresses exhibited between individual grains in the matrix, were measured in the silicon and SiC grains of the composite matrix using Raman spectroscopy. The microstresses were predicted using a modified thermo-elastic model in which the thermo-elastic strain was substituted for the crystallization expansion strain. The ply-level residual stresses, stresses formed between the laminate layers of the composite, were measured using Vickers indentations. A separate thermo-elastic model for determining residual stress as a result of coefficient of thermal expansion mismatch was modified to determine the residual stress accompanying the crystallization expansion of silicon in the matrix. To determine how the stress state changes as a result of processing and use, the residual microstress states were measured after annealing. A concurrent study of the oxidation of the debond layer concluded that thick fiber coatings allowed for oxygen to penetrate deeply into a fully dense composite without the presence of cracking or mechanical loading of any kind.