Damage Accumulation in 2-D Woven SiC/SiC Ceramic Matrix Composites

Abstract

Ceramic matrix composites are envisioned for use at elevated temperatures (1000 to 1400°C) in oxidizing environments. For non-oxide composites, the time-dependent failure stress is dependent on the severity of reactions between the environment and the load-bearing fibers and interphase. One of the most severe pitfalls for these types of materials occurs at intermediate temperatures, 600 to 1000°C. Environmental access to load-bearing fibers occurs through matrix cracks that are bridged by load-bearing fibers. The rate at which these composites lose the ability to carry load is partially controlled by the extent of cracking in the matrix. In this study, the damage accumulation of woven SiC/SiC composites tested in tension was quantified using unload/reload hysteresis tests and modal acoustic emission. The behavior of composites reinforced with ceramic-grade Nicalon™, Hi-Nicalon™, and Sylramic® fibers, with carbon or boron nitride interphases and chemically vapor-infiltrated or melt-infiltrated SiC matrices was investigated. The most significant finding of this study is that the formation of matrix cracks that bridge the load-bearing fibers (0° cracks) occurs at approximately the same strain for all of the woven SiC/SiC composites tested. All of these systems have at least a CVI SiC matrix layer adjacent to the interphase. In addition, the onset stress-strain condition for 0° cracks corresponds to the tensile stress-strain condition above which intermediate temperature embrittlement occurs for BN interphase SiC/SiC composites. For carbon interphase SiC/SiC composites, the critical stress-strain condition for the onset of intermediate temperature embrittlement corresponds to the first cracks in the matrix which can occur at stresses half the 0° crack onset stress.