Subcritical crack growth processes in SiC/SiC ceramic matrix composites

Ceramic matrix composites have the potential to operate at high temperatures and are, therefore being considered for a variety of advanced energy technologies such as combustor liners in land-based gas turbo/generators, heat exchangers and advanced fission and fusion reactors. Ceramic matrix composites exhibit a range of crack growth mechanisms driven by a range of environmental and nuclear conditions. The crack growth mechanisms include: (1) fiber relaxation by thermal (FR) and irradiation (FIR) processes, (2) fiber stress-rupture (SR), (3) interface removal (IR) by oxidation, and (4) oxidation embrittlement (OE) resulting from glass formation including effects of glass viscosity. Analysis of these crack growth processes has been accomplished with a combination experimental/modeling effort. Dynamic, high-temperature, in situ crack growth measurements have been made in variable Ar + O2 environments while a Pacific Northwest National Laboratory (PNNL) developed model has been used to extrapolate this data and to add radiation effects. In addition to the modeling effort, a map showing these mechanisms as a function of environmental parameters was developed. This mechanism map is an effective tool for identifying operating regimes and predicting behavior. The process used to develop the crack growth mechanism map was to: (1) hypothesize and experimentally verify the operative mechanisms, (2) develop an analytical model for each mechanism, and (3) define the operating regime and boundary conditions for each mechanism. A map for SiC/SiC composites has been developed for chemical and nuclear environments as a function of temperature and time.