Accelerated oxidation of SiC CMC's by water vapor and protection via environmental barrier coating approach

Silicon carbide fiber reinforced silicon carbide matrix composites (SiC/SiC CMC's) are attractive materials for use in gas turbine hot sections due to the potential for high temperature mechanical properties and overall lower density than metals. Potential SiC/SiC CMC gas turbine components include combustion liners, and turbine shrouds, vanes, and blades. Engine design with SiC/SiC CMC's will allow optimization for performance, efficiency, and/or emissions. However, SiC/SiC CMC's are silica formers under oxidizing conditions and have been shown experimentally to undergo accelerated oxidation due to exposure to steam in high temperature combustion environments such as found in the gas turbine hot section. Oxidation by steam in a flowing gas stream has been shown to exhibit paralinear behavior and result in unacceptable recession of the surface. Thus, prior to the successful introduction of SiC/SiC CMC's for long life use in gas turbines, the problem of accelerated oxidation needs to be addressed and resolved. To this end, one approach has been the development of the environmental barrier coating (EBC) to prevent accelerated oxidation by limiting oxidant access to the surface of the silica former. This paper will review the accelerated oxidation of silica formers such as silicon carbide, the experimental testing confirming the problem, and EBC approaches resolving the problem.