Mechanical reliability evaluation of silicon nitride ceramic components after exposure in industrial gas turbines

Several studies have recently been undertaken to examine the mechanical reliability and thermal stability of silicon nitride ceramic components that are currently being considered for structural application in industrial gas turbines. Specifically, ceramic components evaluated included a bow-shaped silicon nitride nozzle evaluated in an engine test rig, silicon nitride vanes exposed in an engine field test, and an air-cooled silicon nitride vane that is currently under development. Despite the differences in field test conditions all of the exposed silicon nitride ceramic components exhibited a significant material recession arising from the oxidation of silicon nitride and subsequent volatilization of the oxide (i.e., silica). The fracture strength of exposed airfoils was also decreased due to the formation of a subsurface damage zone induced by the turbine environments. In addition, studies indicated that the properties of as-processed ceramic components, especially in airfoil regions, were not always comparable to those generated from the standard specimens with machined surface extracted from production billets. The component characterization efforts provided an important insight into the effect of gas turbine environments on the material recession and mechanical reliability of materials as functions of exposure time and conditions, which were very difficult to obtain from a laboratory scale test.