CMAS degradation of EB-PVD TBCs: The effect of basicity

Abstract

Turbine entry temperatures of aerospace gas turbine engines will need to increase in future engine designs due to a constant desire to improve fuel efficiency and power whilst decreasing emissions which may be environmentally harmful. At present, thermal barrier coatings (TBCs) are employed together with a cooling film to protect engine hardware from the high temperatures seen within the turbine and combustion zones. Airborne particles such as dust, sand and volcanic ash, otherwise termed ‘CMAS’ (calcium–magnesium–alumino-silicates) are ingested into the engine, where at the elevated temperatures, this deposit can melt and induce degradation of the TBC.The columnar, high porosity microstructure of electron-beam physical vapour deposited (EB-PVD) TBCs, typically found on high pressure turbine blades, makes them particularly susceptible to CMAS molten deposit attack. This paper investigates the effect of modifying the chemical composition of molten CMAS deposits on the degradation mechanisms of EB-PVD TBCs. This paper identifies a parameter termed “basicity index” which can be used to evaluate the severity and damage caused by these molten deposits to TBCs. It is believed that the higher temperatures expected to be observed in next generation aero-engines coupled with sand/dust chemistry varying around the world, and then basicity-driven TBC degradation will become more prevalent.