The degradation of thermal barrier coatings by molten deposits: introducing the concept of basicity

Abstract

Airborne particles such as dust, sand and volcanic ash, otherwise termed ‘CMAS’ (Calcia Magnesia Alumina-Silicate) are ingested into the engine where at the elevated temperatures this deposit can melt and induce degradation of the thermal barrier coating (TBC). The columnar, high porosity microstructure of electron beam physical vapour deposited (EB-PVD) TBCs, typically found on HP 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 and, therefore, the degradation mechanisms of EB-PVD TBCs. This paper identifies a parameter termed ‘Basicity Index’, which modifies the severity and damage caused by these molten deposits to TBCs. It is believed that with the higher temperatures likely to result in next generation aero-engines and with sand chemistry varying around the world, basicity driven TBC degradation will become more prevalent.