CMAS attack – a limiting factor for the application of thermal and environmental barrier coatings

Hot-gas exposed gas turbine components typically are made of Ni-based superalloys with ZrO2 thermal barrier coatings (TBC). In operation TBC experience severe degradation by airborne inorganic particles commonly referred to as CMAS (CaO-MgO-Al2O2-SiO2). CMAS particle ingestion, deposition, and subsequent melt infiltration is a major reason for premature TBC failure. Advanced TBC as well as new ceramic matrix composites (CMC) protected by environmental barrier coatings (EBC) promise higher efficiency due to higher operation temperatures. Increasing operation temperatures, however, will aggravate the CMAS problem since particles are expected to impinge on TBC/EBC in a fully molten and highly reactive state. Therefore, the development of new TBC as well as EBC materials with superior CMAS resistance is mandatory. A promising way to tackle CMAS attack is the use of highly reactive TBC/EBC materials which can trigger rapid crystallization of CMAS melts, thus mitigating melt infiltration. A variety of new TBC/EBC materials such as rare-earth element rich zirconates and silicates exhibit such promising resistance against simplified CMAS compositions. However, partial melting of more complex CMAS compositions as well as de-composition of crystalline reaction products of CMAS and TBC/EBC materials can lead to complex recession scenarios and may eventually limit operation temperatures