6 - Ceramic coatings in turbine applications

Abstract

To increase the efficiency and fuel utilization of turbine engines and reduce their CO2 emissions, it is necessary to enhance the engine operation temperature. Thermal barrier coating (TBC) is a technology capable of achieving this, which is applied onto the surface of the hot metallic section in turbine engines. A typical TBC system is composed of a ceramic top coat, a metallic bond coat, and a thermally grown oxide layer between them. Several technologies including atmospheric plasma spraying, electron beam physical vapor deposition, and plasma spray physical vapor deposition have been developed to prepare TBCs. The material of choice for the ceramic top coat is 6–8 wt.% yttria partially stabilized zirconia. However, it has some limitations, which motivates the research to develop new ceramic topcoat materials. Single stabilizer or co-dopants-doped zirconia, pyrochlore-type zirconates, perovskites, and rare earth phosphates are considered as promising candidates for TBC applications. During service, turbine engines inevitably ingest sand, fine particulates, and volcanic ash, which melt and form the deposit consisting of CaO, MgO, Al2O3, and SiO2 (CMAS) on TBC surfaces. CMAS attack has become a critical issue to cause premature failure of TBCs, which involves thermochemical and thermomechanical aspects. Several CMAS mitigation strategies have been developed, including applying protective layers on TBC surfaces, TBC composition modification, and TBC microstructure tailoring by laser.