Degradation of EB-PVD thermal barrier coatings caused by CMAS deposits

In aero-turbine engines, thermal barrier coatings (TBCs) must be capable to withstand harsh environments, such as high-temperature oxidation and hot-corrosion. Recently, a new failure mode of TBCs caused by calcium–magnesium–alumina–silicate (CMAS) glass has attracted increasing attention. In this paper, yttria stabilized zirconia (YSZ) TBCs produced by electron beam physical vapor deposition (EB-PVD) were exposed to CMAS deposits at 1250 °C. The microstructure evolution and failure mechanism of the coatings were investigated. It has been shown that CMAS glass penetrated into the YSZ ceramic layer along the inter-columnar gaps and interacted with YSZ. As a result, an interaction zone of about 20 μm thickness, which was the mixture of CMAS and YSZ with equiaxial structure, was formed in the YSZ surface layer after 4 h heat-treatment at 1250 °C. Meanwhile, yttria in YSZ layer as a stabilizer was dissolved in CMAS glass and caused accelerated monoclinic phase transformation. After 8 h heat-treatment, degradation of YSZ TBC occurred by delamination cracking of YSZ layer, which is quite different from the traditional failure caused by interfacial cracking at the YSZ/metallic bond coat. Physical models have been built to describe the failure mechanism of EB-PVD TBCs attacked by CMAS deposits.