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 degrees 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 mu m 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 degrees 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. (C) 2012 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.