CMAS corrosion of EB PVD TBCs: Identifying the minimum level to initiate damage

Over the last decade a significant amount of research has been conducted into the durability of thermal barrier coatings (TBCs) focusing mainly on issues of oxidation, erosion and foreign object damage (FOD). However, as the performance and durability of TBCs has improved the temperatures at which they operate has increased. This increase in temperature has resulted in another lifing issue for EB PVD TBCs, namely that of CMAS attack. Calcium-magnesium-alumino-silicate (CMAS) attack occurs when atmospheric dust that has deposited on the surface of turbine blades melts and wicks into the columns of the TBC. This occurs at temperatures above 1240-1260 degrees C and results in the degradation of the columnar microstructure of the TBCs. Due to the fact that TBCs operate in a temperature gradient CMAS only infiltrates part of the coating before solidifying. There are a number of issues associated with CMAS attack, both chemical and mechanical. From a chemical point of view CMAS attack of electron beam (EB) physical vapour deposited (PVD) TBCs can be considered as a form of corrosion; when there is a lot of excess CMAS on the surface of a coated component Yttria diffuses out of the TBC into the molten CMAS resulting in a t' to monoclinic phase transformation in the yttria stabilised zirconia (YSZ), CMAS attack also results in localised melting and subsequent re-precipitation of the coating resulting in a loss of the defined columnar microstructure. While from a mechanical point of view the CMAS, once re-solidified, reduces the strain compliance of the EB PVD and can result in spallation of the TBC on cooling. Furthermore, current studies have indicated that small amount of CMAS infiltration significantly increases the erosion rate of EB PVD TBCs. This paper covers various aspects of CMAS attack of EB PVD TBCs, specifically looking at minimum levels of CMAS required to initiate damage, as well as investigating it from an erosion-corrosion perspective. (C) 2009 Elsevier Ltd. All rights reserved.