Microstructural evolution, mechanical properties and degradation mechanism of PS-PVD quasi-columnar thermal barrier coatings exposed to glassy CMAS deposits

Abstract

Thermal barrier coatings (TBCs) applied in aero-engines tend to be attacked by molten calcia-magnesia-alumino-silicate (CMAS) at high operating temperatures. Yttria-stabilized zirconia (YSZ) coatings with quasi-columnar microstructure were fabricated by plasma spray physical vapor deposition (PS-PVD) technique. The chemical changes, microstructural transformation, mechanical properties and degradation mechanisms of the CMAS-interacted TBCs in the thermal cycling tests were investigated. Feathered YSZ grains were dissolved in the CMAS melts, and then the ZrO2 grains were reprecipitated with spherical shape, accompanying with phase transformation from tetragonal (t) to monoclinic (m). The thermal cycling tests reveal that the YSZ coating fails at the early stage due to the attack of CMAS. The fractures in intra-columns lead to partial spallation of the coatings. The failure of the coating occurs at the interfaces between thermally grown oxides (TGO) layer and YSZ topcoat; especially, the hardness and Young’s modulus of the YSZ coatings increase intensively, as the coatings were infiltrated by the CMAS for a long time.