Thermal Cycling Behavior of Plasma Sprayed Segmented Thermal Barrier Coatings

Abstract

Thermal barrier coatings (TBCs) with different levels of segmentation crack densities (Ds) were sprayed at different substrate temperatures (Ts). Thermal cycling resistance of the TBC specimen was examined. The segmented coating significantly improved the thermal cycling resistance as compared to the traditional non-segmented coating. Maximum thermal cycling lifetimes were achieved in the coating with a crack density level of 2.2 mm � 1 . New segmentation cracks were hardly generated during thermal cycling testing. Spallation of segments within the segmented coating occurred, which is different from spallation of the whole coating from substrate in the case of traditional coating. Oxidation of bond coat and limited phase decomposition of YSZ topcoat were considered as not responsible for the failure of TBCs. Turbine engine components are subjected to high thermal and mechanical loads. Ceramic thermal barrier coatings (TBCs) are used to protect the components from thermal loads by decreasing the surface temperature of the metallic substrate. Generally, plasma sprayed (PS) thermal barrier coatings (TBCs) have exhibited shorter thermal cycling lifetimes than electron beam physical vapor deposition (EB- PVD) TBCs, which makes PS TBCs suitable only for less exacting applications in gas turbines. The failure of plasma sprayed TBCs is mainly caused by the sintering and the progressive destabilization of the zirconia topcoat, due to thermal mismatch stresses applied to the coating, and the formation of thermally grown oxide (TGO), as well as thermal shock. Recently, it was reported that macrocracks in TBC perpendicular to a substrate, namely segmentation cracks, are beneficial to increase the thermal cycling lifetime of plasma sprayed TBCs, by accommodating the stresses arising from the different thermal expansions between the coating and substrate. 1-3) Such macrocracks can be developed by post treatment such as laser glazing, 4,5) or directly introduced during the deposition by applying high heat input to the substrate and high lamellar thickness. 2,3,6) The formation mechanism of segmentation cracks and thermo-physical properties of the segmented coatings have been investigated. 7-9) However, the thermal cycling behavior and associated failure mechanism of the segmented TBCs are seldom reported. In this paper, thick TBCs with different levels of segmentation crack densities (Ds) are produced by controlling the spray parameters. The thermal cycling behavior of traditional non-segmented and modified seg- mented TBCs is studied. The failure mechanisms of these TBCs are also discussed. 2. Experimental