Microstructure and mechanical properties of zirconia-based thermal barrier coatings with starting powder morphology

The effects of starting powder morphology on the microstructure and mechanical properties of thermal barrier coatings (TBCs) prepared by an air plasma spray (APS) were investigated after thermal exposure of the TBCs. Three kinds of powder were prepared by a spray drying process. They showed mixed morphologies and microstructures, consisting of the following ratios of deformed hollow type to filled spherical type: 30:70 vol.% in water solvent (WS-1), 70:30 vol.% in methanol solvent (MS), and fully filled spherical type in water solvent (WS-2). Partial cracking was observed at the interface in the cases of the MS and WS-2 powders after 400 h. After 800 h, the defects such as interlamellar and vertical cracks were newly reproduced with the partial cracking at the interface in all samples tested. In the case of WS-1, there were relatively thin and uniform “splat” boundaries/interlamellar cracks and vertical cracks. The adhesive strength of TBCs with MS was higher than with the WS powders, because of the relatively dense microstructure. The hardness values depended on the plane of the TBC section and surface. Higher values were recorded on both planes of MS. The toughness values on the surface were higher than on the section. The highest toughness values on the section were achieved with MS. The feedstock powder morphology and microstructure affected the microstructure evolution and mechanical properties of the TBCs.