Structure evolution of 7YSZ thermal barrier coating during thermal shock testing

Abstract

Bond coating NiCoCrAlYTa was prepared on nickel-based superalloy by low temperature-high velocity oxygen flame (LT-HVOF). Then 7wt% Y2O3 stabilized ZrO2 ceramic coating was fabricated on bond coating by air plasma spray (APS). Diffusion principles of bond coating were investigated under thermal cycle by using multi-functional flame tester. Sintering and phase transformation of ceramic coating were studied. The experiment results show that with thermal cycle increase, thermally grown oxide (TGO) forms at the interface of ceramic-bond coating and its thickness continuously increases. Besides, the bond coating further diffuses to surface of ceramic coating. Lots of oxides appears on the surface of ceramic coating nearing microcrack. Diffusion of bond coating to surface contributes to sintering of ceramic coating, which leads to increase of microhardness. Additionally, plenty of voids were appeares in bond coating for Kirkendall effect resulting in decrease of microhardness. Large-scale cracks are observed after thermal cycle due to phase transformation and thermal residual stress.