Plasma sprayed thermal barrier coatings with outstanding ultrahigh temperature thermal shock resistances through gradient porosity level optimizations

Abstract

In this study, the YSZ thermal barrier coatings (TBCs) with gradient porosity was developed through design the spraying parameters. The effect of the gradient porosity on the bond strength and thermal shock life was investigated, and the failure mechanism of the TBCs was explored. This article prepared three coatings with different porosities, tested the bonding strength of different porosities using the tensile method, and tested the thermal shock resistance of the coatings using water quenching after thermal shock. By simulating the residual stress inside the coatings after spraying, the failure mechanism of the coatings after thermal shock was analyzed. With the decrease of the porosity from TBC I to TBC III, the residual stress concentration after the spraying raised. Subsequently, the bond strength of the TBCs decreased form 37 MPa–31 MPa owing to the residual stress evolution. All of the three TBCs underwent three thermal shock test without macroscopic spalling. During the thermal shock test, tensile horizontal stress formed and promoted the formation of vertical cracks. Tensile vertical stress concentrated at the top - coat/bond - coat (TC/BC) interface significantly, inducing horizontal crack adjacent to the interface. Additionally, the TC layer surface zone sintered under the oxygen-acetylene flame heating. Then tensile vertical stress concentration was formed in the sintering zone region, and horizontal crack was detected in the TC layer. The connection of the vertical cracks and horizontal cracks was responsible for the spalling and failure of the TC layer.