Simulation Of Stress Accumulation In Plasma Sprayed 8YSZ TBCs During The Deposition

In this paper, a model of plasma spraying process based on pass-by-pass deposition was developed to simulate temperature fluctuation and stress evolution in 8% wtY2O3-ZrO2/NiCoCrAlY thermal barrier coatings (TBCs) on the surface of the cylinder with different ceramic coating thicknesses and spraying paths using the finite element software ANSYS. The effects of ceramic coating thickness and spraying path on the evolution of stress during the spraying process and distribution of the final stress when the coating system was cooled to room temperature were explored.The results show that temperature of the nodes at the interface between the substrate and the bond layer and the interface between the bond layer and the ceramic layer fluctuates remarkably when the spraying gun scans back and forth along the axis of the cylinder. Therefore, a corresponding stress fluctuation is caused by this rapid thermal shock. Axial tensile stress at the interface between the substrate and the bond layer increases first, then decreases and last increases with the increase of coating thickness. Axial tensile stress at the interface between the bond layer and the ceramic layer increases first and then decreases rapidly. Radial tensile stresses at the interface between the substrate and the bond layer and the interface between the bond layer and the ceramic layer increase with the increase of coating thickness. Shear stress fluctuates around zero.When the TBCs with the bond layer thickness of 120μm and the ceramic layer thickness of 240μm are cooled to room temperature, the maximum axial compressive stress exists on the surface of the ceramic layer and the maximum axial tensile stress appears at the interface between the substrate and the bond layer. The maximum radial compressive stress is located at the edge of the substrate close to the bond layer. The maximum shear tensile stress exists at the edge of the interface between the ceramic layer and the bond layer. Stresses at the interface between the substrate and the bond layer and the interface between the bond layer and the ceramic layer fluctuate periodically along the axis of the cylinder and axial stress σx is the major stress.Axial stress σx at the interface between the substrate and the bond layer increases gradually with the increase of the ceramic layer thickness. When the TBCs with the bond layer thickness of 120μm and the ceramic layer thickness of 120μm,240μm, 360μm and 480μm respectively are cooled to room temperature, corresponding axial stress σx at the interface between the substrate and the bond layer fluctuates around 10 MPa,85MPa,150MPa,200MPa along the axis of the cylinder. Thickness of the ceramic layer has little impact on axial stress ax at the interface between the bond layer and the ceramic layer and the surface of the ceramic layer. The maximum axial compressive stress increases first and then decreases, the maximum radial tensile stress increases gradually and the maximum shear stress increases rapidly with the increase of the ceramic layer thickness.Three kinds of spraying paths, the single direction raster scanning(SDRS), the reciprocal direction raster scanning(RDRS) and the cross direction raster scanning(CDRS) are designed based on the angle of the deposition passes between odd and even layers. Distance between peaks of coating temperature under the RDRS scanning path and the CDRS scanning path is smaller than that of under the SDRS scanning path. Difference between adjacent peaks of the coating temperature is smallest and therefore, stress value is lowest under the CDRS scanning path. Stress distributions of the coating systems are generally the same after the coating systems are cooled to room temperature under these three kinds of spraying paths. The maximum axial stress and the maximum radial stress are smallest and stress concentration at the edge of the coating is weakest under the CDRS scanning path, therefore, the CDRS scanning path is the ideal spraying path.