Analysis Of Residual Stress Of Thermal Barrier Coatings System With Curved Substrate By Finite Element Method

Thermal barrier coatings (TBCs) are a kind of ceramic layers with excellent high temperature oxidation resistance and very low thermal conductivity, which are sprayed onto bond coat (NiCrAlY) deposited on the surface of substrate. TBCs can effectively reduce the service temperature of the protected substrate and the requirement for cooling air and improve the thermal efficiency of aircraft engine. So TBCs have been widely used in the fields of space flight, metallurgy and energy industry.The interface coalescence and spallation problems always are the key subjects in TBCs applications. Due to the mismatch in the thermo-mechanical parameters of metals and ceramics, the ceramic coating often detaches from the substrate under the complex residual stresses, which would result in the degradation and fracture of substrate. So the emphasis of this thesis is to analyze and predict the evolution of residual stress fields in TBCs system. The main content in this paper are listed as follows,Firstly, the evolutions of residual stresses of TBCs system during the preparation process were analyzed by finite element method (FEM). The corresponding results were verified by an analytical solution. Effects of coating location, substrate curvature radius, deposition temperature and coating thickness on residual stress distributions in TBCs system were discussed. It is found that the FEM results consist well with the analytical results. We discussed three different cases, respectively, i.e. the ceramic coating located on the concave surface, convex surface and both surfaces of the substrate. The calculated results show that the distributions of radial stresses in TBCs are strongly influenced by substrate curvature radii and different coating deposition locations. However, the substrate curvature radius has a few influences on the hoop residual stress of TBCs system. With the increase of deposition temperature, radial and hoop stresses in TBCs system increase rapidly. When the coating thickness increases, radial stress in TBCs system increases but hoop stress decreases. In order to minimize the mismatch stresses within TBCs system, the preparing temperature should be as low as possible and the coating should be as thick as possible.Secondly, three dimensional distributions of residual stress of TBCs system with cylindrical geometry during thermal cycling were calculated by FEM. Assumed that elastic deformation and high-temperature creep in the ceramic coating, elasto-plastic deformation in the bond coat and thermally grown oxide layers during thermal cycling, we discussed, respectively, the influence of substrate curvature radii, working temperature and coating thickness on the evolutions of residual stresses in TBCs system. With substrate curvature radius increasing, radial stress in the ceramic coating decreases but the corresponding hoop and axial stresses increase. As working temperature increases, all residual stressed in the ceramic coating and thermally grown oxide increase quickly before the 20th thermal cycling. However, after the 20th thermal cycling, these residual stresses gradually stop increasing and then keep a constant.In this thesis, the evolutions of residual stresses in TBCs system during the preparation and thermal cycling had systematically analyzed and discussed by FEM, which are helpful to predicting the distributions of residual stresses in coated-turbine blades and guide vanes in the future.