Interface Design,Bond Coat Microstructure Optimization And Failure Mechanism Analysis Of Air Plasma Sprayed Thermal Barrier Coatings

Thermal barrier coatings?TBCs?are widely deposited on the hot sections of industrial gas turbines such as the combustion chamber and the turbine blade to provide thermal insulation and,thereby,improve the efficiency and extend the lifetime of the turbine.In the absence of mechanical damage,e.g.,foreign object erosion,failure of the TBCs typically occurs in the vicinity of the thermally grown oxides?TGO?layer,following a sequence of crack nucleation,propagation and coalescence process.The composition,growth rate and microstructure have a significant effect on the behavior of the interfacial cracks and affect the stability of the TBCs.However,the properties of the TGO layer depend on the microstructure,composition and morphology of the bond coat.Therefore,this work focuses on:how to impede the propagation and coalescence of the interfacial cracks through interface modification;how to optimize oxidation and spallation resistance of the TGO layer by modifying the microstructure of the bond coat.The main contents and conclusions are as follows:Three-dimensional mesh patterns were fabricated using laser powder deposition?LPD?method at the interface of the air plasma sprayed?APS?TBCs to investigate the effect of the mesh pattern on the stablility of TBCs.The thermal cycling results showed that the spallation degree of the TBCs with mesh pattern?5¹0%?was much higher than that of the conventional TBCs?>40%?.It demonstrated that the mesh pattern is effective in impeding the propagation and coalescence of the interfacial cracks and improving the durability of the TBCs.Meanwhile,the mesh parameters such as the height and spacing length determined the failure behavior of the TBCs.Three-dimensional mesh patterns with different mesh height and spacing length were deposited at the interface between the bond coat and substrate,aiming at studying the influence of the mesh parameters on the durability and failure behavior of the TBCs and proposing the optimized mesh parameters.The study revealed failure sequences of the TBCs with mesh patterns consisting of:?I?initiation of the interfacial and ridge cracks(around the ridge of the meshes;?II?propagation of the cracks and buckling of the YSZ layer between the meshes;?III?interfacial cracks deflection and coalescence with ridge cracks,leading to final spallation.The mesh height determined the stability of the top coat around the ridge of mesh and the deflection angle.The mesh spacing governed the buckling of the YSZ layer between the meshes.Based on the experimental and calculation results,for a typical TBCs with YSZ thickness about 200?m and the bond coat thickness of 150?m,the critical mesh height h and spacing length L are about 110?m and 7 mm,respectively,when the mesh width w is fixed at about 480?m.NiCoCrAlY bond coat with dendritic microstructure,few defects and uniform phase distribution was fabricated by the LPD method,aiming at investigate the defects of the bond coat on the growth behavior and stability of the TGO layer.The results of the isothermal test at 1150°C showed that the LPD bond coat exhibited a lower TGO growth rate and higher lifetime compared with the conventional APS and High velocity air-fuel?HVAF?bond coat.This could be attributed to the higher defect density of the APS and HVAF bond coat,which preferred the formation of the spinels and cracks,leading to the spallation of the TGO layer.For the LPD bond coat with dense microstructure,the TGO layer predominantly consisting of a-Al₂O₃ was dense and uniform.Therefore,the LPD bond coat exhibited better superior oxidation and spallation resistance.It demonstrates that the microstructure and defects of the bond coat play a vital role in the growth,stability and failure behavior of the TGO layer.Different types of the NiCoCrAlY bond coats were fabricated using LPD and HVAF methods,aiming to investigated the b phase size and shape on the nucleation,growth,stress and stability of the TGO layer.Thermal cycling tests at 1150°C showed that the nucleation of the alumina inherited the contours of the underlying?phase.A smaller size and higher aspect ratio?cylindric or disc shape?of the?phase benefited the exclusive formation of the alumina,lower TGO growth rate and residual stress.The TGO lifetime of the LPD coating was almost a two-fold of that on HVAF bond coat,which is attributed to the nanoscale dendritic?phase and dense microstructure.