Study On The Diffusion And Reaction At The Interface Of Thermal Barrier Coatings On Superalloy Substrate

NiCrAlY bond coat and thermal barrier coating (TBC) top coat were deposited on industrial superalloy substrate by plasma spraying. The diffusion and interface reaction among the elements of the industrial superalloy and the TBCs were studied both experimentally and theoretically. The study includes image analysis, phase analysis, chemical microanalysis and interface reaction by means of optical microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD). The simulations of microstructure evolution at the interface between the superalloy and the bond coat were made through using the software packages of Thermo-Calc and DICTRA. TBC failure mechanisms and the relation between the failure and interfacial reaction were investigated, and some measures of improving the TBC were then put forward. It was found that duplex NiCrAlY/TBC satisfying service conditions of turbine engines can be fabricated successfully by using low pressure plasma spraying and adopting an optimized process.Before heat treatment, a melting layer with 2~3um in thickness was found at the substrate/bond coat interface. After heat treatment at 1100C in air, the elements of the substrate and the bond coat diffuse and the diffusion of Ni is an uphill diffusion. The layer of r ' phase and the layer of A12O3 or NiAl2O4 were formed at the interface of the substrate and the bond coat and at the interface of the bond/top coat, respectively. The thickness of the r ' phase and the oxide layer increases with the increase of the time of the heat treatment. The r ' phase layer has good adhesion with the substrate and the bond coat.During heat treatment in air at 1100C, tetragonal (t) phase of ZrO2 transformed to monoclinic (m) phase of ZrO2 and the amount of the m-ZrO2 increased with time. When the sample was heat-treated in vacuum, however, the phase transformation was not observed. After heat treatment for 25h and 50h in air, the r (or r ') phase showed an anisotropy on XRD pattern. The molar fraction of the r ' phase formed at the interface between the substrate and the bond coat was calculated and the profile of composition of all elements was modeled after considering the change of heat treatment temperature and time. The simulation results agree with experimental data, indicating that when useful simulation of microstructure evolution can be obtained. This does help to design practical heat-treatment processes and compositions of coatings.The oxidation and thermal shock are the main mechanisms of TBC failure at high temperature. The oxidation-deoxidization between Al and ZrO2 or Y2O3 is one of the mechanisms of high temperature oxidation failure of TBCs. The failure under thermal shock results from the formation of cracks normal to the bond/top coat interface, due to the different coefficients of thermal expansion between bond coat and top coat and the temperature change.At the interface between the substrate and the bond coat, there are solid-liquid reaction before heat treatment and phase-transformation reaction of forming the Y ' phase during heat treatment. At the interface between the bond coat and the top coat, there are oxidation reaction in the bond coat and oxidation-deoxidization between Al and ZrO2. If r ' and oxide layer are thin, they will improve the adhesion between the substrate and the bond coat. They will reduce hightemperature-oxidation-resistance if they become thick. The solid-liquid reaction of forming melting layer can enhance the bonding between the substrate and the bond coat.