Effect of heat treatment at 900 °C on microstructural and mechanical properties of thermal barrier coatings

The effect of heat treatment at 900 °C in air on the microstructural and mechanical properties of electron beam physical vapour deposited thermal barrier coatings (TBCs) was investigated in this work. To clarify this effect, the phase and the microstructure of the zirconia ceramic top coat (TC) before and after the treatment were first characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively. The changes in Youngʹs modulus and hardness of the TC before and after the treatment were then measured by indentation tests at room temperature. The delamination behavior, especially the evolution of the interfacial load capacity between the ceramic TC and the thermally grown oxide (TGO), was finally examined by the finite element analysis with surface-based cohesive behavior. Correlations between the changes in microstructure and mechanical properties were also discussed. Results showed that after the treatment neither the phase transformation from the tetragonal zirconia to the monoclinic nor the transformation to the cubic was detected, whereas the TC sintered. The combined effect of the sintering related densification and the cracks formed during the treatment led to an initial increase and a subsequent decrease in Youngʹs modulus and hardness of the TC with increasing treatment time. The interfacial load capacity increased with increasing Youngʹs modulus of the TC and increasing thickness of the TGO.