Evaluation of microstructural evolution and corrosion types in ultrasonic assisted laser re-melted thermal barrier coatings under exposure to molten salts

In this study, yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) with improved microstructure and properties were re-melted using the novel ultrasonic assisted laser re-melting technique and its hot corrosion behavior was investigated in 50 wt% Na2SO4+50 wt% V2O5 molten salts at 1100 °C. The results indicated that the microstructure and corrosion types of the laser re-melted TBCs were significantly affected by the ultrasonic vibration power output (UVPO). The increased convection effect of low UVPO (20%) caused the enhanced nucleation of equiaxed grain and improvement in crack distribution and size. High strain tolerance led to the enhancement in the self-healing (eliminate cracks spacing) performance of cracks when exposed to the thermal expansion and phase transformation volume expansion, and seal the macroscopic diffusion channels of particles, thus promoting the propagation of only intergranular corrosion (internal corrosion). However, intensive ultrasonic nonlinear effect of high UVPO (50%) led to the generation of the uneven surface with coarse irregularly distributed cracks that are not entirely self-healing when exposed to volume expansion. Thus, the massive coatings particles could migrate preferentially outward through the cracks and voids to react with molten salts on the surface (external corrosion), leading to formation of the corrosion products.