Microstructural study and hot corrosion behavior of bimodal thermal barrier coatings under laser heat treatment

Abstract

In this study, nanostructured YSZ powders were deposited on the Hastalloy X Superalloy substrate coated with a metallic bond coat by plasma spraying to produce a nanostructured thermal barrier coating with bimodal microstructure. After that, the coated samples were heat-treated using a Nd:YAG laser. Then, the microstructures of the conventional and nanostructured TBCs before and after the laser glazing process were examined using a scanning electron microscope (SEM). The coating phases were studied by X-ray diffractometry (XRD). The high-temperature corrosion behavior of the nanostructured plasma sprayed coating in the presence of Vanadium pentoxide and Sodium sulfate molten salt was compared with that of the conventional coatings before and after laser treatment at 1050 °C. The hot corrosion results showed that the coatings had a similar degradation mechanism based on which the corrosive molten salt reacted with the stabilizer of YSZ, producing hot corrosion products such as YVO4. It led to an unwanted phase transformation from tetragonal (t) to monoclinic (m) Zirconia and the final degradation of the TBC system. However, reducing molten salt penetration, decreasing surface roughness and, reduction of the specific surface area are three important mechanisms that improved hot corrosion resistance, finally extending the lifetime of the glazed samples. The results also showed that the nanostructured TBC had higher hot corrosion resistance in comparison with other samples.