Inter-diffusion effects and tribological behaviour of electron beam evaporated Ni-YSZ nanocomposite coatings subjected to diffusion annealing with borosilicate glass for nuclear applications

Abstract

Nuclear vitrification furnace made of Inconel-690 alloy fails prematurely due to alloy-borosilicate glass interaction by forming Cr rich secondary precipitates which results in the loss of corrosion resistance and incorporation of nuclear waste in the alloy-borosilicate interface. In this work, the development of a novel compositionally graded Ni-YSZ diffusion barrier coating using electron beam physical vapor deposition method was investigated to mitigate the issues related to inter-diffusion and elemental exchange across the alloy-borosilicate melt without the addition of any nuclear waste. A comparative study on the interdiffusion effects of compositionally graded Ni-YSZ nanocomposite coating with increasing Ni content from 5 to 50 wt % subjected to diffusion annealing with and without borosilicate melt at 1373 K in air was carried out. No phase transformation was observed in Ni-YSZ coating subjected to diffusion annealing at 1373 K in air as observed from XRD analysis. FESEM-EDS analysis confirms the formation of NiO and grain growth of NiO in YSZ matrix after heat treatment. Raman spectroscopic studies confirmed the outward diffusion of Ni from the adjacent Ni rich multilayer to YSZ top coat and the formation of NiO at the surface of the Ni-YSZ nanocomposite. In contrast, there was no elemental exchange across the interface between nanocomposite coating and adhered borosilicate layer as there was no traces of NiO on the surface of the glassy layer. The wear behaviour was studied on the compositionally graded Ni-YSZ nanocomposite coatings subjected to high temperature diffusion annealing with and without borosilicate glass using a linear reciprocating tribometer by analyzing different modes of wear scar and the chemical changes inside the wear track using Raman spectroscopy. The observation of higher hardness and wear resistance obtained for the diffusion annealed Ni-YSZ nanocomposite was found to be due to the formation of NiO on the surface of the coating.