Nanomechanical behaviour of Ni – YSZ nanocomposite coatings on superalloy 690 as diffusion barrier coatings for nuclear applications

Abstract

Ni-YSZ nanocomposite coatings with different Ni content in the range of 0–50 wt.% were developed on Inconel 690 substrates using an electron beam physical vapour deposition method to optimise the Ni concentration in order to enhance the durability of coatings for nuclear applications. X ray diffraction confirmed the formation of cubic phases of Ni and YSZ in the Ni-YSZ nanocomposite coatings. Increased addition of Ni was found to increase the crystallite size of Ni which resulted in lower strain. FESEM analysis of cross-sectional view of the compositionally graded Ni-YSZ coating with low concentration of Ni showed dense columnar structure and exhibited increased porosity along the columnar boundaries with higher concentration of Ni. FESEM and XRD analyses suggest that the grains of the columnar growth could be along (111) plane of the YSZ phase. The elemental composition of individual layers constituting the compositionally graded Ni-YSZ was confirmed by Energy Dispersive Spectroscopy analysis. The nanoindentation analysis of the as deposited coatings showed an increase in the hardness from 1.7 to 9.1 GPa, reduced Young’s modulus from 48 to 168 GPa, elastic recovery from 15.03% to 32.78% and resistance to plastic deformation from 0.0021 to 0.027 with the increased Ni content. Scratch test confirmed superior adhesion of the Ni-YSZ (50 wt.%: 50 wt.%) nanocomposite coating with the substrate. Also, investigation on the scratch track of Ni-YSZ (50 wt.%:50 wt.%) coating did not reveal chipping or spallation of the coating throughout the scratch track indicating a good adherence of coating with the substrate. The structural and the nanomechanical properties of Ni-YSZ (50 wt.%:50 wt.%) nanocomposite coating suggest that it could be used as diffusion barrier coating in the components of nuclear vitrification furnaces which are operated at higher temperatures.