Oxide in Ni–Cr Coatings Produced by Thermal Spraying in Air

Abstract

A Ni–19·0 wt-%Cr alloy powder was plasma sprayed and flame sprayed in air onto a mirror polished nickel substrate and a grit blasted steel substrate. Chromium oxide was hardly detected by electron probe microanalysis at the splashed splat/cold nickel substrate interface and the sprayed deposit/steel substrate interface, and was often present at the disc shaped splat/hot nickel substrate interface. The deposits were composed of alternate lamellae of alloy and chromium oxide. The alloy/oxide interface in the as sprayed deposits was smooth. On heat treating the deposits at 1073 K, the interface becomes wavy, and at higher temperatures the oxide layer granulates. A mechanism is proposed for this granulation, which takes into account the balance of the interfacial tensions at the triple point at which a grain boundary in the oxide layer or the metal layer meets the alloy/oxide interface.