Effect of Oxidation on the Bonding Formation of Plasma-Sprayed Stainless Steel Splats onto Stainless Steel Substrate

Abstract

Stainless steel splats were deposited on 304 stainless substrates with different thicknesses of oxide layer to examine the effect of substrate oxidation on splat morphology and splat-substrate interface bonding by inert low-pressure plasma spraying. The cross sections of splats showing the splat-substrate interface were prepared by focus ion beam (FIB). The splat morphology and splat-substrate interface bonding state were characterized by scanning electron microscopy. The interface bonding was also examined by an electrolytic etching process. Results showed that with increasing oxide layer thickness and surface roughness, the morphology of splat changed from disk shape to splashed finger-like shape. The examination into the interface bonding by using FIB-prepared cross-sectional samples revealed that the splat interface bonding depended on the oxide roughness and composition. The interface bonding with a ratio of 44% was formed at the inner part of a splat on the pre-oxidized substrate when iron oxide presented on the surface, and the roughness of oxide scale was <5 nm. When the pre-oxidizing temperature exceeded 800 °C, the surface roughness increased to 14 nm and chromium oxide covered the pre-oxidized surface, resulting in no effective bonding forming at the whole interface. Thus, surface roughness and oxide composition have a significant influence on the splat interface bonding formation.