Microstructural evolution and oxidation behavior of nanocrystalline 316-stainless steel coatings produced by high-velocity oxygen fuel spraying

The microstructural evolution and oxidation behavior of nanocrystalline 316-stainless steel coatings produced by high-velocity oxygen fuel spraying is described. Stainless steel powders with a particle size in the range of 45–11 μm were mechanically milled for 10 h in liquid nitrogen to produce powders with a nanocrystalline grain size of 21±8 nm and an aspect ratio of 1.68. The cryomilled powders were subsequently sprayed onto a stainless steel substrate by high-velocity oxygen fuel spraying. The resultant coating exhibited a superior microhardness, despite an increased porosity, over that of the conventional coating sprayed with the same parameters. Transmission electron microscopy performed on the cross-sections of the nanocrystalline coating revealed the splat formation with a thickness ranging from 40 to 400 nm. Various oxide phases (Cr2O3, FeO, Fe2O3 and γ-Fe2O3) in the stainless steel matrix were identified using selected area diffraction. This observation suggests that in-flight oxidation may have occurred during spraying and/or during splat formation.