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 mu m were mechanically milled for In h in liquid nitrogen to produce po wders 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 coatin g exhibited a superior microhardness, despite an increased porosity, over t hat of the conventional coating sprayed with the same parameters. Transmiss ion electron microscopy performed on the cross-sections of the nanocrystall ine coating revealed the splat formation with a thickness ranging from 40 t o 400 nm. Various oxide phases (Cr2O3, FeO, Fe2O3 and gamma-Fe2O3) in the s tainless steel matrix were identified using selected area diffraction. This observation suggests that in-flight oxidation may have occurred during spr aying and/or during splat formation. (C) 1999 Elsevier Science S.A. All rig hts reserved.