Enhanced wear resistant nickel-based alloy coatings produced by high velocity oxy-fuel spraying

Three Ni-Cr based powders (Metco 700, Armacor C and PSI-1) have been thermally sprayed using the HVOF process. The resultant deposits were then characterised using SEM, TEM and XRD. Microhardness surveys were carried out on the coatings together with abrasive wear tests. Coatings 250 mu m thick, were produced with low oxides and porosity levels. The most consistent coatings were prepared from powder sources with a minimum particle size of 25 mu m. Coating microstructures were either nano-crystalline or mixtures of amorphous and microcrystalline phases. Boron additions to the powder encouraged the formation of amorphous coatings and higher microhardness values i.e. > 675 H nu. The formation of boride precipitates in the coatings does not appear to raise their hardness as the precipitates were either of larger diameter > 500 nm or present in low volume fractions. Limited amounts of oxides were produced in the coatings, e.g. as alpha-Cr2O3 or spinel oxide NiCr2O4 particularly at intersplat boundaries. Abrasive wear resistance of the PSI-1 coating was superior to the other two coatings because it contains more amorphous material. The combination of wear and corrosion resistance associated with these coatings may prove to be attractive in many applications where both these properties are needed, carbides and borides.