| Abstract: | A low alloy steel was coated with WC-Ni by the High Velocity Oxy-Fuel process, in order to increase the wear resistance and corrosion resistance of the material. The microstructure of the coating was determined using various techniques, such as SEM, TEM, EDAX, EPMA and X-ray diffraction methods
During spraying, the WC-Ni powder granules were heated to above the melting point of Ni, before arriving at the substrate surface. The molten Ni dissolves WC rapidly, forming Ni-W-C liquid alloys. Some C and some W are removed from the WC particles by oxidation. When each droplet strikes the steel substrate, a pancake-shaped splat forms, and solidifies with a rapid cooling rate. The structure is built up by the progressive deposition of the splats to produce a continuous coating. The Ni-W-C metallic matrix of the coatings consists of two basic structural types, Ni-rich material with a grain size of 10 - 100 nm, and a FCC crystal structure with a lattice parameter larger than that of Ni indicating the presence of dissolved W and C. The other metallic phase is W-rich, with a grain size less than 10 nm, and in some regions it appears to be amorphous. Dispersed within this material there were sometimes small crystals of W and W2C. In the initial powder the volume fraction of WC was about 75%, but in the sprayed coating the volume fraction of the WC was reduced to about 20%. This is partly due to loss of W by oxidation, but chiefly due to dissolution of W into the metallic phase. |
