MICROSTRUCTURAL CHARACTERIZATION OF HIGH-VELOCITY OXY-FUEL SPRAYED COATINGS OF INCONEL-625

High velocity oxy-fuel (HVOF) thermal spraying is being increasingly u sed to deposit high quality surface coatings. In the present study HVO F spraying was used to de posit coatings of the Ni-based alloy Inconel 625 onto mild steel substrates and the structure of the sprayed coati ngs were related to the processing conditions employed. The microstruc tural characteristics of the deposits were investigated using X-ray di ffraction together with optical, scanning electron and transmission el ectron microscopy. The as-sprayed microstructure was found to consist of Ni-based metallic regions together with oxides exhibiting the Cr2O3 and NiCr2O4 crystal structures. Transmission electron microscopy reve aled that although the metallic regions were predominantly highly allo yed, Ni-rich grains (depleted in solute) were also present in all coat ings. Three processing variables were examined: oxygen to fuel gas rat io, total gas flow rate in the gun and combustion chamber length. All were found to significantly influence the oxide contents of coatings a s measured by X-ray diffraction methods.