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High velocity oxy-fuel (HVOF) thermal spraying is being increasingly used to deposit high quality surface coatings. In the present study HVOF spraying was used to deposit coatings of the Ni-based alloy Inconel 625 onto mild steel substrates and the structure of the sprayed coatings were related to the processing conditions employed. The microstructural characteristics of the deposits were investigated using X-ray diffraction together with optical, scanning electron and transmission electron 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 revealed that although the metallic regions were predominantly highly alloyed, Ni-rich grains (depleted in solute) were also present in all coatings. Three processing variables were examined: oxygen to fuel gas ratio, total gas flow rate in the gun and combustion chamber length. All were found to significantly influence the oxide contents of coatings as measured by X-ray diffraction methods.