Development of fine WC-NiCr powder coatings by optimising HVOF spray parameters

Thermal spray processing parameters need to be optimised to mitigate decarburisation and oxidation of carbide-based coatings that causes unacceptable brittleness. In the present work, a theoretical model was developed to optimise kerosene fuelled high-velocity oxy-fuel (HVOF) flame characteristics and spray parameters for producing high-quality WC-NiCr coatings using finer feedstock of particle size distribution of – 30 + 5 μm. The model suggested an oxygen-to-kerosene ratio of 3.3 and a shorter torch barrel to avoid decarburisation in coatings. In total four parameter sets suggested by the theoretical model were selected to spray the fine-cut powder using a 100 mm long torch barrel. The developed theoretical framework was successful to provide optimised set of HVOF spray parameters to deposit high-quality fine carbide coatings with less decarburisation. Also, the coatings deposited using the optimised set of parameters exhibited the best performance in terms of low porosity, inter-splat cracks, brittleness, as-spray surface roughness, and corrosion resistance. Hence, the optimised parameters can be used to produce a finer as-sprayed finish, thereby demonstrating the potential in reducing grinding efforts.