Optimization of parameters for HVOF spraying of fine WC-NiCr powders

Thermal spray WC-NiCr coatings often require grinding processing to meet the surface finish requirements. The cost associated with grinding can be reduced through the deposition of fine-cut (–30+5μm) feedstock rather than a more conventional commercial (–45+15 or –53+20μm) feedstock. However, the spray parameters need to be optimised to mitigate increased decarburisation which results in unacceptable brittleness, and to produce defect-free dense coatings.

In the present work, a theoretical framework to optimise spray parameters was investigated, which suggested an oxygen-to-fuel ratio slightly more than 3.3 and a shorter barrel to avoid decarburization in coatings. In total four optimised and seven unoptimised parameters were selected to spray fine-cut powder using a 100mm and 150mm long barrel. Scanning electron microscopy, X-ray diffraction, microhardness, and 3D-profilometer were used to analyse coatings. The coatings deposited using optimised parameters exhibited the best performance in terms of low porosity, inter-splat cracks, brittleness, and roughness. Among optimised parameters, coating deposited at lower kerosine and oxygen flow rates, with reduced stand-off-distance, was denser, crack-free, and ductile. Hence, fine-cut powder can used to produce a finer as-sprayed finish thereby reducing grinding efforts. Additionally, reduced heat flux to the substrate, making this an attractive option for thermally sensitive substrates.