Influence of heat treatment on surface properties of HVOF deposited WC and Ni-based powder coatings: A review

Post-spray treatments have recently been popular as a means of improving the overall quality of thermally sprayed coatings, particularly those done using the HVOF technique. Thermally assisted surface treatment of deposited coatings is an effective way to improve the characteristics of coated components for specific applications. The tribomechanical properties of post-treated WC and Ni-based coatings deposited with high-velocity oxy-fuel (HVOF) technique have been addressed. The structure-property correlations concerning the as-sprayed and post-treated coatings have been considered to understand the various mechanisms responsible for improved performance in terms of wear and corrosion resistance. The recent advancement in the post-treatments such as post-processing using microwave hybrid heating, laser-assisted processing and Stationary friction processing have been incorporated in the current review. Comparative studies have been presented to understand the structure-property relationship and performance of WC and Ni-based HVOF sprayed coatings with the help of various characterization techniques in this review article.

Effect of Laser Processing Parameters on Microstructure, Hardness and Tribology of NiCrCoFeCBSi/WC Coatings

In the present study, pulsed laser post-processing was applied to improve the properties of the thermally sprayed NiCrCoFeCBSi/40 wt.% WC coatings. The powder mix was deposited onto a mild steel substrate by flame spray method and then the as-sprayed coatings were processed by Nd:YAG laser. The peak power density applied was between 4.00 × 106 and 5.71 × 106 W/cm2, and the laser operating speed ranged between 100 and 400 mm/min, providing processing in a melting mode. Scanning electron microscopy, energy dispersive spectroscopy, Knop hardness measurements, and “ball-on-disc” dry friction tests were applied to study the effect of the processing parameters on the geometry of laser pass and microstructure, hardness, and tribology of the processed layers. The results obtained revealed that pulsed laser processing provides a monolithic remelted coating layer with the microstructure of ultrafine, W-rich dendrites in Ni-based matrix, where size and distribution of W-rich dendrites periodically vary across remelted layer depth. The composition of W-rich dendrites can be attributed to a carbide of type (W, Cr, Ni, Fe)C. The cracks sensitivity of coatings was visibly reduced with the reduction of power density applied. The hardness of coatings was between ~1070 and ~1140 HK0.2 and correlated with microstructure size, being dependent on the processing parameters. The friction coefficient and wear rate of coatings during dry sliding were reduced by up to ~30% and up to ~2.4 times, respectively, after laser processing.