WC-TI COATINGS DEPOSITED VIA COLD GAS SPRAY AND MODIFIED BY LASER AND FURNACE HEAT TREATMENTS

Introduction This work focuses on furnace and laser post-treatment and characterization of WC-Ti cermet coatings obtained by cold gas spray technique. The main purposes of this study are to find a suitable alternative to cobalt as metallic binder, since it has been listed as critical material, and to investigate the effects of different post-deposition heat treatments, carried out to trigger a self-propagating high-temperature synthesis reaction promoting a partial and controlled decarburization of WC and the consequential in-situ nucleation of TiC micro/nano-precipitates. Material and Methods High-energy ball milled WC50Ti cermet powder was purposely designed for CGS application. The feedstock powder was deposited on AISI 304 substrates, with a gas temperature of 550 °C. A standard furnace heat-treatment at 550 °C and different laser treatments were performed to increase the overall coating microhardness. The effects of the furnace and laser treatments on the WC-Ti coatings were evaluated in terms of microscopic, compositional, and phase analyses and through preliminary block-on-ring sliding wear tests. Results The microstructure exhibited a good distribution of carbides, with an average size ranging from submicrometric to 5 µm, and the XRD analysis revealed the formation of TiC in the laser-treated coatings. In the sliding wear tests, the laser-treated materials showed a significative reduction in the wear rate, with a weight loss up to 75% lower than both the furnace-treated and the as-sprayed coatings. Discussion The heat delivery to the coating is beneficial in terms of Vickers microhardness, as it increases for all the scenarios. The laser post-treatment, performed with medium-to-low powers and high scan rates, proved to be effective in activating the reaction that led to the formation of the TiC phase, with consequent increase in microhardness. While the SEM observation did not highlight important morphological differences between the furnace and the laser-treated coatings, the XRD investigation showed that TiC formed on the laser-radiated surfaces. This result was expected, as the powder feedstock was purposely designed to form finely dispersed TiC microprecipitates into the metal matrix of the cermet coating. The tribological performances of the furnace heat-treated and the as-sprayed coatings are overall comparable to each other, with high amount of material transfer from the counterpart to the wear scars. The laser-treated coatings outperformed the corresponding heat-treated samples under identical sliding wear conditions. It has been observed that the higher the hardness, the lesser the amount of material transfer occurring from the rotating alumina counterpart to the worn-out sample.