Demonstrating circularity in thermal spraying: Converting waste to wealth

Typically, 40–60 % of the injected powder feedstock in most thermal spray processes ends up as overspray waste and is a prominent technology drawback. This unutilized powder not only represents significant raw material loss but also poses disposal challenges. This study examines the prospect of the possible reuse of overspray waste that can considerably enhance the sustainability of thermal spray processes. Overspray waste from high velocity oxy fuel (HVOF) spraying of a WC-20Cr₃C₂-7Ni powder from an industrial spray shop was systematically collected and characterized for morphology, particle size distribution, and phase composition. This overspray powder was considered for reuse as feedstock for both HVOF and high velocity air fuel (HVAF) spraying. The resulting coatings were evaluated for microstructure, phase constitution, hardness and wear performance, and their characteristics were compared with coatings produced from virgin powder deposited via HVOF and HVAF. The results were noted to be extremely encouraging, with HVAF coatings using HVOF overspray outperforming HVOF coatings with virgin powder feedstock, making a strong case to investigate further possibilities to gainfully utilize thermal spray waste from varied sources. Considering that tungsten and chromium are critical raw materials, with tungsten bearing a particularly high carbon footprint, this study also quantitatively assesses the sustainability benefits of reusing the overspray powder. This illustrative exercise reveals a reduction in energy consumption and CO₂ emissions by a factor of two to three, along with a three-to-fivefold reduction in production costs.