Parametric Study and Deposition Mechanism of Sn–Zn Coatings Fabricated by Underwater Cold Spray

Underwater cold spray (UCS) is a promising low-temperature technique for repairing protective coatings on submerged components, but the effects of processing parameters and surrounding water remain poorly understood. Here, blended Sn–Zn powder was deposited onto 304 stainless steel by low-pressure UCS, and the effects of water depth, traverse speed, gas pressure, gas temperature, and stand-off distance were investigated, supported by single-particle impact simulations. At a water depth of 25 mm, the deposition efficiency decreased from 32% in air to 2%, no Zn was detected, and the substrate temperature dropped substantially. Reducing the traverse speed to 5 mm/s yielded a continuous coating. Increasing the gas pressure from 0.4 to 0.6 MPa moderately reduced the deposition efficiency but raised the Zn area fraction from 2% to 23%. The coating delaminated at a gas temperature of 623 K, and no coating formed at a stand-off distance of 10 mm. Simulations showed that a 4 µm water film dissipated approximately 12% and 22% of the kinetic energy of Sn and Zn particles, respectively, before impact. UCS coating formation is thus governed by gas–water interaction at the jet scale and water-film energy dissipation at the particle scale, the latter preferentially suppressing Zn deposition.