Review of impact and solidification of molten thermal spray droplets

The unique properties of coatings created by thermal spray deposition depend on the rapid solidification of individual splats created by impinging molten droplets. However, the impact process has been little studied because of the difficulty of measuring or numerically simulating the process, which occurs very quickly over a small area. Other scientific fields have investigated the impact of liquid droplets on solid surfaces. This paper reviews these studies, along with those conducted specifically on the thermal spray process. Modelers have almost universally ignored droplet solidification during impact; however, some experimental evidence suggests that the solidification process plays a significant role in splat formation. Splashing of impacting liquid droplets, another topic that has been largely ignored, affects deposition efficiency, porosity, and bond strength, and may also affect the amount of oxides incorporated in the coating. The scaling of data from impacting millimeter-size droplets traveling at low velocities to thermal spray conditions is questioned.