Numerical study of pore formation in thermal spray coating process by investigating dynamics of air entrapment

Abstract

When a liquid droplet impacts onto a solid surface, a thin air film is generally entrapped between the droplet and the substrate at the central portion which eventually evolves into a spherical air bubble for minimizing the surface energy. It leads to the formation of pores in the thermal spray coatings. This interesting phenomenon during the impact of molten metal droplets in thermal spray deposition that involves freezing is not clearly understood because of opaque nature of the metal droplet. Earlier works have focused on hydrodynamics of air entrapment without considering the phase change. Without considering heat transfer and phase change, the actual conditions of thermal spray deposition were not represented. The current study numerically analyzes pore formation by investigating the entrapped air film dynamics during the high-speed impact of a micron-sized molten metal droplet onto a solid substrate for thermal spray conditions by considering fluid flow, heat transfer, solidification phase change and free surface evolution. The volume of fluid surface tracking method (VOF) coupled with the solidification and heat transfer model within a continuum formulation is utilized. Results show that initially a thin circular film of air was entrapped in the impacting droplet which subsequently grew into an air bubble. The formation of the air bubble from the entrapped air film involves a complicated evolutionary process consisting of retraction, contraction and toroid formation of the air film. The analysis of heat transfer shows that wall heat flux distribution has a strong relation with dynamic feature of the trapped air. Further, it is shown that wettability of the substrate affects the detachment of the air bubble, which suggests a method for air bubble elimination from the solidifying droplet. Trapped air film tends to detach easily from the surface for low contact angle.