During thermal plasma spraying impacting droplets splash and break-up. Fingers and satellite droplets generated during splashing and breaking up have a strong effect on coating microstructure. Incomplete filling of the space under small solid particles on the substrate is one source of coating porosity.

In this paper, a three-dimensional, time-dependent numerical model of free-surface flows and heat transfer including phase change has been developed to simulate the impact of a liquid droplet on a solid satellite droplet and to predict the size of the void under the satellite droplet caused by incomplete filling by liquid landing on top of it.

The solid satellite droplet is considered to be a protrusion on the substrate, which has the same temperature as the substrate and the same material properties as the liquid droplet. Fluid flow in the impacting liquid droplet was modeled using a finite difference solution of the Navier-Stokes equations in a 3D Cartesian coordinates assuming laminar, incompressible flow. Heat transfer in the liquid droplet was modeled by solving the energy equation, assuming densities of liquid and solid to be constant and equal to each other. The free surface of the liquid droplet was assumed to be adiabatic.

The porosity in this simulation is defined as the volume of the incompletely filled void under the satellite droplet to the volume of the solid satellite droplet. The simulation is repeated with different process parameters, and the results show that the impact velocity, the size of the liquid droplet and the offset distance between the centers of these two droplets play significant roles in determining the amount of porosity. A correlation is found to express the porosity as a function of process parameters such as impact velocity, size of the liquid droplet and the offset distance.