The effect of substrate characteristics on the formation of plasma-sprayed alumina splats was studied using both experiments and numerical simulation. Knowledge of the particle and substrate conditions is critical in understanding coating formation and in validating computational models. The size, velocity and temperature of the alumina particles prior to impact were measured using a particle in-flight diagnostic system. Experiments were performed on two substrate materials: stainless steel and glass. Substrate temperatures were varied in a range of 20-500°C and controlled with an electric heater. For each substrate material, a transition temperature was observed above which there was no fingering/splashing and the splats had a circular disk shape. A 3D computational model of free surface flows with heat transfer and solidification was used to simulate the impact of alumina particles in conditions given by the experiments. The splat shapes from numerical model were comparable to those of the experiments for hot stainless steel substrate. For a cold substrate, the numerical model did not show any fingering/splashing. In the experiments, however, we observed two types of splat shapes: intensive splashing with no central core and circular disk splat. Substrate surface contamination, not considered in the numerical model, was the probable cause of droplet splashing on the cold substrate.

A three-dimensional model of free-surface flows with heat transfer, including solidification, was used to model the build-up of a coating layer in a thermal spray process. The impact of several nickel particles on a stainless steel plate in different scenarios was considered. Particles diameter ranged from 40 to 80 µm and their impact velocity ranged from 40 to 80 m/s. Particles were initially super-heated; their temperature ranged from 1600 to 2000°C. Fast growth of solidification was found to be one cause of particle splashing in thermal spray coatings. Different splat morphologies obtained from the numerical model were comparable with those obtained from the experiments. Simulation of the sequential impact of two nickel particles showed side-flow jetting and particle splashing observed in experiments. The numerical model proved to be capable of simulating different impact scenarios that occur in a thermal spray; this was demonstrated by simulating nine consecutive particles during their impact on the substrate. Several characteristics of a coating layer build-up such as particle splashing and formation of small satellite droplets and rings around the splat could be seen in the numerical results. Particle splashing is one possible cause of porosity formation in thermal spray coatings.

Experiments have shown that the mechanical properties of plasma-sprayed coatings depend to a large extent on the details of the spraying process, in particular, they are strongly dependent on the details of the solidification and deformation history of the individual droplets which are in turn highly affected by the substrate conditions such as its temperature, material, and surface thermal contact resistance. In this study, droplet-substrate interactions are investigated through a complete numerical solution of droplet impact and solidification for a typical thermal spray process. The energy equation is numerically solved for both droplet and substrate regions; the solution is based on the Enthalpy Method for the liquid and solidified parts of the droplet, and the conduction heat transfer in the substrate. The numerical solution for the complete Navier-Stokes equations is based on the modified SOLA-VOF method using rectangular mesh in axisymmetric geometry. The developed model is suited for investigating droplet impact and simultaneous solidification permitting any desired condition at the substrate. The splat shape, the solidification front, and the temperature profile in the entire droplet and substrate regions are obtained at any desired time elapsed after the impact. Through these results, the nucleation and growth of solidification and droplet-substrate interactions are extensively studied.