Abstract

Thermal spray technology is widely used in many industries. The most important purposes of applying coatings are improving functional performance by the ability to work at higher temperature, increasing component life by protecting the surface against degradation, and repairing worn parts without changing the properties of the main part. Nano-structured coatings, which show improved characteristics as compared to conventional ones, have been extensively studied over the recent years. Suspension plasma spray (SPS) is a recently developed process that can produce nano-structured coatings on large surfaces. The technique involves spraying fine molten particles that solidify rapidly in contact with the substrate with grain size less than 100 nm. A suspension of sub-micron or nano-sized particles in a liquid carrier (water and ethanol) is injected in and atomized by the high speed high temperature plasma plume in a DC plasma spray torch. The SPS technique allows us to deposit micron-size particles more efficiently and creates fine grains, and small-sized pore microstructure. The microstructure of SPS coatings results from a series of complicated phenomena influenced by several parameters. Thus, depending on the suspension interaction with the plasma plume, powder types, solid concentration in suspension, substrate preparation method, plasma spray setup, spray distance, powder feed rate, etc. various microstructures can be obtained. Therefore, it is of great interest to predict the SPS coating microstructure within the context of particle conditions upon impact i.e. trajectory, size, and velocity. To understand how the microstructure of SPS coatings relates to particle conditions and predict coating attributes such as porosity, columnar structure, thickness and surface roughness, a three-dimensional predictive model for coating buildup on a substrate has been developed.
In this model, the impact properties of each particle that comprise size, velocity, temperature, as well as particle’s location near substrate are obtained from a computational fluid dynamics (CFD) model simulating the SPS process using a commercial Oerlikon Metco 3MB plasma torch.
Subsequently, the trajectory of each particle close to the substrate surface is calculated to determine its final impact location on the substrate or previously deposited particles. It is assumed that particles stick at the impact location according to some prescribed scenarios. The coating structure is specified using a variable f(i,j,k) which is zero when the cell is empty and equals unity when the cell is filled with the spray material. The numerical results can capture the columnar structures observed in SPS coatings due to the shadow effect.