The Influence of Substrate Topography on the Plasma Jet Flow in the Substrate Boundary Layer under Suspension Plasma Spray Conditions: A Numerical Approach

This study deals with the numerical analysis of the plasma jet behavior close to the substrate surface depending on its topography. It uses a 2D axisymmetric time-dependent CFD model solved with the Ansys Fluent 2020/R1 package. The model takes into consideration the nonlinear thermophysical properties and turbulent phenomena of the plasma jet as well as its interaction with the microtextured substrate. Representative substrate topographies were considered as a boundary condition in the numerical simulations. They correspond to the bond coats used in Thermal Barrier Coating technology, actually APS sprayed NiCrAlY coatings which were experimentally microtextured using various laser unit operational conditions resulting in different substrate topographies. The numerical calculations showed that the substrate topography, modified and controlled in this work by microtexturing, disturbs the homogeneity of the pressure field in the substrate boundary layer resulting in the periodical pressure fluctuation. It was also observed that the relative local pressure disturbance is more significant in the substrate outer regions than close to the centerline. Then, based on the results of numerical calculations, the potential movement of feedstock particles near to the substrate was discussed. It was concluded that the deposition of fine powders, characterized by a low Stokes number, will be influenced by the pressure field distribution near to the substrate and will take place mainly in the local high-pressure zones. Furthermore, the local swirl of plasma taking place in each fine microtexture, created here by laser ablation, privileges the deposition of such particles on the surface asperities. These observations show that the CFD code modeling opens the possibility of predicting the movement and deposition of particles during plasma spraying, which is essential for understanding coating deposition mechanisms in suspension plasma spray.