Improvement of Coating Properties in Three-Cathode Atmospheric Plasma Spraying

The main goal of this work is to improve the coating properties of three-cathode atmospheric plasma sprayed coatings with respect to porosity and residual stresses. This was done by use of numerical simulation coupled with advanced diagnostic methods. A numerical model for the triple injection of alumina feedstock, as well as acceleration and heating of the powder particles in characteristic tree-fold symmetrical plasma jet cross section produced by a three-cathode-plasma torch was developed. The modelling results for the standard injector’s position “0” were calculated and experimentally verified by laser doppler anemometry (LDA). Moreover, the numerical model was applied to investigate the influence of variations of injector positions “1-4”. Based on the criteria defined for concentrated feedstock transport and homogeneous thermal treatment of powder particles in the plasma jet, the optimal injection position was found. In the next step a previously developed, coupled CFD-FEM-simulation model was used for simulations of the coating build-up, describing flattening, solidification and deformation due to shrinkage for alumina particles on a rough substrate surface. Due to the optimised injection improved particle properties were used as input parameters for the calculation of the coating microstructure at different stand-off distances. In this paper, the results are discussed and the positive influence of optimised parameters on the coating quality, with respect to porosity and residual stresses is shown.