Correlations Between In-Flight Particle Concentrations and Coating Properties in Atmospheric Plasma Spraying of Alumina

The plasma spraying process is controlled by various parameters that have an influence on powder particle velocities, temperatures and trajectories just before impact to the substrate. In order to fully utilize the thermal and kinetic energy of the plasma it is important to obtain information from these powder particle properties. In this work an intensified CCD camera has been used to detect in-flight particles in an atmospheric plasma spraying process. Plasma spraying was performed using fused and crushed AI2O3 powder. The powder carrier gas flow rate was varied during the spraying experiments. All the other deposition parameters were kept constant. Coatings produced using relatively new spraygun electrodes are compared with ones produced later with the same electrodes when they were worn out. The particle concentration is determined on a relative scale by the fraction of the area of a CCD camera frame covered by particle images. Further investigations necessary to clearify the relationship between the measured relative particle concentration and the true particle concentration are identified. The coatings are analyzed for wear resistance, degree of melting, deposition efficiency, hardness and porosity. The dependence of these coating properties on the relative particle concentration and the effect of electrode wear on the relative particle concentration are studied.