Simulation of the Particle Melting Degree in Air Plasma Spraying

Abstract

Plasma spraying is a coating process which is widely used for the application of thermal barrier coatings. High plasma jet temperatures allow the processing of ceramic material particles which characteristically exhibit low thermal conductivities. This, in turn, produces high temperature gradients inside the particles and vaporization on the particles' surface during their dwell time in the plasma jet. Thus, a single particle in the plasma-jet can exhibit 3 states of matter simultaneously: solid in the core, molten exterior and gaseous on the surface. The temperature distribution inside the particles is the foremost factor which influences the particles' behavior during their impact on the substrate surface. Experimental investigations can provide only the surface temperature of the particles, which is not a good indicator of the melting degree for ceramic particles due to the high temperature gradients. This study focuses on the determination of the temperature distributions inside the particles, during their flight in the plasma and the free jet, with the help of numerical simulations. For this purpose, a numerical model of the plasma spraying process that describes the plasma and the free jet loaded with sprayed particles is presented. The model includes three sub-models; a plasma torch sub-model, a particle-laden free jet sub-model and a powder particles sub-model. The first sub-model calculates the temperature and velocity fields of the plasma inside the plasma torch, the second sub-model the kinetics of temperature and velocity fields of the turbulent free jet generated by the plasma torch. This information is used in the third sub-model to calculate the kinetics of temperature distribution within the particles, their melting degree and mass losses due to evaporation. The third sub-model also calculates heat and mechanical impulse loses due to the particle-plasma interaction, which in turn is coupled with the free jet sub-model.