Modeling of phenomena occurring in plasma jet during suspension spraying of hydroxyapatite coatings

The phenomena occurring after injection of water–ethanol suspension of fine hydroxyapatite powder were simulated numerically. The simulation started with the calculation of the map of velocity and temperature of working plasma gases. The map was calculated by taking into account the evaporation of the liquids included in the suspension. The content of the chemical species included in plasma jet was also calculated. The suspension was injected through a mechanical injector into the anode-nozzle of SG-100 torch. The plasma was generated with the use of working gases composed of 45 slpm of Ar and 5 slpm of H2 and with the electric power input of 30 kW. The initial droplets of suspension were supposed to be spherical with a diameter equal to that of the injector i.e. 500 μm. The trajectory of suspensionʹs droplets was calculated until the evaporation of liquids. Then, the simulation of movement and heating of solid hydroxyapatite (HA) started. The HA powder was in-house synthesized and had a bimodal size distribution with two maxima around 3 and 10 μm. The equations describing the momentum and heat transfers from hot gas to the solids took into account the small size of solid particles. In particular, the effects of thermophoretic force and of the modification of drag coefficient by non-continuum phenomena were taken into account in the calculation of the trajectories of small particles. Similarly, the non-continuum effect was considered in calculation of heat transfer. The obtained trajectories were tentatively correlated with the microstructure of suspension plasma sprayed HA coatings.