Modelling of plasma particle interactions and coating growth for plasma spraying of hydroxyapatite

Numerical simulations of the interaction between hydroxyapatite (HA) particles and an Ar–H2 plasma were carried out. The particles were injected into the anode nozzle of a plasma torch. A ballistic model was used to describe the phenomena of exchange of momentum and heat transfer, including heating, melting, and evaporation of particle material. The simulations were performed using temperature and velocity fields of the plasma jet obtained from the public GENMIX code. Numerical simulations of different experimental conditions, including variations of carrier gas flow rate and spraying distance, were carried out. Short distances were used for particles sprayed onto the substrate; long distances for particles injected into water. The data obtained in the simulations at short spraying distances were used subsequently to model HA coating growth. The numerical simulations were validated in two ways. Firstly, the fraction of amorphous phase in the sprayed material was predicted and compared with experimental data from semi-quantitative X-ray analysis. To make the comparison, the crystal phase composition of a particle in flight was assumed to be frozen on impact with the substrate or on contact with water, and that the liquid material transforms into a CaO–P2O3 glass. Secondly, the porosity of coatings generated by the numerical simulations was compared to that obtained for the real deposits. Finally, the experimental size distribution of the powder is compared to the calculated one.