Ceramic powders such as zirconia and titania are especially useful in their application as thermal barrier coatings, but are also useful in the production of more advanced coatings such as the dye-sensitized solar cell and functionally gradient prosthetic coatings. Due to their low thermal conductivity large temperature gradients through the powder particles are experienced during plasma spray deposition. As a result the particles often impinge at the substrate in a semi-molten form, which in turn substantially affects the final performance characteristics of the coating. This paper firstly summarizes some simulations and key results for completely solid and completely liquid particles. The modelling of semi-molten droplet impingement is then outlined, which is applicable to ceramic powders. The study examines the semi-solid impingement process during impact, spreading and solidification, and contributes to the growing insight being provided by numerical simulations on the topic of particle impingement in thermal spraying.

An adapted HVOF system has been computationally investigated in order to test the effects of injecting a cooling gas on both the gas phase dynamics and particle behaviour through the system. An existing liquid-fuelled HVOF thermal spray gun is modified by introducing a centrally located mixing chamber. The gas phase model incorporates liquid fuel droplets which heat, evaporate and then exothermically combust within the combustion chamber producing a realistic compressible, supersonic, turbulent jet. The trajectory of each discrete phase powder particle is tracked using the Lagrangian approach, with the inclusion of heating, melting and solidification through each particle. The results obtained give an insight to the complex interrelations present between the gas and particle phases, and demonstrates the usefulness of this modelling approach in aiding the development of thermal spray devices.