Rapid solidification in thermal spray deposition: Microstructure and modelling

Mechanical, thermal, and adhesive properties of thermal spray coatings are primarily determined by the phase and microstructure of single splats, which ultimately depend on rapid solidification of each splat and on the interactions between the splats and between the splat and the substrate. Significant efforts are being made to develop a better understanding of the physical mechanisms underlying these phenomena. This paper reviews a series of work in the area of mathematical modelling of phase and microstructure formation during the rapid solidification of single splats and coatings. The model development has been complimented by special experiments. Conditions under which planar interface solidification occurs, columnar cellular or dendritic growth takes place, or banded structure forms, have been identified. A microstructure map can therefore be built using the model presented here. The process parameters that promote crystalline nucleation and grain structure formation can be isolated and the effect o f interfacial heat transfer, splat substrate temperature difference, and substrate melting and resolidification can be examined using the model. The model predictions agree qualitatively well with the experimental data for alumina, yttria, partially-stabilized zirconia, and molybdenum.