Pure Appl. Chem., Vol. 74, No. 3, pp. 441–445, 2002. © 2002 IUPAC 441 Splat formation in plasma-spray coating process* Javad Mostaghimi ‡ and Sanjeev Chandra Centre for Advanced Coating Technologies, University of Toronto, 5 King’s College Road, Toronto, Ontario M5S-3G8, Canada Abstract: The paper describes recent developments in modeling formation of plasma-spray coatings. This is a stochastic process in which particle-impact conditions, thermophysical properties, substrate topology, and temperature all play important roles in determining the structure of coating. Specific attention is paid to the simulation of droplet impact and solidi- fication under plasma-spraying conditions. Results of 3-dimensional models show that on a flat surface solidification may cause splashing and break-up of the impacting particle. Undercooling effects result in faster solidification and, thus, smaller extent of spreading. More recent works investigate the effect of surrounding gas on dynamics of impact. It is shown that this could result in entrapment of a bubble under the droplet. INTRODUCTION Macroscopic properties of plasma-spray coatings, such as hardness, porosity, and mechanical strength, are determined by their microstructure. Understanding what parameters affect microstructure of coat- ings and, for example, what causes formation of porosity, could result in better design of the process. A fundamental aspect of plasma-spray coating process is the deposition of molten or semi-molten particles on a surface. It is the shape of these individual deposits and their interactions with each other that determines much of a coating’s properties. Individual impacts may result in the formation of round thin disks, disks with fingers, or they may break up. Plasma-spray coating is a stochastic process in which a stream of molten, semi-molten, or even some solid particles strikes the surface of the work piece where they undergo rapid deformation and solidification to form disk-like splats. The shape of these splats plays a crucial role in determining the physical properties of the coating. Splat shapes depend on many factors such as size, velocity, and ther- mophysical properties of the impacting particles, as well as the topology and physical properties of the substrate. On-line measurement of these parameters for all impacting particles would be enormously complex, but their statistical distributions can be easily determined. Modeling formation of plasma-spray coatings requires the development of a stochastic model. Recently, Ghafouri-Azar et al. [1–3] used a Monte-Carlo approach to model coating formation. Given particle size, velocity, and temperature distributions as well as spray gun velocity and substrate condi- tions, they predicted coating thickness, porosity, and surface roughness. Results for porosity, surface roughness, and coating thickness compare favorably with experimentally measured values. The model, however, uses analytical expressions to relate particle impact conditions to the final splat shapes. Additionally, porosity is assumed to be solely due to the curl-up of the splats as a result of thermal *Lecture presented at the 15 th International Symposium on Plasma Chemistry, Orléans, France, 9–13 July 2001. Other presenta- tions are presented in this issue, pp. 317–492. ‡ Corresponding author