Gas and Particle Dynamics in Thermal Spray

In thermal spraying of protective coatings or free-standing parts, the proper control of the trajectories of injected particle or droplet (suspension or solution) and their respective temperature histories in the gas flow represent one of the critical aspects on which the overall success of the operation depends. The slightest deviation from near optimal conditions can easily lead to poor results due to either the lack of melting of particles, insufficient impact velocities, or the modification of their composition due to particle overheating and evaporation or unwanted chemical reactions. In order to fully understand the basic phenomena involved it is essential to deal first with the interaction between a single spherical particle and a high temperature plasma environment. This topic that was covered in Chap. 4 of this book includes such phenomena as plasma-particle momentum and heat transfer, transient heating, melting, and evaporation of particles under plasma conditions, mass transfer, and chemical reactions For liquid or gaseous phases. The objective of the present chapter is to follow up on this analysis introducing the calculations and measurements of particle trajectory and temperature history prior to their impact on the substrate. Splat formation and the building up of the coating will be dealt with separately in Part III of this book in dedicated Chaps. 15 and 16. The case where the precursor material used in the spraying operation is in the form of a wire rod or cord will be dealt with also separately in Part II, Chap. 11.