Heat, mass and momentum transfer in coating formation by plasma spraying

The plasma jets produced by d.c. spray torches exhibit unusual properties: high flow velocities (up to 2 500 m s(-1)), high temperatures (up to 14 000 K), steep temperature and velocity radial gradients (up to 10(tau) K.m(-1) and 5.10(5) s(-1)) and low gas density (1/30 to 1/50 that of the cold gas). They are laminar in their core and turbulent in their fringes. When they exit the torch nozzle, the resulting vortices coalesce inducing an engulfment process of the ambient gas with large scale eddies entraining bubbles of cold gas bubbles of cold gas. The latter do not mix instantaneously with the plasma due to the high density difference. Mixing occurs after the heating of the cold inclusions. In addition, the plasma jets are continuously fluctuating in length and position because of the continuous movement of the are root on the anode wall at frequencies ranging between 3 and 20 kHz. This results in a sort of piston flow. In plasma spraying, the solid particles are injected in the plasma jet through an injector set downstream or upstream of the nozzle exit. In this injector, particles collide between themselves and the injector wall. Therefore, they have trajectory and velocity distributions at the injector exit. It results in a dispersion of their trajectories within the jet. The flow rate of the powder carrier gas has to be adjusted to give the particles about the same momentum as that of the plasma jet at the injection point. The large difference between particle and flow velocity can induce convective movements within the molten droplets resulting in a continuous renewing of the liquid material at the particle surface. For metal or alloy particles sprayed in air this internal movement brings about a high oxidation rate enhanced by the presence of atomic oxygen in the jet. Particles impact on the part to be covered at velocities between 150 and 300 m.s(-1) The liquid material spreads out from the point of impact and forms a lamella called "splat. The flattening time is below a few mus and splat solidification generally starts before the flattening process is completed. The next particle that impacts a few tens of us later, flattens on already solidified particles. The piling up of a few splats forms a pass in less than one millisecond, then, the next pass is deposited a few seconds later. The thickness of a pass varies between 3 and 60 mum. The flow and heat phenomena during the impact and solidification processes control the microstructure and thermo-mechanical properties of coatings. The build-up of a coating in plasma spraying is a multiscale problem with time scales ranging between microseconds and seconds and length scales ranging between a few micrometers and a few hundred micrometers or more. Therefore, models and experiments deal with either the formation of splats or the piling of layers. This paper will review what is our present knowledge of the modeling and measurement of the transient phenomena involved in the various subsystems of the plasma spray process: jet formation, particle injection, particle hearing and acceleration and coating formation. (C) 2000 Editions scientifiques et medicales Elsevier SAS.