Particle size effects during plasma spraying and atomization

Particle size effects during plasma spraying and atomization have been studied both analytically and experimentally. It is shown that for optimal plasma processing the particle size distribution must be tailored, based primarily on the material density and enthalpy of melting, to the thermal and kinematic properties of the plasma jet. Particles smaller than a critical size limit volatilize, as determined in approximate form as 6q{dollar}sp*/Delta{dollar}H{dollar}sb{lcub}rm b{rcub},{dollar} where q{dollar}sp*{dollar} is the total heat transferred across the plasma-particle boundary and {dollar}Delta{dollar}H{dollar}sb{lcub}rm b{rcub}{dollar} is the enthalpy of boiling. Alloyed particles slightly larger than this size limit may undergo extensive compositional changes due to superheating when one of the constituents has a high volatility. Particles larger than a critical size do not completely melt which causes high porosity and reduced adhesion in plasma sprayed coatings. A particle size criterion and model is proposed which determines the necessary particle sizes needed for uniform plasma processing. These ideas may be directly extended to other thermal spray processes.; A numerical simulator, PLASIM, has been developed which solves for the trajectories, temperatures, velocities, phase and compositional changes occurring when a representative particle size distribution is injected at variable velocities, positions and angles into a plasma jet. Simulations have been made for the plasma processing of NiAl, Ni{dollar}sb3{dollar}Al, MoSi{dollar}sb2,{dollar} Fe{dollar}sb3{dollar}Al, Al{dollar}sb2{dollar}O{dollar}sb3,{dollar} Ni, and Mo, and are compared to experimental depositions made under different plasma gun operating conditions. It is shown that the particles classify based on their size and that the larger particles traverse across the plasma jet and travel relatively further away from the injection port than the smaller size particles. Examination of the experimentally sprayed deposits provides direct evidence of the validity of the trajectory solution as well as microstructural information regarding the effect of particle size on the plasma spraying process.; It is concluded that higher density, larger particles need to be sprayed at reduced injection velocity and increased gun current. To process fine particles, the injection velocity must be very high and the jet velocity and plasma enthalpy reduced.