Kinetic spray coatings

Coatings have been produced by entraining metal powders in an air flow which is accelerated by a de Laval type of nozzle, The particles are not melted or thermally softened prior to impingement onto the substrate. The coating process depends primarily on the kinetic energy of the incident powders. The coatings have low oxide content and low thermal stress, and can exhibit relatively low porosity and high adhesion. The mechanism by which the coatings are formed is not well understood, and it is the goal of this work to provide some insights into this mechanism. We have produced a new high-velocity spray apparatus which allows the spray parameters to be controlled and monitored for the first time. This, together with our simulations of air and particle velocities and temperatures, has provided new information on the coating process. Al, Cu, and Fe powders were sprayed onto Al, brass, Cu, and steel substrates. A threshold behavior was observed for coating deposition as a function of nozzle inlet air temperature, with a roughly linear behavior above the threshold. Results are obtained as a function of nozzle inlet air pressure and temperature, powder feed rate, and nozzle-substrate stand-off distance. The effect of the choice of substrate metal was relatively weak in our experiments. Results seem consistent with necessary inelastic processes such as plastic deformation and/or partial melting of the powder particles upon collision with the substrate. More research is needed to define the relative importance of these phenomena or of other possible mechanisms. (C) 1999 Published by Elsevier Science S.A. All rights reserved.