Bryan A. Gillispie

Effects of spray conditions on coating formation by the kinetic spray process

The kinetic spray coating process involves impingement of a substrate by particles of various material types at high velocities. In the process, particles are injected into a supersonic gas stream and accelerated to high velocities. A coating forms when the particles become plastically deformed and bond to the substrate and to one another upon collision with the substrate. Coating formation by the kinetic spray process can be affected by a number of process parameters. In the current study, several spray variables were investigated through computational modeling and experiments. The examined variables include the temperature and pressure of the primary gas, the cross-sectional area of the nozzle throat, the nozzle standoff distance from a substrate, and the surface condition of nozzle interior and the powder gas flow. Experimental verification on the effects of these variables was performed primarily using relatively large-size aluminum particles (63–90 µm) as the feedstock material. It was observed that the coating formation is largely controlled by two fundamental variables of the sprayed particles: particle velocity and particle temperature. The effects of different spray conditions on coating formation by the kinetic spray process can be generally interpreted through their influences on particle velocity and/or particle temperature. Though it is limited to accelerate large particles to high velocities using compressed air or nitrogen as carrier gas, increasing particle temperature provides an additional means that can effectively enhance coating formation by the kinetic spray process.

Vapor deposited thin gold coatings for high temperature electrical contacts

Using electron beam evaporation, thin films of Au over Ni and Au over Pd/sub 80/Ni/sub 20/ have been deposited on stainless steel and copper alloy substrates for high temperature electrical contact studies. The structure and composition of the films were studied in detail using electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS) with sputter depth profiling. The contact properties, such as contact resistance, fretting wear resistance, and thermal stability have been measured. The Ni and Pd/sub 80/Ni/sub 20/ layers of about 200 to 300 nm thickness have been shown to be effective in maintaining high temperature stability up to 340/spl deg/C in air by blocking the diffusion of elements in the substrates to the Au surface. These coatings also show good fretting wear resistance. These desired properties have been achieved with the thickness of the Au, Ni, and Pd/sub 80/Ni/sub 20/ layers substantially less than that of the conventional electroplated coatings.