Improving deposition efficiency in cold spraying chromium coatings by powder annealing

Abstract

Annealing of attrition-milled, electrolytically produced chromium powder has been investigated to improve cold spray coating deposition efficiency of pure chromium on Zr-alloy substrate for light water reactors with the goal of enhancing high-temperature oxidation resistance. The annealing heat treatment at 800 °C for 5 h induced microstructural transitions in the powder such as the development of equiaxed grains and strain relaxation, both associated with a measured decrease in nano-hardness. Deposition efficiency of the annealed powder was about three times more than the as-received electrolytic Cr powder. In addition, the utilization of the annealed powder reduced substrate deformation effects. A qualitative explanation of the effects powder microstructure on the cold spray deposition process in terms of the resulting coating microstructures and deposition efficiencies has been introduced. Finally, high-temperature exposure tests indicated that oxidation resistance of these Cr coatings was comparable to that previously reported for those produced using gas-atomized Cr powder. This study suggests that annealing of electrolytic Cr powder is a practical and economically favorable pathway to produce oxidation-resistant cold spray Cr coatings. More generally, use of the widely available electrolytic and mechanically milled powder vastly opens scope of cold spray deposition process, where atomized powders may not be either available or challenging to produce.