Powder Development and Qualification for High-Performance Cold Spray Copper Coatings on Steel Substrates

This paper presents the development work undertaken to tailor Cu powder size specifications to produce dense cold spray Cu coatings featuring strong adherence on steel substrates. Through review of historical data generated in the course of the development of Cu coatings for corrosion protection of Used (Nuclear) Fuel Containers, it was found that particle size distribution was particularly critical to meet application adhesion requirements of 60 MPa and prevent nozzle clogging during application; D01 and D90 were fixed at 5 and 60 µm, respectively. The effect of powder size on coating microstructure and adhesion was further investigated with four Cu powder lots presenting different granulometries. The four lots were analyzed for composition, hardness, microstructure and morphology. Their in-flight particle velocities were measured using a coldspraymeter and the impact temperatures and velocities were simulated. Using fixed spraying conditions consisting of a He bond coat and a nitrogen buildup step, 3-4-mm-thick coatings were produced and tested for bond strength. Coating cross sections and fracture surfaces of selected coatings were examined. These trials confirmed that although the various powder sizes tested produced dense and sound coatings, tight control of particle size distributions was required to achieve optimal impact velocities that ensure deformation of the steel and good coating adhesion.