In copper cold spray, the feedstock powder particle size can significantly affect the mechanical, electrical and microstructural properties of the coating, as it influences both the particle velocity and the critical velocity. The particle-to-critical velocity ratio, η, is used to predict the coating interparticle bonding and properties while varying the spray parameters. However, its applicability for properties such as ductility and electrical conductivity when comparing powders with different sizes, is limited. In the present work copper coatings are fabricated by cold spraying three copper powders with different sizes ( d 50 =29.5 μm, 45.8 μm and 69 μm, respectively), and their mechanical, electrical and microstructural properties are evaluated. By maximizing the gas pressure and temperature (4.9 MPa – 800 ° C ), the coatings originating from the 29.5 μm-sized and 45.8-μm-sized powders attained electrical conductivities over 90% IACS and predicted ductility around 6%. Despite having attained the same η ratio, the coating originating from the 69 μm-sized powder resulted in 0% predicted ductility and 84% IACS electrical conductivity. This discrepancy is mainly attributed to the better interparticle bonding in the coatings made by the finer powders compared to the coarser ones, achieved by removing interparticle oxides from the particle-particle interfaces (PPIs). In contrast, the coarse powder coatings contained thick interparticle oxides at their PPIs that could not be removed even at the highest possible η ratios. Finally, it is indicated that for the current case the reduction in weakly bonded-PPIs is the key factor for achieving improved coating properties, while the degree of grain refinement/recrystallization plays a less significant role.
Kotsakis et al. (Mon,) studied this question.