Influence of agglomerate morphology on micro cold spray of Ag nanopowders

The micro-cold spray process produces thick patterned films by impacting powders consisting of nano-to-sub-micron particles at velocities from 300 to over 1400 m/s. This process has been used extensively to produce ceramic films but has been employed only on a very limited basis to produce metallic films. The fine powders used in micro-cold spray are invariably agglomerated to some degree, but the influence of agglomerate morphology has not previously been studied for metallic particles. Here we study the effect of pre-treatments of the powder in the temperature range from room temperature to 200 °C on the powder morphology and resulting film properties of Ag. Experiments show that as the pre-treatment temperature is increased, the agglomerated powders do not densify significantly, but changes in the agglomerate morphology increase the fractal dimension. There is also evidence that increased pre-treatment temperatures increase the size of the necks between primary particles. Although the calculated impact velocities are reduced slightly for pre-treated powders compared to films produced from untreated powders, experimental results show that using pre-treated powders produces films with higher relative densities (up to 80%) and increased as-deposited conductivities (up to 3.5 S/μm). It is postulated that the pre-treated agglomerates utilize a larger fraction of the impact energy for deformation of the primary particles rather than fragmentation of the agglomerates due to increased bonding between primary particles.