A nozzle design for mitigating particle slowing in the bow shock region during micro-cold spray of 8 YSZ films

During micro-cold spray deposition, also referred to as the aerosol deposition method or vacuum kinetic spraying, performed with conventional nozzles, the particle impact velocities decrease drastically with particle size for fine particles <500 nm in diameter due to slowing in the stagnation region. A new design for a nozzle that contains pressure relief channels is proposed that allows the pressure in the stagnation region downstream of the bow shock to be reduced. This reduced stagnation pressure results in less particle slowing compared to conventional nozzle geometries, particularly for smaller or less dense particles. The effects of the channel geometry on the particle impact velocity are systematically investigated by independently varying the channel parameters. Calculations show that the impact velocities for 100 and 200 nm yttria stabilized zirconia particles is increased by 111% and 31%, respectively, for a selected pressure relief channel nozzle when compared to a comparable conventional nozzle. Although impact velocities are increased, a tradeoff exists with this nozzle design in that the particle focusing in the nozzle is decreased and some of the particles may be removed from the aerosol by the channels. Experiments using a nitrogen as a carrier gas at ∼40 kPa upstream pressure show that, despite the loss of larger particles into the relief channels, the deposition efficiency is improved by 300% when depositing films from fine 8 YSZ powder with the pressure relief nozzle compared to a conventional nozzle.