The micro-cold spray (MCS) method offers a promising technique for producing dense metal and ceramic films with thicknesses ranging from 1 to 100 μm. Unlike conventional methods, MCS operates by aerosolizing particles (with diameters ranging from 200 nm to 5 μm) in a carrier gas, which are then accelerated through a nozzle into a low-pressure vacuum chamber. Upon impact with a substrate, these particles deform and adhere, forming the desired film. This process is particularly advantageous for substrates sensitive to high temperatures, such as polymers, as they do not require external heating or cooling during deposition.
However, MCS encounters several challenges that limit its efficiency and effectiveness. For instance, MCS depiction of ceramics is challenging due to their intrinsically brittle nature of ceramics. Ceramic particles fracture upon impact, resulting in amorphous films. Additionally, the deposition of large particles increases substrate and film erosion due to high-energy impacts. Therefore, it is crucial to modify the nozzle design to enhance the deposition efficiency and improve the quality of deposited films.
This technology includes a nozzle design for micro-cold spray (MCS) applications by introducing pressure relief channels in the diverging region of the nozzle (Figure 1). These channels continuously alleviate gas pressure within the stagnation region while preserving high gas velocity. Compared to conventional nozzles, variations of particle impact velocities are significantly reduced. Additionally, increasing the channel diameter enhances the velocity of finer particles, minimizing the spread in impact velocities for different particle sizes.
The efficacy of the pressure relief channel design was validated with yttria stabilized zirconia (YSZ) particles, yielding films approximately three times thicker than those produced with conventional nozzles. This substantial increase in deposition efficiency (~300%) arises from finer particles impacting at sufficient velocities to adhere without accelerating larger particles to erosive velocities.
Figure 1: Geometry for the nozzle with a pressure relief channel. Only half of the nozzle is shown because it is symmetric about the horizontal plane defined by the nozzle axis. Solid black lines are the boundaries of the nozzle, dashed green lines represent the inlet and outlet, dotted lines are used as an aid in dimensioning the drawing, the dash-dotted line represents the symmetric nozzle axis, and the substrate is depicted as a solid vertical blue line.
S. Bierschenk and D. Kovar, “A Nozzle Design for Mitigating Particle Slowing in the Bow Shock Region during Micro-Cold Spray of 8-YSZ Films,” Journal of Aerosol Science, 179 106360 (16 pages) (2024). (https://doi.org/10.1016/j.jaerosci.2024.106360)