A Study on the Effect of Nozzle Geometrical Parameters on Supersonic Cold Spraying of Droplets

Supersonic cold spraying of droplets containing functional nanomaterials is of particular interest in advanced thin-film coating, that enabling high-adhesion strength particle deposition. In this method, coating occurs when the particles are accelerated to supersonic velocities in a converging-diverging nozzle, and then impact onto a target surface. Here, the optimum design of the nozzle is essential to deal with low-inertia particles like droplets. In particular, nozzle geometrical parameters (i.e., throat diameter, exit diameter, divergent length) determine droplets’ acceleration and deposition characteristics under supersonic flow conditions. To this end, we thoroughly investigate the influence of nozzle geometrical parameters on droplets acceleration by numerical modeling followed by experimental validation, and a case study on surface coating application. Two-phase flow modeling was used to predict droplets’ behavior in continuous gas flow for different nozzle configurations. The results show that the nozzle expansion ratio — a function of throat and exit diameters — has a significant influence on droplet velocity, followed by divergent length. In particular, to correctly accelerate low-inertia liquid droplets, optimum nozzle expansion ratio for an axisymmetric convergent-divergent nozzle is found to be in a range of 1.5–2.5 for various sets of parameters, which is different than the recommended expansion ratio (i.e., 5–9) for cold spraying of micro-scale metal particles. The findings can help determine the ideal design of a supersonic nozzle to minimize turbulent velocity fluctuation and shock wave formation that in turn assist to effectively spray low-inertia particles like micro-scale droplets. Based on the simulation results, an optimal design of supersonic nozzle is selected and prototyped for the experimental studies. Numerical modeling results are validated by particle image velocimetry (PIV) measurements. Moreover, coating experiments confirm the adaptability of the optimized nozzle for supersonic cold spraying of droplets containing nanoparticles, which thereby has the potential for rapid production of advanced thin films.