Particle velocity is one of the most crucial parameters to determine coating’s properties in cold spray process, such as coating’s densification and bonding strength. To achieve a high particle velocity, the common way is to increase the working gas temperature or pressure, sometime using the extremely expensive helium gas. In this study, computational fluid dynamics (CFD) method was employed to optimize the nozzle dimension in order to improve particle velocity. Particle velocity was also measured with DPV-2000 to verify the simulation results. Furthermore, coatings were deposited with the optimized nozzle, and their mechanical and electric properties were tested. The results show that the optimized nozzle benefited to improve the adhesive strength with substrate and electric properties of cold-sprayed coatings.

The hypothesis of local thermal equilibrium (LTE) in thermal plasma has been widely accepted. Most of the simulation models for the arc plasma torch are based on the hypothesis of LTE and its results indicate a good validity to mimic the pattern of the plasma flow inside the plasma torch. However, due to the LTE hypothesis, the electrical conductivity near the electrodes is significantly low because of the lower gas temperature. Consequently, it is difficult for the flow of electrical current to pass between the anode and cathode. Therefore, a key subject for a model depending on the LTE assumption is to deal with the low electrical conductivity near the electrodes. In this study, two models, determining the electrical conductivity at the vicinity of the electrodes with two different assumptions, were employed to calculate the flow patterns inside a non-transferred DC arc plasma torch. A comparison of the gas temperature, velocity, voltage drop and the heat energy of the plasma arc between the two models were carried out. The results indicate that plasma arc inside the plasma torch fluctuates as simulated by both of the two models. It seems that the model can obtain comparable accuracy compared with the experimental results if the plasma gas electrical conductivity is determined by a nominal electron temperature.

The deposition mechanism of cold spray process has not been fully understood at present. It has been widely accepted that particle velocity prior to impact is one of the most important parameter for cold spray process, and bonding occurs when the impact velocities of particles exceed a critical value. For cold spray, the splat is the basic element of coatings and determines the coating properties, such as porosity, bonding strength. Therefore, the study of splats is helpful to understand the deposition mechanism of cold spray process. In this work, copper powder was utilized to prepare splats on three substrates, aluminum alloy, copper and stainless steel, under different spray conditions. Particle velocities were measured by DPV-2000 system experimentally. The morphologies of copper splats and cross sections were characterized by scanning electron microscope (SEM). In order to control the cross sections of splatted particle passing the center of particle as much as possible, the cross sections were polished by a new method with ion beam. The influences of particle velocity on cold-sprayed splat morphologies and cross-sections were discussed.

Cold spraying is a new emerging coating technology and has been widely used to produce coatings of various materials. It has been widely accepted that particle velocity prior to impact is one of the most important parameters for the cold spray process, and bonding occurs when the impact velocities of the particles exceed a critical value. As we know, particle velocity is influenced by many parameters, such as nozzle design, particle size, particle morphology, working gas pressure and temperature. In this work, three types of commercial stainless steel powders with different sizes or morphologies were employed to prepare coatings. Their respective particle velocities were also measured. With a non-clogging nozzle developed in Plasma Giken Company, Ltd., the particle velocities can be adjusted by changing the working gas pressures and temperatures to values as high as 4 MPa and 1000°C, respectively. The in-flight particle velocity was monitored via the DPV-2000 system. The results show that the particle velocity was influenced by the working gas pressure, temperature, particle diameter and morphology. Much denser coatings can be obtained with higher particle velocities, and consequently higher micro-hardness values of the coatings can also be achieved.