This paper investigates the physical reasons of why the particle is more accelerated in the cold spray nozzle when helium gas is used as a process gas compared to nitrogen gas, by using one-dimensional calculation. In addition, the deceleration mechanism of the particle between a plate shock wave and a substrate is also investigated. The selected particle in the calculation is spherical Cu with the diameters of 5 and 15 μm. The calculated results reveal that 1)the particle injected into the helium gas flow has a larger velocity just upstream of the plate shock wave because a larger drag force keeps acting on the particle from the injection port to the plate shock wave. The larger drag force is due to a larger drag coefficient just downstream of the injection port, followed by a larger squared relative velocity of the particle in the downstream region, 2) the particle accelerated by the helium gas is less decelerated between the plate shock wave and the substrate because the squared relative velocity of the particle is decreased to less than 1⁄10 by passing through the plate shock wave for the helium gas case.

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