Near-substrate plasma characteristics and relationships between heat fluxes into substrates and them of a supersonic ammonia and nitrogen-hydrogen-mixture DC plasma jet for nitriding under a low pressure environment

摘要

Spectroscopic and electrostatic probe measurements were made to examine plasma characteristics near a titanium plate under nitriding for a supersonic direct-current arc plasma jet generator under a low pressure environment. Heat fluxes into the plate from plasma were also evaluated with a Nickel slug and thermocouple arrangement. Ammonia and mixtures of nitrogen and hydrogen were used as the working gas. In the mixture of N_2+nH_2, the H_2 mole fraction n was varied from 0 to 3, in which mole fractions of 0 and 3 corresponded to pure nitrogen and simulated ammonia, respectively. The thermodynamical nonequilibrium plasma came to be a temperature-equilibrium one as approaching the plate because the plasma flow tended to stagnate just in front of the plate. The electron temperature had a small radial variation. Both the electron number density and the heat flux decreased radially outward except for the electron density for N_2, and an increase in H_2 mole fraction raised them at a constant radial position because of increasing tank pressure. Although the electron number density for N_2 was the much highest at a constant radial position, the heat flux was the lowest. This is expected because the enthalpy for N_2 is the lowest owing to no H-atoms and the lowest tank pressure. Although the electron number density for NH_3 was relatively low, the electron temperature was the highest. Therefore, the heat flux for NH_3 is expected to be the highest because of the high enthalpy by a large amount of high temperature H-atoms and the highest tank pressure. In nitriding experiments, in cases with NH_3 and a mixture of N_2+3H_2, hard TiN-rich surfaces were made even at a large radial position of 50 mm. A neutral radical of NH with a radially wide distribution is expected to contribute to the better nitriding as a chemically active and no heating process.