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Spectroscopic and electrostatic probe measurements were made to examine plasma characteristics with or without a titanium plate under nitriding for a 10-kW-class direct-current arc plasma jet generator with a supersonic expansion nozzle in a low-pressure environment. Heat fluxes into the plate from the plasma were also evaluated with a Nickel slug and thermocouple arrangement. Ammonia and mixtures of nitrogen and hydrogen were used as a working gas. The NH3 and N2+3H2 plasmas in the nozzle and in the downstream plume without a substrate plate were in thermodynamical nonequilibrium states. As a result, the H-atom electronic excitation temperature and the N2 molecule-rotational excitation temperature intensively decreased downstream in the nozzle although the NH molecule-rotational excitation temperature did not show an axial decrease. Each temperature was kept in a small range in the plume without a substrate plate except for the NH rotational temperature for NH3 working gas. On the other hand, as approaching the titanium plate, the thermodynamical nonequilibrium plasma came to be a temperature-equilibrium one because the plasma flow tended to stagnate in front of the plate. The electron temperature had a small radial variation near the plate. Both the electron number density and the heat flux decreased radially outward, and an increase in H2 mole fraction raised them at a constant radial position. In cases with NH3 and N2+3H2, a radical of NH with a radially wide distribution was considered to contribute to the better nitriding as a chemically active and non heating process.