Nitrogen species in a thermal plasma under very low pressure (150 Pa): Application to reactive plasma spraying

Abstract

Very Low-Pressure Plasma Spraying (VLPPS) is an emerging process allowing the manufacturing of metallic or ceramic coatings under reduced pressure conditions (i.e., in the 150 Pa range). It aims at vaporizing primary feedstock precursors in the warm core of the plasma flow and at carrying them, through the expanded plasma column, towards the surface to be coated, where they condense to form a deposit. This process also can be operated in a reactive mode. In such a case, a secondary reactive gaseous precursor is injected in the plasma flow where it can react with the primary vaporized precursors. In this work, nitrogen is considered as the secondary gaseous precursor. Thereby, nitriding reactions occur between this secondary precursor and the primary one, titanium in the considered case. Optical emission spectroscopy is implemented to identify the reactive nitrogen species in an Ar-H2-N2 plasma gas ternary mixture together with the reaction mechanisms giving rise to the nitriding processes. Two mechanisms have been identified. At first, a gas-gas reactions occur between titanium vapors and N2, N2+ and N species leading to the formation of TiN nitrides; At second, a solid-gas reactions occur between flattened titanium lamellae and N2 species, leading to the formation of Ti2N nitrides, which as evidenced by XRD analyses. Nevertheless, the number of nitrides is rather low, very likely due to the limited fraction of titanium vapors in the system: this is a consequence of a rather limited plasma energy (i.e., net power of about 26 kW, corresponding to an average plasma mass enthalpy of 15.21 kJ kg−1).