MHCD in N2-H2 mixtures: towards a localized plasma nitriding process?
Résumé
In this contribution, we report the preliminary results on the use of MHCD discharges for nitriding treatments on metallic substrates. The MHCD design employed in this study consists in a matrix made of 39 axisymmetric isotropic cavity etched on a 500 µm thick Si wafer (100 µm circular aperture diameter and 30µm depth). A thin layer SiO2 generated by heat oxidation on the wafer surface electrically isolates the Si cavity. To serve as an electrode, the SiO2 is partially coated with a thin Ni coating (~500nm). The second electrode is made up of a Ni metal layer produced by a PVD method at the rear of the wafer. The MHCD device is placed in a reactor chamber pumped to ~1∙10-3 mbar and then filled with a high purity N2-H2 mixture (90-10%) up to 100 mbar. It is powered by a stabilized DC voltage power supply through a ballast resistor of 36 kΩ. A substrate in austenitic stainless steel (AISI 316L), previously polished like a mirror and cleaned, is facing the MHCD device (5 mm gap) and heated at 400 °C. The nitriding treatment was performed during 30 minutes by applying respectively 335 V and 1.4 mA to the MHCD system. The recorded emissions of N2+ (B2Σu+-X2Σg+) and N2 (C3ΠuB3Πg) lead to the determination of a rotational temperature of 385(22)K and a vibrational temperature of 4064(35)K during the treatment. The results seem to indicate (i) a moderate temperature elevation of the MHCD device during the treatment and (ii) a probably high dissociation degree of N2 [1] (favorable to nitriding processes). After treatment, a clear modification of the surface aspect of the sampled (figure 1) is observed. TEM investigations shown the formation/grow up of
nitrided nanoparticles, with a nitrogen concentration up to 35 at%. These preliminary results evidenced the capability of MHCD device to produce thermochemical treatments. Further studies are necessary to improve the concept and achieve a real nitriding layer.
[1] C. Pattyn, N. Maira, M. Buddhadasa, E. Vervloessem, S. Iseni, N.C. Roy, A. Remy, M.-P. Delplancke, N. De
Geyter, and F. Reniers, R. Soc. Chem. Green Chem. 24(18), 7100–7112 (2022).