Titration of N and C atoms in flowing N₂-CH₄ post-discharge between 300 K and 850 K

¹ HEF R&D, rue Benoît Fourneyron, 42166 Andrézieux-Bouthéon Cedex, France
² Laboratoire de Science et Génie des Surfaces, UMR CNRS 7570, École des Mines, Parc de Saurupt, 54042 Nancy Cedex, France
³ Centre de Physique des Plasmas et Applications de Toulouse, UMR CNRS 5002, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France

Corresponding author: jaoul@mines.inpl-nancy.fr

Received: 30 October 2003
Revised: 22 March 2004
Accepted: 2 April 2004
Published online: 11 May 2004

Abstract

Flowing post-discharges resulting from surfatron microwave discharges in nitrogen and methane gas mixture with low amount of methane (less than 1%) are implemented for nitrocarburising treatment. Atomic nitrogen and atomic carbon densities are measured in post-discharge by combining optical emission spectroscopy and NO titration. The kinetic schema leading to N and C atoms densities is presented with a special attention paid to the effect of gas temperature on the kinetic constants. The influence of different discharge parameters such as methane percentage, total pressure, methane injection upstream or downstream the discharge and gas temperature are studied by these techniques. It is shown that, except at the lowest percentage used in this study (0.02%), methane introduction decreases nitrogen atoms densities. A maximum in carbon atoms density (10¹³ cm⁻³) is found for 0.05% of methane introduced in a nitrogen discharge at 2670 Pa. The corresponding nitrogen atoms density is two orders of magnitude higher (2–3 × 10¹⁵ cm⁻³). We present some arguments showing that gas expansion is the only effect that gas temperature has on nitrogen atoms densities. At present, it is not clear if such explanation is also applicable to the effect of gas temperature on carbon atoms densities.