Plasma monitoring of plasma-assisted nitriding of aluminium alloys

To improve the plasma-assisted nitriding process of aluminium alloys it is necessary to obtain knowledge about the underlying reaction mechanisms. A suitable diagnostic tool to clarify these mechanisms is plasma monitoring, which provides mass- and energy-resolved analysis of the ions hitting the substrate surface in a glow discharge. Application of this technique to plasma-assisted nitriding of pure aluminium and the 2024 aluminium alloy is demonstrated. Mass spectra and energy distributions of the major ions were recorded during sputtering in an argon atmosphere and nitriding in pure nitrogen. The energy distribution of the gas ions are mainly determined by resonant charge exchange collisions, whereas the ions of the sputtered metal atoms reach the cathode almost without interaction. The influence of the process parameters, temperature and working pressure, on the discharge characteristic were examined. Changing the substrate temperature did not significantly affect the ion energy distributions, whereas a reduction of the working pressure increased the collision probability of the ions due to a disproportionate elongation of the cathode fall. (C) 1999 Elsevier Science S.A. All rights reserved.