In situ and ex situ examination of plasma-assisted nitriding of aluminium alloys

Pre-sputtering and plasma-assisted nitriding of pure aluminium and 2024 alu minium alloy were studied in situ by plasma monitoring and ex situ by sever al surface analysis methods. The influence of the process parameters, time, temperature, pressure and gas composition on mass and energy distributions of ions, as well as on topography and chemical composition, was examined. The chronology of the sputtering process could thus be clarified. An oxide film which is relatively thick compared to the native oxide layer and enric hed with magnesium is formed at the initial stage of the sputtering treatme nt, followed by the removal of this film with a subsequent roughening of th e surface. The cleaning effect of the plasma can be enhanced by increasing the argon pressure or by an intermixture of hydrogen. Due to a supplementar y chemical etching, hydrogen admixture reduces the surface roughness and ef fectively decreases the oxygen content of the surface. During the nitriding treatment, nanocrystalline hexagonal aluminium nitride AIN is formed. Grow th rate and nitrogen content of the nitride layer were found to strongly de pend on the gas composition during sputtering and nitriding. The positive e ffect of addition of argon and hydrogen, respectively, arises from an inten sification of the nitrogen plasma, the production of nitrogen-hydrogen mole cular ions and an increase of the ion energy. The nitrogen pressure affects the layer thickness due to higher nitrogen activity and ion energies. The chemical composition of the cathode material influences discharge current a nd, as a result, the ion energies. (C) 2001 Elsevier Science B.V. All right s reserved.