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

Pre-sputtering and plasma-assisted nitriding of pure aluminium and 2024 aluminium alloy were studied in situ by plasma monitoring and ex situ by several 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 enriched with magnesium is formed at the initial stage of the sputtering treatment, followed by the removal of this film with a subsequent roughening of the surface. The cleaning effect of the plasma can be enhanced by increasing the argon pressure or by an intermixture of hydrogen. Due to a supplementary chemical etching, hydrogen admixture reduces the surface roughness and effectively decreases the oxygen content of the surface. During the nitriding treatment, nanocrystalline hexagonal aluminium nitride AlN is formed. Growth rate and nitrogen content of the nitride layer were found to strongly depend on the gas composition during sputtering and nitriding. The positive effect of addition of argon and hydrogen, respectively, arises from an intensification of the nitrogen plasma, the production of nitrogen–hydrogen molecular 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 and, as a result, the ion energies.