PLASMA-ASSISTED NITRIDING OF ALUMINUM

Three aluminium materials, Al 99.5, AlMgSi 0.5 and AlZnMgCu 1.5 (1050, 6063 and 7075), were nitrided in a modified plasma-nitriding unit with a pulsating d.c. power supply. Prior to nitriding, the samples were cleaned in a plasma-assisted sputtering process in gas mixtures of argon and hydrogen. Shiny black aluminium nitride (AlN) layers with hexagonal crystal structure were formed on top of all three substrate materials, but distortion, microcracks and delamination of the AlN layers were observed too. Therefore AlN layers with high hardness and good adhesion could be obtained only by a proper adjustment of the nitriding parameters, especially the nitriding time and temperature, to keep the thickness of the AlN layer below about 3 mum. The wear resistance of the nitrided specimens was significantly increased compared with those without nitriding treatment. On the other hand, a significant increase in corrosion resistance of the AlN layer in 1N HCI could be achieved only by using nitrided Al 99.5 specimens.