Inductively coupled plasma nitriding of aluminium

Abstract

Substrates of aluminium alloy 2011 were plasma nitrided using an inductively coupled plasma source. The plasma nitriding parameters of temperature, length of nitriding and negative dc bias of the substrates were varied in order to optimise the plasma nitriding process. Substrates were characterised by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). All nitriding was carried out at a rf power level of 100 W and pressure of 6.5×10 −3 Torr N 2 . AlN was detected by XPS on all plasma nitrided Al substrates but the relative contribution of AlN to the total Al photoelectron signal was not significant (<15%) for Al substrates treated at low temperatures (50 and 450 °C) and low negative dc bias (100 and 150 V, respectively). AlN was not detected by XRD in these instances. Significant formation of AlN was observed both by XPS and XRD after nitriding for 180 min at a temperature of 575 °C whilst the substrate was biased with a negative dc voltage of 400 V. AlN formed at 575 °C both with and without prior Ar sputtering of the naturally occurring aluminium oxide over layer giving rise to an AlN contribution to the total Al photoelectron signal of 49 and 21%, respectively. XRD analysis confirmed the presence of a surface polycrystalline AlN layer. Both AlN and Al 2 O 3 were also observed by XPS on the surface of the nitrided Al substrates. SEM revealed a nodular morphology of the nitrided surfaces which were black in colour and exhibited a 2–3 μm thick surface layer. Nitriding of an Al substrate at 485 °C with dc bias of −400 V gave rise to a 35% AlN contribution to the total Al photoelectron signal. However, no AlN XRD pattern was observed. This indicates formation of a thin AlN layer only.