A way to decrease the nitriding temperature of aluminium: the low-pressure arc-assisted nitriding process

Abstract

For the purpose of applications in mechanics, nitriding of aluminium has been performed in a high-current (300 A), lowvoltage (20–45 V) and low-pressure (0.8 Pa) thermionic arc. Although nitriding of ferrous materials is efficient in this arc-assisted nitriding process even for unbiased workpieces, an additional negative substrate bias voltage is necessary to process aluminium. Ion bombardment is necessary not only for ion cleaning in an Ar–H2 gas mixture but also for the nitriding treatment in Ar–N2. Under these conditions, a compact and continuous aluminium nitride layer with hexagonal AlN phase is formed on pure aluminium at 450°C. The kinetics of aluminium nitride formation at low temperature (between 340 and 460°C) is characterized by a two-stage mechanism comprising first the nucleation and growth of nodular AlN grains, followed by the formation of a continuous AlN layer. The growth rate of the aluminium nitride layer seems to be controlled by the rate of the chemical reaction to form AlN, rather than the rate of nitrogen diffusion. Some tribological tests performed on the aluminium nitride layers are also reported in order to evaluate the improvement in friction and wear behaviour.