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

For the purpose of applications in mechanics, nitriding of aluminium has be en performed in a high-current (300 A), low-voltage (20-45 V) and low-press ure (0.8 Pa) thermionic are. Although nitriding of ferrous materials is eff icient in this are-assisted nitriding process even for unbiased workpieces, an additional negative substrate bias voltage is necessary to process alum inium. Ion bombardment is necessary not only for ion cleaning in an Ar-H-2 gas mixture bur also for toe nitriding treatment in Ar-N-2. Under these con ditions, a compact and continuous aluminium nitride layer with hexagonal Al N phase is formed on purl aluminium at 450 degrees C. The kinetics of alumi nium nitride formation at low temperature (between 340 and 460 degrees C) i s characterized by a two-stage mechanism comprising first the nucleation an d growth of nodular AlN grains, followed by the formation of a continuous A lN layer. The growth rate of the aluminium nitride layer seems to be contro lled 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 fr iction and wear behaviour. (C) 1999 Elsevier Science S.A. All rights reserv ed.