Nitriding of stainless steel and aluminium alloys by plasma immersion ion implantation

Stainless steels show excellent corrosion resistance, which is lost during conventional hardening processes at temperatures above 500 degrees C, like gas or plasma nitriding, to improve the low hardness and to reduce the high wear rate. For aluminium alloys it is impossible to improve the mechanical properties by traditional nitriding. Plasma immersion ion implantation (PIII) of nitrogen is successfully used for hardening both austenitic stainless steels and aluminium alloys. Compared to the untreated materials the hardness and the wear resistance can be improved significantly. For austenitic stainless steels an 'expanded austenite' layer of up to several tens of micrometer thickness and for aluminium alloys an AlN layer of more than 10-mu m thickness were formed over a few hours. Corrosion tests showed no or only small changes in the corrosion behaviour for stainless steel. The AlN layer can be used as a supporting layer for the deposition of hard materials like CrN on aluminium alloys. It is shown that the partial pressure of oxygen in the residual gas in the vacuum chamber plays an important role for the nitriding of both stainless steel and aluminium alloys. (C) 2000 Elsevier Science S.A. All rights reserved.