Nitriding at low temperature

This paper reports advances in the use of low-pressure rf plasmas for nitriding with an emphasis on treatments at temperatures of 250–450°C; i.e. well below those used by more conventional methods. The treatment of austenitic stainless steel AISI-316 was chosen to represent the efficacy of such plasmas for nitriding over a wide temperature range, producing thicker nitrogen-rich layers at low temperature than more conventional methods in the same process time. This is due to a lower activation energy. Application of high-voltage pulses to the workpiece (plasma-immersion ion implantation, PI3) increases the thickness of the nitrogen-rich layer but does not significantly alter the activation energy. Other aspects of the process investigated include the role of hydrogen, various regimes of plasma-based cleaning, process gas purity and the variation of workpiece bias, from zero up to the 10s of kV characteristic of PI3.