Effects of pulse frequency and temperature on the nitride layer and surface characteristics of plasma nitrided stainless steel

This work was initiated to examine the effects of pulse frequency and treatment temperature on the formation of nitrided layers and surface characteristics during plasma nitriding. Plasma nitriding experiments were performed with 316L austenitic stainless steel at the temperatures of 400 and 500°C using a pulsed d.c. plasma with various frequencies in an atmosphere of N2–H2 gas mixture. The microstructure and thickness of the nitrided layers and morphology of the nitrided surfaces were investigated using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and atomic force microscopy (AFM). XRD patterns revealed that the surface layer formed at 400°C consisted of S-phase only and CrN+Fe4N nitrides at 500°C, and no change of phase composition with frequency was observed. The compound layer thickened slightly with increased frequency. In addition, the surface roughness also increased as the frequency of the pulsed plasma was increased from 50 to 5000 Hz.