Microstructural changes by ion in the upper surface of austenitic steel induced sputtering and gas nitriding

The results of investigations of the influence of ion sputtering parameters applied as a pre-treatment on the formation of a nitrided layer during gas nitriding are presented. Previous work had shown that a low reproducibility of layer composition occurred after gas nitriding, thought to be the result of uncontrolled variation of sputtering parameters during the plasma initiation. In the present studies a two-stage sputtering process in nitrogen at a pressure of 3-5 Pa was used: firstly for a few seconds to 1 min with the plasma parameters 1.8 kV and 3 mA/cm(2), and secondly followed by sputtering with the parameters 1.35 kV, 0.5 mA/cm(2) for 15 min. The samples were gas nitrided immediately following these processes in 100% ammonia. It was found that even a few seconds of the first plasma treatment enabled magnetic "expanded austenite" to be formed in the layer. Surface layers were analysed using grazing angle X-ray diffraction (GXRD), light microscopy (LM), scanning electron microscopy (SEM), electron back scattered diffraction (EBSD), magnetic and atomic force microscopy (AFM and MFM) and electron probe micro analysis (EPMA) that revealed a complex microstructure. The results have emphasised the need for good control over plasma sputtering parameters in the initial process stage. (c) 2007 Published by Elsevier Ltd.