Importance of surface activation for nitrided layer formation on austenitic stainless steel

Gas nitriding of austenitic steel is only possible if the passive oxide layer is removed. Therefore, surface activation forms an important part of the treatment. The main objective of the present study was to investigate the influence of activation on nitrided layer's growth. Various activation treatment methods have been applied – physical (ion sputtering with various current–voltage parameters and gases) and chemical ones (phosphating and chemical etching). The effectiveness of activation has been evaluated after gas nitriding in different conditions of atmosphere composition. The layers have been investigated using transmission electron microscopy, X-ray diffraction and grazing angle X-ray diffraction, magnetic and atomic force microscopy, electron probe microanalysis. Physical activation turned out to be the most effective method. At the same time, it made it possible to reduce the quantity of ammonia in nitriding atmosphere necessary to obtain layers composed of ‘expanded austenite'. Moreover, depending on the sputtering parameters and the gas used, it was possible to obtain layers with various kinds of morphology.