Characterization of gas-nitrided austenitic steel with an amorphous/nanocrystalline top layer

Nitriding is an important surface treatment for improving the hardness and wear resistance of steel. The application of this process for austenitic steel, which is characterised by very poor tribological behaviour, unfortunately reduces its corrosion resistance due to the precipitation of nitrides. However, it has been found that if this treatment is conducted below 500 °C, the formation of so-called “expanded austenite” can lead to improvement of the corrosion resistance of the steel even to above that of the untreated material. This phase also has a very high hardness (1600–2000 HV) and good wear resistance. Most investigations into this material concern nitrided layers produced by plasma or ion techniques. However, it is also possible to obtain layers containing “expanded austenite” by gas nitriding. From a practical point of view it is important that the wear and corrosion resistance are optimised and these are strongly related to the microstructure of the layers. The morphology of the layers was investigated using several methods: grazing incident angle X-ray diffractometry (GXRD), electron probe microanalysis (wavelength disperse x-ray spectroscopy, WDX), secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), atomic and magnetic force microscopy (AFM/MFM), and transmission electron microscopy (TEM). A reciprocating ball-on-plate test was used to evaluate the friction characteristics of the layers in air and vacuum.The tribological behaviour of the layers is discussed in terms of their morphology. Special attention is paid to the formation of an amorphous (nanocrystalline) top layer, the presence of which is not always observed.