Influence of surface morphology and roughness on water wetting properties of low temperature nitrided austenitic stainless steels

Low temperature nitriding is known as a useful treatment to increase surface hardness and corrosion resistance of austenitic stainless steels, but little information is reported about its effect on surface wettability of the nitrided alloys. In this preliminary study the modifications of water wetting properties due to this treatment were investigated and they were related to the surface morphology and roughness changes. Samples of AISI 316L and AISI 202 austenitic stainless steels were nitrided using the glow-discharge technique, and modified surface layers consisting mainly of the so called S phase were produced. The treatments affected also the surface morphology and caused the formation of peculiar features, which were due to both ion bombardment during sputtering and nitriding, and local plastic deformations promoted by the high amount of solubilised nitrogen. As a consequence, roughness increased markedly on nitrided samples in comparison with untreated ones. Water wetting tests, performed with the sessile drop method, showed that the apparent contact angle values were higher on the nitrided samples in respect of the untreated ones, up to a hydrophobic behaviour. According to the Cassie–Baxter model, it may be supposed that the drops lie on solid and air pockets, formed thanks to the rougher surfaces. Additional tests suggest that this state is metastable: when the water drop was pushed on the nitrided surface, filling of almost all the cavities and valleys was obtained, and smaller contact angles were measured, also in respect of the untreated samples, suggesting that a state near to that hypothesised by Wenzel was achieved.