Surface Modification of Austenitic Stainless Steel by Means of Low Pressure Glow-Discharge Treatments with Nitrogen

Abstract

When low temperature nitriding of austenitic stainless steels is carried out, it is very important to remove the surface passive layer for obtaining homogeneous incorporation of nitrogen. In the glow-discharge nitriding technique this surface activation is performed by cathodic sputtering pre-treatment, which can heat also the samples up to nitriding temperature. This preliminary study investigates the possibility of producing modified surface layers on austenitic stainless steels by performing low pressure glow-discharge treatments with nitrogen, similar to cathodic sputtering, so that surface activation, heating and nitrogen incorporation can occur in a single step having a short duration (up to about 10 min). Depending on treatment parameters, it is possible to produce different types of modified surface layers. One type, similar to that obtained with low temperature nitriding, consists mainly of S phase and it shows improved surface hardness and corrosion resistance in 5% NaCl solution in comparison with the untreated steel. Another type has large amounts of chromium nitride precipitates, which cause a marked hardness increase but a poor corrosion resistance. These surface treatments influence also water wetting properties, so that the apparent contact angle values become >90°, indicating a hydrophobic behavior.

Effects of Surface Modification by Means of Low-Temperature Plasma Nitriding on Wetting and Corrosion Behavior of Austenitic Stainless Steel

“…% H 2 , and the following three different treatment conditions were chosen. Low-pressure treatment, similar to cathodic sputtering, named Type A, was carried out at 130 Pa fixing the discharge current density and increasing it with two constant steps, while the voltage drop and the treatment temperature increased freely, as previously reported [48]. At the beginning, the current density was fixed at 2.2 ± 0.1 mA•cm −2 ; when the measured temperature was 140 • C, current density was increased and fixed at 2.6 ± 0.1 mA•cm −2 .…”

“…Treatments were performed with laboratory plasma equipment according to the procedures that were described in previous papers [48,49]. For all the treatment types, the used gas mixture was 80 vol.…”

“…These changes are able to influence the wetting behavior of liquids, but in the international literature this this topic, which should be particularly interesting for extending the possible uses of components subjected to this treatment type, has been neglected, and the studies about the wetting behavior of low-temperature nitrided austenitic stainless steels are few, regarding mainly water wettability [41][42][43][44][45][46][47][48]. The obtained results were not univocal, and the contact angle of water drops on nitrided specimens was reported to decrease [42][43][44]46,47] or increase [41,45,47,48], in comparison with that for untreated samples. The changes of wetting behavior were investigated mainly for biocompatibility applications on samples having a smooth surface finishing before the treatments [42][43][44]46,47].…”

“…The changes of wetting behavior were investigated mainly for biocompatibility applications on samples having a smooth surface finishing before the treatments [42][43][44]46,47]. The studies on the effects of nitriding parameters on surface modification and thus on changes of wettability are few [41,42,44,48], and for the most part [41,42,44] they also do not report the influence of the treatments on the corrosion resistance, which remains of paramount importance for these sample types. A lack of information exists also on the influence of the initial surface finishing of the specimens and on the wetting behavior of the modified surfaces by different liquids.…”

“…Treatment conditions employed in the present study were chosen on the basis of our previous research [41,49,50]. A set of samples was subjected to a low-pressure process, similar to cathodic sputtering, so that surface activation, heating, and nitrogen incorporation could occur in a single step with a duration of about 8 min [48], in order to limit both plastic deformations due to the formation of the S-phase layer and the phenomena related to use a plasma process. Nitriding treatments were performed using two different pressures, a high one (1000 Pa), in order to have a less energetic ion bombardment, and a low one (130 Pa), in order to have also an enhanced ion bombardment.…”