Characteristics of the nitrided layer formed on AISI 304 austenitic stainless steel by high temperature nitriding assisted hollow cathode discharge Yang Li a,⇑ , Shangzhou Zhang a , Yongyong He b , Lei Zhang a , Liang Wang c a Department of Materials Science and Engineering, Yantai University, Yantai 264005, PR China b State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, PR China c Department of Materials Science and Engineering, Dalian Maritime University, Dalian 116026, PR China article info Article history: Received 23 February 2014 Accepted 7 August 2014 Available online 17 August 2014 Keywords: Plasma nitriding Austenitic stainless steel Hollow cathode discharge abstract A series of experiments have been conducted on AISI 304 stainless steel using a hollow cathode discharge assisted plasma nitriding apparatus. Specimens were nitrided at high temperatures (520–560 °C) in order to produce nitrogen expanded austenite phase within a short time. The nitrided specimen was character- ized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, potentiody- namic polarization and microhardness tester. The corrosion properties of nitrided samples were evaluated using anodic polarization tests in 3.5% NaCl solution. The nitrided layer was shown to consist of nitrogen expanded austenite and possibly a small amount of CrN precipitates and iron nitrides. The results indicated that rapid nitriding assisted hollow cathode discharge not only increased the surface hardness but also improved the corrosion resistance of the untreated substrate. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Austenitic stainless steels offer good resistance to general corro- sion due to the formation of a passive surface film [1]. They are widely used in the food and chemical processing industries as well as in biomaterial applications [2]. However, they can suffer from pitting corrosion in chloride ion containing solutions [3]. Neverthe- less, at the same time they have found little use in mechanical engineering applications because of their low hardness and poor wear resistance [4,5]. For these reasons, surface hardening by nitriding has long been a well-known thermo-chemical treatment for improving the surface properties of austenitic stainless steels [6,7]. Low temperature plasma nitriding can produce a new phase with high hardness and good corrosion resistance on austenitic stainless steel surfaces [8]. It does so by the formation of a non- equilibrium supersaturated layer, what is called ‘S-phase’ or expanded austenite ‘c N -phase’ [7]. Zhang and Bell [9] described this surface layer as being hard and corrosion resistance with anticor- rosion proprieties equivalent to the original material. Nitrogen remains in solid solution inside the c N phase without removing chromium from the austenitic structure by precipitation of Cr nitrides. Since then, many researchers have produced such modified layer on austenitic stainless steels by various nitriding processes at low temperatures (<450 °C) [8]. These production methods include glow discharge plasma nitriding [10], plasma immersion ion implantation [11], plasma source ion nitriding [12], active screen plasma nitriding [13], and reactive magnetron sputtering [14]. It is generally thought that higher temperatures may cause pre- cipitation of CrN, thus removing Cr from the solid solution and degrading the corrosion resistance of the materials [15]. Li [16] sta- ted that the precipitation of CrN occurs, above the nitriding treat- ment temperatures of 420–440 °C for the AISI 304L austenitic stainless steel. In plasma immersion ion implantation, Collins et al. [17] found that the c N phase layer was formed on the 450 °C implanted sample whereas at 520 °C, the precipitation of CrN and a-ferrite dominated. In the investigation of Lei [18], austenitic stainless steels were plasma source ion nitrided at vari- ous process temperatures from 230 to 480 °C for 4 h. The c N phase with the different nitrogen contents was formed in the range of 300–450 °C, however, the c N phase decomposed with precipitation of chromium nitride CrN above 450 °C. Therefore, in order to avoid the detriment to the corrosion resistance of austenitic stainless steels, these nitriding treatments performed at temperatures lower than 450 °C. However, a long nitriding time is necessary to obtain a sufficiently thick c N phase layer by low temperature nitriding techniques. http://dx.doi.org/10.1016/j.matdes.2014.08.023 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +86 535 6706050; fax: +86 535 6706038. E-mail addresses: liyang@ytu.edu.cn, metalytu@163.com (Y. Li). Materials and Design 64 (2014) 527–534 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes