On the materials properties of thin film plasma-nitrided austenitic stainless steel L. Trabzon * , M.C. I ˙ g ˘dil Istanbul Technical University, Mechanical Engineering Department, Materials Science Division, I ˙ stanbul, Turkey Received 25 October 2004; accepted in revised form 22 December 2004 Available online 26 January 2005 Abstract A series of experiments were performed to study the composition and mechanical properties of the surface layers formed on the austenitic stainless steel after plasma nitriding in the temperature range of 400–500 8C with different N 2 –H 2 gas mixtures. The thin layer at the nitrided surface was examined by the glancing-angle XRD and the differential load penetration from the microhardness measurements. The formation of the expanded austenite phase was detected at temperatures below 450 8C and/or 10% N 2 in the treatment gas. The distortion of equivalent lattice constant after plasma nitriding was as high as 10%. The modulus of elasticity in the nitrided surface was increased by 33% after the plasma nitriding. The coating-only hardness was measured and it was equivalent to the hardness of coating containing CrN. Thus, it is possible to obtain thin coatings with superior resistance to corrosion and high hardness on the austenitic stainless steel. D 2004 Elsevier B.V. All rights reserved. Keywords: Austenitic stainless steel; Plasma nitriding; Expanded austenite; Nano-indentation 1. Introduction One of the main goals to improve the surface properties of the austenitic stainless steel without deterioration of its excellent corrosion properties is increasing the surface hardness to withstand the conditions inducing excessive wear [1–5]. One method among others is the plasma nitriding, which is a common surface processing method used to introduce nitrogen into steel to improve the surface hardness at temperatures higher than 400 8C [2,6–9]. Since there is a deterioration of corrosion properties when CrN precipitation occurs at certain processing conditions, there is a great interest of introducing nitrogen into the austenite matrix without forming CrN phase [10,11]. Thus, the nitrided steels having high surface hardness, good wear and corrosion properties can be obtained through formation of the expanded austenite phase which is seen in the plasma nitriding at temperatures lower than 500 8C. The mechanical and materials properties of the expanded austenite have been studied extensively [11,12]. However, it is also important to understand the materials characteristics of the expanded austenite which is grown in the short duration of plasma nitriding. In this study, the materials properties of thin nitrided layer by plasma nitriding at low temperatures were investigated. The true nature of the expanded austenite was studied extensively by the glancing-angle XRD and the microhardness measurements to eliminate any perturbations from the bulk of austenitic stainless steel. 2. Experiments The specimens used in the study were standard austenitic stainless steel that contains only 0.01 C% by weight (316 L). The size of the samples was 10Â3Â30 mm and they were cleaned by trichloroethylene to free any kind of oil and dust. The plasma nitriding was carried in a dc-discharge plasma system which is a 200-mm-diameter quartz glass vacuum tube. Prior to locating the specimens on the 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.12.012 * Corresponding author. Tel.: +90 212 29313 00/2437; fax: +90 212 2450795. E-mail address: levent.trabzon@itu.edu.tr (L. Trabzon). Surface & Coatings Technology 200 (2006) 4195– 4200 www.elsevier.com/locate/surfcoat