Comparison of conventional and active screen plasma nitriding of hard chromium electroplated steel M. Keshavarz Hedayati, F. Mahboubi * , T. Nickchi Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Hafez Ave., PO Box 15875, Tehran 4413, Iran article info Article history: Received 29 September 2008 Received in revised form 4 February 2009 Accepted 4 February 2009 Keywords: Conventional plasma nitriding Active screen plasma nitriding Hard chromium electroplating CPN ASPN abstract This paper considers the nitriding behavior of hard chromium electroplated steel by conventional plasma nitriding (CPN) and active screen plasma nitriding (ASPN) methods. Indentation test along the cross- section of the treated samples reveals that duplex treatment performed by two methods exhibits almost the same hardnesses. Furthermore, an increase in the time of plasma nitriding from 5 h to 10 h restores 30% of the hardness decline. Morphological studies show that surface particles formed on active screen plasma nitrided samples have orderly formed geometrical shapes while in conventional plasma nitriding they are in cauliflower shape. The reason for reaction between chromium and nitrogen seems to be the difference between thermal expansion coefficient of chromium oxide, chromium and steel substrate which results in partial breakdown of the oxide film. Moreover, the reducing of chromium oxide by hydrogen promotes the process. It looks as if nitriding treatment changes the corrosion behavior of the chromium coating from severe localized to uniform corrosion. Also active screen plasma nitriding treatment lowers the anodic dissolution 50–100 orders of magnitude which would be the result of full closure of surface microcracks. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Despite its characteristics, that are high hardness and wear resistance, electroplated coatings have microcracks which allow corrosive agent to pass through the microcracks and attack the substrate, resulting in a comparatively low corrosion resistance [1– 4]. Indeed, decrease in hardness of electrodeposited hard chro- mium coating at elevated temperatures hinders the application of this coating above 500  C: the hardness decreases from 800HV to lower than 400HV at this temperature [1,5,6]. One of the possible solutions to the problems mentioned above is plasma nitriding of this coating [1–4,6–8]. Through this process, nitrogen diffuses to the surface of the coating and forms chromium nitride phases like CrN and Cr 2 N. The increase in corrosion resis- tance of plasma nitrided hard chromium layer is attributed to occlusion of surface microcracks by chromium nitride phases [9,10]. The nitride phases have hardnesses of about 900HV up to 1200HV, which would maintain at elevated temperatures. The hardness value of such a duplex process is dependent on temperature and duration of plasma nitriding treatment. Higher hardness and higher thickness of nitrided layer are achieved by an increase in treatment time and temperature [4,6–8]. However the conventional plasma nitriding (CPN) has some restrictions, the most important would be the ‘‘edge effect’’ and ‘‘hollow cathode’’ [11–19]. These two effects result in heterogeneity of mechanical properties of the treated surface and impose restrictions on nitriding of complex shaped parts. Recently a method named ‘‘active screen plasma nitriding’’ (ASPN) is introduced which does not suffer from restrictions mentioned above [11,16–19]. Moreover, it is possible to nitride non- conductive specimens like ceramics [19] and polymers [20] by this method. In ASPN, the specimens are placed inside a cylindrical steel screen on which cathodic potential is applied, leading to formation of plasma on the screen not on the specimens, and the required heat for the process is supplied from the screen radiation [11–16]. Studies show that the hardness enhancements obtained by the two methods are the same [11,16,17]. In spite of the improvements observed in terms of wear and corrosion properties using ASPN method for stainless steel, this alloy remains practically the same after CPN treatment [14,15]. Also the morphology of the surface, treated by these two methods, exhibits some differences. These differences are attributed to different mechanisms involved in the two processes [11,12,16,17]. To the authors’ knowledge, no research has been done on ASPN of electrodeposited hard chromium * Corresponding author. E-mail address: mahboubi@aut.ac.ir (F. Mahboubi). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.02.005 Vacuum 83 (2009) 1123–1128