Effect of nature of nitride phases on sliding wear of plasma nitrided sintered iron C. Binder a , T. Bendo a,n , G. Hammes a , A.N. Klein a , J.D.B. de Mello a,b a Universidade Federal de Santa Catarina, Pós-Graduação em Ciência e Engenharia de Materiais, Laboratório de Materiais, CP 476, 88040-900 Florianópolis, SC, Brazil b Universidade Federal de Uberlândia, Faculdade de Engenharia Mecânica, Av. João Naves de Ávila, 2121 – Bloco 1M sala 1M 218, CP 593, 38400-902 Uberlândia, MG, Brazil article info Article history: Received 24 November 2014 Received in revised form 19 January 2015 Accepted 29 January 2015 Keywords: Tribology Plasma nitriding Powder metallurgy abstract In the present investigation, the effect of the nature of the iron nitrides (γ 0 -Fe 4 N or ε-Fe 2-3 N) on the sliding wear of plasma nitrided unalloyed sintered iron is studied. Plasma nitriding was performed in an industrial scale plasma reactor using two different sets of operative parameters to produce micro- structures composed of different dominant iron nitride phases, i.e., γ 0 -Fe 4 N and ε-Fe 2–3 N. Microstruc- tural characterisation was performed using optical microscopy, phase analysis was performed via X-ray diffraction, and topographical analysis was performed using laser interferometry. Dry, ball-on-flat and reciprocating sliding tests were used to perform the tribological characterisation. The wear loss was determined by volumetric loss of the wear scar using laser interferometry. The wear rate of the counter- body was also evaluated. Wear mechanisms were characterised using scanning electron microscopy. Compact, free of porosity compound layers with the same thickness were formed, all of which were topographically similar. The analysis of the worn surfaces showed that several wear mechanisms operate simultaneously during the wear process, with the most significant mechanisms being oxidative and abrasive wear. Based on the wear mechanisms, a model for the wear of the nitrided layers (composite layer and diffusion layer) was proposed. The tribological tests indicate that, for the tribo-system evaluated, the ε-nitride layer exhibited a superior wear performance. The volume loss for the ε-phase was two times lower than that for the γ 0 -phase. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Powder metallurgy (PM) of ferrous components has gained outstanding importance over the past three decades. PM has been proving to be an alternative lower cost process for small parts when compared with machining, casting, stamping, forging and other similar metal working technologies [1]. In particular, iron based sintered parts have been used for many years in load bearing parts in sliding contacts, such as plain bearings, gears, cams, compressor parts (in particular connecting rods), chain pinions, pulleys, sprock- ets and brake linings [2]. In addition to reduction in strength and, as a consequence, in load bearing capacity, the inherent presence of porosity might influence the wear mechanisms acting on the surfaces of a PM part in different ways [3]. In order to improve load bearing capacity, fatigue and corrosion resistance, wear behaviour, and even aes- thetic aspects of PM iron based alloys, heat and surface treatment (e.g. steam oxidation [4–7], phosphating [8] plasma nitrocarburis- ing [9] and, in particular, plasma nitriding [9–12]) are often carried out. During the sliding wear process of metals, an oxide layer formed by frictional heating is known to prevent excessive adhesive damage to the underlying metal, resulting in mild wear. Mild wear produces fine, oxidised wear debris. If the oxide is removed faster than it is formed, the resulting wear is severe, as characterised by a rough, torn wear surface with ploughing by hard asperities and the formation of large, metallic debris [13]. As discussed by Kato et al. [14], the nitriding shifts the onset of severe wear to higher loads and sliding speeds and reduces the wear rate compared with non-nitrided steel within the same wear mode. Among the existing heat and surface treatment, nitriding is quite extensively used for the treatment of PM components. In the plasma- assisted nitriding process, N 2 ,H 2 and Ar gas mixtures are typically used. Dissociation of molecular nitrogen is one of the most important reactions because it provides atomic nitrogen that diffuses into the sample and forms a nitride layer. The PM parts are usually placed on the cathode of a plasma discharge (cathode configuration); as a consequence, the surface cleaning process can be enhanced by Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/wear Wear http://dx.doi.org/10.1016/j.wear.2015.01.083 0043-1648/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ55 48 37219268; fax: þ55 48 37219938. E-mail address: tatiana.bendo@labmat.ufsc.br (T. Bendo). Wear 332-333 (2015) 995–1005