Tribo-mechanical and electrochemical properties of plasma nitriding titanium F.M. El-Hossary a , N.Z. Negm a , A.M. Abd El-Rahman a, ⁎, M. Raaif a , A.A. Seleem b , A.A. Abd El-Moula a a Physics Department, Faculty of Science, Sohag University, Sohag, Egypt b Zoology Department, Faculty of Science, Sohag University, Sohag, Egypt abstract article info Article history: Received 20 November 2014 Revised 16 May 2015 Accepted in revised form 1 June 2015 Available online 4 June 2015 Keywords: Titanium nitride RF plasma Tribo-mechanical properties Surface wettability Electrochemical performance Titanium nitrides have good tribo-mechanical and biomedical properties. They are employed to harden and protect cutting and sliding surfaces for industrial purpose and as a non-toxic outer-surface for bio-medical appli- cations. In this study, pure titanium was nitrided using RF plasma technique. The microstructural, mechanical, tri- bological, electrochemical and biomedical properties of nitrided titanium were investigated. The X-ray diffraction demonstrates the formation of ε-Ti 2 N and the cubic δ-TiN phases after plasma nitriding. The microhardness of the nitride samples increases as the plasma-processing power increases up to 1300 HV 0.1 . That represents approxi- mately 7-fold increment in the microhardness in comparison with the untreated titanium. High nitriding rate of 0.17 μm 2 /s was recorded for the sample that was treated at 650 W. The wear and corrosion resistance are improved after plasma nitriding. Moreover, the friction coefficient is reduced from nearly 0.75 for the untreated titanium to 0.25 for the nitride one. An enhancement in the biocompatibility of the nitrided titanium has been achieved. The number of grown mesenchymal stem cells was higher for nitrided substrates compared to that of the untreated titanium. The improved tribo-mechanical and electrochemical performance of the nitrided titanium can be attributed to the formation of super-hard titanium nitrided phases. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Titanium and titanium alloys have excellent properties including lightweight, high strength-to-weight ratio and outstanding corrosion performance [1–3]. Further, they are suitable to work under high stress conditions and indicated no toxicity effects with biological environ- ments [4]. For these attractive properties, titanium and its alloys are widely used for aerospace, chemicals and petrochemicals, automotive, orthopedic implants, dental and endodontic instruments and other in- dustrial and biological applications. However, they are still suffering from tribological drawbacks; mainly the low wear resistance and high friction coefficient that limit some of their practical applications [5]. Dif- ferent surface treatment techniques have been successfully developed to overcome these drawbacks and to provide these surfaces with more desired properties and functionalities for exceptional applications. In this regard, plasma-based nitriding and ion nitriding are well-known technologies used for many years among various plasma surface engi- neering techniques. Plasma nitriding [6–9], plasma carburizing [10] and plasma carbonitriding [11] are typical methods used for surface treatment purposes. The significant advantages of plasma nitriding over conventional nitriding methods include a reduce in operating coast (gas and energy consumption), and a complete elimination of environmental pollution. Further, controlling the treatment tempera- ture during the process leads to control the formation of nitrided layer with a specific phase composition and less shape distortion with free porous zone [12,13]. As reported in a previous literature, the surface treatment of titanium is better to be performed at low temperature range, up to ~ 950 °C in order to reduce the fatigue strength of the treat- ed titanium [14]. Plasma nitriding of titanium based on thermal diffu- sion mechanism produces a compound layer formed from δ-TiN on top and ε-Ti 2 N beneath; giving a hardness of about 1500–3000 HV [15]. A diffusion layer of solid solution phase α-Ti(N) can create under- neath as a consequence of incorporation of nitrogen into titanium ma- trix which results in hardening of dislocation-pinning effects [16]. It has been found that, the nitrided titanium surfaces lead to significant changes in surface topography beside the physiochemical features and tribo-mechanical properties [14,17,18]. A reduction in the wear rate to a value of 4.8 × 10 -7 mm 3 /Nm has been reported for nitrided titanium [19,20]. From another side, such nitrided layers have interesting features in biomedical applications especially towards cell adhesion, proliferation, and differentiation and ultimately the interfacial tissue formation [21–25]. The current study focuses on improving the tribo-mechanical prop- erties of commercial titanium by RF plasma nitriding. Further, it was ex- tended to study the surface energy characterization and corrosion behavior of the nitrided layers as a function of plasma processing power. Furthermore, cell adhesion and cell spreading were correlated Surface & Coatings Technology 276 (2015) 658–667 ⁎ Corresponding author. http://dx.doi.org/10.1016/j.surfcoat.2015.06.003 0257-8972/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat