ORIGINAL PAPER Lubrication of CrN Coating With Ethyl-Dimethyl-2- Methoxyethylammonium Tris(pentafluoroethyl) Trifluorophosphate Ionic Liquid as Additive to PAO 6 D. Blanco • A. Herna ´ndez Battez • J. L. Viesca • R. Gonza ´lez • A. Ferna ´ndez-Gonza ´lez Received: 2 August 2010 / Accepted: 6 October 2010 / Published online: 24 October 2010 Ó Springer Science+Business Media, LLC 2010 Abstract This paper studies ethyl-dimethyl-2-methox- yethylammonium tris(pentafluoroethyl)trifluorophosphate ionic liquid [(NEMM)MOE][FAP] as 1 wt% additive to a polyalphaolefin (PAO 6) in the lubrication of CrN PVD coating. The tribological behavior of this mixture has also been compared with a traditional oil additive, such as zinc dialkyldithiophosphate (ZDDP). Friction and wear tests were performed by means of a ball-on-plate reciprocating tribometer, and XPS was used to analyze wear surfaces. The experimental results showed that both additives sub- stantially improve the anti-friction and anti-wear perfor- mance of the base oil. However, the tribological behavior of the ionic liquid as oil additive does not reach that of ZDDP. The interactions of each additive with the surface and tribofilm formation contributed to improve the tribo- logical behavior of the lubricants. Keywords Ionic liquids Á PVD coatings Á Lubricant additives Á Friction coefficient 1 Introduction In recent years, there has been an explosive growth of studies on ionic liquids due to their potential value in diverse industrial applications as catalysts, liquid crystals, electrochemistry, extraction technology, development of new materials, green solvents in organic synthesis, and tribology [1–4]. Ionic liquids (ILs) are salts formed by a weakly coordinating anion and an organic cation that have melting points that are lower than room temperature. Since they do not emit volatile organic compounds, they are regarded as ‘‘green’’ lubricants; they also present unique characteristics, including high thermo-oxidative stability, non-volatility, non-flammability, high ionic conductivity, and controlled miscibility with organic compounds, all of which meet the demands of high-performance lubricants [5, 6]. Some authors state the possibility of using ILs as neat lubricants under severe conditions for which conventional lubricants fail, such as aerospace and spacecraft applica- tions where the study of tribological systems based on high-temperature and high-corrosion-resistant materials becomes necessary [2, 7–9]. On the other hand, ionic liq- uids, even without any additives, have shown better lubrication and anti-wear properties than normally used synthetic, mineral, or engine oils [4, 10–12]. Finally, some studies have focused on the efficiency of ionic liquids as lubricant additives, studying the surface interactions that take place at the interface [1, 10–13]. The numerous advantages of ILs as novel lubricants or additives have been reported over the years [4, 14–16], leading to testing of numerous combinations. Normally, most works in this field use a variety of BF 4 and PF 6 im- idazolium salts that are commercially available at reason- able costs [17, 18]. However, water may cause unfavorable chemical reactions and physical changes due to the for- mation of hydrogen fluoride, a corrosive and highly toxic gas that may damage tribology systems [19]. Therefore, current research efforts are focused on hydrophobic salts, with the aim of avoiding these risks and D. Blanco Á A. H. Battez (&) Á J. L. Viesca Á R. Gonza ´lez Department of Mechanical and Civil Engineering, University of Oviedo, Oviedo, Asturias, Spain e-mail: aehernandez@uniovi.es A. Ferna ´ndez-Gonza ´lez Department of Physical and Analytical Chemistry, University of Oviedo, Oviedo, Asturias, Spain 123 Tribol Lett (2011) 41:295–302 DOI 10.1007/s11249-010-9714-1