Contents lists available at ScienceDirect Tribology International journal homepage: www.elsevier.com/locate/triboint Axle gear oils: Friction, wear and tribofilm generation under boundary lubrication regime Maroua Hammami a,b, ⁎ , Nuno Rodrigues c , Carlos Fernandes a , Ramiro Martins a , Jorge Seabra c , Mohamed Slim Abbes b , Mohamed Haddar b a INEGI, Universidade do Porto, Campus FEUP, Rua Dr. Roberto Frias 400, 4200-465 Porto, Portugal b Unit of Mechanics, Modelling and Manufacturing, National Engineers School of Sfax, University of Sfax, Sfax W-3038, Tunisia c FEUP, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal ARTICLE INFO Keywords: Boundary lubrication Friction and wear Tribofilms XPS surface analysis Surface topography Ferrographic oil analysis ABSTRACT The internal friction torque measurements in Cylindrical Roller Thrust Bearings (RTB) lubricated with axle gear oils under boundary film conditions were performed using an axial rolling bearing test rig generating tribofilms. The X-ray photoelectron spectroscopy (XPS) was used to characterize the tribofilms formed on the bearing rollers and raceways. After the tests, wear debris were found on the oil samples which lubricated the surface of the roller bearing raceways for that surface topography measurements and oil analysis (ferrography) were mandatory to measure and to visualize the occurring wear. The results obtained indicate that axle gear oil formulations and their additive packages have got a significant influence in controlling roller bearing friction and wear under boundary film lubrication. 1. Introduction The automotive industry has become increasingly interested in reducing the fuel consumption. Many efforts are employed on the increase of vehicle fuel economy due to its significant environmental impact since it will directly contribute towards reduction of CO 2 polluting emissions to the atmosphere [1–4]. Transportation is one of the key areas for this legislation as it accounts for a large proportion of energy consumption and of carbon dioxide emissions. According to the technical report published by the Environmental Protection Agency (EPA) in 2015, the transportation activities accounted for more than one-third (33.4%) of U.S carbon dioxide emission in 2013, where passenger cars and light-duty trucks are responsible for 60% of all transportation emissions [5]. In studies of standard automobiles used in urban and highway driving, it was found that around 15–22% of the energy produced actually is used to drive the wheels, and that a significant portion of the remaining energy is dissipated as heat [6,7]. A continuously tightening legislation is imposed on the automotive industries and lubricant and additives suppliers to improve fuel efficiency and reduce emissions [4]. Mandated legislation in the four largest automobile markets, US, EU, China and Japan, are demanding the automotive and lubricant manufacturers to meet certain fuel economy standards like the corporate average fuel economy (CAFE) in US and the New European Driving Cycle (NEDC) in the European Union [8–11]. Not only governments require a strong drive towards better fuel economy but also consumers are demanding energy efficiency in order to save energy, reduce expenditure and minimize the effect of global warming [4,9]. To achieve significant reductions in automotive emissions, the efficiency of all driveline components should be improved. That is the case of the axle transmission, which is a key component of the vehicle powertrain and is focused by the present research [12]. Of course, these targets cannot be reached only by tribological measures through overcoming the friction forces which take place in tribological contacts in axle components like rolling bearings and gears. Important addi- tional steps have to be implemented based on including axle lubricants with enhanced durability, protection and lower operating tempera- tures. An effective lubrication of all axle tribological contacts is needed [13]. Since 1/3 of the total friction losses occur in the mixed film or boundary film lubrication regimes, the reduction of friction and wear is of particular importance when such lubrication regimes prevail [2]. According to Bartz et al. [2], this ratio is valid for the relative influences of friction modifiers or lower viscosities and it can be modified by changing the viscosity of the lubricant. Under these lubricating conditions, the chemical composition of the axle lubricants, i.e. the additive package is fundamental, while the rheological properties of the http://dx.doi.org/10.1016/j.triboint.2017.04.018 Received 2 February 2017; Received in revised form 30 March 2017; Accepted 11 April 2017 ⁎ Corresponding author at: INEGI, Universidade do Porto, Campus FEUP, Rua Dr. Roberto Frias 400, 4200-465 Porto, Portugal. E-mail address: mhammami@inegi.up.pt (M. Hammami). Tribology International 114 (2017) 88–108 Available online 14 April 2017 0301-679X/ © 2017 Elsevier Ltd. All rights reserved. MARK