Tribological response of fresh and used engine oils: The effect of surface texturing, roughness and fuel type Tiago Cousseau n , Juan Sebastian Ruiz Acero, Amilton Sinatora LFS - Laboratory of Surface Phenomena, University of São Paulo, Av. Prof. Mello Moraes 2231, 05508-970 São Paulo, SP, Brazil article info Article history: Received 1 August 2015 Received in revised form 6 November 2015 Accepted 8 November 2015 Keywords: Surface texturing Engine oils Oil degradation Friction coefficient MoDTC abstract Smooth and rough anisotropic surfaces were used to evaluate the influence of surface roughness and texture on the in situ formation of MoS 2 from MoDTC in reciprocating sliding tribotests. In order to also assess the influence of the fuel type contamination (gasoline and ethanol) on the oil properties and tribological performance, the experiments were carried out with an additive free base oil and with a fully formulated 5W30-SM engine oil, as new and after running in dynamometers tests fuelled with ethanol and gasoline. Both texture and fuel types were shown to influence the friction coefficient and wear by changing the tribofilm formation. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Since the earliest developments in human history, friction has been a major issue. From the invention of the wheel and the use of the first lubricants to the studies of coated and microtexturized surfaces, significant effort has been put on improvements that can overcome the resistance to motion [1]. Internal combustion engines (ICE) for example have been a significant source of friction losses resulting in fuel consumption and pollutant emissions since their conception. Nowadays, due to the massive use of passenger cars, it is estimated that about 72,000 million litres of fuel is used yearly to overcome ICE internal friction [2]. This data shows the utmost importance of improving engines efficiency. This task however is not easy. Engines have changed sig- nificantly due to improvements on oil formulation, surface fin- ishing and development of new materials, but its friction losses are still quite high, ca. 11.5% [2]. One of the difficulties faced to improve ICE can be depicted by the lack of understanding between oil and surface interaction. Molybdenum disulphide (MoS 2 ) for instance has been commercially available and used as a friction modifier additive (FM) since the late 40s [3]. From then on its interaction with the contacting surfaces and friction reduction capacity have been studied by many. Currently, MoS 2 dispersed in oil as molybdenum dialkyldithiocarbamate (MoDTC) is one of the most commonly used additives as FM in engine oils. Despite it all, there is still an uncertainty about the needed requirements for its activation and thus, for its efficiency [4]. Extensive effort has been put into the understanding of MoDTC performance; its lubrication capability as a function of load, tem- perature, entrainment speed, slide-to-roll ratio, surface roughness, MoDTC type, base oil, additive package and oil usage has been reported by many [5–8]. However, surface texture, which is extensively studied in terms of hydrodynamic film formation, has been little studied in terms of MoDTC performance. In which concerns the in situ formation of MoS 2 from MoDTC among the most accepted theories lies the need of nascent metal surfaces due to oxide removal through surface wear or a combi- nation of high flash temperature and contact pressure at the asperity level or a catalytic process [5]. With regard to MoDTC durability throughout the engine oil drain interval, it has been reported that its lubrication capabilities decrease sharply under typical lubricating conditions of ICE [7,9]. Such behaviour is mostly related to MoDTC oxidation due to thermal degradation, as demonstrated by many [7,8,10]. Another factor interfering on MoDTC degradation and activation, but con- stantly disregarded, is fuel oil contamination, which occurs during engines operation. Currently, the most used fuels are ethanol and gasoline, the former being greatly used in South America [11]. This work attempts to contribute to both, the understanding of surface topography (roughness and texture) and fuel type effect on engine oils performance, by studying the friction response of fresh and aged oils contaminated during engine operation with ethanol and gasoline. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/triboint Tribology International http://dx.doi.org/10.1016/j.triboint.2015.11.016 0301-679X/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: tiagoegm@gmail.com (T. Cousseau). Please cite this article as: Cousseau T, et al. Tribological response of fresh and used engine oils: The effect of surface texturing, roughness and fuel type. Tribology International (2015), http://dx.doi.org/10.1016/j.triboint.2015.11.016i Tribology International ∎ (∎∎∎∎) ∎∎∎–∎∎∎