Two phosphonium cation-based ionic liquids used as lubricant additive: Part I: Film thickness and friction characteristics
Introduction
Ionic liquids (ILs) have become an increasing research topic in tribology since 2001 [1]. The interest of the lubrication field for the ionic liquids is due to their exceptional properties such as high thermo-oxidative stability, non-flammability, non-volatility, ashless character and controlled miscibility with organic compounds [2], [3], [4], [5], [6], [7]. The good lubricating properties of the ionic liquids are linked to their high polarity [8] and the adsorbed tribofilms formed on the metal surfaces, which contribute to reducing friction and wear [9], [10], [11], [12], [13].
In general, the ionic liquids have a low solubility in non-polar hydrocarbon oils. Due to this fact, many research works have been developed in the last decade using the ionic liquids as a lubricant additive or as a component of emulsions [14], [15], [16], [17]. Since 2009, the commercial availability of phosphonium cation-based ionic liquids has increased interest for their potential use in lubrication [18], [19], [20]. Some of these phosphonium based ILs have a good solubility in oils, so their application as a lubricant additive has become a current research topic [15], [16], [17], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Most of the above-mentioned works studied the tribological behaviour of these ILs using reciprocating sliding tests under a mixed or boundary lubrication regime. However, the study of the lubricant film thickness of IL-containing mixtures and their friction properties under different testing conditions should also be addressed.
About 33% of the fuel energy is wasted by cars due to friction losses and from that amount the engine and the transmission systems represent the 35% and 15%, respectively [30]. These systems work under the three general lubrication regimes: boundary, mixed and elastohydrodynamic/hydrodynamic [15]. Among the potential mechanisms to reduce friction in these systems are the use of low-viscosity and low-shear lubricants as well as the development and use of novel additives [30], [15]. Recently, the improvement of mechanical efficiency during engine cold start by increasing the oil temperature more quickly has been proved by using different solutions under the New European Driving Cycle (NEDC) [31]. For all the studied solutions, the oil temperature reached values from 95 and 105 °C. Roberts [32] reported that 11 min was the average trip duration and two-thirds of the trips did not go over the line of 11 min, and for this duration, the oil temperature was between 70 and 95 °C under the NEDC.
This work studies the film forming and friction properties of a mineral oil used in the formulation of fuel economy motor oils and its mixtures with two phosphonium cation-based ionic liquids used as additive. These lubricant samples were tested at room temperature in [29]. However, the main goal of this study is to understand the influence of the type and concentration of the ionic liquid on the lubricant film thickness and coefficient of friction under different temperature and sliding/rolling conditions. Stribeck curves will be used to characterise the tribological performance of the lubricant samples.
Section snippets
Base oil and ionic liquids
The base oil used in this work was a hydrocracked mineral oil (Yubase4/Group III) provided by Repsol S.A. and the two ionic liquids trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate or (coded as IL1) and trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate or (coded as IL2) were provided by Ionic Liquid Technologies GmbH. The base oil was mixed with at concentrations of 0.5 wt% (coded as 05IL1) and 1.0 wt% (coded as
Film thickness
All lubricant samples showed similar film thickness values during the tests performed under full film lubrication (see Figs. 2a and b) according to their similar rheological properties listed in Table 1. Although the viscosity and the pressure–viscosity coefficient have a similar influence on the film thickness, different tribological behaviours were observed under mixed and boundary lubrication regimes for low viscosity base oils and their mixtures with substances with higher polarity and
Conclusions
The film forming properties and friction characteristics of a hydrocracked mineral oil and its mixtures with two phosphonium cation-based ionic liquids used as additive at concentrations of 0.5 and 1 wt% were studied under two SRR values and three different temperatures. From the results obtained, the following conclusions can be drawn:
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The addition of the ionic liquids at low concentrations hardly influenced the rheology of the base oil used; therefore, all the lubricant samples (base oil and
Acknowledgements
The authors want to thank to the Spanish Ministry of Education, Culture and Sport for funding (ref: PR2015-00458) the research stay of Antolin Hernández Battez at Universidade do Porto (Portugal) within the programme ‘Salvador de Madariaga’. They also thank to the Spanish Ministry of Economy and Competitiveness and the Foundation for the Promotion in Asturias of Applied Scientific Research and Technology (FICYT) for supporting the research projects DPI2013-48348-C2-1-R and GRUPIN14-023,
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