Ionic liquids as oil additives for lubricating oxygen-diffusion case-hardened titanium
Introduction
Titanium alloys have extensive applications in aerospace, bioengineering, and automotive because of their good corrosion resistance, excellent fatigue strength, and strength-to-weight ratio (1.7 times that of steel) [[1], [2], [3], [4], [5]]. Particularly, the grade-5 Ti-6Al-4V (Ti64) alloy is widely used in aerospace turbines as well as racing cars and high-end motorcycles [[6], [7], [8]]. However, the poor abrasion and adhesion resistance of titanium alloys limits their application as tribological materials [9,10]. Several studies had reported to use an oxygen diffusion process to enhance the surface hardness of titanium alloys [[11], [12], [13]]. Previous tribological evaluation of oxygen diffused Ti64 (OD-Ti64) was largely focused on dry sliding conditions, and favorable impact on the wear resistance of Ti64 alloy was observed [11,12,14]. Our earlier investigation found that the OD treatment could induce wear reduction by up to six orders magnitude for Ti64 in a lubricant containing a conventional anti-wear additive ZDDP [15], as a result of enabling the formation of a ZDDP-based protective tribofilm. Understanding the friction and wear properties and mechanisms of OD-Ti64 in oil-lubricated conditions with advanced anti-wear additives, such as ionic liquids, is of great significance to broaden the use of titanium alloys.
The unique physical and chemical characteristics of ionic liquids (ILs), including inherent polarity for strong surface adsorption, high thermal stability, and low volatility, make them potential lubricant additives [[16], [17], [18]]. Several groups of oil-soluble ILs have been developed and demonstrated encouraging friction and wear reducing characteristics [18,19]. The promising anti-wear performance of ILs for steel and cast iron contacts was attributed to the formation of an IL-induced tribofilm [[20], [21], [22], [23], [24]]. Phosphonium-based IL additives exhibited effective wear protection in lubricating steel-steel [[25], [26], [27]] and steel-aluminium contacts [28,29]. Specifically, the lubricating performance of alkylphosphonium–organophosphate ILs has been reported to be superior to that of the conventional ZDDP [23] and phosphorus-containing ashless additives [28]. Later, alkylphosphonium–organophosphate ILs have been discovered with synergistic effects with ZDDP in friction and wear reductions [30]. The alkylphosphonium–carboxylate ILs exhibited better friction reducing efficiency than a commercial tricresyl phosphate additive under high temperature conditions [27]. So far, most studies of ILs lubrication have been focused on ferrous and aluminium alloys [18]. While there were a few attempts of using ILs to lubricate untreated titanium alloys with limited progress [31,32], to our best knowledge, no report is available on using oil-miscible ILs as lubricant additives to lubricate the oxygen-diffused titanium alloys.
This paper aims to investigate the tribological behavior of various phosphorus-containing ILs in a poly-alpha-olefin (PAO) lubricant for an OD-Ti64 and steel sliding contact under boundary lubrication. The morphology and chemical composition of the OD-Ti64 worn surfaces were examined through SEM, TEM, EDS, and XPS to gain insights of the anti-wear mechanism and compatibility between IL chemistry and the OD-Ti64.
Section snippets
Materials
A synthetic PAO 4 cSt base oil (kindly provided by Exxon Mobil Corp.) with a density of 0.80 g/cm3 at room temperature and kinematic viscosities of 17.6 and 3.7 cSt at 40 °C and 100 °C, respectively, was used as the base stock. The molecular structures of selected ILs and a secondary ZDDP (baseline anti-wear additive) are shown in Fig. 1. The four ILs were synthesized in our laboratory: trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate ([P66614][DEHP]), trihexyltetradecylphosphonium
Tribological performance
The friction coefficient traces of steel against OD-Ti64 in lubrication of PAO containing different ILs or ZDDP or their combinations are illustrated in Fig. 2. Friction coefficient of PAO base oil was in the range of 0.09–0.13 during the sliding process. The friction coefficients of the IL- or ZDDP-additized PAO were considerably lower (0.05–0.09) compared to that of the neat PAO. The lubricant with 1.0% [P66614][DEHP] achieved the lowest steady-state friction coefficient (0.05), followed with
Conclusions
Four different ionic liquids were used as anti-wear additives in lubricating an oxygen-diffusion case-hardened titanium alloy sliding against a bearing steel. The effectiveness of ILs in friction reduction and wear protection was benchmarked against that of a commercial ZDDP. The morphology and chemical composition of selected OD-Ti64 worn surfaces were examined to gain insights of the anti-wear mechanism and compatibility between the IL chemistry and the contact surface. Results of this study
Notice
This manuscript has been authored by UT-Battelle, LLC under Contract No. DEAC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will
Acknowledgements
The authors thank Dr. A.G. Bro and C. Dubin from ExxonMobil for providing the PAO base oil, E. Bardasz from Lubrizol for providing the ZDDP, Drs. J. Dyck and E. Conrad from Cytec Industries for providing phosphonium cation feedstocks, and D. Coffey from ORNL for preparing the STEM sample, respectively. Research was sponsored by the Vehicle Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE). Electron microscopy characterization was in part
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