New Functionality of Ionic Liquids as Lubricant Additives: Mitigating

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Surfaces, Interfaces, and Applications

A New Functionality of Ionic Liquids as Lubricant Additives: Mitigating Rolling Contact Fatigue Benjamin Stump, Yan Zhou, Huimin Luo, Donovan N Leonard, Michael B Viola, and Jun Qu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10001 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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ACS Applied Materials & Interfaces

A New Functionality of Ionic Liquids as Lubricant Additives: Mitigating Rolling Contact Fatigue Benjamin C. Stump1, Yan Zhou1, Huimin Luo2, Donovan N. Leonard3, Michael B. Viola4, Jun Qu1,* 1Materials

Science and Technology Division, Oak Ridge National Laboratory & Transportation Science Division, Oak Ridge National Laboratory 3Center for Nanophase Materials Sciences, Oak Ridge National Laboratory 4Research & Development Center, General Motors

2Energy

Abstract Oil-soluble ionic liquids (ILs) have recently been demonstrated as effective lubricant additives in friction reduction and wear protection for sliding contacts. However, their functionality in mitigating rolling contact fatigue (RCF) is little known. Because of the distinct surface damage modes, different types of surface protective additives are used in lubricating the sliding and rolling contacts. As a result, the lubricating characteristics and mechanisms of ILs learned in sliding contacts from the earlier work may not be translatable to rolling contacts. This study explores the feasibility of using phosphoniumphosphate, ammonium-phosphate, and phosphonium-carboxylate ILs as candidate additives for rollingsliding boundary lubrication, and results suggested that an IL could be either beneficial or detrimental on RCF depending on its chemistry. Particularly, the best performing phosphonium-phosphate IL at 2% addition made a low-viscosity base oil significantly outperform a more viscous commercial gear oil in reducing the RCF surface damage and associated vibration noise. This phosphonium-phosphate IL generated a thicker, smoother, and more homogeneous tribofilm compared with other additives, which is likely responsible for the superior RCF protection. Results here suggest good potential for using appropriate IL additives to allow the use of low-viscosity gear and axle fluids for improved efficiency and durability.

Keywords: Ionic liquids, lubricant additives, rolling contact fatigue, friction and wear, micropitting, gear

*Corresponding

author, P.O. Box 2008, MS-6063, Oak Ridge, TN 37830-6063, tel: (865) 576-9304, fax: (865) 5744913, [email protected]

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1. Introduction The power losses of a gear or axle system include mechanical (load-dependent) and spin (loadindependent) loss components 1. The mechanical loss is primarily caused by the boundary friction between the gear contacts while the spin loss is largely proportional to the oil viscosity 2. The current trend in lubrication is chasing better fuel economy by using less viscous lubricants, and an earlier study had demonstrated up to 1.4% increased axle efficiency by using a SAE 75W-90 rear axle fluid to replace the traditional, more viscous SAE 75W-140 3. However, further reduction in oil viscosity faces an increased risk of rolling contact fatigue (RCF) surface damage as well as sliding wear loss. Therefore, there is need to develop more effective additives with combined anti-wear, anti-RCF, and friction reducing functionalities to (i) reduce the load-dependent loss by a lower boundary friction coefficient and (ii) reduce the load-independent loss by allowing the usage of lower-viscosity oils with improved RCF and wear protection. Ionic liquids (ILs) possess unique physicochemical properties such as inherent polarity for strong surface adsorption, low flammability, and high thermal stability and have shown great potential in lubrication applications 4-6. The main efforts prior to 2011 were to study the feasibility of ILs as neat or base lubricants 4-6. In the earlier attempts to use ILs as oil additives 7-10, the test lubricants basically were oil-IL emulsions because those ILs had rather low oil solubility. Since the breakthrough in ILs’ oil miscibility in 2012 11,12, using ILs as lubricant additives has become the new central research focus 13. Many ILs have demonstrated superior friction reduction and wear protection when used as oil additives in pure lubricating sliding contacts 11,12,14-21 and certain ILs even exhibited synergistic effects with the conventional anti-wear additive zinc dialkyl dithiophosphate (ZDDP) 22 and an organic frictional modifier 23.

ILs’ superior lubricating performance had been attributed to the formation of a protective, self-healing

tribofilm as a result of tribochemical interactions among the IL molecules, contact surfaces, and wear debris 24-28. Ionic liquids are very different from traditional, neutral oil additives because ions have stronger adsorption to the metallic contact surfaces to allow more efficient tribofilm formation, they offer

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more possibilities in tailoring the properties by combining different cations and anions, and they possess higher thermal stability to better resist aging. Unlike the extensive research on sliding contacts, there are only a few studies on using ILs as oil additives in lubricating either pure rolling 29-31 or rolling-sliding contacts 32-34, and none of them investigated the impact of ILs on RCF. The dominant surface damage in sliding and rolling contacts are distinct: while the former experiences 2-body and 3-body abrasive wear, the latter often is primarily caused by RCF causing microcracking and micropitting. As a result, different surface protective agents are needed in the lubricants for sliding and rolling tribo-systems. For example, the most common antiwear additive, zinc dialkyl dithiophosphate (ZDDP), widely used in engine oils and hydraulic fluids (mostly pure sliding) is excluded from the gear or axle oil formulations because the patch-like ZDDP tribofilm increases the localized contact pressure and promotes micropitting as a result of RCF 35-38. Instead, ashless (no metal content) phosphor containing anti-wear additives often are used in gear oils, however their effectiveness in mitigating RCF is controversial 37,39,40. While the ashless nature of ILs is a good starting point, the IL chemistry developed for sliding contacts in the earlier work and the associated lubricating behavior as well as friction and wear reducing mechanism may not be directly translatable to rolling contacts. Beyond nearly pure rolling in the conventional spur gears, many advanced gear and axle systems, such as the automotive rear axles, actually have nonzero sliding even at the pitch line and 𝑢1 ― 𝑢2

experience as high as >100% slide-to-roll ratio (𝑆𝑅𝑅 = (𝑢1 + 𝑢2)/2, a 200% scale, where u1 and u2 represent the two tooth surface speeds at the contact). A desired IL additive for rolling-sliding lubrication would possess the ability of protecting the contact surfaces from both sliding wear and RCF. This study conducted rolling-sliding boundary lubrication tests of several oil-soluble ILs composed of different cation and anion chemistries when used as lubricant additives, investigates the ILs’ influences in mitigating friction, vibration, wear, and RCF cracks, and provides fundamental insights of ILs’ surface protection mechanism based on tribofilm characterization.

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2. Materials and Methods 2.1 Ionic liquids and lubricants The candidate ILs include a phosphonium-phosphate IL ([P8888][DEHP], denoted as IL1) 18, a protic ammonium-phosphate IL ([N888H][DEHP], denoted as IL2) 21, and a phosphonium–carboxylate IL ([P66614][C17H35COO], denoted as IL3) 17. The ILs’ structures are illustrated in Table 1. The three ILs have previously demonstrated superior wear protection when used as additives in sliding contacts of ferrous alloys 17,18,21, and they were selected here intentionally with either the same anion (IL1 vs. IL2) or similar cations (IL1 vs. IL3) to allow understanding the impact of the cation and anion separately on the lubricating behavior in rolling-sliding. All three ILs were synthesized in our organic chemistry lab using the protocols reported previously 17,18,21. The IL purity was above 98%. The densities and viscosities of the three ILs have been reported previously 17,18,21 and are listed in Table S1 for readers’ convenience. ILs 1-3 are thermally stable in atmosphere up to 300, 210, and 308 oC, respectively 17,18,21, and they all have >5% solubility in nonpolar hydrocarbon lubricating oils, either mineral oil- or synthetic-based. A synthetic base oil, VHVI8 with a nominal viscosity of 8 cSt at 100 oC, was used to mix with the candidate ILs. The nominal weight concentrations of [P8888][DEHP], [N888H][DEHP], and [P66614][C17H35COO] were 2.07%, 1.74%, and 1.98%, respectively, to have the same molar concentration for fair comparison. An additional test oil, VHVI8 containing 0.52% [P8888][DEHP] and 0.57% a commercial primary ZDDP (1:1 molecular ratio), was studied to check whether the synergistic effect reported for pure sliding lubrication 22 is applicable to the rolling-sliding lubrication. A fully-formulated SAE 75W-90 gear oil (14.5 cSt at 100 oC), a commercial automotive rear axle fluid, was chosen as the baseline lubricant for comparison. This gear oil has a blend of hydrocarbon polymers and mineral oil as the base oil and contained olefin sulfide and amine phosphate as the extreme pressure and anti-wear additives, respectively. Kinematic viscosities of the test fluids were measured at 50 and 85 oC (the main operation temperatures in the testing protocol, shown in Table 2) using a Petrolab MINIVIS II viscometer, and results are shown in Table S2. Evidently, adding the ILs or IL+ZDDP into the base oil at the above4


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described concentrations had little impact on the oil viscosity and therefore did not affect the lubrication regime.

Table 1. Molecular structures of the candidate ILs. [P8888][DEHP]

[N888H][DEHP]

[P66614][C17H35COO]

2.2 Tribological bench testing and analysis Rolling-sliding bench tests were carried out on a PCS InstrumentsTM Micropitting Rig (MPR), as shown in Fig. S1, using three rings rolling-sliding against a roller at the center (all made of AISI 52100 bearing steel). The rings have a nominal diameter of 54 mm and thickness of 8 mm and the roller’s 5



contact band has a nominal diameter of 12 mm and a nominal width of 1 mm. The Vickers hardness (microindentation at 100 g-f) of the ring and roller are 770 and 602 HV, respectively. The roughness is about 0.40 μm (Ra) for both the roller and ring contact surfaces. The above described IL-additized lubricants and baseline oils were evaluated using a test protocol derived previously 41. Such a protocol was designed to match the oil temperature, lubrication regime, and contact pressure of the ~2 million ‘heavy-duty’ cycles (contact pressure > 1.1 GPa) experienced in an industrial full-scale rear axle dynamometer test 41. Table 2 shows the detailed testing parameters. For each fluid, two tests were first run, one at -1.5% and the other at -30% SRR for the roller (or +1.5% and +30% SRR for the rings) simulating the contacts near and away from the pitch line, respectively. Then, tests were repeated for the best-performing VHVI8+IL1 and the commercial SAE 75W-90 gear oil to confirm the improvement. It was previously determined that the -ratio (i.e. the ratio between the oil film thickness at the contact interface and the composite roughness of the contact surfaces 42) of running the SAE 75W-90 oil in the test protocol was around and less than 1.0 at 85 oC or above 41. The VHVI8 base oil without or with the ILs have much lower viscosities than the SAE 75W-90 and, as a result, the -ratio of testing them in the same protocol was determined to be less than 1.0 during the 2.13 million cycles at 85 or 120 oC, indicating boundary lubrication in the majority of the test. The friction coefficient and vibration behavior were monitored in situ and the roller wear volume was quantified based on dimension change as described previously 41.

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Table 2. MRP bench test protocol 41.

Test stage 1 2 3 4 5 6 7 8 9 10

Lubricant temperatu re (oC) RT50 50 5085 85 85 85120 120 120 120 120

Load (N) 25165 165 25 165 165 25 165230 230 230 265350

Hertzian contact pressure (GPa) 0.51.1 1.1 1.1 1.1 1.11.3 1.3 1.3 1.31.6

Rolling speed of the roller (m/s) 13.478 3.478 1.0 3.478 3.4781.5 1.0 1.53.478 3.478 3.4781.5 1.5

SRR of the roller 0-1.5 or -30% -1.5 or -30% 0% -1.5 or -30% -1.5 or -30% 0% -1.5 or -30% -1.5 or -30% -1.5 or -30% -1.5 or -30% total

Test duration (min) 30 30 30 60 60 60 7 8 15 2 302

Contact cycles on the roller (million) 0.50 1.00 0.72 0.08 0.13 0.18 0.01 2.63

2.3 Surface Characterization The roller’s RCF would be more significant than the rings’ because the roller experiences 15 times more contact cycles and the negative sliding is known to promote microcracking and micropitting 36.

Therefore, surface characterization was focused on the roller wear track. The worn surface morphology

was examined using a Hitachi TS-4800 field emission scanning electron microscope (SEM). Scanning transmission electron microscopy (STEM) samples were extracted from selected worn roller surfaces using a Hitachi NB-5000 dual-beam focused ion beam (FIB) system with a gallium source. A protective carbon film was deposited on the tribofilm prior to the FIB process. Cross sectional STEM imaging and EDS elemental mapping of the tribofilms were then conducted to reveal the film thickness, nanostructure, and composition.

3. Results and Discussion 3.1 Comparison among selected ILs Figures 1 and 2 summarizes the MPR rolling-sliding test results of the VHVI8 base oil, VHVI8 containing different ILs, and VHVI8 containing IL1+ZDDP. The roller surfaces tested in the neat VHVI8

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base oil showed a network of RCF cracks at -1.5% SRR and a combination of abrasive wear and micropitting at -30% SRR, as shown in Fig. 2a. IL1 appeared to be the best performer among the three ILs. The 2% addition of the phosphoniumphosphate IL1 ([P8888][DEHP]) slightly reduced the friction and vibration and, more importantly, effectively protected the roller surface from both sliding abrasion and RCF cracking. The IL1 reduced the roll wear loss by 80% and 50% for -1.5% and -30% SRRs, respectively (Fig. 1c). Examination of the worn roller surface morphology clearly showed that both the size and density of micro-cracks produced by VHVI8+IL1 (Fig. 2b) were significantly less than those by the neat VHVI8 base oil. IL2 ([N888H][DEHP]) is in the same chemical family as the amine-phosphate anti-wear additives that are commonly used in commercial gear oils, and IL2 had previously demonstrated excellent wear protection in pure sliding 17,22. However, it performed poorly in the rolling-sliding tests in this study. Particularly, at -30% SRR, the addition of IL2 increased the friction coefficient by 30% and caused the vibration to rise sharply higher when the oil was heated to 120 oC (Note that vibration signal is plotted in log scale in Fig. 1a). Even worse was the wear performance: instead of expected protection, the presence of IL2 made the roller material loss 25% and 190% higher than the neat base oil at -1.5% and -30% SRRs, respectively (see Fig. 1c). The micro-cracking on the worn roller surfaces lubricated by VHVI8+IL2 (Fig. 2c) appeared to be more crowded than the neat VHVI8, especially for the test at -30% SRR. The phosphonium-carboxylate IL3 ([P66614][C17H35COO]) seemed to be effective in reducing the abrasive wear by 75% and 47% at -1.5% and -30% SRRs, respectively, as shown in Fig. 1c. However, it produced higher friction and vibration at -30% SRR and had more severe RCF surface damage at -1.5% SRR (promoted micro-cracking to micro-pitting, as shown in Fig. 2d). While IL1 shares the identical anion with IL2 and similar cation with IL3, they have exhibited distinct performance as presented above, which indicates that both the cation and anion play important roles in rolling-sliding lubrication. Our previous studies (pure sliding) have shown that the tribofilms formed by IL1 and IL2 both are composed of iron phosphates and oxides but possess different nanostructures: the IL1 tribofilm 18 basically is an amorphous layer with just a few relatively round 8


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nanocrystals (1-10 nm) scattered around but the IL2 tribofilm 21 is full of nanocrystals (many appeared to 5-10 nm in irregular shapes) embedded in the amorphous matrix. If their tribofilms remain similar nanostructures in rolling-sliding (confirmed for IL1 in Figure 5), the large number of nanocrystals (especially their sharp corners) in the IL2 tribofilm would cause stress concentrations leading to crack initiations under RCF, which could possibly explain IL2’s detrimental effects observed in this work. The IL3 tribofilm in pure sliding 17 was reported to consist of iron oxides and carboxylate complexes and provided slightly less wear protection than the IL1 tribofilm. It also contains a lot of nanocrystals but the particles look more round and somewhat smaller (most

New Functionality of Ionic Liquids as Lubricant Additives: Mitigating

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