In-situ formation of an ultra-low wear boundary tribofilm from carboxylic acid molecules

doi:10.21203/rs.3.rs-2131113/v1

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In-situ formation of an ultra-low wear boundary tribofilm from carboxylic acid molecules

https://doi.org/10.21203/rs.3.rs-2131113/v1

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This paper describes series of four-ball tribological and atomic force squeezing test results obtained under different contact pressures and sliding speeds (or squeezing speeds) and performed with poly-alpha-olefin suspended 0.5% weight percentage of different types of carboxylic acids. Results indicated that under a confined zone (contact pressures varied from 2.4~3.9 GPa), there is a critical turning speed at which the friction performances of carboxylic acid change dramatically. When the sliding speed exceeds the critical turning speed, carboxylic acid molecules or their incomplete decomposition products would adsorb, deposit, and spread in the friction surface, thus form an island–like aggregate tribofilm with an approximately thickness of 150 nm achieving a super–low wear.

Carboxylic acid

confined liquid

critical turning speed

tribofilm

Amphiphilic molecules that contain both a polar head group and a nonpolar hydrocarbon tail group is one type of friction modifier additives except functional polymers, soluble organic–molybdenum additives and dispersed nanoparticles. ^[1] It has made significant contributions to reducing friction between asperity contacts and increasing the energy efficiency of mechanical systems. The classical mechanism proposed to explain the friction–reducing properties of amphiphilic molecules at boundary and mixed lubrication conditions that they self–assemble on polar solid surface to form vertically oriented, close–packed monolayers or multilayers. ^[1–3] These reduce friction is caused by the easy slip between the resultant, opposing methyl end group. In the last few decades, much attention has focused on the influences of amphiphilic molecule structure (in particularly referring to carboxylic acid with different head groups or hydrocarbon tail groups) ^[4–8], adsorbed density ^{[4, 5, 8, 9]}, operating condition ^[10], and substrate type ^[5] on the friction behavior of self–assemble film.

Although the tribological properties of self–assemble tribofilm from amphiphilic molecule solution are largely determined by the adsorption between functional groups and metal–based surfaces, and van der Waals force between closely packed hydrocarbon chains, ^{[1, 5]} it is also closely related to the asperity–asperity shear speed. ^[10] Under a static contact condition, the self–assemble tribofilm in many cases only very weakly bound to the surface and thus fragile when extracted from the liquid phase. ^{[7, 11]} However, this adsorbed tribofilm that can withstand certain pressure could be activated by a shear process. ^{[1, 5, 10]} Lots of amphoteric molecules, such as stearic and elaidic acid, give very low friction (0.03–0.05) at low sliding speeds (3.16×10^− 7–5.62×10^− 3 m/s) that increases approximately linearly with log(speed). ^[10] However, oleic acid gives very different behavior–the friction is still reasonably low but is almost constant with speed. ^[10] It was considered that unsaturation in the hydrocarbon chain results in steric effects that inhibit the formation of well–organized monolayers and increase friction. ^{[1, 5, 7, 10]} The applicability of this mechanism, however, needs to be further investigated. Besides, numerous researches suggest that the shear process favors the occurrence of chemisorption or chemical reaction between metal surface and carboxylic acid modules, thus the formation of metal carboxylate. ^[12–17] Based on the above overview, explore the friction–speed behavior and the friction–reducing effect at a wider sliding speed range of amphoteric organic molecules are of increasing importance.

In addition to the influence of shear speeds on the friction–speed behavior of amphoteric molecules, the maximum pressure, and the load–bearing mechanism that the self–assemble tribofilm can support are other currently research topic. Under a boundary lubrication state, the liquid lubricant film is in a highly restricted state between two friction surfaces. Under these confined conditions, the thin lubricant’s rheological behavior differs from its bulk state due to the liquid–to–solid phase transition phenomenon. ^{[18, 19]} Phenomena including a solid–like transition characterized by an enormous increase in shear viscosity, the epitaxial ordering of surface–adjacent molecules, and the interfacial velocity slip have been observed for linear alkanes. ^{[8, 9, 19]}

The tribological performances of amphiphilic molecules were generally investigated using dedicated boundary friction experiments which employ relatively low sliding velocities and high pressures to maintain boundary lubrication conditions. Meanwhile, the detailed structural information of self–assemble tribofilm and/or monolayer can be obtained from simulation tools ^{[8, 9, 19]} and experimental techniques including sum frequency spectroscopy (SFS) ^[20], polarized neutron reflectometry (PNR) ^[21], the surface force apparatus (SFA) ^[22], and in situ atomic force microscopy (AFM) ^{[7, 11]}. Although the above research methods have made outstanding contributions to the properties of amphiphilic organic molecules as lubricant friction modifier additive, there are still some questions necessary to be further explored, such as (і) the origins of monolayer–monolayer friction and/or monolayer–metal friction (more essential friction–speed and friction–load behavior of amphiphilic organic molecules under the boundary lubricant state); (ⅱ) the wear–resistance property rather than just friction–reducing property; (ⅲ) whether friction reduction is produced by the original material or by a decomposition product, and the effects of operating load and shear speed on the level of decomposition; (Ⅳ) the film shear information or film structural information not yet possible to be obtained immediately in the current research methods. In this research, we focused on the detailed friction–reducing and anti–wear results for carboxylic acid in poly–alpha–olefin (PAO) under a higher pressure (2.4 ~ 3.9 GPa) and a higher sliding speed (3.8×10^− 2~9.1×10^− 1 m/s) which could restrict the friction in the boundary lubrication state. Insight into the tribological properties of such lubricants is also beneficial for revealing the anti–wear mechanism of self–assembled monolayers.

2.1 Friction and wear tests

The oil samples used in the friction and wear tests were based on PAO 8 and were executed on a four–ball tester (MRS–10A). The weight percentage of oleic acid (AR, 99%), stearic acid (AR, 90%) and elaidic acid (AR, 98%) (all supplied by Shanghai Aladding Bio–Chem Technology Co., Ltd) were 0.5% (termed as PAO 8–OA, PAO 8–SA and PAO 8–EA, respectively). As stearic acid and elaidic acid are solid at ambient temperature, the corresponding oil samples were agitated at 80 ℃. Table 1 shows the viscosities of tested oil samples which were not affected at any measurable level. The experimental conditions included applied loads of 100, 200, 300, 400, 500 and 600 N (corresponding to maximum Hertzian contact pressures of 2.4, 2.7, 3.1, 3.4, 3.7 and 3.9 GPa, respectively), rotational speeds of 100, 300, 600, 900, 1200, 1800 and 2400 rpm (corresponding to linear speeds of 0.038, 0.124, 0.228, 0.342, 0.456, 0.684 and 0.912 m/s, respectively), temperature of 75℃ for 1 h. The employed ball samples were made of AISI 52100 steel with a diameter of 12.7 mm, hardness of 62 ~ 64 HRC, and roughness of 0.02 µm. All sets of tests were repeated three times. The average wear scar diameter (WSD) of three lower fixed balls was measured after the tests was finished, and the coefficient of friction (COF) was obtained by real–time monitoring.

Table 1

The viscosities of oil samples used in four–ball friction and wear tests.
Oil samples	PAO 8	PAO 8–OA	PAO 8–SA	PAO 8–EA
Viscosities at 25 ± 2℃ (mPa·s)	72.60	71.53	73.07	89.97
Viscosities at 75℃ (mPa·s)	11.52	11.39	11.37	11.72

2.2 AFM force–distance curve measurement

AFM force–distance curve measurement was carried out to assist in revealing the effects of shear speeds on the effective viscosity of confined lubricant. In view of the inaccuracy of bulk viscosity on the force–distance curve, PAO 2 with a smaller viscosity of 6.14 mPa·s at 25 ℃ was used in AFM measurements. The weight percentage of oleic acid in PAO 2 was also 0.5% corresponding to viscosities of 5.28 mPa·s at 25 ℃. Force–distance curves were performed using an AFM (Dimension Icon, Bruker Corp., USA) at room temperature (25 ± 2 ℃). The liquid probe holder was rinsed with alcohol and deionized water before it was used to measure the force in a PAO–based environment. A silica colloidal microsphere (diameter of 21 µm, contact angle of 30° estimated by a plate of the similar material) was glued onto the AFM tip–less probe (TL–CONT, Nano sensors Inc., USA, with normal spring constant equals to 0.196 N/m calibrated by thermal fluctuation method). Force–distance curves were obtained when the AFM tip approached the polished AISI 52100 steel surface (hardness of 62 ~ 64 HRC and roughness of 0.02 µm) immersed in oil samples with different squeezing velocities including 1, 10, 20, 40, 87, 153 and 203 µm/s.

2.3 Characterizations

Scanning electron microscopy (SEM, Hitachi SU8020) equipped with energy–dispersive X–ray spectroscopy (EDS) was used to detect the morphology and elemental distribution of the boundary tribofilm formed under different experimental conditions (corresponding to the minimum and maximum anti–wear rate, respectively). Subsequently, X–ray photoelectron spectroscopy (XPS, PHI Quantera Ⅱ), Fourier transform infrared spectroscopy (FTIR, Nicolet 6700) and time–flight secondary–ion mass spectrometry (ToF–SIMS, TOF.SIMS 5–100) were used to characterize the structure and chemical composition of the boundary tribofilm. Meanwhile, the tribofilm thickness were obtained using focused–ion beam (FIB) approaches.

Figure 1 and Fig. S1 show the wear rates of pure base oil and base oil added different carboxylic acid molecules from which the anti–wear rates for PAO 8–OA, PAO 8–SA and PAO 8–EA were calculated. Oleic acid, stearic acid or elaidic acid with a weight precent of 0.5% all can significantly improve the anti–wear property of base oil over a wide range of contact pressures and sliding speeds. What is noteworthy is that the anti–wear effects of these carboxylic acid molecules were speed–dependent, and there was a critical turning speed at which the friction performances of carboxylic acid changed remarkably. The following sections will focus on the tribological properties of PAO 8–OA for simplifying analysis. With the rotational speed ranging from 100 rpm to 900 rpm, the wear volumes of ball sample lubricated with base oil and base oil contained oleic acid did not change conspicuously, and the anti–wear rate of PAO 8–OA remained at 50.97 ~ 74.33% (except for the anti–wear rate of 96.22% at condition 600 rpm and 100 N). As the rotational speed gradually increased to 1200 ~ 2400 rpm, the addition of oleic acid lowered the corresponding wear rate of base oil up to 95.73 ~ 98.88% and caused a super–low wear that can be observed by a three–dimensional white light interference shown in Fig. S2.

Figure 2 and Fig. S3 summarize the average friction coefficients for base oil containing carboxylic acid during the whole friction process. The friction–reducing performance of carboxylic acid molecules was also speed–dependent, and a critical turning speed of 1200 rpm for oleic acid (same as the turning value for wear–resistance capacity) can be observed. The COFs from a steel test pair subjected to friction testing in PAO 8–OA decreased with the increase of sliding speeds no matter how the applied load changed in the range of 100 ~ 600 N. However, in the range of lower and higher than the critical turning speed, the fitted slope factors of COF to rotational speeds varied greatly ranging from 2.5×10^− 5 to 3.21×10^− 5 and 0.12×10^− 6 to 8.67×10^− 6, respectively. Three reasons might cause COF decrease with the rise of rotational speeds. Firstly, the actual contact pressure was slighter as the ball specimen’s wear became sever at higher rotational speed (as shown in Fig. 1). Secondly, the hydrodynamic effect was enhanced. Thirdly, the shear character of formed boundary tribofilm varied fundamentally. Nevertheless, when the test rotational speed was greater than 1200 rpm, almost no wear occurred for specimen lubricated with base oil added oleic acid (Fig. S2), the friction decrease was attributed to the latter two factors.

The worn surfaces obtained under test conditions of 600 rpm, 300 N and 1800 rpm, 100 N (corresponding to minimum anti–wear rate of 50.97% and maximum anti–wear rate of 97.85% for PAO 8–OA, respectively) were selected to analyze their morphologies and chemical compositions, as shown in Fig. 3. Numerous scratches and a scattered dark tribofilm deriving from the breakdown of hydrocarbon chain appeared on the contact region for samples lubricated with pure base oil, which indicated sever wear occurred on the friction process. This phenomenon also conformed to the morphology and elemental distribution for sliding surface tested in lubricant oil containing oleic acid at experimental condition of 600 rpm and 300 N (Fig. 3c). However, the surface became even extraordinary smooth under 1800 rpm and 100 N, which consisted with the better anti–wear performance of PAO 8–OA shown in Fig. 1. Meanwhile, a large number of island–like aggregations that simultaneously consisted of oxygen and carbon formed on the sliding surface as the test conditions converted to 1800 rpm and 100 N (as shown in Fig. 3d and e). Because element oxygen and carbon were in the same region, it was speculated that the hydrocarbon chain of base oil or carboxylic acid molecules might not completely decompose or reacted with iron to produce carboxylate during the sliding process, but aggregate and deposit in a certain structure.

XPS spectra were executed to explore the chemical composition of the scattered dark tribofilms and the island–like aggregation tribofilms as shown in Fig. 4, 5 and Table S1. The atomic concentrations of Fe in these rubbed surfaces were all relatively low maintaining at 0.11 ~ 6.34%. Meanwhile, the atomic concentrations of O and C were in range of 7.89 ~ 33.87% and 59.78 ~ 92.00%, respectively. This indicated that both lubricating oil molecules and carboxylic acid molecules decomposed to a certain extent under the combination of severe frictional shear forces and high flash temperatures. Elements of O, C, and Fe that within the dark tribofilms obtained by the lubrication of pure base oil and PAO 8–OA under experimental conditions of 600 rpm and 300 N mainly existed in forms of iron oxides (Fe₂O₃ and FeOOH), carbides (CH₂–CO) and iron carbon compounds (Fe₃C) ^{[13, 23, 24]} (Fig. 4a). This appearance was also observed for the worn surface tested by pure base oil under 1800 rpm and 100 N (Fig. 5a). However, the content of iron oxides in the island–like aggregations corresponding to PAO 8–OA lubrication was significantly reduced, and C was mainly in the form of –(CH₂–CH₂)_n and C–C ^{[13, 23]} (Fig. 5b). This was consistent with the absence of wear for PAO 8–OA as sliding speed exceeds the critical turning speed, and indicated that the island–like aggregates were mainly composed of oxygenated hydrocarbons.

To further confirm the chemical structure of the island–like aggregates, FTIR analysis was performed as shown in Fig. 6. The scattered dark tribofilm on the worn surface lubricated with PAO base oil was composed of C = C, C–H, C–O, C–H and aliphatic compounds ^{[25, 26]}. This was caused by the decomposition of lubricating oil molecules and the subsequently reactions between the decomposer and iron substrate. Unfortunately, the distribution density and percentage of this tribofilm were not enough to play an excellent anti–wear performance. When the friction surface was lubricated with PAO 8–OA and tested at 600 rpm and 300 N, the corresponding tribofilm was also mainly composed of small fragments containing C and O (O–H, C–CH₂–C–, C = O, C–H, C–O and aliphatic compounds). However, under the experimental condition of 1800 rpm and 100 N, the peak intensity of these fragments on the friction surface increased significantly. Besides, there were –COOH group and –CH₂ long–chain organic compounds with carbon atomic number greater than 4 in the wave number range of 1611 ~ 1713 cm^− 1 and 600 ~ 1013 cm^− 1, respectively ^{[25, 26]}. This indicated that oleic acid molecules do not completely decompose to ion fragments and react with iron to produce carboxylate under such friction conditions, but partially decompose and accumulate to forming island–like tribofilms on the friction surface which demonstrates excellent wear resistance.

Crosse–sectional SEM image and ToF–SIMS spectra were performed to acquire the island–like aggregates’ thickness and depth elemental information, as shown in Fig. S4 and Fig. 7, respectively. This type of tribofilm was about 150 nm. Form the negative ions ToF–SIMS plots in Fig. 7b, peaks assigned to molecular ions of C₄H₈O₂⁻ (87.74 amu) and C₅H₁₂⁻ (71.75 amu) can be observed which are normally used to characterize the structure of organic compounds. Due to the lack of molecular ion peaks for some aldehyde–containing or hydroxy–containing compounds, the island–like tribofilm originated from the lubrication of PAO 8–OA cannot be precisely determined composing of C₄H₈O₂⁻ and C₅H₁₂⁻. It might consist of hydrocarbon chain with carbon atomic number greater than 5. ^[27] Simultaneously, the observation of ion fragments including ¹H⁻, ¹²C⁻, ¹⁶O⁻, CH⁻, OH⁻, C₂⁻, O₂⁻, C₃H₅⁻, C₃H₆⁻, C₂H₅O⁻, C₃H₈O⁻ in the spectra verified the organic composition of the tribofilm. Although the same results were also be observed in the negative-ion ToF-SIMS spectra acquired on the friction surface lubricated with PAO 8 and PAO8-OA under 600 rpm and 300 N (Fig. S5), the amount of C₃H₅⁻, C₃H₆⁻, C₂H₅O⁻, C₃H₈O⁻, C₄H₈O₂⁻ and C₅H₁₂⁻ were greatly reduced, while the content of ¹⁶O⁻ and OH⁻ was apparently increased. Combined with the above analysis and the elemental distribution mappings, it can be concluded that the island-like aggregations tribofilm were mainly composed of hydrocarbons with carbon atomic number greater than 5.

AFM force-distance curves were conducted to assist in revealing the boundary tribofilm-forming ability of oleic acid. Figure 8 shows the typical experimental and theoretical fitting curves of the normal force as the microsphere probe approaching the steel surface immersed in PAO 2 and PAO 2–OA at different nominal velocities. In the theoretical fitting process, the Spikes model considering the hydrodynamic process, the slip length and Van der Waals force was used to speculate the interfacial viscosity of the oleic acid adsorption layer. ^[28–35] Detailed information can be found in the supported material. The effective viscosity of PAO 2 in the confined zone was in the same order of magnitude as the bulk viscosity (6.14 mPa·s), so it was inferred that the change of viscosity was not enough to cause the change of the anti-wear performance of PAO. However, when a certain amount of oleic acid molecules was added to the lubricating oil, the effective viscosity of the liquid in the confined range was increased by nearly 2 to 4 orders of magnitude compared with the bulk viscosity (5.28 mPa s). This indicated that oleic acid molecules play an active role in anti-wear during the shearing process of friction roughness peaks. In addition, the effective viscosity of PAO 2–OA in the restricted interval was related to the tip approaching speed. With the increase of the approaching speed, the effective viscosity of the oil samples gradually decreased. However, this did not signify the anti–extrusion ability was weakened for the thickness variation of the confined layer must be considered.

Figure 9 gives schematic diagrams of the load- and speed- dependent boundary tribofilm’s formation for carboxylic acid molecules. Under a relatively slight load, amphiphilic organic molecules such as oleic acid can self-assemble on polar solid surfaces to form monolayers or multilayer tribofilms that are closely arranged in the vertical direction (Fig. 9a). This physical adsorption friction film can be activated by the shearing process, so that it can withstand a certain pressure. The shearing process is conducive to the chemical adsorption or chemical reaction between the metal surface and organic carboxylic acid molecules, thereby forming metal carboxylate. With the further increase of shear rate, load or temperature, amphoteric organic molecules begin to decompose, and a loosely distributed tribofilm composed of hydrocarbon group fragments, metal oxides, metal carbides will form on the friction surface, as shown in the Fig. 9b. When the four-ball friction test pressure was maintained within 2.2 ~ 3.17 GPa and the sliding speed exceeded the critical speed, carboxylic acid molecules’ partial decomposition would gather on the friction surface, and finally form an island-like distributed tribofilm with a thickness of about 150 nm, as shown in Fig. 9d.

In conclusion, we have investigated the tribological performance of carboxylic acids including oleic acid, stearic acid and elaidic acid at severer boundary lubrication conditions (a higher pressure of 2.4 ~ 3.9 GPa and a higher sliding speed of 3.8×10^− 2~9.2×10^− 1 m/s). Results show that the tribological properties of carboxylic acids are speed- and load- dependent, moreover, there is a critical turning speed at which the tribofilm formation behavior change dramatically and even achieve “zero” wear. Speculated from the comprehensive analysis of the tribological and interference test results, this substantial reduce in wear is attributed to the formation of island-like aggregation tribofilms. This tribofilm mainly consists of characteristic oxygenated hydrocarbons with the number of carbon atom greater than five.

Acknowledgements

This research used resources of the National Natural Science Foundation of China (no. 52005284).

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No competing interests reported.

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In-situ formation of an ultra-low wear boundary tribofilm from carboxylic acid molecules

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Abstract

Figures

1. Introduction

2. Experimental Procedure

2.1 Friction and wear tests

2.2 AFM force–distance curve measurement

2.3 Characterizations

3. Results And Discussion

4. Conclusion

Declarations

Acknowledgements

References

Additional Declarations

Supplementary Files

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