Investigation of the Interaction, Rheological and Tribological Properties of Bis(pinacolato)diboron with Lithium Grease

The lithium-based grease was prepared via the same method reported by Xu et al. [15, 19] and the weight percent of lithium soap was about 12 wt%. Then bis(pinacolato)diboron (B₂Pin₂, purity ≥ 98%, J&K Scientific Ltd., Beijing, China) was introduced into the base grease. After vigorous mechanical mixing, each sample was ground five times through a three-roll mill. Typical properties of the tested grease with various contents of B₂Pin₂ are summarized in Table 1.

Thickener fibers in the greases could be obtained by the following method: 10 g grease was thoroughly washed with petroleum ether (3 × 20 mL) to remove the poly-alpha-olefin (PAO 10) base oil and washed several times with dichloromethane to remove the unreacted biboron. Then the resulting soap fibers (LHS + B₂Pin₂) were dried under vacuum at 50 °C overnight.

FTIR of the thickener fiber was performed with an IFS 66v/S using the KBr disk method. TEM images were taken on a JEOL JEM-F200 microscope in TEM mode as well as in annular dark-field (ADF) mode form, operated at 200 kV. XPS of the soap fiber was conducted on a Thermo Scientific K-Alpha spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV) operating at 100 W.

Rheological measurements of the tested grease were conducted on an Anton Paar MCR 302 rheometer (Austria) using 1 mm gap for a plate-to-plate geometry (24.985 mm diameter). Oscillatory shear experiments were carried out at a fixed angular frequency (ω = 10 rad/s) and various shear strains (ranging from 0.01 to 100%) and temperatures (0, 30 and 60 °C) to evaluate the structure strength of thickener fiber during mechanical aging. Shear measurements were performed at different temperatures (0, 30 and 60 °C) and shear rates (0.01 and 100 s⁻¹) to reveal the evolution of viscosity as a function of time for the lubricating greases.

To investigate the tribological performances of B₂Pin₂ as additives for the lithium-based grease, high contact stress ball-on-disc oscillating reciprocating tribological tests were carried out with an Optimal-SRV-IV tribometer. The AISI 52100 steel balls (ø10 mm with hardness of about 626 HV) reciprocated against stationary AISI 52100 steel discs (ø24.00 × 7.88 mm, hardness 700 HV) at a stroke of 1 mm and an oscillation frequency of 25 Hz. The friction coefficient was collected automatically by a computer connected to the SRV tribometer. Wear volumes of the steel disc were measured with a MicroXAM 3D surface profilometer after each tribological test. The surface composition of the wear disc was obtained with XPS. Note that all experiments were repeated at least three times.

The proposed Lewis acid–base interaction of B₂Pin₂ with LHS was shown in Scheme 1. Infrared band assignments of the thickener fibers are reasonable to confirm the formation of Lewis acid base complex (as shown by the red dashed box in Scheme 1). Figure 1a displays the IR spectra between 800 and 2000 cm⁻¹ of various soap fibers (LHS, LHS + 1, 3, 5, and 8 wt% B₂Pin₂) isolated from the base grease containing different concentration of B₂Pin₂. Two strong peaks located at 1580 cm⁻¹ and 1559 cm⁻¹ in the spectrum of neat thickener fibers are assigned to CO₂⁻ stretching bonds of LHS. When the concentration of B₂Pin₂ is 1 wt%, the IR spectrum of LHS + 1 wt% B₂Pin₂ is similar with that of neat LHS, particularly no obvious CO₂⁻ stretching bonds change is observed. When the concentration of B₂Pin₂ reaches over 5 wt%, a new broad peak is clearly seen to be centered at around 1636 cm⁻¹, which may be due to the formation of Lewis acid–base complex between boron atom in B₂Pin₂ and CO₂⁻ of LHS. In this reaction, the boron atom tends to donate an electron pair to the carbonyl oxygen atom resulting in electron-rich CO₂⁻ group, so the CO₂⁻ stretching band is broadened and shifts to higher frequency with increasing B₂Pin₂ concentration. TEM characterization was performed to explore the influence of B₂pin₂ on the fibrous microstructure of lithium-based lubricating grease. Although the fiber structures of LHS and LHS + 5 wt% B₂Pin₂ are similar to each other (Fig. 1b, the TEM image of the fiber morphology of LHS is not shown), the corresponding C, O, and B elemental mapping of LHS + 5 wt% B₂Pin₂ (Fig. 1c–e) display that B phase mapping distributes over the area of C and O phase mapping, indicating boron containing layer on the surface of soap fiber structure. Additional evidence for the presence of B₂Pin₂ in the soap fiber structure comes from the B 1s XPS spectrum for LHS + 5 wt% B₂Pin₂ (Fig. 1f). As expected, it can be observed that an obvious B 1s peak located at about 192.3 eV, which is similar to that of pure B₂Pin₂. Quantitative analysis of LHS and LHS + 5 wt% B₂Pin₂ is reported (inset Tables of Fig. 1f) and shows the boron level of 0.92 atomic% in LHS + 5 wt% B₂Pin₂, confirming the interaction of B₂Pin₂ with LHS to form the Lewis acid–base complex in the lubricating grease.

Shear can result in mechanical breakdown of the thickeners [20]. In order to study the reinforcing effect of B₂Pin₂ on the fibrous structure of lithium grease, the rheological properties of fresh grease with varying B₂Pin₂ content were evaluated by shearing the greases in a rheometer at low and moderate temperature. In oscillatory shear measurements, shear modulus and shear stress versus strain are reported in Fig. 2 for the base grease with 0, 1, 3, 5, and 8 wt% B₂Pin₂ at 0 °C, 30 °C and 60 °C. When the modulus at strains outside the linear viscoelasticity (LVE) regime, all these greases exhibit transitions from G′-dominant to G″-dominant behavior, corresponding to the structure transformation from the solid-like to liquid-like, and the transformation point indicates the decomposition point of grease crosslink structure. It is evident that at all shear strains, both storage (G′) and loss (G″) for the grease with the addition of B₂Pin₂ are markedly higher than for the base grease at different temperatures, particularly for the grease with 5 wt% B₂Pin₂. Meanwhile, the corresponding shear stress deduced from these measurements is also greater than the base grease. These results mean that the lithium lubricating grease with the addition of B₂Pin₂ is both much stronger and more viscous than the base grease [21], which might be attributed to the formation of Lewis acid–base complex of LHS with B₂Pin₂ and the enhancement of the structure strength of lithium grease. In addition, it is seen that 5 wt% B₂Pin₂ is the optimum concentration to provide significant improvement for grease structure strength. In the shear experiment, evolution of viscosity with time at different shear rate and temperature for the base grease and the grease with 5 wt% B₂Pin₂ is shown in Fig. 3. At low value of shear rate (0.01 s⁻¹), the viscosity of the grease with 5 wt% B₂Pin₂ is substantially higher than the viscosity of the base grease (Fig. 3a), especially for the viscosity at low temperature (0 °C), which manifests a value around two orders of magnitude greater than that of the base grease, further demonstrating that soap fiber structure strength is significantly reinforced by the addition of B₂Pin₂. However, at high shear rate (100 s⁻¹), the grease with 5 wt% B₂Pin₂ shows a slightly increase in viscosity compared to the base grease under low and moderate temperature. This can be explained by the fact that the grease thickener will not enter the contact surface but will be pushed to the sides at higher shear rate [22], leading to the formation of lubricating film primarily governed by the base oil [9].

The lubrication performance of the lithium grease additized with B₂Pin₂ were investigated by SRV under variable temperatures, frequencies and loads. Firstly, the friction coefficient of the base grease with 0, 1, 3, 5, and 8 wt% B₂Pin₂ measured at a constant load of 200 N, a fixed frequency of 25 Hz, and different temperature of 30 and 60 °C are displayed in Fig. 4. It is seen that the base grease has a relatively long running-in time (around 0–300 s) with very high friction coefficient (> 0.2) at 30 °C (Fig. 4a). However, the addition of B₂Pin₂ can dramatically reduce the running-in period and shows lightly reduce friction coefficient. When the temperature is increased to 60 °C (Fig. 4c), the friction coefficients are noticeably lower than that obtained for the base grease. In particular, 5 wt% B₂Pin₂ is the optimum concentration to reduce the friction coefficient (up to 19%) compared to that of 1 wt% (8%), 3 wt% (14%), and 8 wt% (16%) B₂Pin₂. Moreover, B₂Pin₂ additivated grease also exhibits exceptional AW performance, as shown in Fig. 4b and d. The addition of 5wt% B₂Pin₂ reduced the wear volume of the base grease by 12 times and 3 times at 30 °C and 60 °C, respectively, providing stronger protection against wear than other concentrations of B₂Pin₂. Figure 5 also show the 3D morphology of the wear scars in Fig. 4b and d. Compared with the base grease, the addition of B₂Pin₂ generated shallower and narrower wear scars, while the wear scars generated by base grease are deeper and wider, so it is significant that the use of B₂Pin₂ prevents the formation of wear scars larger and deeper. The excellent friction and wear reduction properties could probably be ascribed to the boundary lubrication film formed by B₂Pin₂ adsorbed on the sliding surface and/or tribochemical reaction between borate esters and steel surfaces [12, 14].

The lubricating performance of 5 wt% B₂Pin₂ added in the grease was further evaluated by changing the reciprocating frequencies and applied loads. As shown in Fig. 6a and b, the addition of 5 wt% B₂Pin₂ could contribute efficiently to the friction reduction of base grease during frequency ramp test from 10 up to 70 Hz stepped by 15 Hz at different temperatures, particularly as the frequency below 40 Hz. In addition, Fig. 6c and d display the load-carrying capacity of 5 wt% B₂Pin₂ additivated grease increased remarkably from 100 and 150 N to 500 N with increase in applied load from 50 to 500 N at 30 °C and 60 °C, respectively. The improvement of anti-shear and load-carrying capacities of lithium grease may be attributed to the reinforcement of the grease structure strength and the formation of boundary lubrication film by the addition of B₂Pin₂.

To investigate the friction reduction and AW mechanism of B₂Pin₂ in lithium grease, high-resolution (HR) XPS spectra of B, O and Fe on the worn surface of the steel discs were obtained. As shown in Fig. 7a, the binding energies of B 1s on the worn scars lubricate by the base grease with the addition B₂Pin₂ at 30 °C and 60 °C are similar to each other, and the peaks centered at 193.5 eV and 192.0 eV might be ascribed to B–B and B–O binds related to B₂Pin₂ residue and B₂O₃ [15, 23]. The O 1s XPS spectra for the wear scars lubricated with the base grease at 30 °C and 60 °C are also very similar, and one of them can be deconvoluted into five peaks, which appear at 529.1 eV, 529.8 eV, 530.8 eV, 531.7 eV and 532.7 eV, are identified to Fe₃O₄, Fe₂O₃, and LHS (Fig. 7b) [23]. After lubricating with the grease additized B₂Pin₂ at 30 °C and 60 °C (Fig. 7b), the O 1s spectra including three and five peaks, which might be assigned to LHS (530.8 eV, 531.7 eV) and B₂Pin₂ (532.6 eV) residues, and Fe₂O₃ (529.8 eV), Fe₃O₄ (530.1 eV), LHS (530.8 eV, 531.7 eV) and B₂Pin₂ (532.6 eV) residues, respectively [23, 24]. In addition, the XPS spectra of Fe 2p display four peaks and five peaks when the lubricating grease with B₂Pin₂ were evaluated at 30 °C and 60 °C (Fig. 7c), these peaks were corresponded to FeO (709), Fe₃O₄ (709.8 eV), Fe₂O₃ (710.8 eV and 724 eV), and Fe(OH)O (712 eV) [23, 25], respectively. These results demonstrate that the addition of B₂Pin₂ in the lithium grease could adsorb on the contact surfaces and generate tribochemical products to form boundary lubrication film at 30 °C and 60 °C. The film contains FeO, Fe₃O₄, Fe₂O₃, Fe(OH)O, B₂O₃, and B₂Pin₂ residue, which is beneficial to improving the tribological properties of the lithium grease during friction and wear process.