3.1 Spectroscopic characterization
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 8wt % 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 1wt %, the IR spectrum of LHS + 1wt % B2Pin2 is similar with that of neat LHS, particularly no obvious CO2− stretching bonds change is observed. When the concentration of B2Pin2 reaches over 5wt %, 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 + 5wt% 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 + 5wt % B2Pin2 shown in 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 + 5wt % B2Pin2 is reported (inset Tables of Fig. 1f) and shows the boron level of 0.92 atomic% in LHS + 5wt% B2Pin2, confirming the interaction of B2Pin2 with LHS to form the Lewis acid–base complex in the lubricating grease.
3.2 Rheological properties of lithium grease with diboron
Shear can result in mechanical breakdown of the thickeners.  In order to study the reinforcing effect of B2Pin2 on the fibrous structure during mechanical aging, 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 8wt % B2Pin2 at 0 oC, 30 oC and 60 oC. 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 5wt% 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,  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 5wt% 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 5wt% B2Pin2 is shown in Fig. 3. At low value of shear rate (0.01 s− 1), the viscosity of the grease with 5wt% B2Pin2 is substantially higher than the viscosity of the base grease (Fig. 3a), especially for the viscosity at low temperature (0 oC), 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 during mechanical aging. However, at high shear rate (100 s− 1), the grease with 5wt% 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,  leading to the formation of lubricating film primarily governed by the base oil. 
3.3 Tribological properties of lithium grease with diboron
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 8wt % B2Pin2 measured at a constant load of 200 N, a fixed frequency of 25 Hz, and different temperature of 30 and 60 oC 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 oC (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 oC (Fig. 4c), the friction coefficients are noticeably lower than that obtained for the base grease. In particular, 5wt% B2Pin2 is the optimum concentration to reduce the friction coefficient (up to 19%) compared to that of 1wt% (8%), 3wt% (14%), and 8wt% (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 oC and 60 oC, 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 probably owing to the fact that the addition of B2Pin2 greatly improved the structure strength of grease thickener, resulting in the suppression of mechanical degradation of lubricating grease during friction and wear process.
The lubricating performance of 5wt% 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 5wt% B2Pin2 could contribute efficiently to the friction reduction of base grease during frequency ramp test from 10 Hz 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 5wt% B2Pin2 additivated grease increased remarkably from 100 N and 150 N to 500 N with increase in applied load from 50 N to 500 N at 30 oC and 60 oC, respectively. These results indicate that the reinforcement of the grease structure strength by the addition of B2Pin2 is beneficial for improving the anti-shear and load-carrying capacities of lithium grease.
3.4 Surface analysis of wear scars lubricated by lithium grease with diboron
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 oC and 60 oC 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.  The O 1s XPS spectra can be deconvoluted into three and five peaks as the contact surfaces were lubricated at 30 oC and 60 oC (Fig. 7b), 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. [21, 22] In addition, the XPS spectra of Fe 2p display four peaks and five peaks when the lubricating grease with B2Pin2 were evaluated at 30 oC and 60 oC (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), [21, 23] respectively. These results demonstrate that the presence of B2Pin2 in the lithium grease did not generated tribochemical products to form boundary lubrication film at 30 oC and 60 oC, and the excellent friction reduction and AW performances of the base grease with the addition of B2Pin2 might be due to the fact that the combination of B2Pin2 with LHS leads to enhancement of the soap fiber structure strength, which is beneficial in preventing the mechanical degradation of the lithium grease during friction and wear process.