3.1 Characterization of the core-shell materials
SEM images of neat SiO2, SiO2@Cu and SiO2@MoS2 nanocomposites at different magnifications were shown in Fig. 2. The morphology of SiO2 spheres was shown in Fig. 2a-b, it can be seen that SiO2 spheres possessed a perfectly spherical structure, and the average diameter was about 623 nm according to the measurement results (Fig. 2c-d). The average diameter of SiO2@Cu microspheres was about 694 nm. Apparently, copper nanoparticles were uniformly coated on the surface of SiO2 spheres, forming a uniform copper shell layer with a thickness of about 35.5 nm. Likewise, as shown in Fig. 2e-f, SiO2 spheres were surround by an irregular MoS2-nanolayer. The results show that the average diameter of SiO2@MoS2 microspheres was about 683 nm, and the thickness of MoS2-nanolayer was about 30 nm. It is worth noting that the MoS2 shell prepared by the hydrothermal method do not exhibit traditional floral crystal structure but an amorphous structure, which can be attributed to magnetic stirring during the synthesis process . Consequently, the differences in morphology and shell thickness between the two kinds of core-shell particles were basically eliminated, which was beneficial to compare lubrication performances between Cu shell and MoS2 shell .
In order to further prove that the materials adsorbed on the surface of SiO2 in the above figures are Cu and MoS2, the surface elemental compositions of spherical particles were analyzed by EDS. As can be seen from Fig. 3a, Si, O and Cu elements were detected, and the peak value of each element was strong. Combined with the SEM image, it can be concluded that SiO2@Cu core-shell structure composites were successfully synthesized. Similarly, the characteristic peaks of Si, O, Mo and S elements were detected in the microspheres from Fig. 3b, which fully demonstrated that SiO2@MoS2 nanocomposites were successfully synthesized.
Homogeneous dispersion plays a vital role in lubrication. Visual observations were used to determine the qualitative dispersion stability of the core-shell microspheres in PAO 40. As shown in Fig. 3c-d, the 1.0 wt.% SiO2@Cu and 1.0 wt.% SiO2@MoS2 nanospheres could be uniformly dispersed in oil after sonication. The oil containing SiO2@Cu appeared to be brownish-yellow in color, and the oil containing SiO2@MoS2 was black. After seven days of storage, the two core-shell microspheres still exhibited admirable dispersity, which could be due to their small size and the presence of oxygen-containing groups on their surface [26, 32, 36]. These results show that these nanoparticles have good dispersive stability, and can meet the requirements of the friction test.
3.2 Tribological behavior
The friction coefficient curves and average friction coefficient of SiO2, Cu and SiO2@Cu core-shell microparticles as lubricating additives under mixed lubrication regimes are shown in Fig. 4. Clearly, the friction curve of PAO 40 lubricating oil was relatively unstable, and even fluctuates wildly under 40 N and 60 N loads. The friction coefficient of submicron particles with SiO2 compound lubricating oil was relatively stable, which could be due to the high bearing capacity of SiO2 microspheres. However, the friction coefficient of SiO2 increases gradually with the prolonging of experiment time, which was because the silica particles gradually gather during the friction process and cause serious abrasive wear. In addition, when SiO2@Cu core-shell microparticles as additives, the friction coefficient was not only lower than pure oil, but also very stable. In contrast, the overall friction curve of the lubricating oil with copper as an additive was smoother, and the average friction coefficient was lower than SiO2@Cu. Obviously, SiO2 microparticles cannot effectively reduce the wear under mixed lubrication conditions, but the copper layer wrapped in the outer layer of SiO2@Cu core-shell microspheres can make the friction coefficient tend to lower and more stable.
The tribological properties of MoS2, SiO2@MoS2 core-shell composite lubricants under mixed lubrication regime were also investigated. As can be seen from Fig. 5, MoS2 performed well under low load conditions, with the lowest friction coefficient. But its friction coefficient continues to rise with the increase of load, which was closely related to the characteristics of soft texture and low bearing capacity of MoS2. The friction coefficient of SiO2@MoS2 core-shell microparticles was much lower than that of SiO2, indicating that soft shell was beneficial to reduce abrasive wear caused by hard particles. Unfortunately, the friction reduction effect of the SiO2@MoS2 core-shell microspheres as lubrication additives was not better than onefold materials, which maybe because it is difficult for core-shell particles to form stable tribo-film in the mixed lubrication state [17, 36].
By comparing the size of wear tracks and scars generated at 40 N (Fig. 6), it is found that using Cu and MoS2 as soft shells could effectively reduce the wear caused by SiO2 microspheres under a mixed lubrication regime. Especially, when SiO2@Cu spheres were used as lubricating additives, the wear scar diameter decreased 9.1% comparing with SiO2. In addition, when Cu or MoS2 particles were used as lubricating additives, its tribological properties of them were better than core-shell particles. This may be due to the competitive mechanism of the abrasive wear of SiO2 and the self-repairing property of soft shells. Specifically speaking, the self-repairing property of Cu was better than that of MoS2 according to the comparison of the wear scar diameter of SiO2@Cu and SiO2@MoS2.
Figure 7 shows the friction coefficient curves and wear volume of the substrates lubricated by different oil samples at 100 N and 200 N. The friction experiments were still under mixed lubrication conditions at 100 N, so the friction coefficient and wear of the hybrid oil with various solid additives are greater than that of the pure oil. But the wear volume of SiO2@Cu hybrid oil was still less than that of SiO2@MoS2, which was consistent with the results in Fig. 6. By contrast, SiO2@Cu and SiO2@MoS2 core-shell materials as lubrication additives have obvious anti-wear and anti-friction effects under boundary lubrication regimes (200 N). Compared with pure oil, the friction coefficient of SiO2@Cu and SiO2@MoS2 was reduced by 32.47% and 30.98% respectively, and the wear volume is reduced by 67.86% and 52.24%, respectively. It was worth noting that the anti-wear effect of the core-shell materials is better than that of any onefold additives (Fig. 7d). Under boundary lubrication regime, the wear resistance of Cu microspheres and MoS2 was very poor when they were used as single solid lubrication additive, even the wear volume of MoS2 was much larger than that of pure oil. This may be because the insufficient carrying capacity of these soft particles causes the oil film to rupture, then causing adhesive wear under high contact pressure and high friction heat. In addition, the wear volume of SiO2@Cu and SiO2@MoS2 decreased by 48.45% and 23.39% compared with SiO2, which indicates that the Cu shell has better wear reduction properties than SiO2. Especially, no matter SiO2@Cu or SiO2@MoS2, the wear-resistance of them was better than onefold additive (SiO2, Cu and MoS2). Core-shell microspheres as solid lubricant additive can achieve low friction and good service reliability under harsh working conditions.
Figure 8 shows the 3D surface morphologies and cross-sectional profiles of the wear track at 200 N lubricated with PAO 40, SiO2, SiO2@Cu and SiO2@MoS2 hybrid oil. From Fig. 8a we can see the cross-sectional profile from PAO 40 lubrication displays maximum wear track depth (1.364 µm), which could be attributed to contact fatigue and adhesive fatigue during the tribo-test. Whereas, the deep of wear track was decreased to 1.127 µm when 1 wt.% SiO2 microspheres were added to the pure oil, suggesting that the SiO2 microspheres filled the gaps in the worn surface and was beneficial for reducing wear. By contrast, the wear track depth of lubricant with SiO2@Cu and SiO2@MoS2 microspheres was reduced to 0.752 µm and 0.748 µm, respectively. Consequently, those results indicated that the soft shell of core-shell structure microspheres improves the anti-wear properties of SiO2. which were consistent with the change of friction coefficient and wear volume mentioned above.
3.3 Analysis of worn surface
After the friction test, we observed the wear tracks and wear scar morphology by optical microscope, then analyzed the EDS element diagram under mixed lubrication conditions (40 N). As can be seen from Fig. 9a, there are a lot of furrows on the PAO 40 lubricated worn surface, the width of the wear tracks reaches the maximum of 228 µm, which indicated that the poor bearing capacity of pure lubricating oil leads to seriously wear on the contact surface. Figure 9f shows that EDS detected Si element signal, which indicated SiO2 particles were adsorbed on the surface of the wear track. There are obviously deeper furrows on the wear surface of SiO2 lubricating oil (Fig. 9b), due to the three-body abrasion of SiO2 microparticles, which have high hardness and brittle texture. Moreover, it can be seen from Fig. 9c-d that SiO2@Cu and SiO2@MoS2 core-shell materials hybrid lubricating oils have a lower wear track width than SiO2, and obvious black-brown transfer film was generated on the wear mark. EDS detected Cu and S element signal at their wear tracks even though the content of these elements was relatively low (Fig. 9g-h), which indicates that the synergistic effect of SiO2 and Cu or MoS2 plays a role in the friction process. Additionally, compared to the conditions which were lubricated with SiO2 hybrid oil, the wear scar diameter was smaller when lubricated by SiO2@Cu hybrid oil, while the wear scar diameter of SiO2@MoS2 was almost same as SiO2. Hence, the abrasive wear reduction of Cu shell under mixed lubrication conditions was better than MoS2.
Moreover, the worn surface after sliding for 1 h at 200 N was characterized by SEM and EDS (Fig. 10), the results can be used to analyze the effect of different additives under boundary lubrication conditions. The SEM results were basically consistent with the 3D morphology (Fig. 8). As shown in Fig. 10a, when pure oil was used for lubrication, there are large spalling pits and O element (14.95 wt.%) on the wear marks. That is because pure oil cannot reduce high contact pressure and interfacial flash temperature effectively, leading to serious adhesive wear and oxidation on the worn surface. Interestingly, when SiO2 microspheres were added, spalling pits on the worn surface were fewer, but deep furrows were produced by abrasive wear (Fig. 10b). EDS analysis shows that Si (0.69 wt.%) and O (11.10 wt.%) elements exist on the wear track, indicating the particles can be transferred and deposited on the worn surface. There are reasons to believe that SiO2 microparticles could improve the anti-wear ability of base oil to a certain extent under boundary lubrication conditions, but will cause abrasive wear, because it is difficult to produce stable tribo-film . Figure 10c-d show the morphology of the worn surface when SiO2@Cu and SiO2@MoS2 microspheres were added, respectively. As we can see, the worn surfaces lubricated by soft-shell@hard-core microparticles all display shallower and smoother wear tracks. Even more importantly, the friction contact surface was the smoothest when SiO2@Cu particles were added, and covered with a stable and robust tribo-film which containing a large amount of copper (37.13%). On the contrary, the wear track of SiO2@MoS2 hybrid oil exhibited deeper furrows and fewer Mo element (16.29 wt.%), suggesting that the tribological transfer film formed by the MoS2 shell was weaker than Cu shell. It implies that using Cu as shells are better than MoS2 in the self-repairing property. On the other hand, the ratio of O element in SiO2@Cu lubricated surface decreased sharply than SiO2 hybrid oil, this is due to that the core-shell particles can promote the formation of a compact tribo-film in the contact area, which is conducive to preventing oxidation of the worn surface . Thus, soft-shell@hard-core microparticles were proved to be an admirable composite material with better self-repairing property, and Cu as soft shells are likely better than MoS2 for protecting contact surfaces .
Furthermore, in order to study the properties of the tribo-film formed by SiO2@MoS2 microspheres hybrid oil, the worn surfaces at 200 N are investigated by Raman spectra analysis as shown in Fig. 11. According to the Raman spectrum, peaks at 223, 295, 411 and 663 cm− 1 (Fig. 11a) can be assigned to the spectra of Fe2O3 and Fe3O4 , which indicated that there is a large amount of iron oxide on the worn surfaces. In fact, the occurrence of oxidational wear is one of the reasons for the poor anti-wear effect of pure oil under boundary conditions . In Fig. 11b, two weak peaks at 373 and 410 cm− 1 (E12g and A1g mode) of MoS2 were discovered on the wear traces tested by SiO2@MoS2 hybrid oil, indicating that the SiO2@MoS2 microspheres in base oil could enter the friction surfaces to form the tribo-film on the contact areas . However, the intensity of E12g and A1g mode peaks were relatively low, this could be due to the weak reaction with the worn surfaces [41, 42]. Besides, peaks at 292 and 670 cm− 1 also can be assigned to the spectra of different iron oxides, the absence of peaks at 223 and 411 cm− 1 may be because the decrease of oxidational wear . As the main component of the tribo-film, iron oxides and MoS2 play a vital role in boundary lubrication. Eventually, the analysis results of Raman spectra show that the soft-shell@hard-core microparticles were beneficial to the reduction of oxidational wear, but MoS2 as the soft shell of SiO2 cannot give aid to the formation of the robust tribo-film during the frictional process .
3.4 Lubrication mechanism
The topographies of microspheres are related to the lubrication mechanism and follow a certain evolution law, which provides an effective weapon for us to study the effect of core-shell particles on different lubrication regimes. Therefore, the morphology and microstructure of SiO2@Cu and SiO2@MoS2 microparticles after the test could shed more insight into the lubrication mechanism. As shown in Fig. 12a-b, most the microspheres maintain original morphology after friction test at low contact pressure (1.593 GPa) for 1 h, and only a few of the SiO2@Cu and SiO2@MoS2 microspheres were broken. The spherical structure of the crushed core-shell microspheres was deformed into irregular spheres. Moreover, EDS results in Fig. 12a-b show that Cu and MoS2 still exist on the surface of the SiO2 particles, demonstrating that the part of the shell has not worn completely during the friction. Correspondingly, the morphology and EDS analysis of SiO2@Cu and SiO2@MoS2 spheres after tribo-tests at 200 N for 1 h were shown in Fig. 12c-d, respectively. We can see clearly that original SiO2@Cu and SiO2@MoS2 spheres were crushed into smaller spheres at high contact pressure (2.724 GPa). Surprisingly, EDS results show that the surface of the crushed particles did not contain Cu or MoS2, indicating the particles suffered serious wear, leading to the exposure of internal SiO2 core part. By observing the change of particle morphology after test under different loads, we found that whether SiO2@Cu or SiO2@MoS2 particles, low pressure could make the surface of the spheres become rough, while high pressure could make the particles completely broken and the shell peeled off . The breaking degree of SiO2@Cu microspheres was lower after the friction experiment, demonstrating that soft metal shell can equip the microspheres with a higher load-carrying capacity. Under boundary lubrication, using microparticles of soft-shell and rigid-core as lubricant additives can not only take advantage of the easy formation of a transfer film, but also bring the advantages of small size and bearing advantage of the hard nanoparticles into full play . The two-phase composite particles were gradually peeled off during the rolling process, which promote the uniformity and integrity of the transfer film and enhance the self-healing capacity .
Combining with the analysis of the worn surface and microparticles after the test, the lubrication mechanism of two kinds of soft-shell@hard-core microspheres under different lubrication regimes was deduced, as depicted in Fig. 13. To begin with, the molecules of lubricant and core-shell microspheres were adsorbed on the frictional interfaces and formed a tribo-film during the friction process. During mixed lubrication conditions, the core-shell microspheres were acted as ball-bearings on the nanometer scale, and only a few particles enter the worn surface to act as a third body material filling the gap on the worn surface . The surface of the particles which have entered the contact area would be slightly deformed. Meanwhile, the core-shell microspheres were sheared off on the worn surface due to the role of shear force, and a weak physical tribo-film was formed at the same time . By contrast, during boundary lubrication conditions, the contact between the microbulges leads to the oil film piercing. Then core-shell particles broken up completely into smaller spheres and the shell of those particles peeled off under high load. Besides, the debris formed by the wear and core-shell particles would fill the pits on the worn surface to repair the damaged region and reduce the surface roughness. At the same time, for SiO2@Cu hybrid oil, microspheres gradually deposit to the friction interface during the friction process, then Cu shell become melt and spread to form a dense protective film under high temperature and pressure, and repair the damaged interface . Then the crushed SiO2 turn into a carrier to support worn surface , and form a composite boundary lubrication film with Cu shell . For SiO2@MoS2 hybrid oil, the thermal induction during sliding can increase the size and crystallinity of MoS2, and the parallel sliding of MoS2 along the section induced by high pressure was beneficial to reduce the friction. However, the peeled MoS2 nanosheet cannot form a stable adsorbed film and a robust tribo-film, that is the reason why the tribological properties of SiO2@MoS2 were worse than that of SiO2@Cu. On the whole, the physical deposition of core-shell microparticles, self-repairing of the Cu shell, and chemical reaction of base oil together construct a synergistic effect, which can contribute to the optimal lubrication performance of SiO2@Cu.