3.1 Characterization of LaF3 Nanoparticles
Figure 1 shows the XRD pattern of LaF3 nanoparticles surface-modified by D2EHPA (D2EHPA-LaF3). There are eight diffraction peaks at 2θ of 24.76°, 27.61°, 43.60°, 44.73°, 50.48°, 52.39°, 70.46° and 80.86°, respectively. They correspond to the crystal planes (110), (111), (300), (113), (302), (221), (411) and (413) of hexagonal LaF3 (JCPDS standard card No. 76-1500), which indicates that the as-obtained product is nano-LaF3 with a high purity and the encapsulation by the modifier D2EHPA does not disturb the crystal structure of LaF3.
Figure 2 shows the FTIR spectra of D2EHPA and D2EHPA-LaF3 nanoparticles. From the graph, it can be seen that the C-H bond symmetric and antisymmetric stretching vibration absorption peaks of - CH3- and - CH2- appear at 2956 cm− 1 and 2871 cm− 1, while the bending vibration absorption peak of the C-H bond appears at 1464 cm− 1.The stretching vibration absorbance peaks of P = O and P-O bonds of D2EHPA emerge at 1226 cm− 1 and 887 cm− 1; and they exhibit a slight blue shift after being incorporated onto the surface of LaF3 nanoparticles. It indicates that the polar end group in D2EHPA interacts with La on the surface of LaF3 nanoparticles, and a new Chemical bond is formed.This proves that LaF3 nanoparticles have been successfully surface-modified by D2EHPA.
The TG curves of D2EHPA-LaF3 and D2EHPA are shown in Fig. 3. D2EHPA and D2EHPA-LaF3 begin to lose weight around 220 ℃. The difference is that D2EHPA-LaF3 experiences severe weight loss at about 275 ℃ while D2EHPA undergoes severe decomposition at 250 ℃. This demonstrates that the thermal stability of LaF3 nanoparticles is improved at a certain degree after the encapsulation by D2EHPA, which could be because the organic modifier functions to retard the transfer of heat to the inorganic nanoparticles. Besides, D2EHPA-LaF3 leaves about 28% (mass fraction) of residue after being heated around 600 ℃ and above, which indicates that D2EHPA-LaF3 has an organic modifier content of about 72%.
Figure 4 shows the TEM image of nano-LaF3. It can be seen that the as-prepared D2EHPA-LaF3 nanoparticles have a sheet-like shape and an average size of approximately 10 ~ 20 nm, and they show no obvious agglomeration. The reason might lie in that the introduction of the modifier D2EHPA contributes to reducing the surface energy of LaF3 nanoparticles and retarding their interactions[22].
3.2 Dispersion Stability of Nano-LaF3 in Base Oil
Nano-LaF3 was added to 150N, PAO6 and DIOS with the maximum addition (1 wt%) of four-ball friction test. Figure 5 presents the optical photos of nano-LaF3 dispersed in different base oils. The base oils with the dissolved nano-LaF3 is colorless and transparent, which indicates that the as-prepared nano-LaF3 is oil-soluble and exhibits good solubility in the three kinds of base oils. Moreover, a small amount of white flocs appeared at the bottom of DIOS base oil after standing at room temperature for 2 days, while no precipitates emerged in 150N and PAO6 base oils after standing under the same condition for 1 month. This demonstrates that the LaF3 nanoparticles surface-modified by D2EHPA have good dispersibility in the three kinds of base oils, which is because the organic modifier dwelling at the surface of LaF3 nanoparticles can improve their compatibility with the base oils and reduce their polarity as well. Therefore, D2EHPA-LaF3 nanoparticles can be stably dispersed for a long time in low polarity base oils such as PAO6 and 150N. In high polarity base oil like DIOS, however, the interaction between the organic modifier and DIOS base oil molecules is relatively weak[23, 24], and the oil-soluble LaF3 nanoparticles undergo soft aggregation therein during storage for a relatively long-term.
3.3 Tribological Properties of Nano-LaF3 as Lubricant Additives
Figure 6 shows the friction coefficient and wear scar diameters under the lubrication of 150N, PAO6 and DIOS base oils with different dosage of nano-LaF3 (four-ball machine, load: 392 N, speed: 1200 rev/min, oil temperature: 75 ℃, time: 60 min). It can be seen that after adding nano-LaF3 to the three kinds of base oils, the friction coefficient and wear scar diameters decrease to varying degrees; and the base oils with 0.2% of nano-LaF3 exhibit the best tribological properties. In this case, the friction coefficient for base oils PAO6, 150N and DIOS is reduced by about 28%, 33% and 49%, respectively; and the wear scar diameters is decreased by about 38%, 53% and 46%. With the increase of the dosage of nano-LaF3, the friction coefficient and wear scar diameters tend to gradually augment. This could be because the probability of contact between the nanoparticles increases with increasing amount of nano-LaF3 added and the nanoparticles with high surface activity are easier to agglomerate, thereby forming large-size clusters. The large-size clusters could cause abrasion during rubbing [25] while they also could disturb the entrance of the lubricating oil into the contact surface of the sliding pair as well as the formation of continuous lubricating film, thereby resulting increase in the friction torque and deteriorating the lubricating performance [26]. Therefore, the optimal addition amount of nano-LaF3 in the three kinds of base oils is selected as 0.2%.
Figure 7 presents the friction coefficient-time curves of the steel-steel contact lubricated by PAO6, 150N and DIOS base oils containing 0.2% nano-LaF3 (test condition the same as that of Fig. 6). In the later stage of sliding, the friction coefficients of the sliding pair lubricated by various base oils containing 0.2% D2EHPA (the control groups) tend to rise with extending time [27, 28]. This is because, as the sliding test continues, the temperature of the rubbed metal surfaces rises continuously, and D2EHPA with relatively lower thermal stability would lose lubricating effect under high enough temperature, due to decomposition thereat. After adding nanoscale D2EHPA-LaF3 into the base oils, the friction coefficients tend to decrease slightly with extending time, which is because the decomposed species of the organic modifier and the released LaF3 nano-core are deposited on rubbed steel surfaces to form a protective layer with a relatively low shear strength. As the sliding test continues, the temperature of the friction interface rises, which is more conducive to the deposition and film formation of nano-LaF3 on the worn surface [16].
Figure 8 shows the variations of friction coefficients and wear scar diameters of the steel-steel contact lubricated by PAO6, 150N and DIOS base oils containing 0.2% nano-LaF3 with the applied load (test condition the same as that of Fig. 6). The introduction of the nano-additive into the three kinds of base oils always results in minor decreases in the friction coefficient and wear scar diameters, which indicates that the nano-additive as the lubricant additive exhibits good tribological properties. Besides, the friction coefficients under the lubrication of PAO6 and 150N base oils gradually decrease with the increase of the applied load; and those under the lubrication of base oils PAO6 and 150N containing 0.2% nano-LaF3 fluctuate to some extent therewith. This could be because nano-LaF3 hinders the flow of lubricating oil into the contact gap and destroys the continuity of the lubricating oil film. Such a fluctuation of the friction coefficient with rising applied load becomes negligible under the lubrication of DIOS containing 0.2% nano-LaF3. This is possibly because the DIOS base oil has a slightly lower viscosity than base oils PAO6 and 150N, which refers to less disturbance to the lubricating oil flow and the formation of the oil film by the nano-additive. Moreover, the increase of normal load will lead to direct contact between some asperities and adhesion as well as increased shear force to cut adhesive nodes, which corresponds to gradual increase in the friction coefficient with augmenting load. In addition, it should be pointed out that the plastic deformation of the contact surfaces would occur and the actual contact area would increase with the gradual increase of the applied load, which means continuous increase in the wear scar diameters of the steel balls therewith. Fortunately, after adding nano-LaF3 to base oils PAO6, 150N and DIOS, the anti-wear ability of the base oils is significantly enhanced in the load range of 100 ~ 400 N, due to the deposition of LaF3 nano-core on the rubbed steel surfaces. In one word, under the action of shear force and normal load, the nano-LaF3 surface modified by D2EHPA forms a stable low-shear strength lubricating film on the contact surface, which contributes to reducing the friction and wear of the steel-steel sliding pair to some extent and adding to the load-carrying capacity of the base oils.
3.4 Worn Surface Analysis and Discussion on Tribomechanism
The wear scar morphology of the steel balls was observed with a scanning electron microscope and a three-dimensional (3D) optical profiler, respectively. Figure 9 shows the SEM images and 3D topography of the worn steel surfaces (test condition the same as that of Fig. 6, except for nano-additive concentration of 0.2%). Under the lubrication of the base oil alone, there are signs of serious damage and deformation as well as deep and wide furrows on the wear scars (Fig. 9(a, c, and e), which demonstrates that the three kinds of base oils exhibit relatively poor friction-reducing and anti-wear abilities for the steel sliding pair.
After the addition of nano-LaF3 to the base oils, the worn surfaces of the steel balls are relatively smooth and flat and contain no obvious furrow and spalling along the sliding direction (Fig. 9(b, d, and f); and corresponding 2D profiles of the worn steel surfaces are smaller in size and shallower in depth than those lubricated by the base oils alone (Fig. 10). This is consistent with the wear scar diameters of the steel balls given in Fig. 6 and Fig. 8.
The element distributions on the worn steel surfaces (determined by energy dispersive spectrometry (EDS)) are presented in Fig. 11 (test condition the same as that of Fig. 6, except for nano-additive concentration of 0.2%). Under the lubrication of various base oils containing 0.2% nano-LaF3, large amounts of Fe, C, and O elements originating from the steel substrate and organic modifier as well as a small amount of P, F and La elements originating from the organic modifier and inorganic nano-core are present on the worn surfaces of the steel balls. This gives good evidences to the deposition of surface-capped LaF3 nanoparticles on rubbed steel surfaces.
With PAO6 and DIOS base oils containing 0.2% nano-LaF3 as the examples, the chemical states of major elements in corresponding tribofilms were further analyzed by XPS. As shown in Fig. 12 and Fig. 13 (test condition the same as that of Fig. 6, except for nano-additive concentration of 0.2%), the worn steel surfaces lubricated by both oil samples exhibit La3d-XPS peaks at 836.7 eV and 834.5 eV as well as F1s-XPS peak at 684.9 eV. A comparison with the XPS peaks of LaF3 (La3d: 836.7 eV; F1s: 684.3 eV) demonstrates that, aside from the deposition onto the rubbed steel surfaces, the D2EHPA-LaF3 nano-additive could also participate in tribochemical reactions to form FeF2 and La2O3 thereby causing changes in the chemical states of La and F [29]. The P2p peak at 133.4 eV belongs to PO43−, which is due to the formation of phosphate in the tribofilm via the degradation of the P-containing modifier D2EHPA. Similarly, Fe species including Fe3O4/Fe2O3 (Fe2p: 710.4 eV and 708.3 eV, O1s: 530.2 eV), FePO4 (Fe2p: 712.8 eV, O1s: 532.1 eV), and FeF2 (Fe2p: 711.6 eV) in the tribofilm also come from tribochemical reactions.Particularly, as compared with PAO6 base oil, it seems that the relatively dilute and polar DIOS base oil is more favorable for the deposition of nano-LaF3 on the rubbed steel surfaces and for promoting the inorganic nano-core to participate in tribochemical reactions. This well conforms to our previous observation that the DIOS base oil with lower viscosity causes less disturbance to the flow of the lubricating oil at the friction interface and the formation of the lubricating oil film thereon. Moreover, this may also be due to the weak interaction between D2EHPA surface modified LaF3 and strongly polar base oil DIOS molecules, resulting in the formation of soft aggregates of nano-LaF3 in DIOS base oil, corresponding to the dispersion observed in Fig. 5. Therefore, during the friction process, aggregates are more likely to deposit on the contact surface, so in DIOS strong polar base oil, LaF3 deposits more on the worn surface of the steel ball.
Based on the above experimental results, we speculate that the possible tribomechanism of the surface-capped LaF3 nanoparticles can be schematically illustrated in Fig. 14. Namely, D2EHPA-capped LaF3 nanoparticles enter the contact area of the steel-steel sliding pair during the friction process to allow the release and deposition of LaF3 nano-core therein, while D2EHPA modifier undergoes tribochemical reactions with the freshly exposed metal surfaces and the inorganic nano-core to form tribofilm composed of phosphate, iron oxides, lanthanum fluoride, and lanthanum oxide, thereby significantly improving the anti-wear ability of the base oils [30, 31]. In the meantime, the LaF3 nano-core released and deposited on the worn steel surfaces can fill up the grooves thereon to exert self-repairing effect, which is also favorable for reducing the friction and wear of the steel-steel sliding pair. In summary, D2EHPA-capped LaF3 as the lubricant additive in the three kinds of tested base oils can participate in tribochemical reactions to form a complex boundary lubrication film composed of iron phosphate, iron oxide, lanthanum fluoride, and lanthanum oxide on the contact surfaces of the frictional pair, which, in combination with the deposition of the nano-additive thereon to exert self-repairing effect, jointly functions to reduce the plowing effect as well as the friction and wear of the steel-steel sliding pair [32].