3.1 Characterization of OA-LBNs nanofluid
Figure 2 presents the TEM images of unmodified lanthanum borate and OA-capped lanthanum borate. The unmodified lanthanum borate shows obvious sign of agglomeration (Fig. 2a), while the nanosheet-like OA-LBNs have a diameter of about 22 nm and a thickness of 6 nm and exhibit no obvious sign of agglomeration (Fig. 2b). It is generally recognized that oleic acid as the surface modifier can effectively prevent the growth of lanthanum borate nanoparticle in the reaction solution, and its long-chain alkyl group with excellent lipophilicity can improve the compatibility of the surface-capped inorganic nanoparticle with lubricant base oil.
XRD analysis of the as-prepared samples was carried out to identify their species and crystal form. As illustrated in Fig. 3, the diffraction peaks of OA-LBNs are very similar to those of the unmodified lanthanum borate, which demonstrates that the surface modifier OA has little effect on the crystal structure of lanthanum borate. Besides, the diffraction peaks of the unmodified lanthanum borate and OA-LBNs are weak, which is because their drying temperature (80°C) is much lower than the crystallization temperature of lanthanum borate (about 900°C). Moreover, the diffraction peaks of OA-LBNs in the 2θ range of 5 ~ 90° are broadened, which could be due to their small particle size associated with surface-modification by oleic acid or the formation of amorphous structure of lanthanum borate [23].
High-resolution XPS spectra were recorded to further analyze the element composition of the as-prepared OA-LBNs nanofluid. Figure 4 shows its B1s, C1s, O1s, and La3d XPS spectra. The strong double peaks at 838.0 eV and 853.0 eV are assigned to La3d5/2 and La3d3/2, respectively; the peak at 194 eV is attributed to B1s; and the peak at 528 eV is ascribed to O1s. Moreover, the C1s peak at 282.5 eV originates from oleic acid modifier and contaminant carbon. These XPS results further confirm the successful synthesis of the target product.
The interaction between the modifier and lanthanum borate was analyzed by FTIR spectrometry. As shown in Fig. 5, both OA-LBNs and LBNs have absorbance peaks at 3438 cm− 1 and 3402 cm− 1, due to the stretching vibration of hydroxyl groups in adsorbed water [24]. The absorbance peaks of OA-LBNs nanofluid at 1307 cm− 1 and 718 cm− 1 and those of LBNs at 1384 cm− 1 and 735 cm− 1 correspond to the symmetrical stretching vibration of B(4)-O and the stretching vibration of B(3)-O, respectively [25, 26]. In addition, the absorbance peaks of oleic acid and OA-LBNs nanofluid at 2925 cm− 1 and 2856 cm− 1 are attributed to the C-H stretching vibration of ‒CH3 and ‒CH2‒, and the unmodified lanthanum borate does not show corresponding absorbance peaks. It is worth noting that the peak at 1709 cm− 1 is caused by the carbonyl stretching vibration of OA, and this peak disappears in the FTIR spectrum of OA-LBNs nanofluid. Furthermore, the absorbance band at 1112 cm− 1 refers to the carbonyl stretching vibration of carboxylate in OA-LBNs nanofluid, which proves that OA is modified onto the surface of LBNs via chemical bonding. These FTIR data demonstrate that the long alkyl chains of OA are successfully grafted onto the surface of lanthanum borate nanosheets [16].
Figure 6 illustrates the TGA curves of OA-LBNs nanofluid and oleic acid. Oleic acid begins to lose weight at about 210°C, and it completely loses weight at 360°C. However, OA-LBNs nanofluid begins to lose weight at 310°C, higher than that of oleic acid. This indicates that the grafting of oleic acid onto the surface of lanthanum borate by chemical bonding is favorable for improving the thermal stability of the inorganic nanosheet. Moreover, the complete weight loss of OA-LBNs nanofluid around 570°C is about 63.5%, corresponding to its content of surface grafted OA.
3.2 Tribological properties
Figure 7 depicts the relationship between the friction coefficient and additive concentration in PAO, DIOS and RO base oils. After the addition of OA-LBNs nanofluid in PAO4, the friction coefficient first increases and then decreases with the increase of the additive concentration, and it reaches the lowest value of 0.07 when the additive concentration is 0.3% (mass fraction), being equal to the friction coefficient under the lubrication of PAO4 alone (0.07). This indicates that OA-LBNs nanofluid has a negative effect on the friction-reducing behavior of the PAO4 base oil. On the one hand, the OA-LBNs nanofluid and the PAO4 base oil undergo competitive adsorption on the rubbed steel surfaces to form a boundary lubricating film thereon. On the other hand, OA-LBNs nanofluid may participate in tribochemical reactions with the freshly exposed steel surfaces during the sliding process, forming an adsorption film or tribofilm thereon. The shear strength or roughness of the as-formed composite film, however, is higher than that of the steel sliding pair, which results in an increase in the friction coefficient [27]. When OA-LBNs nanofluid is introduced into DIOS and RO, the friction coefficient curves are relatively stable with the increase of the additive concentration. This indicates that the friction-reducing effect of the OA-LBNs nanofluid as the additive in DIOS and RO is insignificant and nearly independent of its concentration. The reason might lie in that the DIOS and RO base oils have a stronger polarity than the OA-LBNs nanofluid, which is favorable for their preferential adsorption on the rubbed steel surfaces to inhibit the adsorption of the nanofluid thereon.
Figure 8 presents the relationship between the WSD and the concentration of OA-LBNs nanofluid in PAO4, DIOS, and RO base oils (four-ball friction and wear tester; load: 392 N; speed: 1200 rev/min; time: 60 min; temperature: 75oC). Under the lubrication of PAO4 containing OA-LBNs nanofluid, the WSD decreases rapidly with the increase of the additive concentration and then levels off. When the additive concentration is 0.3%, the WSD reaches the lowest value of 0.36 mm, much smaller than that under the lubrication of PAO4 alone (0.76 mm), and also smaller than the one under the lubrication of PAO4 containing 0.3% of oleic acid (0.66 mm). On the one hand, the introduction of an appropriate amount of OA-LBNs nanofluid leads to a significant improvement in the antiwear ability of PAO4, since the additive can easily enter the rubbed steel surfaces to form a deposited film thereon under local high flash temperature and high contact pressure. The as-formed surface protective film contributes to preventing the direct contact between the rubbing surfaces and greatly reducing wear [28]. On the other hand, the best antiwear ability occurs at a certain additive concentration at which the adsorbed film could be arranged more closely and densely on the rubbed steel surfaces. Beyond that concentration, however, the adsorption and desorption of the lubricant additive on the rubbed steel surfaces reach a dynamic balance, and hence the WSD levels off with further increasing additive concentration [29].
When OA-LBNs nanofluid is used as the additive of DIOS base oil, the WSD reaches a minimum of 0.63 mm at an additive concentration of 0.1%, lower than that under the lubrication of DIOS alone (0.76 mm), and also lower than the one under the lubrication of DIOS containing 0.1% of oleic acid (0.76 mm). As the additive concentration further increases, the antiwear performance deteriorates, possibly due to the agglomeration of the too high concentration of OA-LBNs with a high surface energy and a high intermolecular force. The aggregates of OA-LBNs nanofluid in the DIOS base oil could cause three-body abrasive wear, thereby resulting in increase in wear [30]. However, the antiwear performance does not change significantly when OA-LBNs nanofluid is introduced into RO base oil. For example, the WSDs under the lubrication of RO with 0.3% of OA-LBNs nanofluid and with 0.3% of oleic acid (0.71 mm and 0.75 mm) are close to that under the lubrication of RO alone (0.81 mm). In general, OA-LBNs nanofluid in DIOS and RO base oils is less effective than in PAO4 base oil in terms of its antiwear ability for the steel-steel sliding pair. This could be related to its competitive adsorption with DIOS and RO base oils on the rubbed steel surfaces. Namely, both DIOS and RO with a high polarity can be chemically adsorbed on the rubbed steel surfaces to generate a relatively thick oil film with a low shear stress. When the additive concentration is low, DIOS and RO are preferentially adsorbed on the rubbed metal surfaces as the main adsorbates, resulting in a stable tribofilm thereon to reduce the friction and wear of the steel-steel sliding pair. While the additive concentration is too high, OA-LBNs nanofluid tends to agglomerate, thereby damaging the compactness and continuity of the oil film as well as antiwear ability [31]. Previous researches demonstrate that the lubricant additives such as oil-soluble nano-Cu with a relatively low melting point [32, 33] and functionalized BN nanosheets [34, 35] have good friction-reducing and antiwear properties. This is because the surface modifier of the oil-soluble nano-Cu plays a role in reducing the friction coefficient while the Cu nanocore plays a role in load-carrying during the friction process [32, 33]. Similarly, molybdenum dialkyldithiocarbamate (MoDTC), a commonly used lubricating oil additive, exhibits excellent friction-reducing ability in PAO base oil, because it forms a surface layer of MoS2 nanocrystal with a low shear strength; its antiwear ability in the same base oil, however, is relatively poor [36, 37]. Different from MoDTC, traditional engine oil additive zinc dialkyldithiophosphate (ZDDP) [38], borate ester [39], ZnO nanoparticle [40], and CeO2 nanoparticle [41] have significant antiwear ability but poor friction-reducing ability, since they often form tribofilms with a high shear strength.
The plot of wear scar diameter as a function of applied load is shown in Fig. 9. Under the same load, the WSD under the lubrication of pure PAO4 is relatively large. Under the lubrication of PAO4 containing 0.3% OA-LBNs nanofluid, the WSD decreases to a certain extent within the tested load range. This indicates that OA-LBNs nanofluid can improve the antiwear ability of PAO4 under the applied load from 196 N to 490 N. On the whole, the variation trend of WSD versus load under the lubrication of PAO4 alnoe and PAO4 containing 0.3% OA-LBNs nanofluid is similar: the WSD initially decreases and then increases with increasing load. When the load is lower than 294 N, the WSD decreases with the increase of the load, because the elastic fluid effect of the base oil is significant at low loads while the additive can form chemically adsorbed film to reduce wear thereat. When the load is above 294 N, the WSD tends to increase with further increase of the load, which is due to the decrease in the thickness of the tribofilm in association with the increase in the direct contact area of the friction pair thereat [42].
Figures 10 and 11 exhibit the 3D and 2D profiles of the worn surfaces of lower steel balls lubricated by different base oils without or with 0.3% OA-LBNs nanofluid. The WSD of the lower steel balls lubricated by pure base oils is large, and there are many wide and deep furrows along the sliding direction (Fig. 10a1, b1and c1); under the lubrication of the base oils containing OA-LBNs nanofluid, the WSD is relatively small. In particular, under the lubrication of PAO4 base oil containing 0.3% of OA-LBNs nanofluid, the WSD is the smallest. This is mainly because OA-LBNs nanofluid as the additive in PAO4 base oil can be chemically adsorbed onto the friction surface to form a protective tribofilm, thereby avoiding the direct contact between sliding steel surfaces and reducing wear. Similar phenomenon is also visible in the 2D profiles of the worn surfaces: the depth of the wear scar under the lubrication of PAO4 alone is larger than that under the lubrication of PAO4 containing 0.3% OA-LBNs nanofluid.
To clarify the action mechanism of OA-LBNs additive in different base oils, we analyzed the morphology of the worn surfaces of the steel balls and the element composition of the tribofilm thereon by SEM-EDS. Figure 12 shows the SEM images and EDS element distributions of the worn surfaces of the steel balls lubricated by different base oils without or with OA-LBNs nanofluid. Similar to what is seen in Fig. 10, the WSDs of the steel balls lubricated by PAO4 (Fig. 12a1), DIOS (Fig. 12b1), and RO (Fig. 12c1) base oils are large; and deep furrows and plastic deformation are visible on the worn steel surfaces. This indicates that the sliding pair is dominated by abrasive wear and galling under the lubrication of the base oil alone. In other words, the lubricating film formed by the base oil alone has poor antiwear ability, possibly due to its poor strength. Compared to the worn steel surfaces lubricated by the base oil alone, those lubricated by the base oil with OA-LBNs nanofluid are relatively smooth and do not show signs of severe galling along the sliding direction (Fig. 12a2, b2, c2). This indicates that the as-synthesized OA-LBNs nanofluid as the additive in PAO4, DIOS, and RO base oils can reduce the wear of the steel balls to a certain extent.
It is worth mentioning that the wear scar of the steel ball under the lubrication of PAO4 with 0.3% OA-LBNs nanofluid is dominated by slight wear in association with narrow furrows and shallow scratches (Fig. 12a2), and it is smaller than those under the lubrication of DIOS or RO with OA-LBNs nanofluid (Fig. 10b2 and c2). As seen in Fig. 12a2 and Table 2, there is an obvious tribofilm as well as a large amount of O, La, and B elements on the worn surface of the steel ball lubricated by PAO4 containing 0.3% OA-LBNs nanofluid. Nevertheless, no obvious EDS signals of La and B elements are visible on the worn steel surfaces lubricated by DIOS or RO with 0.3% OA-LBNs.This further proves that OA-LBNs nanofluid in DIOS and RO base oils are not easily enriched on the rubbed steel surface, corresponding to the friction and wear test data provided in Fig. 8 and Fig. 10.
Table 2
Element composition of worn steel surface
Lubricant
|
Element composition (%)
|
Fe
|
C
|
O
|
B
|
La
|
Other
|
PAO4 + OA-LBNs
|
51.31
|
5.88
|
15.84
|
7.35
|
18.44
|
1.17
|
DIOS + OA-LBNs
|
91.44
|
4.45
|
1.47
|
0.78
|
0.06
|
1.81
|
RO + OA-LBNs
|
84.14
|
9.59
|
3.36
|
1.10
|
0.00
|
1.82
|
The chemical composition of the tribofilm on the worn surface was further analyzed by XPS to reveal the tribomechanism of OA-LBNs nanofluid as the additive in different base oils. Figure 13 depicts the XPS spectra of typical elements on the worn steel surface under the lubrication of PAO4 containing 0.3% OA-LBNs nanofluid. The B1s peak at 192.3 eV is assigned to B2O3 [43], which indicates that B participates in tribochemical reaction during the friction process. It should be pointed out that the atomic number of element B is small, which corresponds to the less intense B1s spectrum. The C1s peaks at 284.8 eV and 285.5 eV correspond to C = C and C-O derived from the oleic acid modifier [44]. The XPS spectrum of O1s is fitted into three peaks located at 529.5 eV, 531.2 eV, and 533 eV, corresponding to the Fe-O bond, C = O/C-O bond, and B-O bond [45]. The Fe2p peaks at 710.2 eV, 711.4 eV, 714.4 eV and 724 eV are attributed to Fe3O4, FeOOH, FexBy and Fe2O3, respectively [46, 47], which demonstrates that the steel sliding pair also participates in tribochemical reactions. The La3d peaks at 835 eV and 838.6 eV are attributed to La2O3. These XPS results reveal that OA-LBNs nanofluid in PAO4 is initially adsorbed on the rubbed steel surface, then it takes part in complicated tribochemical reactions to generate a protective tribofilm containing Fe2O3, B2O3, La2O3, etc, thereby improving the antiwear performance of the PAO4 base oil.
As to the worn steel surfaces lubricated by DIOS with OA-LBNs nanofluid or by RO with OA-LBNs nanofluid, the B1s and La3d XPS signals are very weak, which indicates that the OA-LBNs nanofluid in DIOS and RO base oils is poorly adsorbed onto the rubbed steel surface or prevented from participating in the tribochemical reactions. Relevant strong XPS signals of C1s, O1s and Fe2p (Fig. 14) suggest that the as-formed tribofilms consist of only iron oxides and C species derived from DIOS or RO base oil, as evidenced by corresponding EDS data in Table 2. The XPS spectra of the rubbed steel surface lubricated by DIOS with 0.3% OA-LBNs nanofluid are similar to those of the rubbed steel surface lubricated by RO with 0.3% OA-LBNs nanofluid.
Base on the above mentioned, it is evident that OA-LBNs nanofluid as the additive in different base oils functions via different tribomechanisms; and its tribomechanism is closely dependent on the nature of the base oil. PAO4 base oil belongs to alkane synthetic oil and has a low polarity, while DIOS and RO are esters and have higher polarity than alkane. Especially, RO contains double bonds and carboxylic acids, which results in further increase in its polarity. Therefore, OA-LBNs nanofluid is well dissolved in PAO4 but poorly dissolved in RO, due to the similarity and intermiscibility principle. The tribomechanisms of OA-LBNs nanofluid in different base oils are schematically illustrated in Fig. 15. There is a competitive adsorption between the additive and the base oil molecules on the steel sliding surface. OA-LBNs nanofluid has a higher polarity than PAO4 base oil, and hence OA-LBNs nanofluid rather than PAO4 base oil is preferentially adsorbed onto the surface of the steel sliding pair. In the meantime, OA-LBNs nanofluid participates in trbochemical reactions in the presence of friction-induced heat and applied normal load to form a tribofilm composed of Fe2O3, B2O3, La2O3, ect, thereby improving the antiwear properties of the PAO4 base oil. Contrary to the above, DIOS and RO with polarity higher than that of OA-LBNs nanofluid become the dominant adsorbates on the rubbed steel surface during the competitive adsorption, while a small amount of OA-LBNs nanofluid adsorbed on the rubbed steel surface might disrupt the continuity of the oil film and damage the antiwear performance.