3.1 Morphology and structure for RGO-MoS2 and OA-RGO-MoS2
Fig. 2 shows the morphology of RGO-MoS2 and OA-RGO-MoS2 composite materials. As shown in Fig. 2 (a), (b) TEM images, and (e), (f) shown in SEM topography, MoS2 is in the shape of nano-flower balls and is wrapped in slightly wrinkled, tulle-like RGO. The surface of the nanosphere is relatively smooth, and the flakes overlap each other and grow in all directions, and the oleic acid modified of RGO layer surface more smooth level off. MoS2 is highly dispersed on the lamellar RGO, mainly because MoS2 and RGO have similar two-dimensional layered structure, and RGO plays a role of nucleus and epitaxial growth substrate in the formation of molybdenum disulfide. Under this kind of hydrothermal conditions, GO is gradually reduced to RGO, while MoS2 precursor is adsorbed on the defect position on the surface of RGO and reacts with the vulcanizing agent and a reducing agent to nucleate, and then molybdenum disulfide is attached to RGO for epitaxial growth to form a layer [22]. The HRTEM images in Fig. 2 (c) and (d) can intuitively display the layered structure of RGO-MoS2 and OA-RGO-MoS2 composites. The lattice spacing of the MoS2 (002) crystal plane of the RGO-MoS2 composite is about 0.67 nm, and the OA-RGO-MoS2 material is about 0.70 nm, its slightly increase after OA modification. According to the above analysis, RGO-MoS2 composite was successfully prepared by the hydrothermal method.
XRD spectra of RGO-MoS2 and OA-RGO-MoS2 are shown in Fig. 3. It can be seen from the figure 3 that the two diffraction peaks of 2θ=23.3 ° and 2θ=43.1 ° belong to the (002) and (100) lattices of graphene respectively. The diffraction peaks near 33.5 ° and 57.4 ° belong to (100) and (110) planes of MoS2, respectively. The diffraction peaks of OA-RGO-MoS2 and RGO-MoS2, which belong to the (002) crystal plane of MoS2, appear at 13.2 ° and 13.7 ° respectively, which is inconsistent with the interlayer spacing of standard MoS2 (002) diffraction peak at 14.4° (0.62 nm). This phenomenon is attributed to MoS2 material with low stacking degree and extended layer spacing synthesized by hydrothermal reaction [23-24]. Compared with MoS2, the (002) crystal plane of the two has a low angle shift, and the (002) crystal plane was the position of the S-Mo-S layer of molybdenum disulfide, and its peak intensity is proportional to the stacking degree of lamellae. The MoS2 (002) peak strength of OA-RGO-MoS2 is weaker than that of RGO-MoS2. It indicates that the layered stacking degree of OA-RGO-MoS2 is inferior to the RGO-MoS2. RGO-MoS2 (002) crystal plane of OA-RGO-MoS2 shifts to a lower angle, indicating that the layer spacing of OA-RGO-MoS2 is larger than that of RGO-MoS2 after modification by OA. In other words, the successful insertion of the O atom into the MoS2 skeleton causes the interlayer expansion of S-Mo-S, which leads to the increase of interlayer spacing of composite materials [25]. This result is consistent with SEM and HRTEM.
To further characterize the structure and composition of the composite nanomaterials, Raman spectroscopy was used to analyze the composite nanomaterials. Fig. 4 displays the Raman spectra of RGO, RGO-MoS2 and OA-RGO-MoS2. The peaks near 1347 cm-1nd 1579 cm-1 correspond to the D peak of sp3 hybrid disordered carbon or defective carbon and the G peak of sp2 hybrid fossil ink carbon respectively. Generally speaking, the ratio of peak intensity ID/IG represents the defect degree of carbon materials. ID/IG of RGO is 1.1234, that of OA-RGO-MoS2 is 1.1845, and that of RGO-MoS2 is 1.2137. The ID/IG values of RGO-MoS2 and OA-RGO-MoS2 are higher than those of RGO, indicating that the volume of sp2 atom decreases in the redox process. It shows that the hydrothermal reaction reduces the graphene oxide to a greater extent [26]. The value of ID/IG of OA-RGO-MoS2 is lower than that of RGO-MoS2, which indicates that its graphitization degree is higher and its surface defects are reduced, which is beneficial to improving its tribological properties [27].
RGO-MoS2 and OA-RGO-MoS2 shown E12g and A1g characteristic Raman peaks of MoS2 near 374 cm-1 and 400 cm-1, respectively, while the E12g and A1g characteristic Raman peaks of MoS2 were not observed in RGO. The research shows that the stronger peak value of A1g, the stronger van der waals force between MoS2 layers is [28]. It can be seen that the van der Waals force between MoS2 layers in OA-RGO-MoS2 is weaker than RGO-MoS2. Due to the weak interaction force between layers, MoS2 is proned to slide between layers during friction, showing excellent lubrication performance.
4.2 Thermogravimetric analysis of RGO-MoS2 and OA-RGO-MoS2
In an air atmosphere, the thermogravimetric analysis results of MoS2, RGO, RGO-MoS2, and OA-RGO-MoS2 are shown in Fig. 5. The component mass ratio of the composite material is calculated by the mass percentage of the sample at 700 ℃. The calculation formula is [29]:
The result data are shown in Table 3 :
Table 3 TGA data of MoS2, RGO, RGO-MoS2 and OA-RGO-MoS2
Sample
|
Residual mass ratio wt %
|
MoS2 wt %
|
RGO wt %
|
MoS2
|
86.0
|
/
|
/
|
RGO
|
2.6
|
/
|
/
|
RGO-MoS2
|
60.0
|
56.7
|
43.3
|
OA-RGO-MoS2
|
64.0
|
63.3
|
36.7
|
Combined with table 3 and Fig. 5 (a), it can be seen that RGO-MoS2 and OA-RGO-MoS2 are heated and burned in the air atmosphere, and the remaining mass is 60.0% and 64.0% respectively. The MoS2 is heated in an air atmosphere and undergoes an oxidation reaction, and some S atoms are replaced by O atoms, thus reducing the mass. The vast majority of RGO is oxidized to CO2 by heating and combustion, and only a small part of amorphous carbon is difficult to be oxidized, so there is still 2.6% mass residue after 700 ℃. The MoS2 loading in RGO-MoS2 and OA-RGO-MoS2 composites is calculated to be 56.7% and 63.3% respectively. The weight loss rate of RGO-MoS2 is higher than that of OA-RGO-MoS2. The reason is that OA-RGO-MoS2 is modified by oleic acid, which makes the RGO carrier load more MoS2. Because MoS2 has good thermal stability, it only loses a small amount of mass when heated and burned in the air atmosphere. Therefore, the weight loss rate of OA-RGO-MoS2 loaded with more MoS2 is lower than that of RGO-MoS2, and first reach a stable mass around 550 ℃.
It can be seen from Fig. 5 that the adsorption water is mainly removed at about 100 ℃, the main mass loss is attributed to the decomposition of unstable oxygen-containing functional groups between 100 ℃ to 250 ℃, and the decomposition of relatively stable oxygen-containing functional groups between 250 ℃ to 450 ℃. Mass loss from 450 ℃ to 600 ℃ is ascribed to RGO combustion. The maximum mass-loss rates of OA-RGO-MoS2 and RGO-MoS2 are 2.865 %/min and 2.067 %/min, respectively, and the temperature at this time is 497.13 ℃ and 524.56 ℃, respectively. The analysis reason is that after OA modification, more organic functional groups are grafted on the surface of OA-RGO-MoS2, and it is more fully heated and burned in the air atmosphere, which also indicates the thermal stability of RGO-MoS2 is better than that of OA-RGO-MoS2.
4.3 Elemental analysis of RGO-MoS2 and OA-RGO-MoS2
To quantitatively analyze the kinds and contents of elements in RGO-MoS2 and OA-RGO-MoS2, a comprehensive analysis is made by using an element analyzer (total element content) and XPS (surface element content). The elemental analysis of RGO-MoS2 and OA-RGO-MoS2 is shown in table 4. Table 4 XPS surface content data shows that the content of C, O, and Mo in OA-RGO-MoS2 is higher than that in RGO-MoS2, while the content of the S element is lower than that in RGO-MoS2. The content of C and H elements in OA-RGO-MoS2 is higher than that in RGO-MoS2, and the content of S and N elements is lower than that in RGO-MoS2. The increase of molybdenum and oxygen content may be due to OA modification of OA-RGO-MoS2, which leads to more MoS2 being loaded on the RGO carrier. A large number of O atoms are inserted into the skeleton of molybdenum disulfide, resulting in a Mo-O bond, which leads to the expansion of the S-Mo-S interlayer. And H and N mainly come from -COOH and -OH in OA or NH4+ in raw material ammonium molybdate. The experimental results are in good agreement with XRD, XPS, and TG analysis results.
Table 4 Elemental analysis results of RGO-MoS2 and OA-RGO-MoS2
Sample
|
XPS
|
Elemental analyzer
|
C/ %
|
O/ %
|
S/ %
|
Mo/ %
|
C/ %
|
S/ %
|
H/ %
|
N/ %
|
RGO-MoS2
|
29.94
|
16.66
|
33.21
|
20.19
|
13.20
|
29.76
|
0.91
|
1.94
|
OA-RGO-MoS2
|
32.02
|
17.53
|
29.78
|
20.67
|
17.33
|
24.80
|
0.94
|
1.83
|
4.4 XPS analysis of RGO-MoS2 and OA-RGO-MoS2
Fig. 6 shows the XPS spectra of RGO-MoS2 and OA-RGO-MoS2. For C1s spectra (a) and (e), the peak located at 284.8 eV belongs to the C-C bond; The characteristic peaks located at 285.96 eV and 286.39 eV belong to C-O-C and C-O; The characteristic peaks located at 288.53 eV and 288.71 eV belong to O-C=O and C=O.
For O1s spectra (b) and (f), the characteristic peaks located at 530.43 eV or 530.95 eV are ascribed to Mo-O. The binding energy peaks of C-O-C and C-O are located at 531.50 eV and 532.04 eV, respectively belong to; The peaks located at 532.69 eV and 533.40 eV are attributed to O-C=O and C=O. It can be seen from the figure that, after OA modification, the content of the Mo-O bond increases. Mo-O bond mainly comes from the insertion of O atoms into the MoS2 skeleton, which causes the interlayer expansion of S-Mo-S and further increases the interlayer spacing of composite materials. This result also verifies the XRD analysis.
For S2p spectra (c) and (g), the peaks located at 161.22 eV and 161.92 eV belong to S2p3/2 orbit. The binding energy at 162.42 eV and 163.14 eV belong to S2p1/2 orbit, indicating that S element mainly exists in the form of S2- in the composite.
For Mo3d spectra (d) and (h), the peaks located at 228.25 eV and 229.01 eV belong to Mo3d5/2 orbit. The peaks located at 231.73 eV and 232.16 eV belong to the Mo3d3/2 orbit. The peaks located at 225.67 eV and 226.44 eV belong to S2s orbit. This indicates that the Mo element mainly exists in the form of Mo4+. In addition, characteristic peaks located at 235.19 eV and 236.09 eV belong to Mo6+ and are mainly formed by oxidation on the surface of products in the air [30-31].
4.5 Tribological properties of 10# WO+RGO-MoS2 and 10# WO+OA-RGO-MoS2 oil samples
Fig. 7 shows the friction coefficient (FC) variation with time diagram of RGO-MoS2 and OA-RGO-MoS2 oil samples with different mass fractions. Fig. 8 shows the average friction coefficient (AFC) variation with additive content diagram of RGO-MoS2 and OA-RGO-MoS2 oil samples. Combined with Fig. 7 (a) and Fig. 8, it can be seen that AFC first declined and then increased with the increase of the added concentration of RGO-MoS2 in 10# WO. When 0.4 wt% RGO-MoS2 is added, FC was significantly lower than that of blank 10# WO, and the running-in period is shorter and the curve trend was relatively stable. At this time, AFC is 0.068, 21.8% lower than that of 10# WO (0.087). This may be because a small amount of RGO-MoS2 entered the friction interface, and gradually adsorbed on the surface of the contact surface micro-concave, fill the micro-pits of the friction interface, friction chemical reaction occurs in the friction process and then form a complex lubricating film to reduce friction effect. After increasing the concentration of RGO-MoS2, AFC began to rise gradually. When 1.0 wt% is added, AFC is 0.096, which is 10.3% higher than that of 10# WO. In addition, the curve fluctuated sharply, the run-in period is longer, and the anti-friction effect is not obvious. This may be because RGO-MoS2 with a higher added concentration is easy to aggregate into large particles in 10# WO, which breaks through the oil film thickness and enters the friction interface to form abrasive wear, failing to achieve the effect of friction reduction.
From Fig. 7 (b) and Fig. 8, It can be seen that AFC decreases first and then increases after OA-RGO-MOS2 is added. AFC of all oil samples added with OA-RGO-MOS2 is less than that of pure 10# WO, and the curve is relatively stable. The reason may be that the expansion of layer spacing reduces the inter-layer interaction. Thus, the anti-friction performance of the material is improved. When 0.2 wt% OA-RGO-MOS2 is added, the friction reduction effect is the best. The AFC is 0.058, 33.3% lower than that of pure 10# WO. When 0.6 wt% and 0.8 wt% are added, the AFC is almost the same, 0.07556 and 0.07548 respectively. However, it is obvious from Fig. 7 (b) that the FC curve of 0.8 wt% has a more obvious upward trend after 1000 s. At 1.0 wt%, the AFC of OA-RGO-MoS2 is 0.085, which is lower than that of RGO-MoS2 (0.096) at this concentration, indicating that after OA modification, the dispersion of OA-RGO-MoS2 in 10# WO become better for the surface grafting of oleophilic groups. Even in a larger concentration, it is not easy to aggregate, and because OA-RGO-MoS2 is adsorbed on the interface of friction pair, it participates in the formation of lubricating film and enhances the strength of oil film, thereby to reduce friction [32].
4.6 Effects of RGO-MoS2 and OA-RGO-MoS2 on wear
After the friction and wear experiment, removed the steel ball and ultrasonically cleaned for 30 min in a beaker containing acetone. After it is naturally dried, using a universal tool microscope to photograph the scar morphology of the test ball and measure its wear scar diameter, to observe the change of wear amount with the increase of additive concentration. The average wear scar diameter(AWSD) variation with additive content and the wear scar morphology are shown in Fig. 9. Fig. 9 shows that the wear scar of the lower test ball after rubbing the oil sample of OA-RGO-MoS2 are shallower and narrower than those of RGO-MoS2. With the increase of RGO-MoS2 and OA-RGO-MoS2 addition concentration, the changing trend of the AWSD of steel balls is firstly declined and then increased. On the other hand, when the concentration of RGO-MoS2 is 0.2 wt%, the AWSD decreases to 0.559 mm, which is 12.4% lower than the AWSD of 10# WO of 0.638 mm. The AWSD of OA-RGO-MoS2 at 0.2 wt%, 0.4 wt%, and 0.6 wt% is 0.548 mm, 0.550 mm, and 0.551 mm, respectively. The minimum AWSD value is 14.1% lower than that of 10# WO. If the concentration of OA-RGO-MoS2 is 1.0 wt%, it can be easily seen that the diameter of the wear scar is obviously larger than that of 10# WO. When the additive content is less than 1.0 wt%, compared with OA-RGO-MoS2, OA-RGO-MoS2 has smaller wear scar diameter, shallower and finer wear marks than RGO-MoS2. The analysis reason may be that OA-RGO-MoS2 has been modified by OA, and its physical and chemical properties are better. Moreover, it has lipophilic groups which are better to disperse in 10# WO. Therefore, the wear resistance of RGO-MoS2 is improved compared with that of unmodified RGO-MoS2. Nevertheless, when the addition concentration is too higher, it is easier to form large agglomeration particles, break through the oil film thickness, form abrasive wear, and unable to play an anti-wear role.
4.7 Analysis of anti-friction and anti-wear mechanism
To explore the anti-friction and antiwear mechanism of RGO-MoS2 and OA-RGO-MoS2, the upper test balls with the best effects were selected after the friction experiment with the addition concentration of 0.2 wt%, the addition concentration of 1.0 wt% and the blank 10# WO, which were characterized magnified 1000 times by SEM. From Fig. 10 (a) can see that under the lubrication of 10# WO, the furrow on the wear scar surfaceis wide and irregular, and a large number of irregular peeling pits of adhesive wear appears on the surface of the wear mark, indicating that the wear mode is mainly adhesive wear. It can be seen from Fig. 10 (b) that when 0.2 wt% RGO-MoS2 is added, the wear marks become thinner, shallower, and more regular, with a few peeling pits appear on the surface of the wear marks. It can be seen from Fig. 10 (c) that when 0.2 wt% OA-RGO-MoS2 is added, the surface of wear marks is smooth and flat, without peeling pits, and the existence of wear marks can hardly be seen, which shows that OA-RGO-MoS2 has more excellent anti-friction and anti-wear effects. It can be judge from Fig. 10 (d) and Fig.10 (e) that when the additive amount is 1.0 wt%, the furrows on the wear scar surface of increasing RGO-MoS2 and OA-RGO-MoS2 become thicker and denser. Meanwhile, the irregular spalling pits and agglomerated wear fragments were appeared. This indicates that the composite material has been added excessively, and the excessive composite material is converted into abrasive grains in the lubricating oil system, which increases the wear of abrasive grains and cannot play a role.
Table 5 Analysis of types and contents of wear trace elements on the surface of steel balls
Sample
|
Element content
|
C/ %
|
O/ %
|
S/ %
|
Cr/ %
|
Fe/ %
|
Mo/ %
|
0.2 wt% RGO-MoS2
|
1.80
|
3.83
|
0.02
|
1.37
|
92.98
|
/
|
0.2 wt% OA-RGO-MoS2
|
8.17
|
12.79
|
0.22
|
2.22
|
76.60
|
/
|
1.0 wt% RGO-MoS2
|
6.32
|
7.39
|
/
|
0.96
|
85.18
|
0.15
|
1.0 wt% OA-RGO-MoS2
|
9.17
|
7.26
|
/
|
1.62
|
81.68
|
0.27
|
Fig. 11 and Table 5 show the EDS analysis results of the wear scar surface of the steel ball. A friction film including C, O, Fe, and Cr elements is detected in the wear scar of all samples, among which Fe and Cr elements all are from the steel ball itself, and C and O elements stem from the lubricating film formed by the interaction of additives and lubricating oil during the friction process. At the same addition concentration, the content of C and O elements on the surface of wear scar with OA-RGO-MoS2 is higher, while it is short of the content of Fe element. In addition, S and Mo elements appeared on the wear scar surface, which indicated that the additives participated in the friction chemical reaction occurred on the wear scar surface to form a friction reaction film. The proof of this result also can be found in the XPS analysis.
To further analyze the anti-friction and anti-wear mechanism of RGO-MoS2 and OA-RGO-MoS2, the XPS analysis carried out on the ware scar surface after friction test with the best anti-friction and anti-wear effect at the addition concentration of 0.2 wt%, and the results are shown in Fig. 12.
Fig. 12 (a) and Fig. 12 (f) are C1s spectra of wear scar on the surface of the upper test ball after adding 0.2 wt% RGO-MoS2 and OA-RGO-MoS2 to the WO for friction test.
The largest characteristic peak at 284.8 eV belongs to the C-C bond, and the middle characteristic peak at 285.20 eV or 285.26 eV belong to the C-O-C bond and C-O bond. The smallest peak at 288.16 eV or 288.18 eV belong to the O-C=O bond and C=O bond [33]. Its main source is organic matter in lubricating oil or lubricating film formed by the oil and additives during friction.
Fig. 12(b) and Fig. 12 (g) are O1s spectra of wear scar. The peaks of 529.84 eV or 530.03 eV belong to the metal oxides, mainly Fe oxides. The characteristic peaks near 531.34 eV-531.48 eV belong to the C-O bond and C-O-C bond. The peaks near 532.33 eV-532.73 eV belong to the C=O and O-C=O bond.
Fig. 12(c) and Fig. 12(h) are Fe2p spectra of wear scar. Among them, nearby 710.31 eV-710.38 eV peaks belong to the FeO characteristic peaks, while 711.19 eV-711.58 eV peaks belong to the Fe2O3 characteristic peaks, and 724.95 eV-725.34 eV neighbouring characteristic peaks belong to the Fe2p1/2 orbit. The peak areas of FeO and Fe2O3 are designated as A1 and A2 respectively. By comparing the ratio of Fe2+ to Fe3+ (the area of A1 to A2 in the map), it can evaluate the wear situation of the steel ball surface [34-35]. When the content of RGO-MoS2 is 0.2 wt%, A1:A2 in the map is 0.95. When the content of OA-RGO-MoS2 is 0.2 wt%, A1:A2 value is 1.28. The above results show that the anti-friction and anti-wear effect of OA-RGO-MoS2 is better than that of RGO-MoS2, and the addition of OA-RGO-MoS2 to 10# WO improves the anti-friction and anti-wear effect of lubricating oil.
Fig. 12(d) and Fig. 12 (I) are Mo3d spectra of wear scar. The peak of 232.30 eV or 232.46 eV belongs to the Mo-S bond, which indicates the existence of MoS2 on the friction surface. The peak nearby 235.40 eV-235.61 eV belongs to the Mo-O bond of MoO3, that is, Mo is oxidized during friction.
Fig. 12(e) and Fig. 12(j) are S2p spectra of wear scar. The feature peak at 162.68 eV-163.48 eV belongs to S2- in MoS2. The characteristic peak vicinity 168.15 eV-168.95 eV belongs to S6+, which indicates that there is not only Fe oxide, but also FeSO4 or Fe2(SO4)3 on the surface of the wear scar. That is, S is oxidized during friction.
Based on the above analysis, the lubrication mechanism of composite materials as additives is put forward. Firstly, the additive enters the contact surface of the friction pair with 10# WO, and under the action of higher contact pressure, the additive is adsorbed on the surface of the friction pair to form an adsorption protective film, which fills the defect areas such as micro cracks and micro pits on the surface of the friction pair, playing a role in repairing the worn surface during friction. Secondly, in the process of friction reaction, MoS2 and RGO in the composite material play a synergistic lubricating effect, and a lubricating film containing iron oxide, iron sulfur oxide, molybdenum oxide, molybdenum disulfide and carbonaceous materials is formed on the surface of the friction pair, which avoids the direct contact of the friction pair and plays a role in reducing friction and antiwear. For RGO-MoS2 and OA-RGO-MoS2 composites, their sizes are smaller than the oil film thickness at low additive concentration, which can cause interlayer slip between friction interfaces and play a role in reducing friction. When the additive concentration is too high, excessive composite materials will agglomerate and turn into abrasive particles in the lubricating oil system, which breaks through the oil film thickness, increasing the wear of abrasive particles, and cannot play an anti-wear role. However, on the whole, OA-RGO-MoS2 has better comprehensive lubricating performance.