3.1 Characterization of NGOQDs, NGOQDs-MoS2, NGOQDs-Al2O3
Figure 4(a) shows TEM images of NGOQDs, NGOQDs-MoS2, NGOQDs-Al2O3. The HRTEM image shows clear lattice fringes of NGOQDs corresponding to the (002) plane of GO with a lattice spacing of 0.33 nm. NGOQDs-Al2O3, whose lattice spacing is 0.213 nm and corresponds to the (104) plane [35], while NGOQDs- MoS2, corresponds to the (100) and (102) planes with lattice spacings of 0.211 nm and 0.719 nm, respectively. XRD further confirms the crystalline nature of the NGOQDs, NGOQDs-MoS2, and NGOQDs- Al2O3 materials. The XRD image of NGOQDs gives an obvious peak at 31.7˚. In addition, NGOQDs-MoS2 and NGOQDs- Al2O3 materials also display characteristic peaks at 33˚, 34˚, and 42.6˚, respectively (Fig. 4b), representing nanometric lattice spacings that are in accordance with the TEM results (Fig. 4a). This reduction in the interlayer spacing is attributed to the π-π stacking of smaller GO layers with minimal structural defects or potential hydrogen bonding occurring among the O-containing functional groups encompassing the edges of the NGOQDs. The images in Figs. 4(a) and (b) indicate that NGOQDs possess a nanocrystalline center consisting of sp2 carbon atoms in GO. The Raman spectrum of NGOQDs exhibited Raman bands at approximately 1355 and 1555 cm− 1, which are known as the D and G bands, respectively. The ID/IG ratio of the intensities of these bands was 0.97, as shown in Fig. 4c. The amplification in the D band's intensity and the shift of the G band to a lower frequency were suggestive of the existence of a large number reactive groups and a high edge-to-area ratio. Liu et al. proposed that the electron donor ability of nitrogen heteroatoms leads to a downward shift of the G peak. The absence of a higher-order 2D band in the NGOQDs suggests that they possess a multilayered structure, which confers excellent lubricating properties [32]. The ID/IG values of NGOQDs-Al2O3 and NGOQDs-MoS2 increased from 0.97 to 1.04 and 1.06, respectively, indicating that the synthesis processes of NGOQDs-Al2O3 and NGOQDs-MoS2 resulted in an increase in surface defects on NGOQDs [35]. The FT-IR spectrum in Fig. 4(c) provides detailed information about the reactive groups present in the NGOQDs. FT-R spectroscopy shows that the peaks in the range of 3000–3500 cm− 1 correspond to stretching vibrations of N-H and O-H. The peaks appearing between 2854 and 2925 cm− 1 correspond to stretching vibrations of C-H, while the peaks corresponding to stretching vibrations of C-O and C-C appear at 1738 and 1571 cm− 1, respectively. The stretching vibrational peaks corresponding to C-H, C-OH, and C-O appear at 1406, 1072, and 917 cm− 1, respectively. The stretching vibrational peaks corresponding to C-N/N-H and C-O/C-N appear at 738 and 543 cm− 1, respectively. The most significant characteristic peak at approximately 385 cm− 1 correspond to the vibration of the Mo-S bond of NGOQDs-MoS2. The bending vibration of the Mo-S bond at 450 cm− 1, the stretching vibration of the Mo-S bond at approximately 520 cm− 1, and the stretching vibration of the S-S bond at approximately 670 cm− 1[36]. NGOQDs-Al2O3 has an Al-O vibration peak at 400 cm− 1, which is corresponding to the vibrations of the bonds between oxygen atoms and aluminum atoms.
XPS analysis provided additional information on the chemical state and composition of NGOQDs-MoS2 and NGOQDs-Al2O3 and given in Fig. 5. Based on the fitting results, NGOQDs-Al2O3 was consisted of C, N, O, and Al elements, while NGOQDs-MoS2 consisted of C, N, O, Mo, and S elements. These results indicated that NGOQDs were successfully synthesized with nanometer-sized Al2O3 and MoS2 using an improved solvent-thermal method. The C 1s spectrum was decomposed into three peaks at 284.4 eV, 282.4 eV, and 288.6 eV, corresponding to C-C/C = C, C-N/C-OH, and C = O [37, 38], respectively. During the synthesis of NGOQDs-Al2O3, the doping of N atoms destroyed the GO structure, which could have led to an increase of defects on the GO surface. This presumption was further validated by Raman spectroscopy findings shown in Fig. 4(c). Additionally, the introduction of N atoms also boosted the reactive sites on the GOQD surface, making it more favorable for adsorption to occur on metal surfaces. Meanwhile, the O 1s peak corresponded to C = O (530.5 eV) and C-O (532.3 eV) functional groups. Meanwhile, the Al 2p peak at 82.7 eV was combined with the O 1s peak at 531.4 eV, verifying the existence of nanometer-sized Al2O3 [39]. For NGOQDs-MoS2, the C 1s spectrum was decomposed into three peaks at 283.5 eV for C-C/C = C bonds, 282.3 eV/283.1 eV for C-N/C-OH bonds, and 284.9 eV for C = O bonds. Mo 1s and S 2p corresponded to Mo-S bond (229.3 eV) and S-S bond (163.6 eV), respectively, verifying the presence of nanometer-sized MoS2.
3.2 Tribological behavior of NGOQDs, NGOQDs-MoS2and NGOQDs-Al2O3
As shown in Fig. 6, the friction coefficient versus time curve and the mean friction coefficient and wear rate of the base fluid and nanofluids with various amounts NGOQDs concentrations are presented. It can be observed that the base fluid exhibits inadequate lubricating efficiency, with the highest coefficient of friction and wear velocity. The curves fluctuate greatly, with the coefficient of friction increasing consistently. When NGOQDs are incorporated into the base fluid, the tribological performance of the lubricant is enhanced to a certain extent. The introduction of NGOQDs reduces the coefficient of friction and wear velocity obviously. For NGOQDs nanofluid lubricants, the most suitable concentration of NGOQDs is 0.1 wt%. At the concentration of NGOQDs exceeds the optimal concentration of 0.1 wt%, the coefficient of friction marginally rises, and the wear rate increases comparatively (Fig. 6a and 6b). This is because the high concentration affects the dispersed stability of nanometer particles in nanofluids. Then these large-sized particles tend to aggregate, and these large particles act as abrasives, deteriorating the tribological performance of the lubricant. Compared with the results under NGOQDs lubrication conditions, the friction coefficient curve is smoother and the friction coefficient is significantly reduced when using a mixture of 0.1 wt% NGOQDs-MoS2 and NGOQDs-Al2O3 and shown in Fig. 6c. At this time, the friction coefficient and wear rate are decreased by approximately 32.3% and 54.50%, respectively. This is mainly because when using NGOQDs-Al2O3 nanofluid lubricants, spherical nano-Al2O3 converts sliding friction into rolling friction. While under NGOQDs-MoS2 nanofluid lubrication, it experiences sliding friction when in contact with other material surfaces and moving relatively. The synergistic use of NGOQDs-MoS2 and NGOQDs-Al2O3 in water-based lubricants shows significant advantages and its competitiveness in reducing friction coefficients compared to other studies.
3.3 Analysis of the worn surface
2D and 3D topography of the worn surfaces in various lubrication settings, as well as the corresponding surface profile curve average linear roughness (Ra), are characterized and shown in Fig. 7. When using NGOQDs nanofluid lubrication, the metal has a slight adhesion, and there are significant gouges and raised peaks caused by abrasive wear. When using 0.1wt% NGOQDs-MoS2, 0.1wt% NGOQDs-Al2O3, nanofluid lubrication, and their mixed lubrication, the surface wear is improved. The Ra value of the worn area decreases by about 4%, 39.50%, and 69% compared to the value under NGOQDs fluid lubrication. The 3D topography shows that when the mixed nanofluid of NGOQDs-MoS2 and NGOQDs-Al2O3, has both sliding and boundary lubrication, the wear loss of the worn interface is reduced additionally, and the surface quality is significantly improved, with the gully becoming sparse and the protrusion weakening [40].
By cutting the cross-section of the wear track with FIB and characterizing it using HRTEM, it was found that a multi-layered tribofilm was formed at the friction interface, and given in Fig. 8. The TEM morphology, SAED patterns of regions I, II, and III, and HRTEM image of region III all provided evidence that the tribofilm was composed of a physical adsorption layer and two chemical reaction films. The adsorption layer (region I) was uniform and dense in structure having a thickness of approximately 20 nm. It mainly consisted of nano-scaled NGOQD, MoS2, Al2O3 fragments, and organic components from the water-based fluid. The SAED pattern of regions I and II displayed amorphous rings accompanied by regular diffraction spots, which matched with the (104), (100), (102), (021), (110), and (420) crystal planes of α-Al2O3 [41]. The dark colored regions II and III close to iron supporting surface were the reaction layers. The depth of these layers was uneven, with region III being approximately 3 nm thick and mainly composed of MoS2, Al2O3, and Fe3O4. The physical adsorption layer and chemical reaction films generated on the interface during the sliding process effectively protected the Fe substrate.
3.4. Physicochemical reactions at friction interface
Through XPS analysis (Fig. 9), it is further understanding the friction chemical reaction between NGOQDs-MoS2 mixed NGOQDs-Al2O3 nanofluid and the Fe surface. Firstly, for the Fe element, the local high temperature and pressure conditions during friction caused it to react with atmospheric O2, resulting in the formation of FeO, Fe2O3, and Fe3O4. This oxidation reaction changed the valence state of Fe from 0 to + 2 and + 3. In terms of binding energy, the Fe2p3/2 and Fe2p1/2 peaks were situated at 706.3 eV and 719.5 eV, respectively, that which confirmed the oxidized state of the Fe element. In addition, wear surface was also found to have the occurrence of FeSO4 and Fe2(SO4)3, indicating a chemical reaction between Fe and SO42−. Secondly, for the N element, during the friction process, some nitrogen-containing groups were oxidized to -NO2 or other nitrogen oxides, causing a change in the valence state of N. In terms of binding energy, the N 1s peak was found at 402.6 eV, which was associated with nitrogen-containing oxidants. This suggested that during the friction process, nitrogen-containing groups in NGOQDs nanoparticles were oxidized. In addition, NGOQDs nanoparticles may also occupy some adsorption sites through covalent bonding or physical adsorption to interact with the metal surface. The binding energy of O1s spectrum at 531.6 eV and Al2p spectrum at 74.5 eV confirmed the presence of nano-Al2O3 [42]. However, Al2O3NP is highly resistant to chemical reactions and will not interact with the Fe matrix.
Finally, for the Mo and S elements, during the friction process, MoS2 nanoparticles reacted with the metal surface to form new MoO3 particles. In terms of binding energy, the S 2p peak of MoS2 was located at 162.6 and 163.8 eV, while the Mo 3d peak of newly formed MoO3 was located at 228.7 eV. This suggested that during the friction process, the valence state of Mo changed from + 4 to + 6, while the valence state of S changed from − 2 to 0 or higher. In addition, wear surface was also found to have the occurrence of FeSO4 and Fe2(SO4)3, indicating a possible chemical reaction between MoS2 nanoparticles and the metal surface during friction. The XPS analysis results provide valuable information for understanding this friction chemical reaction. In summary, the effect of NGOQDs-MoS2 mixed NGOQDs-Al2O3 nanofluid on tribochemical and friction film formation at the friction interface is significant. Through complex physical and chemical processes, these nanoparticles can improve wear and friction resistance.
The MD simulation outcomes showed the distribution of MoS2, Al2O3, and NGOQDs on the Fe surface in the simulated terminal status (1000 ps). It is obvious that Al, S, N, and O atoms all displayed a propensity to migrate towards the Fe surface. The potential energy changes of both systems stabilized after about 200 ps, indicating that the simulation system attained a state of relative equilibrium. The interactions among the added various nanoparticles and the Fe surface are stable. In equilibrium, the average potential energy values of MoS2 + Al2O3 + NGOQD systems was negative, at -2353.6 kcal·mol− 1. Therefore, compared to MoS2 + N-CQD [11], there existed greater attractive interplay within the MoS2 + Al2O3 + NGOQD system. It can be inferred that the friction film formed by the mixed nanofluid has more stable and dense properties, which can help improve lubrication properties. The adsorption characteristic of iron atoms to the molecules, which is crucial to the chemical interactions and film-forming processes near the contact interfaces, can be observed in their migration to the substrate. Therefore, MD simulation method can be used to study the interactions and lubrication performance between nanoparticles and metal surfaces. The use of mixed nanofluids may provide new solutions for the field of tribology, helping to form more stable and dense films and thereby improving lubrication performance.
3.5 Lubrication mechanism of NGOQDs-MoS2 mixed NGOQDs-Al2O3 nanofluid
Based on the exploratory and MD simulation findings, the lubricating process of NGOQDs-MoS2 mixed NGOQDs-Al2O3 nanofluids was proposed and discussed. The mechanism includes the following three aspects: during friction, nanometer-sized particles move rapidly between metal surfaces, improving the surface quality through a polishing effect. When the load is within a certain range, this effect is enhanced. MoS2 nanoparticles exhibit interlayer sliding effects under pressure and shear forces, partially offloading the friction force to the internal friction of the nanosheet. Simultaneously, spherical NGOQDs and Al2O3 nanoparticles roll between the friction surfaces, converting sliding friction into rolling friction. Consequently, the MoS2 nanosheets' interfacial moving action is enhanced by this action of rolling. In terms of physical and chemical properties, spherical NGOQDs and Al2O3 nanoparticles separate different MoS2 single layers, weakening phenomena such as chemical reactions, entanglement, and others. MoS2 nanosheets also prevent harder NGOQDs and Al2O3 particles in adhering on the Fe substrate, ensuring the inherent roll of NGOQD parts and reducing wear on their sliding interface. These consequences combine to contribute to the excellent lubricating properties of nanofluids. In addition, friction on the interface results in the deposition and adsorption of nanometer-sized particles and organic molecules from the nanofluid onto the metal surface, forming a protective tribofilm. This film is composed of several broken and peeled-off nanoparticles, followed by MoS2 particles attaching to the metal surface through S atoms. NGOQDS contains O-based groups (hydroxy group, carboxyl group, ester group) and N-based groups (C–N and –NO2) that may utilize van der Waals forces and hydrogen bonds to attract iron atoms, thereby improving the adsorption strength of NGOQDS on the Fe matrix. Moreover, the metal surface generates a large number of positive charges, making it easier for these O- and N-based groups to adsorb. Therefore, the tribofilm formed by NGOQDs-MoS2 mixed NGOQDs-Al2O3 nanofluids is denser and more robust. This adsorption ability helps improve lubricating performance. In summary, the lubrication mechanism of NGOQDs-MoS2 mixed NGOQDs-Al2O3 nanofluids includes high-speed movement, interlayer sliding effects, rolling friction, physical and chemical properties, as well as tribochemical effects. These effects combine to contribute to the excellent lubricating performance of nanofluids and protect metal surfaces from severe wear.