a) Characterisation of MoS2 nanoparticle and nanolubricant
i. Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive X-Ray Spectroscopy (EDS) of MoS2 nanoparticle.
Figure 2 shows the morphology of MoS2 nanoparticles in (a) 50000x and (b) 100000x magnifications. The nanoparticles are equally distributed, well-faceted, multiple densely grown, semi-vertically and interleaving lamellar nanosheets with rough edges. These images confirmed the nanosheet morphology of the formed MoS2. Figure 2 (b) displays the non-uniformed nanosheets with sizes approximately 150nm-300nm. However, several nanosheets are stacked up and seen agglomerated. The uniform and homogeneous distribution of molybdenum and sulfur across the nanosheet are shown in high-resolution EDS elemental mapping in Figures 2 (c) and (d). Figure 3 shows the sample's EDS spectrum of Mo (c) and S (d). Table 1 reveals the quantitative surface analysis of EDS carried out on the MoS2 nanoparticles reveals the existence of sulfur and molybdenum in terms of atomic and weight percentage of constituents.
Table 1: The elemental distribution of MoS2 nanoparticles
Element
|
Weight%
|
Atomic%
|
S
|
38.04
|
64.75
|
Mo
|
61.96
|
35.25
|
Totals
|
100.00
|
ii. X-ray Diffraction (XRD) of MoS2 nanoparticle
Figure 4 shows the XRD diffraction peaks of MoS2 at 2 = 14.5°, 33.0°, 39.3°, 58.5°, and 69.7°, which can be referred to as the (002), (100), (103), (110), and (201) peaks of pure hexagonal MoS2 phase according to JCPDS card no.371492, which are in accordance with previous studies [14, 15]. Peak broadening is seen, implying that the crystalline size is very small. For (100) and (103) XRD peaks, the magnitude dissimilarity between the reference pattern in the JCPD card and the synthesised nanoparticle is due to differences in texture of crystallite size difference and the size of the scattering domains. Other peaks of separate phases or impurities are not found in the XRD patterns, indicating that the crystal structure of MoS2 nanosheets is in high purity form.
iii. Fourier-Transform Infrared Spectroscopy (FTIR) of MoS2 nanoparticle.
Figure 5 displays the FT-IR spectra of the MoS2 nanoparticle. The peaks were calculated or confirmed using the FTIR application library and the journals. Both samples have strong absorption bands at 485 cm-1, 905 cm-1, 1120 cm-1, and 1665 cm-1. The Mo-S bond is responsible for the band at 485 cm-1, while the S-S bond is responsible for the band at 905 cm-1. The stretching vibrations of the hydroxyl group and Mo-O vibrations are responsible for the absorption band between 1120 cm-1 and 1665 cm-1 [16]. By revealing the functional groups present in the study, the FT-IR spectra further confirm the formation of MoS2.
iv. Visual Observation and Zeta Potential of MoS2 nanolubricant
The stability of the MoS2 nanolubricants against sedimentation via visual observation showed that the four various concentrations of 0.1wt. %, 0.05wt. %, 0.01wt. %, and 0.005wt. % of MoS2 based nanolubricants were stable against sedimentation for 21 days (Figure 6). The zeta potential is significant as its magnitude is used to determine the stability of colloidal dispersions. As shown in Table 2, the zeta potential value of the MoS2 nanolubricant with 0.05wt. %, 0.01wt. % and 0.005wt. % of MoS2 concentrations is higher than 60 mV, indicating the nature of MoS2 nanoparticle to be extremely stable in the nanolubricant. While 0.1wt. % shown lower zeta potential value, indicating poor stability in the engine oil as the nanoparticle concentration is the highest.
Dispersion with a higher zeta potential (negative or positive) is electrically stable, whereas those with a lower zeta potential agglomerate or flocculate. In general, the arbitrary value of 25 mV (positive or negative) distinguishes low-charged exterior from a highly charged exterior. Dispersion with a zeta potential of 40 to 60 mV is thought to be fairly consistent, whereas those with a zeta potential of more than 60 mV are considered to be extremely stable. The value of the zeta potential is directly proportional to the dispersion stability of the materials.
Table 2: Zeta potential magnitude of the MoS2 nanolubricant with different concentrations
MoS2 concentration in nanolubricant (wt. %)
|
Zeta potential (mV)
|
0.005
|
372.8
|
0.01
|
279.7
|
0.05
|
158
|
0.1
|
37
|
b) Tribological Analysis of MoS2 nanolubricant
The coefficient of friction of MoS2 nanolubricant with varying concentrations of MoS2 nanoparticle wt. % in the base oil is shown in Figure 7. Without any nano-additives, the friction coefficient of the base oil was 0.0946. The friction coefficient of base oil with MoS2 nanoparticles was found to be lower than pure base oil. In comparison to the base oil, the COF was reduced to 2%, 10.25 %, 19.24 %, and 11.73 % for 0.1wt. %, 0.05wt. %, 0.01wt. %, and 0.005wt. %, respectively. When the MoS2 percentage in the nanolubricant was increased from 0.01wt. %, some MoS2 nanoparticles agglomerate, resulting in larger secondary particle size. As a result, friction and wear would worsen, resulting in an increase in COF. The lowest concentration of MoS2 nanoparticles, 0.005wt. % was insufficient to cover the entire contact surface, resulting in a greater COF than 0.01wt. % MoS2. This suggests that 0.01 wt.% of MoS2 nanolubricant is the best concentration for reducing COF. The sliding of nanosheets causes this phenomenon at asperities and deformed surfaces of individual nanosheets at interfaces to produce a protective layer known as the tribofilm, which decreases the COF[17-19]. The development of a tribofilm comprising nanosheets aids in reducing the friction caused by the individual layers of nanosheets slipping.
The findings show some damage caused by adhesive wear under the applied stress due to continuous sliding friction. Because of their higher surface energy and many dangling S bonds, MoS2 nanoparticles can readily react and produce an abrasion-resistant protective coating at contacting surfaces. The MoS2 nanosheets will be captivated into frictional surfaces, generating an adsorbed film and forming S-O or S-Fe bonds. The oxide layer on the substrate's surfaces provided the O and Fe. The adsorbed coating eliminated direct contact between frictional contacts and increased tribological characteristics [20]. The effect of firm boundary lubrication between the frictional pairs develops a protective tribofilm. Due to the adequate lubricity, this may result in an excellent ability to withstand shear failure.
Figure 8 presents wear scar diameter details of MoS2 nanolubricant with various MoS2 nanoparticle wt.% in base oil concentrations. The graphic of wear scar diameter created on the ball bearing during the tribological trials is depicted in figure 9. Without nanoparticles, the WSD for the base oil was 0.0953. When the ball bearing was tested for tribological aspects, the addition of MoS2 nanoparticles in the base oil minimises the WSD. In comparison to the base oil, the WSD is reduced by 1.8 %, 10.6 %, 19.52 %, and 16.5 % for 0.1wt. %, 0.05wt. %, 0.01wt. %, and 0.005wt. %, respectively. The aforementioned information demonstrates that 0.01wt.% MoS2 produces the best WSD in tribological analysis. Figure 9 shows that nanolubricant with higher concentrations of base oil (A), 0.1wt. % (B) and 0.05wt. % (C) exhibited darker concentric grooves, indicating abrasive wear, but smaller percentages of MoS2, such as 0.01wt. % (D) and 0.005wt. % (E) exhibited smoother wear tracks, indicating decreases in the contact surfaces between the steel balls. The darker furrow is deeper, whereas the brighter furrow is shallower. Suresha et al. [21] made a similar observation. These ridges are responsible for depositing the MoS2 nanoparticles firmly on the wear surface, which results in a reduction in wear. Huang et al. addressed a similar process with graphite sheets [22]. Hernandez et al. demonstrated that nanoparticles aggregate in the wear scar region in an experiment [23]. In comparison to the base oil containing MoS2 nanoparticles, the wear scar image of the ball bearing lubricated by the base oil displayed many broad and deep ridges. The many MoS2 nanosheets, we believe, penetrate more easily into the lubricant contact. Furthermore, nanosheets can create a continuous layer on rubbing surfaces because of their excellent adherence to contacts, improving tribological qualities. This phenomenon is known as the mending effect, where MoS2 nanoparticles settle and occupy the grooves on the worn surface scratches of the rubbing surfaces, avoiding direct contact between the two surfaces and lowering the WSD. Those described above experimental tribological results imply that with an ideal concentration of 0.01wt% MoS2 in the engine oil, both COF and WSD can be significantly improved.
According to the initial study, the formation of tribofilm and the mending effect is the fundamental mechanism for decreasing the frictional wear in the case of MoS2 based nanolubricant. Due to the planar geometry of MoS2, it may readily slide between the oil's surfaces. Furthermore, MoS2 will cluster or agglomerate together and coagulate as the concentration increases, increasing wear and friction between surfaces. The segregation of interlayers into distinct layers is attributed to the wear process of MoS2 nanosheets due to weaker van der Waals or Coulombic repulsive interactions at contact compulsion [24, 25]. These findings most likely indicate that adding MoS2 to the lubricant significantly improves the nanolubricant's tribological capabilities.
c) Oxidation Analysis of MoS2 nanolubricant
In the automobile industry, lubricants endure oxidation caused by intense temperature, high load, and air. Oxidation accelerates the degradation process of the base oils and additives, which decreases its performance, efficiency and life-span. The results of the OIT of the nanolubricants are shown in Figure 10. In comparison to the base oil, the OIT was improved by 12.17 %, 65.68 %, 61.15 % and 25.46% for 0.1wt. %, 0.05wt. %, 0.01wt. %, and 0.005wt. %, respectively. The nanolubricant with 0.05wt. % of MoS2 nanoparticles showed the highest OIT compared with other concentrations of nanolubricant.
Due to the synergistic effect of MoS2 with Zinc-dialkyl dithiophosphate (ZDDP), the nanolubricant's anti-oxidant characteristics were increased. The ZDDP additive is one of the most extensively employed additives in the automotive sector. It is most recognised for its antiwear characteristics, but it also contains anti-oxidant and extreme pressure characteristics. Phosphate glasses' capacity to digest oxides appears to be linked to ZDDP antiwear capabilities. Several authors [26-28] have demonstrated a synergistic interaction between MoDTC and ZDDP due to MoS2 production. Lubricants undergo a three-step oxidation process. A free radical is produced at the first stage, initiation. The free radical combines with oxygen to produce peroxide radicals in the second stage called propagation. After combination with other lubricant components, these radicals have additional radicals. Two radicals join to form a stable molecule in the third phase, known as termination. The synergistic effect of MoS2 with ZDDP promotes hydrogen donation, which stops the radical propagation process. It causes the nanolubricant’s OIT to be higher. The nanolubricant with 0.1wt.%, 0.01wt.% and 0.005wt.% of MoS2 nanoparticles possesses lower OIT than 0.05wt.% and 0.01wt.% as the mentioned concentration are not optimum in provide higher OIT in the nanolubricant. The substantial improvement in OIT of nanolubricants shows that the synergistic effect of MoS2 nanoparticles and ZDDP can exhibit good oxidation stability, enhancing the antioxidant properties of the nanolubricants.
d) Thermal conductivity Analysis
From the tribological and oxidation analysis, the nanolubricant with 0.01wt. % MoS2 nanoparticles provide the best result compared to other concentrations of MoS2 nanoparticles in nanolubricant. Thus, it was further investigated for its thermal conductivity using the laser flash analysis method to compare the base oil. The addition of MoS2 within the base oil demonstrates an improvement in thermal conductivity, as shown in figure 11. The thermal conductivity of the nanolubricant showed approximately~10 % of improvement as compared to the base oil. Due to the lower concentration of MoS2 nanoparticles (0.01wt. %), the noted improvement in thermal conductivity was caused bt the molecular collisions among the base oil and nanoparticles [29-33]. Moreover, perceived thermal conductivity behaviour during the investigation indicates that this enhancement is due to the percolation mechanism and the involvement of the Brownian motion of the nanosheet [34-36]. In addition, the nanoparticles' phonons get scattered in the active nanostructures, improving the contact conductance [37].
Subsequently, thermal conduction channels are developed, which improves thermal conductivity. This scenario is known as the percolation mechanism. In addition, the thermal transfer among the colliding nanoparticle rose the thermal conductivity of the nanolubricant. For example, from figure 11, the thermal conductivity of the nanolubricant rises more than the base oil after the temperature of 60 °C as a more intense Brownian motion of the nanoparticles occurs [38]. This thermal transport phenomenon in the nanolubricant was due to the physio-chemical attribute of the base oil, as well as collaboration with the reinforcement nanoparticles.