3.1. Reduced Graphene Oxide Characterization
Transmission electron microscopy (TEM) at 50,000× magnification revealed that reduced graphene nano platelets aggregated randomly, as shown in Figure 3
. The magnified view shows that few layers of rGO are attached in the hexagonal lattice. The rGO was evaluated as very fluffy and lightweight nano powder. Multiple layers of rGO were identified, as shown by the arrows in the 150,000× magnified view. Due to fine nano particles, rGO was able to provide high surface area compared to graphene oxide, and this can be used to accelerate the nano platelets’ penetration into the mating surfaces and enhance tribological properties.
Large particles can damage the filter of a hydraulic system, which can lead to inefficient performance. In order to investigate rGO particle size, an FTIR test was conducted. FTIR test results show the total number of particles per 1 ml. As shown in Figure 4
, most of the nano particles range from 4 µm to 70 µm in size. Smaller particle sizes can breach between the surfaces and reduce wear and friction. It is necessary to consider particle size and filtering class in future applications as some larger particles would be filtered out in lubricant cleanliness of the 16/14/12 or better class (according to ISO 4406).
To characterize the chemical composition of reduced graphene oxide in terms of its graphitic nature, size of crystalline and structural disorder, Raman spectroscopy was conducted. A 514 nm laser spectra was used to obtain D, G, 2D, and D+D’ bands using monochromatic light. From past research, the D band indicates defects in the domain, where increasing the intensity of the D band leads to the formation of more sp2 lattices because of oxidation. The G band stands for the first order dispersion of E2g phonon, which increases C–C tangential vibration or stretching [14
]. The ratio of the D band over the G band is crucial to identify defects in the graphitic structure. As shown in Figure 5
, Raman spectra show the D band at 1357 cm−1
and the G band at 1601 cm−1
. The intensity ratio of the D band and the G band (ID/IG) represent the lattice size. A higher ratio leads to bigger lattice size and as the size of the lattice increases, defects of the sp2 carbon domain. However, for rGO this ratio was found as 0.84, which is low enough to have fewer domain defects [23
]. The 2D peak is attributed to the number of layers in reduced graphene oxide. In the spectra, the 2D peak attributed at 1845 cm−1
and D+G peak at 3178 cm−1
. Reduction also increases the intensity of the 2D peak, and as shown in the spectra, the 2D peak intensity was found to be greater, which illustrates that rGO was highly reduced [23
]. As explained, rGO is hydrophobic and does not allow more oxygen groups between the layers, which enables it to generate gaps between the layers and frictionless conduits in the microstructure. Further, it can accommodate liquid lubricants between the layers. It has been found that mechanical, tensile, and compressive strength are directly proportional to the degree of reduction. The rGO used in this study was highly reduced, which leads to great mechanical, and tensile and compression strength [23
]. This is very important for rGO nano additives to mitigate metal-to-metal contact and enhance wear preventive properties.
To provide fault-free machine life, estimation of anti-oxidation capability of lubricants can be investigated using numerous methods such as FTIR, RPVOT, odor, and color inspection. Among these methods, RPVOT is widely used by industry to identify the antioxidant capability of a lubricant [29
]. While transforming graphene oxide into its reduced form, there are always a small number of oxygen groups that remain in the lattice, and to understand the effect on anti-oxidation capability because of those oxygen particles, an RPVOT test was conducted as per ASTM D2272 standards. As shown in Table 3
, pressure drop time is noted as 27.1, 26.1, 25.8, and 25.7 minutes for pure oil and all three samples, respectively. Comparative studies indicate that there is no noticeable change in anti-oxidation life of all three samples having nano additives compared to the pure oil.
To evaluate the effect of the temperature fluctuation of the pure oil due to the rGO nano additive, a kinematic viscosity test was performed at 40 °C and 100 °C, as per ASTM D445 standards. As shown in Table 4
, viscosity at 40 °C and 100 °C are 41.5 mm2
/s and 6.3 mm2
/s, respectively, for the pure oil. For the other three samples, viscosity is around 41.2 mm2
/s and 6.4 mm2
/s which indicates a negligible decrease in viscosity at lower temperatures and insignificant increase at higher temperatures.
The viscosity index was calculated to investigate the change in viscosity with respect to temperature. Results show that for pure oil, the index was around 98, and for nano lubricants it was 104 at lower concentrations (S-1 and S-2), which reduced to 103 for the more highly concentrated sample (S-3). To recapitulate, the rGO nano lubricant’s low viscosity and higher viscosity index enhance shear thinning at solid point contact and reduce frictional losses.
Friction and wear tests were performed in situ to examine the friction behavior of lubricants before adding nano additives to the pure oil, and after adding nano additives at the three concentrations. To obtain precise friction coefficients, data points were taken at 20 ms time intervals. As shown in Figure 6
, the friction coefficient for pure oil reduced for fewer revolutions where the rotating speed was low; however, as speed increased logarithmically with the number of revolutions, friction went up and then normalized around 0.2. In the case of S-1 and S-3, the study found the same trend, where friction increased at fewer revolutions and then normalized around 0.3 and 0.35 respectively. The reason behind this could be that S-1 had an uneven rGO nano lubricant film. For S-3, higher conglomerations of the particles might have created a wall at the joint, which can reduce the circulation of lubricant. However, S-2 shows a different trend where friction increased a bit at low speed and revolutions but started normalizing closer to 0.1 at higher speed and revolutions.
To understand the wear mechanism of the disk under different lubricants, the wear track of the disk was further studied using hole area analysis, scanning electron microscopy (SEM), and optical profilometer. As shown in Figure 7
, the wear rate for the pure oil was calculated as 48.33 × 10−5
/Nm which is comparatively higher than the other three nano lubricants. S-1 reduced wear by 22.06%, S-2 by 51.85% and S-3 by 25.64% in comparison to the pure oil, which allows the lubricant to enhance tribological surfaces.
shows the schematic view of the interaction of ball, disk, and nano additives used for these experiments to understand the behavior of nano particles under the test conditions. For S-1, only 0.01 wt % nano additives were amalgamated into the pure oil, which indicates low wear compared to pure oil due to penetration of rGO platelets in the contact regime. In the case of S-2, 0.05 wt % nano additives were mixed into the pure oil, which showed the lowest wear among all four samples as well as a reduction in friction after a perfectly hydrodynamic condition was achieved. This allows the lubricant to mitigate metal-to-metal contact, which can be considered as optimal concentration. On the other hand, in S-3, 0.1 wt % concentration was prepared, which led to reduction in wear; however, friction for this sample was highest. This can be due to a high concentration rGO nano platelets starting to conglomerate around the contact patch. Conglomerated nano platelets will generate lower suspension and will inhibit the rGO nano platelets from entering into the patch; but due to high concentration and lower viscosity it will require more energy, which can result in increasing friction. Conversely, some particles will enter the tribo contact and shear the rGO layer, which helps to reduce wear compared to the base oil.
The wear of the disks is affected by a number of factors, such as the concentration of the additive rGO, contact stress, mechanical properties of the material, temperature, and surface-moving speed, etc. The beneficial effect of adding rGO nano platelets on wear can be justified from the performance of the disk under different rGO concentration.
Wear tracks for all four disk samples were investigated at 500 µm and 120× magnification, as shown in Figure 9
. At high magnification, worn material debris was investigated on the wear track for all four samples. There are some deep grooves on the worn surface of the pure oil (control sample) and S-1 and the width of the wear scars are about 890 μm and 746 μm, respectively. Both control sample (pure oil) and low rGO sample (S-1) suffered severe wear and the deep grooves are parallel with the sliding direction. The wear mechanism can be adhesive wear. Sample-2 (S-2) shows a smooth wear track (605 μm) which does not have any deep scratches, significant plastic deformation, or deep sliding direction marks. The rGO nano platelets in S-2 reduce friction by preventing sliding contact interfaces from severe or more frequent metal-to-metal contacts. By increasing the rGO concentration in S-3, the rGO nano platelets agglomerated together to form abrasive particles which resulted in increasing the width of the wear scar for S-3 to 709 μm. In the future of this study, further theoretical and experimental analysis need to be conducted to investigate the mechanism of wear at different concentration of nano additives (rGO).
Profilometry data was obtained in situ to understand the effect of nano lubricants on surface roughness using a non-contact type optical probe. ISO 25178 was used to obtain height parameters such as S
a (arithmetic mean height of the scar) and S
z (maximum height of surface). As shown in Table 5
, pure oil has an arithmetic mean height of 1.386 µm and the maximum height of the surface is 15.19 µm, compared to that of S-2, which has an arithmetic mean height of 0.6091 µm and the maximum height of the surface is 11.89 µm, which leads us to conclude that, except for S-2, all three samples, including pure oil, have high adhesive wear.