In this study, the tribological effects of three different forms of reduced graphene oxide (rGO)-2D nano-additives in base oil were investigated. Reduced graphene oxide nanoplatelets were manufactured using a modified Hummers’ method. However, different filtration methods were used to obtain rGO nanoplatelets at three different bulk densities. After adding nano-additives to the base oil at 0.01%w/w concentration, physical and chemical characterization tests were performed such as viscosity test, four-ball wear test, rotating pressure vessel oxidation test (RPVOT), resistivity test and friction coefficient test. The presented results show that material-1 with the lowest bulk density and less lattice defect can perform better by reducing wear of the material by 10.63% as well as the coefficient of friction (COF) by 6.3% with respect to the base oil and under test conditions. The presented results show the promising effect of rGO as nano-additives to fluid lubricants on wear preventive properties without compromising the physical and chemical characteristics of the lubricants.

Statistics exhibit that 23% of global energy is consumed to overcome friction between mating surfaces and to remanufacture the worn parts.1 Industries are increasing alignment capability and controlling stress fluctuation by introducing different types of bearings; however, historical data proves that 90% of bearings fail prematurely due to insufficient lubrication capability.3 To mitigate this problem, the proper lubricant needs to be applied to the contact surfaces, which can provide low shear strength film and eliminate direct contact between surfaces.2 

Lubricants are classified by their material states such as liquid, gaseous, solid, or semi-solid. Determination of the optimum combination of lubricants and their application is crucial in tribology. It is important to enhance the properties of a lubricant, such as shear life, range of viscosity and the behavior under extreme pressure, and also to maintain important properties such as friction and wear control. To address these challenges, researchers have proposed various advanced materials to improve lubricant performance by amalgamating nanomaterials such as carbon nanotubes (CNTs), fullerene and graphene.4,5 Among carbon-based materials, graphene has attracted a great deal of attention during the past few years due to its thermal, electrical, optical, mechanical and hydrophobic properties.6 Studies have confirmed that graphene is one of the strongest materials ever measured and from a tribological point of view, it is effective for deterring wear.7 Moreover, higher electric conductivity of graphene generates more demand for energy storage applications, biomedical applications, supercapacitors, and printable graphene electronics. Reduced graphene oxide (rGO) could provide a suitable tribological solution, since it is easier to manufacture in large quantities than perfect single-layer or few-layer graphene, and the quality can be maintained for bulk manufacturing. Along with that, chemical,8,9 thermal10 or irradiation (UV or IR)11,12 methods are used to reduce graphene oxide into rGO form.

Multilayered rGO can be used by a wide range of industries due to their intrinsic mechanical properties, which act as wear preventive additives by eliminating direct metal-to-metal contact. In addition, 2D geometry, small particle size, and high surface area allow better dispersibility of rGO with getting affected of gravitational forces in the lubrication chamber. Along with that, greater dispersion rate allows particles to float in the lubricant and enter between the mating surfaces. In the past, different multilayered nanoparticles such as graphene, graphite, and boron had shown excellent tribological characteristics as solid lubricant.13,14 Furthermore, graphene was tested at different concentrations in liquid lubricant, water-based lubricant, and engine oil where it has shown exquisite enhancement of friction, wear, and mechanical properties.15–17 Multilayered nano additives are more focused as a tribo-improver due to the strong covalent bond and weak van der walls bonds which allows these materials to shear their layers under the loading conditions. But, the higher concentration of graphene starts conglomerating in the lubricant and causes abrasive wear as well as increases friction.18 rGO nanoplatelets have been used by researchers as friction and wear modifier previously;19–21 nevertheless, the proposed rGO nanoplatelets are chemically inert and a stack of material can cause agglomeration at the contact surface. Furthermore, rGO used for anti-wear properties evaluation has higher lattice defect and disorder (ID/IG) because of oxygen atoms between the layers which can damage oxidation stability of the lubricants.22 On the other hand, application of composites of rGO such as Cu/rGO, MoSe2-rGO, polyacrylonitrile-rGO has improved tribological properties of liquid lubricant.23–25 Although, composites of rGO require higher density defect in lattice for better aggregation. Study also reveals that, defect in graphene based material is most important to quantify the performance of the rGO which means material with less defect has better electrical and mechanical properties.26 

In this study, applications of rGO nano-additives are explored using pure liquid lubricant. rGOs used in this study were differentiated based on the manufacturing process, lattice defect and volumetric bulk density. Furthermore, nanolubricants were prepared by adding rGO additive at 0.01%w/w concentration. To accomplish comparative study, kinematic viscosity, oxidation stability, electrical resistivity, wear, and friction coefficient tests were conducted on each sample according to ASTM standards. Use of pure mineral oil without any additives allowed us to evaluate the direct effects of the rGO additives on the performance of fluid lubricant. The lower concentration of rGO, without any additives in the base oil has proven great wear resistive properties and impressively reduces friction coefficient, which encourages the application of rGO in commercial oil as friction and wear modifier.

Multilayered 2D rGO nanopowder was supplied by Digi-Key Electronics in association with Kennedy Labs. The rGO used in this project was produced using a modified Hummers’ method, which is cost effective and can reduce the emission of toxic gas; this supports the application of this method to manufacture different forms of rGO.27,28 To convert graphene oxide to its reduced form, a single sealed jar was used. One of the parameters used to differentiate the rGO samples from each other is their density, as presented in Table I. The base oil used for this experiment was 200 Neutral Medium HT, which is produced by Petro Canada and claimed as 99.9% pure. Crystal clear and bright lubricant has 0.865kg/l weight density and 20.2 ppm water content as per material information sheet provided by Petro Canada. The oil belongs to Groups II and III as per the American Petroleum Institute (API). Samples were characterized as displayed in Table II using the base oil with different concentrations of additives.

TABLE I.

rGO powder used in this project.

MaterialsDensityMethod of fabricationProduct No.
Material-1 2.0 mg/cm3 Modified Hummers’ 10045412 
Material-2 2.9 mg/cm3 Modified Hummers’ 10045861 
Material-3 7.5 mg/cm3 Modified Hummers’ 10045867 
MaterialsDensityMethod of fabricationProduct No.
Material-1 2.0 mg/cm3 Modified Hummers’ 10045412 
Material-2 2.9 mg/cm3 Modified Hummers’ 10045861 
Material-3 7.5 mg/cm3 Modified Hummers’ 10045867 
TABLE II.

Sample classification.

AdditiveWeightConcentration ofSample
Materialof Oiladditive (%w/w)Number
NA 150g NA P.O. 
Material-1 150g 0.01 1.1 
Material-2 150g 0.01 2.1 
Material-3 150g 0.01 3.1 
AdditiveWeightConcentration ofSample
Materialof Oiladditive (%w/w)Number
NA 150g NA P.O. 
Material-1 150g 0.01 1.1 
Material-2 150g 0.01 2.1 
Material-3 150g 0.01 3.1 

Density was measured using a 0.15cc micro spoon and a calibrated digital scale having a readability of 0.0001g. The weight of each sample was measured four times and the average value was determined. A visual inspection revealed that material-1 was very fluffy, material-3 was in the form of dense powder and material-2 was somewhere in between. Density is inversely proportional to volume, so material-1, which has a lower density due to its fluffy characterization, has higher porosity compared to material-3, which was observed as a packed powder. Material with more particles more effectively covers the mating surfaces and provides good tribological properties. To evaluate further, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were performed to investigate the morphology of these materials under 30kV high vacuum mode. To avoid conglomeration of rGO particles in SEM analysis, ethanol (90% w/w) solvent was utilized. A holey grid was utilized to capture rGO particles after the ethanol evaporated. The SEM image of multilayered reduced graphene oxide is found as shown in Fig. 1. To further evaluate the chemical defects in the reduction method for all three forms of rGO, Raman spectroscopy was conducted using a Renishaw Raman imaging microscope system-2000 at room temperature. 514nm laser excitation was employed to obtain Raman spectra.

FIG. 1.

Multilayer rGO particle.

FIG. 1.

Multilayer rGO particle.

Close modal

Due to a conglomeration of the rGO, analysis of all three forms under TEM became tedious; for this reason, the powders were placed in molds, which were filled with spur epoxy resin and polymerized for 12 hours in an oven at 65°C. Once the mixture solidified, 90nm-thick sections were cut using a diamond knife on a Leica UC7 micrometer. After preparing the samples, an FEI Tecnai 20 transmission electron microscope was utilized to take images at a higher magnification of 200nm, as revealed in Fig. 2(a,b,c). In addition, SEM was deployed to characterize the morphology of the material as the SEM electron beam does not pass through the material structure. Fig. 2(A) signifies, transparent nanoparticles are escalated on the plate due to nanoholes and cavities on the particles surfaces. Additionally, multilayers of rGO material-1 can be characterized from the Fig. 1. Although, Fig. 2(C) reveals that material-3 was highly aggregated and very dense compare to rest of two materials. Furthermore, Fig. 2(B) evident that material-2 has shown highly curved structure in which some areas are highly dense and widely scattered on the plate. Along with that, high magnification TEM images are also aligned to the SEM assumption. In which, Fig. 1(a) manifest less wrinkled and thin structure of material which shows higher porosity on the surface. Moving from material-1 to 3, images disclose higher dense, aggregated, and crumpled structure of material morphology.

FIG. 2.

TEM images of three materials a=Material-1, b= Material-2, c= Material-3 at 1µm and SEM images- A=Material-1, B=Material-2, C=Material-3.

FIG. 2.

TEM images of three materials a=Material-1, b= Material-2, c= Material-3 at 1µm and SEM images- A=Material-1, B=Material-2, C=Material-3.

Close modal

For sample preparation, the weight of the rGO was calculated as per concentration and using a calibrated weight scale the material was transferred into a beaker for mixing. The mechanical mixture ran at a constant speed for at least 20 minutes for each mixture. Besides, all the samples were ultra-sonicated for 20 minutes to break pellets into the mixture as shown in Fig. 3.29 

FIG. 3.

Process Illustration.

FIG. 3.

Process Illustration.

Close modal

Once all four samples were prepared, we ran different tests on each sample to see the change in the properties in comparison to the pure oil (P.O.) as per standards as presented in Table III. A viscometer was used for the kinematic viscosity test (ASTM D445) and the viscosity index was calculated as per ASTM D2270. A rotating pressure vessel oxidation test (RPVOT) was used to analyze oxidation stability as per ASTM D2272, and resistivity was measured to understand contaminants in the oil using ASTM D1169. To compare wear properties of the samples, the ASTM D4172-Wear Preventive test was performed using the four-ball test on four samples including the pure oil. To run this test, three balls, each having a size of 12.7mm were used at the bottom of the tester and a fourth ball of the same size was placed on top of the others, under a load of 147N. Tests ran at 1200RPM for 60 minutes and were performed at room temperature. For each test, new balls were utilized and after each test the machine was ultra-sonicated. To evaluate the effect on the friction coefficient of the lubricant, the shaft on the plate machine was executed. All friction tests were performed under 30N load for 20 minutes’ duration. To obtain accurate data each test was performed three times and average results were considered for the study.

TABLE III.

Test evaluation.

TestsMethodPurposeASTM number
Viscosity Viscometer Kinematic viscosity D445, D2270 
RPVOT Rotating pressure vessel Oxidation stability D2272 
Resistivity Specific resistance Insulating properties D1169 
Scar Value Four balls Wear preventive characteristics D4172 
Friction Shaft test Average friction coefficient NA 
TestsMethodPurposeASTM number
Viscosity Viscometer Kinematic viscosity D445, D2270 
RPVOT Rotating pressure vessel Oxidation stability D2272 
Resistivity Specific resistance Insulating properties D1169 
Scar Value Four balls Wear preventive characteristics D4172 
Friction Shaft test Average friction coefficient NA 

To differentiate rGO based on crystalline defects and size, and degree of hybridization, Raman spectroscopy was employed to obtain the D and G bands for the three forms of reduced graphene oxide. The D band stands for the out-of-plane breathing mode due to defects on the corners or sides occurring in reduction, and the G band represents the scattering of out-of-plane E2g phonon from sp2 domain. The important parameter is the ratio of the intensity at both shifts (ID/IG), which is directly proportional to the degree of disorder. As shown in Fig. 4(a), for material-1, the D and G bands shifted at 1371 and 1601 respectively; for material-2, the D band shifted at 1361 and the G band at 1598; and for material-3 the D band and G bands broadened at 1354 and 1594 respectively. The intensity ratio for all three forms of reduced graphene oxide was calculated as 0.83, 0.85 and 0.87 for material-1, -2, and -3 respectively. Reduction in the ratio leads to conversion of sp3 to sp2 carbon. Fig. 4(B) shows the ratio of intensity at the D band and G band for three forms of reduced graphene oxide. For material-1, it is lowest and for material-3 it is increased due to the variation in reduction time of the reduced graphene oxide.36 Reduction in the ratio of intensity leads to restoration of the carbon lattice. Furthermore, study reveals that, intensity of G band is inversely proportional to the number of layers.38 As shown in spectra, G’ band was observed around 2905cm-1. In addition, intensity peaks for G’ for all three materials illustrate that, moving from material-1 to 3 intensity increases which evidence that material-1 has highest number of layers, material-2 has intermediate and material-3 has lowest number of layers. Additionally, material-1 has the lowest intensity ratio, which can indicate the fewest rGO defects compared to material-2 and -3.37 

FIG. 4.

A-Raman spectroscopy of three forms of reduced graphene oxide at room temperature. B-Intensity ratio of D band over G band (ID/IG) for three reduced graphene oxide.

FIG. 4.

A-Raman spectroscopy of three forms of reduced graphene oxide at room temperature. B-Intensity ratio of D band over G band (ID/IG) for three reduced graphene oxide.

Close modal

An important requirement for a lubricant is to conserve its thickness and maintain hydrodynamic lubrication through a given temperature range.11,34 To reduce friction and wear, it is important to keep rubbing surfaces apart at all temperature levels. The value of the friction coefficient changes according to the Hersey number, which is a function of viscosity, load, and velocity.34 As additives increase, the number of particles in the oil can possibly accelerate the sludge formation characteristic of oil at higher thickness. To understand the effect of additives on viscosity properties, we performed a viscosity test as per ASTM -D445 and calculated the viscosity index as per the ASTM-D2270 standard chart.30 

As shown in Table IV, we can compare the results of viscosity and viscosity index (VI) for four samples. The table demonstrates that there is a minor reduction in the viscosity at 40° on the other side it increases the viscosity at 100° which cumulatively increases the viscosity index compared to the base oil. As an end result increment in viscosity index allows lubricant to work in a broader temperature range without changing its viscosity. However, as per the American Petroleum Institute, if the VI is between 80 and 120, then additives can be dispersed easily in the base oil.32 The results are shown in Table IV indicate that the samples are in the acceptable range.

TABLE IV.

Kinematic viscosity at 40°C and 100°C and viscosity index.

P.O.1.12.13.1
Viscosity at 40° (mm2/s) 41.5 41.2 41.2 41.2 
Viscosity at 100° (mm2/s) 6.3 6.4 6.4 6.4 
Viscosity index 98 104 104 104 
P.O.1.12.13.1
Viscosity at 40° (mm2/s) 41.5 41.2 41.2 41.2 
Viscosity at 100° (mm2/s) 6.3 6.4 6.4 6.4 
Viscosity index 98 104 104 104 

As the oxidation numbers increase, a lubricant will typically start producing acids and other insoluble oxidants, which can hinder the operating capability of a component. In addition, a more highly oxidized lubricant can cause varnish and sludge, which will inhibit the lubricant from performing the functions of cooling and removing debris from contact surfaces.5 To evaluate the oxidation resistance of the oil, the RPVOT test was performed as per ASTM D2272, in which the apparatus had an axially rotating vessel at 100RPM, kept at 30°C and 150°C. After this, 5g of distilled water and new copper coils were placed in the liquid and the pressure was increased from 700kPa to 1400kPa. In the next step, the time for the pressure to drop from 1400kPa to 175kPa was measured and results show that the oxidation time for pure oil is 27.1 min and for the three different rGO samples, the times are 26.1, 26.8 and 25.4 minutes for material-1, -2 and -3 respectively as shown in Table V. This shows that rGO additives do not affect the anti-oxidation properties of a lubricant.33,34

TABLE V.

Rotating pressure vessel oxidation time in min.

MaterialsAntioxidation time in min
P.O. 27.1 
1.1 26.1 
2.1 26.8 
3.1 25.4 
MaterialsAntioxidation time in min
P.O. 27.1 
1.1 26.1 
2.1 26.8 
3.1 25.4 

Lubricants can be utilized for many applications, and additives have a significant effect on the conductivity and temperature of the lubricants. Therefore, if conductivity increases significantly, there is a chance of micro sparks;31 as rGO is conductive and hydrophobic, there is the possibility of reduced resistivity. The resistivity of the mixtures was measured as per ASTM D1169. Results show that the resistance value was higher than 50000 G Ω-cm for all the samples including the base oil, which indicates that the additives were not causing a noticeable reduction in resistivity of the lubricant.27 

When two surfaces are in a sliding, rolling, or impact motion or when they are directly exposed to the environment, which gradually affects the surface by adhesive, abrasive, fatigue, erosion or chemical wear, these conditions can lead to damage to the apparatus. Wear is not a material property but is directly dependent on the design of the tribological system. The high cost of producing machinery motivates designers to consider the proper lubrication system, as lubricants work like a ball in a ball bearing to transfer motion. Nanotribology provides an opportunity not just to analyze a system at the micro level but to understand the behavior of wear surfaces at the nano level; this opens a path to improve lubricants by using wear preventive additives. To obtain results about rGO wear preventive characteristics, the four-ball wear preventive test was performed as per ASTM D4172 standard. Four Grade 25 extra polished balls 12.7mm in diameter were compressed at 147N±2N at 75°C and run at 1200r/min±60r/min for 60min±1min.35 Before running the test, each component of the equipment was cleaned to ensure it was dust and debris free, and all samples were ultrasonicated for another 20 minutes to guarantee a stable and homogenous mixture. In this study, three different materials were tested at 0.01% w/w concentration to understand the effect of surface area, density, porosity of material and lattice defect on wear preventive properties. Each test was performed twice and the average value was calculated to obtain accurate data about wear diameter. The average wear scar diameter for the control sample (base oil) is 0.94mm at 147N. Furthermore, control sample exhibit the highest wear scar diameter compare to all three nanolubricants. Table VI publishes that material-1 reduced the wear scar diameter by 10.63% compared to the base oil. Along with that, material-2 and material-3 reduces wear scar diameter by 7.45% and 3.19% as compare to base lubricant. This tests results leads this study to investigate the morphology of worn substrate to examine deposition of nanoparticle on the surface.

TABLE VI.

Average wear scar value in mm (Diameter measurement equipment capability – ±0.01mm).

Wear scarReduction in WSD
SamplesMaterialsdiameter (WSD)compared to base oil
1.1 Material-1 0.84mm 0.10mm 
2.1 Material-2 0.87mm 0.07mm 
3.1 Material-3 0.91mm 0.03mm 
Wear scarReduction in WSD
SamplesMaterialsdiameter (WSD)compared to base oil
1.1 Material-1 0.84mm 0.10mm 
2.1 Material-2 0.87mm 0.07mm 
3.1 Material-3 0.91mm 0.03mm 

As shown in Fig. 5, worn surface morphology of material-1 was examined at 500μm (Fig. 5A) and higher magnification of 20μm (Fig. 5B) using SEM. Fig. 5A reveals the smooth scar without exposing any deep groove, or sliding direction mark on the surface. The smooth surface wear can be caused by adhesive wear due to higher loading conditions. However, Fig. 5B expose the closure view of the section in which deposition of rGO flakes is shown by yellow lines and arrows on the scar. Fig. 6 represents how three forms of rGP penetrates between the contact surfaces. Which clearly shows material-1 has more particles which leads towards more surface area with respect to rest of two material. Furthermore, this nano particles can work as a nano bearing between the contact patch and mitigate the metal to metal contact which results in reduction of wear. Material-1 rGO had a less defect as explained in Raman spectroscopy which gives material higher strength compare to rest of two materials. In case of rGO multilayer material friction can be caused due to two reasons such as friction between the interlayer shear and friction between the two balls. As disclosed in Fig. 7, test results of friction coefficient expose that material-1 and 2 has less average friction coefficient compare to the pure oil. Although, material-3 displayed highest friction coefficient in all 4 materials. The higher friction of material-3 can be caused due to higher aggregation and dense particle composition as explained in Fig. 2C. In addition, due to this cluster formation, surface area of nano particle can be greatly affected, in further this clustered particles required higher force to shear between the surfaces and start increasing higher friction as revealed in Fig. 8. Conversely, incessant reduction of friction along with time can be grasped in Fig. 8 (material-1) due to accumulation of nanoparticles on the substrate which generates protective layer (film). On the other hand, higher surface area allows lubricant to convert the sliding friction into rolling friction. Though, material-2 has also shown less average coefficient of friction which seem increasing as particles started accumulating along with time as shown but due to less density, they were able to maintain surface area without producing knots of particles.

FIG. 5.

SEM image of the ball surface of sample-1.1 (dashed-line: scar line; arrows: accumulated rGO film).

FIG. 5.

SEM image of the ball surface of sample-1.1 (dashed-line: scar line; arrows: accumulated rGO film).

Close modal
FIG. 6.

Schematic view of lubrication mechanism and nanoparticles behavior.

FIG. 6.

Schematic view of lubrication mechanism and nanoparticles behavior.

Close modal
FIG. 7.

Average friction coefficient data for pure oil and nano lubricant.

FIG. 7.

Average friction coefficient data for pure oil and nano lubricant.

Close modal
FIG. 8.

Friction coefficient curves of different samples with respect to time (s).

FIG. 8.

Friction coefficient curves of different samples with respect to time (s).

Close modal

Lubricant properties can be enhanced by different types of nano-additives, and carbon-based material is found to be the most economical. In this study, rGO demonstrates an effective wear-resistive property by reducing the wear of a steel ball substrate under 147N load by 10.63% compared to the base oil. On the other side, the shaft on disk friction test results shows that material-1 nano lubricant reduces the friction coefficient by 6.3% with respect to the base oil under laboratory conditions. Higher reduction of friction and wear can be achieved due to lower density particles can easily corrugate on the tribo surface which mitigate the metal to metal contact on the steel balls. Furthermore, Raman results shows that by moving from material-1 to -3 lattice defect increases and number layers are decreasing which leads to higher wear and friction due to defected surface morphology, higher aggregation rate due to oxygen particles in defect. Likewise, less number of layers in material-3 will not shear it down as much as material-1 and that will become additional cause of higher wear under loading condition. Besides, resistivity tests clearly show that rGO nanoparticles do not significantly affect the physical and chemical properties of the oil. None of the three additives showed a significant effect on the anti-oxidation characteristics, which indicates that while improving wear resistive properties, the life of the lubricant is not compromised; this results in no economic burden on lubrication system. In addition, SEM analysis of the four-ball wear test shows the accumulation of rGO particles on the scar surface, which proves the generation of rGO tribofilm between the surfaces at high load and act as a bearing. Due to this, direct metal-to-metal contact can be eliminated and the wear rate can be reduced even at higher loads. Although graphene has had many applications in the past, this study adds one more application of different density reduced form of graphene as a wear preventive and friction modifier additive in the industry.

For future work, we can consider a higher concentration of rGO material-1, as it shows a positive effect on the base oil wear preventive properties. We strongly believe in using rGO as an economic, environmentally friendly nano -additive. Moreover, it can lead to be capable solution to synthesis of high-performance commercial oil, with enhanced frictional and wear properties to make a system more efficient and durable.

This research was partially funded by the National Sciences and Engineering Research Council (NSERC).

The authors declare that they have no conflict of interest.

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