Enhancing Lubrication Performance of Plastic Oil Lubricant with Oleic Acid-Functionalized Graphene Nanoplatelets and Hexagonal Boron Nitride Solid Lubricant Additives

Abstract

The study explored the viability of using waste plastic oil (PO) as an alternative lubricant to petroleum-based lubricants in industrial settings. To enhance the lubrication performance of the PO, this study incorporated cost-efficient, oleic acid-modified, graphene nano platelets [GNP (f)] and hexagonal boron nitride [hBN (f)] nano solid lubricant additives into the PO in various concentrations, forming functionalized nano lubricants. The PO and its functionalized nano lubricant’s rheological, dispersion stability, thermal degradation, friction, and wear performance were investigated. Results manifest that incorporating GNP (f) and hBN (f) into the PO significantly enhanced the viscosity and dispersion stability. In addition, it was seen that GNP (f) and hBN (f) nano lubricants lowered the coefficient of friction (COF) by 53% and 63.63% respectively, compared to the PO. However, the GNP (f) and hBN (f) nano lubricants demonstrated a 3.16% decrease and a 50.08% increase in wear volume relative to the PO. Overall, the GNP (f) and hBN (f) nano lubricants displayed a synergistic friction behavior, while they exhibited an antagonistic behavior pertaining to the wear volume. The study elucidated the mechanisms underlying friction and wear performance of the nano lubricants.

1. Introduction

The efficient and smooth performance of machinery components is critically influenced by their friction and wear behavior. The majority of energy losses that take place in these components are due to the friction and wear produced by these moving mechanical components [1]. This critical matter can be overcome by lubricating the contacting and moving surfaces. The usage of liquid lubricants is one of the most successful ways of controlling friction and wear losses in machinery equipment. The exact moment in time when lubricant was first used for friction reduction is not clearly known. In Egypt, during pre-historic times, there was evidence of grease being used as a lubricant [2]. Over time and during the era of the Industrial Revolution, large-scale machinery was employed that was lubricated with animal-based oils. In the subsequent years, petroleum-based oils became more popular owing to their bulk production and affordable costs. As a result, they were widely used for various industrial applications. However, with the advent of time, environmental concerns, and governmental regulations, there was a quest for suitable alternative forms of lubricants.

Plastics are a vital part of the modern world. Globally, in 1964, production of plastic was around 15 million metric tons [3]. However, by the year 2019, this number exponentially increased close to 370 million metric tons [4]. Moreover, by 2022, worldwide plastic production rose to 400 million metric tons, and it is expected to reach approximately 600 million metric tons by 2050 [5]. The rampant production of plastics and their unsuitable disposal have generated waste products that are harmful to living beings and cause environmental challenges [6]. Out of all the waste plastics generated, only around 9% were recycled, and approx. 12% were incinerated. The remaining waste was improperly disposed of in landfills and the environment [7]. One of the realistic techniques for mitigating the waste products from plastics is to convert them into useful chemical products and lubricant oils [8]. In this circumstance, plastic oil (PO) extracted from disposed waste plastics can be a prospective substitute lubricant for various machinery applications. A group of researchers has experimentally shown that a specific lubricant obtained from the conversion of waste plastics exhibited 44% lesser wear compared to petroleum-based Group III mineral oil [9]. Moreover, these PO lubricants with proper modifications have the capability to be blended with synthetic oils. This would thereby provide enhanced lubrication performance, at a lesser expense, as well as aid in mitigating the harmful effects of plastic waste.

The study on the application of nanotechnology and nanoparticle materials, such as graphene nano platelets (GNPs), hexagonal boron nitride (hBN), and copper oxide (CuO) to name a few, has significantly aided in improving the lubrication performance of various lubricant oils [10]. Nanoparticles are deemed suitable lubricant oil additives due to their unique special physical, chemical, and mechanical properties. Attributes such as significant surface energy, size, structure, and specific surface area are considered as reasons impacting the mechanical properties of the nanoparticles [11]. GNP are of vital interest for lubricant formulations, owing to their higher surface area, low density, high thermal conductivity, and ease of chemical modification. hBNs are also useful for lubricant usage owing to their benefits of chemical stability and thermal resistance to oxidative degradation.

When small quantities of nanoparticle additives are incorporated in base oil lubricants, they can support in enhancing their rheological properties, thermal conductivity, electrical conductivity, and tribological properties [12,13]. Some of the advantages of incorporating nanoparticle additives in base oil lubricants for enhanced performance are as follows [14]: (a) the ability of nanoparticle additives to remain insoluble in the non-polar-based oils, (b) lower chances of reactivity with other additives incorporated in the same lubricant, (c) prospective chances of formation of a film on different material surfaces, (d) significant durability, and (e) lesser chances of volatile reaction at high temperature.

However, nanoparticle additives incorporated in lubricants experience poor dispersion stability [15]. This is because their high surface energy can result in agglomeration and sedimentation [16,17]. The sedimentation over time reduces a lubricant’s performance [18]. Moreover, an excessive concentration of nanoparticle additive incorporated in a lubricant can increase friction, thereby negatively impacting lubrication performance [12,19].

In order to mitigate the disadvantages of sedimentation and agglomeration, nanoparticles can be chemically modified with post-treatment techniques [20]. The chemical modification techniques are classified as single-step and two-step methods. Although the one-step method is economical, the additives synthesized by this process are not that compatible with low-vapor pressure-based lubricants. Additionally, the one-step method is not appropriate for bulk production [21]. As a consequence, the two-step method is relatively better for bulk production and synthesis of different types of nano lubricants. The two-step method consists of two steps. In the first step, the chemically modified nano additive is synthesized. In the second step, the synthesized nano additive is incorporated into a base lubricant oil. Despite the advantages of the two-step method, the nano lubricants synthesized by this method experience some limitations. One of the limitations of the synthesized additives by the two-step method is that they may exhibit higher instability (owing to the higher surface energy of the additives produced in bulk) when incorporated in base oil. Second, processes such as drying, storing, and transportation are not completely inevitable by this technique. Hence, one of the potential ways to solve the instability exhibited by the two-step method is to incorporate suitable surfactants like oleic acid (OA). OA is a simple, inexpensive, and commonly available in nature chemical that can help enhance the dispersion stability of nanoparticle additives incorporated in a base oil [22]. The benefit of the OA is that when incorporated in small amounts, it does not change the chemistry of a lubricant. The OA aids in the dispersion of nanoparticles incorporated in a base oil by electrostatic stabilization. The hydrophilic head of the OA contains the carboxylic acid group -COOH-, whereas the hydrophobic tail contains the hydro-carbon group from the lubricant [23]. The combination of the hydrophilic head and hydrophobic tail from the OA helps in improving the lubrication performance of nanoparticle additives when incorporated in a base lubricant oil. Often a dispersing agent is added along with a surfactant in a lubricant system to make the nano additives more stable [24].

Table 1. Attributes of tribological experiments carried out with a few functionalized lubricants.

Base Lubricant	Additives	Tribological Experiment Set-Up	Inferences from Results	Lubrication Mechanisms	References
Synthetic 5W-40	Oleic acid-functionalized MoS2 and hBN	Four ball tester	Reduction in COF by 13%, 21.5%, and wear rate by 90.9%, 91.6%	Protective-tribo film	[10]
PAO 40	Oleic acid-functionalized ZnO	Rotating ball on three pins configuration	0.25 wt.% nano lubricant reduced friction by 25% and wear by 82%	Rolling and Mending	[25]
500 SN	Oleic acid-modified hBN	Four ball tester	0.2 wt.% nano lubricant reduced COF by 19% and 0.4 wt.% nano lubricant lowered wear scar diameter by 22%	Formation of tribo-film	[26]
500 SN	Oleic acid-modified MoS2	Four ball tester	0.04 wt.% nano lubricant lowered COF by 41.2% and wear scar diameter by 17.1%	Protective film formed	[27]
Paraffin	Oleic acid-modified SiO2	Ball on ring tester	0.5 wt.% nano lubricant exhibited the lowest COF and 0.2 wt.% nano lubricant exhibited the lowest wear scar diameter	Formation of tribo-film	[28]

displays some of the functionalized solid lubricant additives incorporated in different base oil lubricants, their beneficial results, and associated lubrication mechanisms.

In a previous study conducted by the authors, it was noted that incorporating GNP and hBN solid lubricant additives did enhance the lubrication performance of the PO [17]. However, these additives over time are sedimented, thereby possessing a detrimental effect on the PO’s lubrication performance for a longer duration. Considering this research gap, in the present study, the authors examined the influence of OA functionalized GNP and hBN solid lubricant nano additives on the rheological, dispersion stability, thermal degradation pattern, and lubrication performance of the PO.

2. Experimental Details

2.1. Materials

In this research, PO and eight different nano lubricants were formulated. These nano lubricants comprised functionalized GNP [GNP (f)] and hBN [hBN (f)] nano lubricants. The PO was obtained from an outside vendor (details restrained owing to confidentiality request). A 99% pure OA was purchased from Home Science Tools, Billings, MT, USA. A 99.5% pure anhydrous Ethanol and a 98.5% pure Hexane were procured from Sigma Aldrich, St. Louis, MO, USA. A 99.5% pure GNP was purchased from Acros Organics, Waltham, MA, USA, and 99.5% pure hBN was obtained from Lower Friction, Mississauga, ON, Canada. Whatman filter paper, grade 1, Marlborough, MA, USA, was utilized for the wear debris analysis. For formulation of the GNP-based nano lubricants, GNP was incorporated in 0.5, 1.0, 1.5, and 2.0 wt.% to the PO. Similarly, for formulating hBN nano lubricants, hBN was incorporated in 0.5, 1.0, 1.5, and 2.0 wt.% to the PO. The relative density of the PO as mentioned by the provider was 0.77. The density of the OA provided by the supplier was 895 kg/m³. The major chemical constitutions of the PO are illustrated in

Table 2. Main chemical compositions of the PO.

Chemicals	Weight (%)
Complex hydrocarbon mixture	93.9
n-Heptane	1.5
n-Hexane	0.1
n-Nonane	1.4
n-Decane	1.4
2,2,4-Trimethylpentane	0.8

.

2.2. Characterization Tests

A scanning electron microscope [SEM] (JEOL JSM-601LA), JEOL USA, Peabody, MA, USA, was employed to characterize the morphology of the GNP and hBN solid lubricant nano additives. The parameters employed for the SEM tests were magnification: 1000× to 2000×, beam energy: 15 KV, and working distance: 10.0 mm.

2.3. Nano Lubricants Preparation

For the present study, nano lubricants were prepared by separately incorporating functionalized GNP and hBN into the PO.

Table 3. Properties of GNP and hBN nano additives.

Additive	Dimension (nm)	Purity (%)	Density (g/cm3)	Shape	Color
GNP	2 to 10	99.90	2.25	Layered	Grey
hBN	70	99.85	2.30	Hexagonal	White

elaborates on the properties of the nano solid lubricant additives deployed in this research.

For the creation of the GNP and hBN nano lubricants, two-step method was employed. In the two-step method, OA was used as a surfactant to lower the sedimentation of additives. The process for preparation of the 1.0 wt.% GNP (f) and hBN (f) additives was as follows:

(a)

Initially, OA was weighed to be 0.8 g and then added to two 100 mL beakers.

(b)

Then, 25 mL of ethanol was added to the OA in each of the beakers. Both these chemicals in the beaker were subjected to magnetic stirring, forming a solution.

(c)

After which, 0.2 g of GNP and hBN was separately added to the solutions in the two beakers.

(d)

The new solutions were subjected to heating (70 °C) using magnetic stirring. The speed employed using a heating plate cum magnetic stirrer equipment during the initial stages of the nano lubricant preparation was 100 rpm at 70 °C. The process was carried out until the entire ethanol had evaporated. During the functionalization process, ethanol was heated at a temperature slightly lower than its boiling point. This was undertaken to ensure that ethanol evaporated slowly, which helped the OA deposit, mix thoroughly, and obtain uniform functionalization with the solid lubricant additives. On completion of evaporation, the obtained product is OA-capped functionalized GNP and hBN nano additives.

(e)

Similarly, following the same procedure, the other concentrations of GNP and hBN were synthesized. The final content of the OA in each of the formulated nano lubricants was 0.8 g.

For the nano lubricants preparation, the functionalized GNP and hBN additives were separately added in varying weight fractions inside the beakers and mixed with the PO. In this study, 10 mL of the PO was separately mixed with 0.5, 1.0, 1.5, and 2.0 wt.% of GNP (f). Likewise, 10 mL of PO was individually blended with 0.5, 1.0, 1.5, and 2.0 wt.% of hBN (f). These specific weight fractions were selected based on some prior trial experiments conducted. Each nano lubricant was formulated individually. For the 0.5 wt.% concentration of a specific nano lubricant, 0.05 g of the SLA was mixed with 0.8 g of OA, and 11.95 mL of the PO. For the 1.5 wt.% concentration of nano lubricant, 0.15 g of the SLA was combined with 0.8 g of OA, and 11.95 mL of the PO. For the 2.0 wt.% concentration nano lubricant, 0.2 g of the SLA was blended with 0.8 g of OA, and 11.95 mL of the PO.

Later, the GNP and hBN-based nano lubricants were blended and mixed thoroughly using a vortex mixer (VWR, Radnor, PA, USA) to form homogeneous lubricants. Using the vortex mixer, the homogenous lubricants were shaken for around 30 min at room temperature. The speed chosen for mixing the nano lubricants using the vortex mixer was approx. 2340 rpm. This high speed made sure that the solid lubricant additives were homogeneously dispersed in the PO. Later, these lubricants were kept in an ultrasonicator for 2 h at 60 °C. This process made sure that the additives incorporated were homogeneously and uniformly dispersed in the PO. For the ultra-sonication process, the frequency involved during the mixing process of the nano lubricants was a constant value (as set up by the manufacturer) of 40 kHz.

On the synthesis of the functionalized GNP and hBN incorporated into the PO, the hydrophobic tail (from the oleic acid) becomes attached to the hydrocarbon group of the lubricant and consequently forms micelles. These formed micelles help in reducing van der Waals interaction between functionalized additives, thereby preventing the additives from sedimentation [29].

2.4. Viscosity Measurements

Viscosity is an important property of a fluid that is influenced by temperature conditions. The formulated lubricant’s kinematic viscosity was computed at 40 °C and 100 °C. To measure the kinematic viscosity of the nano lubricant samples, a Brookfield Ametek DV2T rotary viscometer was employed. Spindle number 62 of the viscometer was used at 40 rpm to perform the viscosity tests. The quantity of lubricant utilized during each of the viscosity tests was 10 mL. Initially, on obtaining the dynamic viscosity values of each of the lubricants from the viscometer, they were divided by the respective densities to obtain the kinematic viscosity. The kinematic viscosities of the lubricants obtained at 40 °C, and 100 °C, were useful to calculate the viscosity index (VI) of each of the lubricants. Viscosity index (VI) of a fluid is a critical parameter as it quantifies how much the fluid’s viscosity changes with its variation in temperature. A high VI indicates that the fluid’s viscosity changes comparatively less with temperature. On the contrary, a low VI indicates that the viscosity fluctuates significantly with temperature. A higher VI of a fluid is useful for machinery application purposes.

2.5. FTIR Tests

The different chemical groups present in the PO and the OA chemicals were analyzed using a Nicolet iS 50^TM spectrometer, Waltham, MA, USA. To analyze each of these chemicals, the background was noted using OMINIC software, Version 9.11. In this test, a minuscule drop of each of the chemicals was put on the head of the pressure tower to collect information pertaining to the entire spectrum of the chemical samples. The head and the disk of the spectrometer were thoroughly cleaned with acetone to avoid contamination.

2.6. UV Spectroscopy and Dispersion Stability Tests

To study the UV spectra at room temperature of the PO as well as that of the GNP (f) and hBN (f) nano lubricants, a Ultra Violet (UV-2550) spectrophotometer, Shimadzu, DC, USA, equipment was employed. Also, the dispersion stability of the GNP and hBN nano lubricants was examined at room temperature. There are two identical cuvettes utilized for the UV spectra and the dispersion stability studies. One of the cuvettes is filled with hexane and is regarded as a reference. The other cuvette is filled with the formulated nano lubricants separately [30]. The absorbance level of visible light was computed with this equipment for different intervals of time. The PO and the functionalized GNP, hBN nano lubricant’s dispersion stability were evaluated using ultraviolet-visible (UV) spectrophotometry absorbance. Prior to carrying out the UV tests, all the cuvettes were thoroughly cleaned with acetone. The importance of the UV test is that it aids in understanding the dispersion stability of additives incorporated in fluids and the additive’s effect on lubrication performance. A higher dispersion stability of nano additives incorporated in a lubricant implies significantly enhanced lubrication performance. In this UV study, the wavelength range of the UV spectra test was selected to be between 200 and 800 nm.

2.7. TGA Tests

Thermogravimetric analysis (TGA) aids in measuring a lubricant’s change in weight with regard to modification in temperature. In this research, the TGA test was utilized to determine the decomposition of PO, OA, GNP (f), and hBN (f) nano lubricants. The temperature range employed for the TGA test was between room temperature and 700 °C. The TGA test was performed in a nitrogen environment utilizing a TA instrument (QA 50 model) analyzer. During the test, the flow rate of nitrogen was kept at 10 °C/min.

2.8. Tribological Tests

For studying the friction and wear tests, a ball-on-disk setup (Rtec, MFT Tribometer, San Jose, CA, USA) was utilized. A 52100 alloy steel (McMaster-Carr, Elmhurst, IL, USA) of 6.35 mm diameter was employed as the ball material. Additionally, 50 mm diameter cylindrical-shaped Al6061 alloy was used as the disk material. The disk materials were also supplied by McMaster-Carr, USA. The Vickers hardness value of the disk was 107 HV, and that of the ball was 848 HV. The chemical composition of the ball and the disk materials as provided by the supplier is mentioned in

Table 4. Chemical composition (wt.%) of the ball and the disk samples supplied by vendors.

Sample Alloy	Fe	Cr	C	Mn	Si	S	P	Al	Mg	Cu
52100	96.5 to 97.32	1.3 to 1.6	0.98 to 1.1	0.25 to 0.45	0.15 to 0.3	0.025	0.025	-	-	-
Al6061	-	0.20	-	-	0.6	-	-	97.9	1.0	0.28

.

The Al6061 disks were polished in random directions to generate random surface textures. In the beginning, the disks were polished with 120, 240, 400, 600, and 1000 grit sandpapers (Buehler, Lake County, IL, USA). Then, the disks were cloth polished using diamond paste (1 and 0.05 microns). After the polishing process, the disks were noted to have an average surface roughness value, R_a = 0.1 ± 0.05 µm.

The ball-on-disk tribological tests were performed post-formulation of the nano lubricants. Prior to executing the ball-on-disk tests, all the test samples were washed. Later they were cleaned with an acetone solution. This was followed by cleaning the samples in the ultrasonicator. Lastly, all the samples were dried utilizing an air blower. Additionally, post-completion of individual ball-on-disk tests, the test specimens were completely cleaned utilizing hexane solution in the ultrasonicator. This was followed by drying the specimens in hot air using the air blower. The ball-on-disk tests were conducted following the test parameter criteria mentioned in

Table 5. Parameters for ball-on-disk tests.

Test Descriptions	Mathematical Value
Sliding distance (m)	100
Angular Velocity (rpm)	21.5
Applied Load (N)	30
Test duration (seconds)	2800
Maximum Hertzian Contact Pressure (GPa)	1.25

. The test criteria for the ball-on-disk tests were selected in such a way that they followed the boundary lubrication regime.

During the tribological experiments, the ball and the disk were totally submerged in the lubricant. This was undertaken to make sure that the tribo-pair was continuously lubricated throughout the complete experiment. The quantity of lubricant used during each of the ball-on-disk tests was 10 mL. The ball-on-disk tests were executed at room temperature (around 25 °C) and lab humidity environments (approximately 15% relative humidity). Individual tests were conducted three times under the same testing environment to show the reproducibility of the experimental data. Table 5 illustrates the parameters for the ball-on-disk tests.

The testing parameters mentioned in Table 5 were chosen based on previous research performed by the authors for non-functionalized GNP, and hBN-based nano additives incorporated in the PO lubricant [17]. The test parameters helped us understand the lubrication performance of PO and its nano lubricants under the boundary lubrication regime. Demirsoz [31] investigated wear characteristics of a bearing material using a pin-on-disk module under lubricated conditions. The lubricant was incorporated with a number of SLAs. The study was intended for application in the rolling process. The test conditions selected were a sliding velocity of 15 mms⁻¹ and a normal load of 30 N. It was noted that an increase in the number of SLAs caused an increase in friction coefficient as well as wear loss. Cabrera et al. [32] performed a pin-on-disk equipment using blended jatropha bio-lubricant to simulate wet clutch operation. They used a sliding velocity of 0.05 ms⁻¹ with a contact pressure of 2.3 MPa. The bio-lubricant displayed favorable anti-shudder performance, and was found to be suitable for automobile applications. Gujar et al. [33] utilized high-velocity oxygen fuel (HVOF) coating on SS316L substrate to investigate the coating’s benefit during hydraulic fracturing operation. They used a pin-on-disk setup using parameters of sliding speed of 0.05 ms⁻¹ and a normal load of 20 N. The hydraulic fracturing operation is employed for extraction of fuels and oils from deep ground of the earth. Results showed that the HVOF coating helped enhance the substrate’s work life by almost three times. Our present research utilized a similar range of testing parameters for potential industrial applications.

2.9. Post-Test Characterization

Post completion of the ball-on-disk tests, the wear track profiles of the disks were recorded and investigated using a noncontact Nikon 3D optical profilometer with 10× magnification. The profiles were useful to compute the wear volume of the disks. The 3D profiles of the worn tracks were later modified into 2D data to obtain the wear area. The wear track radius (r) was 23 mm. The wear area was multiplied by the circumference of the wear track (2πr) to obtain the Wear Volume data. After completion of the tribological tests, the worn tracks were investigated with a Raman microscope (Renishaw Equipment, Kane County, IL, USA) to analyze both the GNP (f) and hBN (f) additives deposited.

3. Results and Discussion

3.1. Morphological Analysis of GNP and hBN Additives

The GNP and hBN nanoparticles in powder form were investigated using a scanning electron microscope (SEM) to understand their morphology. Figure 1 shows the morphology and surface characteristics of the nano GNP and hBN nanoparticles. The image reveals the layered structure of the GNP and hBN nanoparticles. This knowledge of the nanoparticle’s structure will be beneficial to explain the proposed lubrication mechanism.

3.2. Viscosity and Film Thickness

Viscosity is a key factor that is utilized to determine the internal friction and resistance to flow exhibited by a fluid. Figure 2 illustrates the kinematic viscosity of PO, GNP (f) (Figure 2a), and hBN (f) (Figure 2b) nano lubricants at 40 °C and 100 °C. The figure explains the influence of the concentrations of the GNP (f) and hBN (f) nano additives on the kinematic viscosity of the PO with regard to temperature. In this study, it was noted that the kinematic viscosity values of the PO, the GNP (f), and the hBN (f) nano lubricants were relatively lower. This confirmed the fact that these lubricants were of a low viscous nature. It is observed from the figure that with a rise in the concentration of the GNP (f) and hBN (f) nano additives, there was an increment in the kinematic viscosity of the PO. The reason for this increment could be explained on the basis that when small quantities of GNP (f) and hBN (f) nano additives are incorporated into the PO separately, they tend to attract each other owing to van der Waals forces. The increment in concentration of the additives in the PO will cause layers between the oil to have restricted mobility. This will cause an increment in the kinematic viscosity [34]. It was also noted that with an increment in temperature, the kinematic viscosity of the nano lubricants decreased. Moreover, it was observed that the increment in kinematic viscosity, with an increase in the additive’s concentration, was not very noteworthy.

Viscosity index (VI) was measured for all the studied lubricants to understand their contrast in viscosity with respect to modification in temperature. To compute the VI of all the studied lubricants, as per ASTM D-2270 standard [35], the following standard equation was used (Equation (1)):

$$VI=\frac{L-U}{L-H}\times 100$$

(1)

where L depicts a fluid’s kinematic viscosity at 40 °C with VI of 0; H is the fluid’s kinematic viscosity at 40 °C possessing VI of 100. U is the kinematic viscosity of the studied lubricant at 40 °C. The numerical values of L and H are reasonable when the fluid’s kinematic viscosity is less than 70 cSt at 40 °C. For a fluid, possessing kinematic viscosity of more than 70 cSt, L and H are computed by the below equations (Equations (2) and (3)):

$$L=0.8353Y^2+14.67Y-216$$

(2)

$$H=0.1684Y^2+11.85Y-97$$

(3)

where Y is the kinematic viscosity of the fluid at 100 °C.

Table 6. Kinematic viscosity and viscosity index (VI) of PO, GNP (f), hBN (f) lubricants.

Lubricant Sample	Kinematic Viscosity (cSt) at 40 °C	Kinematic Viscosity (cSt) at 100 °C	Viscosity Index (VI)
PO	10.63	3.13	171
PO + 0.5 GNP (f)	10.86	3.43	221
PO + 1.0 GNP (f)	10.98	3.54	237
PO + 1.5 GNP (f)	11.12	3.59	240
PO + 2.0 GNP (f)	11.37	3.71	251
PO + 0.5 hBN (f)	10.99	3.53	234
PO + 1.0 hBN (f)	11.11	3.62	246
PO + 1.5 hBN (f)	11.25	3.71	257
PO + 2.0 hBN (f)	11.53	3.79	259

depicts the kinematic viscosity and VI of all the studied lubricants. It was noted here that with the incorporation of the GNP (f) and hBN (f) additives, the VI of the PO was significantly enhanced. With the increase in the concentration of both the additives, the VI of the PO was further increased. For both the GNP (f) and hBN nano lubricants, the highest VI was observed at 2.0 wt.% concentrations. It could be inferred that the incorporation of the additives was beneficial for improving the VI of the PO.

For the purpose of better explaining the interaction of lubricant with surfaces, it is important to determine its lubrication regime. The Hamrock and Dowson equation is utilized to estimate the film thickness of the lubricant. The fluid film thickness parameter (λ) is calculated by the Equation (4) [36]:

$$\lambda =\frac{{\mathcal{h}}_0}{\sqrt{{R_{q_1}}^2+{R_{q_2}}^2}}$$

(4)

In the above equation, ${\mathcal{h}}_0$ is the minimum film thickness (m); R_q1 and R_q2 are the root mean square (rms) roughness of the tribo-pairs. The above equation provides information on the nature of the lubricating regime of the lubricant by examining the lubricant fluid film and combined asperity heights of the considered tribo-pair. Using mathematics, the minimum fluid film thickness (${\mathcal{h}}_0$) can be calculated employing the below-mentioned Equation (5):

$${\mathcal{h}}_0=3.63{\mathcal{R}}^{\prime }{\left(\frac{U{\eta }_0}{E^{\prime }{R}^{\prime }}\right)}^{0.68}{\left(\alpha {\mathrm{E}}^{\prime }\right)}^{0.49}{\left(\frac{w}{E^{\prime }{{\mathcal{R}}^{\prime }}^2}\right)}^{-0.073}\left(1-{\mathcal{e}}^{-0.68k}\right)$$

(5)

In the above equation, ${\mathcal{h}}_0$ is the minimum film thickness (m), and $R^{\prime }$ is the reduced curvature radius. Additionally, 1/$R^{\prime }$ = 1/R_x + 1/R_y are the reduced curvature radius in the x and y directions. $U$ is known as the entraining surface velocity (m/s), and $U$ = (U_A + U_B)/2, where U_A and U_B highlight the velocity of the pin and the disk. ${\eta }_0$ is the viscosity of the studied lubricant at atmospheric pressure (Pa s). Also, $E^{\prime }$ (Pa) is the reduced Young’s modulus from the pin, and the disk. The term $\alpha$ is represented as the pressure viscosity coefficient (m²/N). This term is mathematically represented as $\alpha$ = (0.6 + 0.965 log₁₀ ${\eta }_0$) × 10⁻⁸. In the same equation, $w$ is the constant applied load (N). The ellipticity parameter, $k$ is represented as k = a/b, where a (m) is the semi-axis of the contact ellipse in the transverse direction. Further, b (m) is the semi-axis in the direction of the motion.

If the value of $\lambda$ < 1, it indicates a boundary lubrication regime. If 1 ≤ $\lambda$ ≤ 3, it indicates a mixed lubrication regime. If $\lambda$ > 3, it indicates a hydrodynamic lubrication regime. On conducting calculations, we have values as follows: $R^{\prime }$ = 0.0125 m, $U$ = 0.0518 m/s, ${\mathrm{E}}^{\prime }$ = 115.95 × 10⁹ Pa, $\alpha$ = 2.74 × 10⁻⁸ m²/N, ${\mathcal{h}}_0$ = 3.5768 × 10⁻⁸ m, and $\lambda$ = 0.2529. These values confirm that our chosen test parameters result in a boundary lubrication regime.

From the obtained value of dynamic viscosity, and the calculated value of pressure viscosity coefficient, the plastic oil can be classified as a medium VI oil with a sub-classification of Light Machine Oil [37].

3.3. FTIR Analysis

FTIR analysis was conducted pertaining to the PO and the OA chemicals. The observations from the FTIR test imparted data on the presence of various functional groups present in the PO as well as the OA.

Table 7. The FTIR spectra for PO and OA.

Chemicals	Wavenumber Range (cm−1)	Functional Groups	Plastic Oil (PO)	Oleic Acid (OA)
Zone 1	3700–3200
Alcohol	3650–3200	-O-H	×	×
Alkyne	3340–3250	≡C-H	×	×
Zone 2	3200–2690
Aryl or Vinyl sp2 C-H bond	3100–3000	≡C-H	×	×
Alkyl sp3 C-H bond	2960–2580	≡C-H	√	√
Carboxylic acid	3000–2500	-COOH	×	×
Zone 3	2380–1990
Alkyne	2260–2000	-C≡C-	×	×
Zone 4	1850–1650
Ketone	1750–1705	=C=O	×	×
Aldehyde	1740–1720	>C=O	×	×
Carboxylic acid	~1700		×	√
Zone 5	1650–1450
Alkene	1650–1620	=C=C=	×	×
Benzene	~1600 and ~1500	Benzene ring	×	×
C-H bond stretching vibration in presence of alkenes	1470–1450	≡C-H	√	√

“×” means Absent, and “√” means Present.

depicts the information obtained from the FTIR test analyzed employing the five-zone analysis method [38]. The table shows five zones that are sub-categorized into various functional groups. In the table among the various functional groups mentioned, Zone 4’s Carboxylic Acid group was the most distinguishable one between PO and OA. The functional groups pertaining to Zone 4 were only present in OA. In Zone 2, the functional group of alkyl bonds was omnipresent in both the PO and the OA. The presence of the OA is advantageous for lubrication as it supports the functionalization process. Moreover, Zone 5’s C-H bond structures are present in both the PO and the OA. The remaining functional groups pertaining to the other Zones are not present in the PO and the OA, as noted in the table.

Figure 3 shows the FTIR spectra of the PO and the OA. It was observed that the PO and OA contain an Alkyl sp³ C-H bond and C-H bond stretching groups. In the OA, it was noted that the peak at 1750 cm⁻¹ indicated the existence of an ester group. However, this peak was absent in the PO. In the wavelength range between 2500 and 3500 cm⁻¹, there were three distinguished peaks observed for the PO. In the same range, two peaks were observed for the OA. The presence of carboxylic acid has an advantage in improving the lubricating property of the PO. The COOH’s polar nature helps it effectively adhere to a metallic surface as well as develop a monolayer, thereby reducing friction and wear of the interacting tribo-pair. Wavenumbers lower than 1500 cm⁻¹ were not significant and hence not studied.

3.4. Dispersion Stability Analysis

Figure 4 illustrates the UV spectra of the studied lubricants and their dispersion stability up to 24 h. Figure 4a depicts the UV spectra of PO, 1.0 wt.% GNP (f), and 1.0 wt.% hBN (f) nano lubricants. Here, absorbance is contrasted against wavelength for the studied lubricants. It was noted from the figure that PO had the highest peak, followed by the 1.0 wt.% GNP (f) nano lubricant, and the lowest by the 1.0 wt.% hBN (f) nano lubricant. The majority of the three lubricant’s peaks were observed to be between the 300 and 350 nm range of wavelength. It could be inferred that the functionalization process on the GNP and hBN was beneficial in lowering the absorbance level of their nano lubricant. To better study the dispersion stability of the GNP (f) and hBN (f) nano lubricants, they were observed for 24 h. Figure 4b elucidates the dispersion stability of the 1.0 wt.% GNP (f), and 1.0 wt.% hBN (f) nano lubricants. With the progress of time, it was noticed that the GNP (f) nano lubricant remained stable throughout for 24 h. On the other hand, the hBN nano lubricant did sediment, but it was not significant, and was still considered to be stable enough. The sedimentation of the hBN nano lubricant was more prominent after 8 h. This was confirmed by the absorbance values shown on the y-axis. The repulsive forces created by the OA surfactant in this study were responsible for the stability exhibited by the GNP (f) and hBN (f) nano lubricants [24].

3.5. TGA Analysis

The thermal stability of the PO, OA, 1.0 wt.% GNP (f), and 1.0 wt.% hBN (f) lubricant samples were investigated using the TGA test, as shown in Figure 5. It was noted that among these samples, the OA exhibited the highest thermal stability, followed by the hBN (f) and GNP (f) lubricants, and the least by the PO. It was observed that the OA exhibited an initial decomposition temperature of around 150 °C. Moreover, the PO had an initial decomposition temperature of less than 50 °C. The functionalization process was beneficial and helped the GNP (f) and hBN (f) nano lubricants slightly enhance the initial decomposition temperature in contrast to that of the PO. The GNP (f) and hBN (f) nano lubricants exhibited an initial decomposition temperature greater than 50 °C. The final decomposition temperature of the OA was observed to be around 275 °C. The final decomposition temperature of the PO was approximately 210 °C. Moreover, the final decomposition temperature of the GNP (f) and hBN (f) nano lubricants was enhanced relative to the PO. The final decomposition temperatures of the hBN (f) and GNP (f) nano lubricants were approximately 250 °C and 220 °C. From the TGA plot, it could be inferred that the working temperature of the PO-based nano lubricants for application purposes is less than 300 °C. The addition of the GNP and hBN additives separately into the PO provided a few advantages [39]. The first advantage was that the oxidation of the PO could be slowed by a specific temperature range. The second advantage was weight loss of the PO was delayed by a particular temperature range. The volatility/thermal stability of the PO was modified by the incorporation of the functionalized nano additives. Solid lubricant additives of GNP and hBN (incorporated in lubricants) possess desirable thermal conductivity and chemical stability, which helps in the efficient dissipation of heat [40,41]. These properties of the additives along with the OA surfactant lower the thermal degradation of the PO, and aid in maintaining its lubricating properties over an extensive range of temperatures (Figure 5).

3.6. Friction Analysis

Figure 6 explains the coefficient of friction (COF) data of the PO, GNP (f) nano lubricants. Figure 6a depicts the COF with respect to time for the studied lubricants. Here, the COF data are obtained from PO, 0.5, 1.0, 1.5, and 2.0 wt.% GNP (f) lubricants. It could be observed that initially, the lubricants exhibited a higher COF. However, with the progress of time, it was perceived that the GNP (f) nano lubricants generally lowered the COF compared to the PO. It was noticed from the same figure that 0.5 wt.% GNP (f) nano lubricant illustrated the highest COF, followed by the 1.0, 2.0, and 1.5 wt.% GNP nano lubricants. It was noticed that almost toward the end of the test, the 0.5 wt.% GNP (f) nano lubricant demonstrated a higher COF contrasted to the PO. This could be inferred as modest lubrication performance demonstrated by the GNP nano lubricant at lower concentrations. The 1.0 and 2.0 wt.% GNP (f) nano lubricants were seen to exhibit relatively lowered fluctuations pertaining to the COF during the entire test duration. This can be reasoned by the fact that these nano lubricants developed a suitable protective layer that helped in effective shearing, thereby lowering the COF [42]. The oleic acid adsorbed on the solid lubricant additive’s surface helped suppress the van der Waals attractions between the additives through repulsive forces. The functionalized nano lubricants exhibited a synergistic behavior. Also, this observation explained the fact that these nano lubricants were relatively more stable compared to the others. Figure 6b indicates the COF data at the end of the test for the PO, and the functionalized GNP (f) nano lubricants. The 1.0, 1.5, and 2.0 wt.% GNP nano lubricants were observed to reduce the COF by 20.0, 53.0, and 29.5%, respectively, in contrast to the PO. However, the 0.5 wt.% GNP nano lubricant was noted to increment the COF marginally by 1.58% compared to the PO. This led to the understanding of the fact that a too-low concentration of the GNP (f) was not beneficial for improving the lubrication performance. From Figure 6b, it can be inferred that the 1.5 wt.% GNP (f) lubricant exhibited the lowest COF among them all. Moreover, it was noted that an increase in concentration beyond the 1.5 wt.% GNP (f), which is the 2.0 wt.% GNP (f) nano lubricant, led to an increase in the COF. This observation is attributed to the fact that the GNPs slightly agglomerated owing to their high surface energy [43,44]. It was realized that an excess concentration of nano additive incorporated in a lubricant could have a detrimental effect on the COF. Overall, it can be inferred that for the layered-like structure of graphene nano platelets meant for enhancing the lubrication performance of the PO, the optimal concentration providing the lowest COF was 1.5 wt.%.

Figure 7 explains the COF data of the PO, hBN (f) nano lubricants. Figure 7a indicates the lubricant’s variation in COF with time from the ball-on-disk tests. It was observed here that small concentrations of 0.5, 1.0, 1.5, and 2.0 wt.% of hBN (f) nano lubricants could modify the PO’s COF. It was noted that the 1.0, 1.5, and 2.0 wt.% hBN (f) nano lubricants exhibited almost identical COF plots compared to the PO. The 1.5 wt.% hBN (f) nano lubricant was seen to exhibit the highest COF. The 0.5 wt.% hBN (f) nano lubricant exhibited superior lubrication performance by providing the lowest COF. This 0.5 wt.% hBN nano lubricant helped in improving the lubrication performance by lowering the asperity–asperity contacts, and shearing action [45]. It could be also observed from the graph that this nano lubricant experienced lower fluctuations and remained almost stable throughout the entire test. However, the other lubricants experienced more fluctuations throughout the entire test. Figure 7b highlights the COF values of the studied hBN (f) nano lubricants at the end of the test. The 1.0 and 1.5 wt.% hBN (f) nano lubricants were observed to increase the COF by 2.38 and 3.96% compared to the PO. However, the 0.5 and 2.0 wt.% hBN (f) nano lubricants were noted to lower the COF by 63.63% and 11.40% relative to the PO. The lowest COF exhibited by the 0.5 wt.% hBN (f) nano lubricant can also be ascribed to the modification in its surface energy. This is due to this incorporated functionalized hBN nano additive proving better binding properties with the PO, thereby improving dispersion and reducing agglomeration. A rise in the concentration of the nano lubricants did increase the COF slightly compared with the PO; however, the increase was not significant. It can be inferred that the higher concentrations of the functionalized hBN nano lubricants exhibited antagonistic behavior.

3.7. Worn Surface Analysis

Figure 8 shows the 3D worn disk surface profiles acquired by an optical profilometer. Figure 8a corresponds to the worn track of PO, Figure 8b corresponds to 0.5 GNP (f), Figure 8c corresponds to 1.0 GNP (f), Figure 8d corresponds to 1.5 GNP (f), and Figure 8e corresponds to 2.0 GNP (f). These figures illustrate the width and height of the wear track from each of the lubricant samples. It was seen that nearly all the lubricant samples exhibited similar types of wear tracks. This explained the fact that the nano lubricants could not lower the wear track area compared to the PO. However, neither did they significantly increase the wear area with an increase in their concentrations. These observations explained the benefits of the functionalized GNP nano lubricants. The analysis from this study was advantageous to compute the wear volume of each of the functionalized GNP lubricant samples discussed in the subsequent Section.

Figure 9 shows the 3D worn disk surface profiles acquired by an optical profilometer. Figure 9a corresponds to the worn track of PO, Figure 9b corresponds to 0.5 hBN (f), Figure 9c corresponds to 1.0 hBN (f), Figure 9d corresponds to 1.5 hBN (f), and Figure 9e corresponds to 2.0 hBN (f). These figures illustrate the width and height of the wear track from the PO and the hBN (f) lubricant samples. It was observed that the hBN (f) nano lubricants demonstrated a higher wear area in contrast to that of the PO. The 0.5 wt.% hBN (f) nano lubricant sample was seen to exhibit the smallest wear area compared to the others. This explained the fact that the hBN (f) nano lubricants and their increase in concentrations were not beneficial for reducing the wear area. The analysis from this study was useful for calculating the wear volume of each of the functionalized hBN lubricant samples, as discussed in the next section.

3.8. Wear Volume Analysis

Figure 10 illustrates the changes in wear volume for the PO, GNP (f), and hBN (f) lubricants. Figure 10a explains the contrast in wear volume with a change in concentration of GNP (f) nano lubricants. It was observed that the 0.5 wt.% GNP (f) nano lubricant lowered wear volume by 3.16% relative to the PO. However, the 1.0, 1.5, and 2.0 wt.% GNP (f) nano lubricants increased wear volume by 26.73, 2.47, and 12.91% compared to the PO. It could be inferred that the 0.5 wt.% GNP (f) nano lubricant provided the smallest wear volume. Moreover, concentrations beyond 0.5 wt.% GNP (f) nano lubricants were observed to increase the wear volume insignificantly. Except for the 1.0 wt.% GNP (f) nano lubricant, the 1.5 wt.% and 2.0 wt.% GNP (f) nano lubricants demonstrated a minimal increase in the wear volume. The beneficial micelles formed from the functionalization process and surface active organic oleic acid leading to corrosive type wear were responsible for the insignificant increase in the wear volume [46].

Figure 10b exhibited the variation in wear volume with a rise in the concentration of the hBN (f) nano lubricants. It was noted that the 0.5, 1.0, 1.5, and 2.0 wt.% hBN (f) nano lubricants exhibited 50.08, 208.69, 354.11, and 598.63% higher wear volume compared with the PO. It was understood that the oleic acid-functionalized hBN nano lubricants exhibited an antagonistic behavior pertaining to wear volume when studied for the PO. It was observed that among all the nano lubricants, the 0.5 wt.% hBN (f) nano lubricant showed the lowest increase in wear volume. The increment in concentration of the hBN (f) nano lubricants was observed to significantly increase the wear volume. The increment in concentration of hBN (f) nano lubricants causing an increased wear volume could be attributed to three reasons. One is the existence of surface-active organic oleic acid leading to corrosive type wear [47]. Second is the larger size of the hBN (f) additives, as per the steric stabilization theory that could not allow it to remain stable in the PO [48]. Third is the increase in the size of the hBN additives after the functionalization process that did not allow them to effectively cover the asperities. The used test parameters correspond to the small sliding distance and time. These parameters result in modest wear of the disks used, and it indicates the beginning of the wear phenomenon. On increasing the number of test cycles, friction will remain more or less constant. As per Archard’s theory, by increasing the number of sliding distances and time, the disks will wear out more when compared to the existing operational test parameters.

4. Lubrication Mechanisms

Figure 11 describes the reaction process involved in the functionalization of the studied nanoparticle additives incorporated in the PO. The figure highlights the schematic of the functionalization process pertaining to the GNP/hBN nanoparticle additives. During the functionalization process, the GNP and hBN additives become embedded with the oleic acid. As a result, an oleate layer is formed. The functionalized hBN and GNP additives are later incorporated into the PO. It is expected that the hydrophilic head from the OA is adsorbed with the GNP and hBN additives separately. Moreover, the hydrophobic tail of the OA becomes attached to the hydro-carbon chain from the PO. This hydrophobic tail attached to the PO helps in the formation of micelles. The micelles being slippery in nature aid in shearing, thereby helping in improving the lubrication performance of the PO.

Raman spectra analysis was performed for the nano lubricants that showed the lowest COF (1.0 wt.% GNP (f) and 1.0 wt.% hBN (f)). Figure 12 highlights the Raman spectra for a section of the wear track 1.0 wt.% GNP (f) nano lubricant and 1.0 wt.% hBN (f) nano lubricant. It was observed from Figure 12a, pertaining to the wear track of the 1.0 wt.% GNP (f) nano lubricant, that the wavelength recorded was between a range of 100 and 3200 cm⁻¹. It was noted that this wear track provided three major peaks at 1385, 1584, and 2717 corresponding to D, G, and 2D modes of graphene. The ratio between these modes provides an idea of the structural defects and number of layers present within the GNP [49]. The ratio of I_D/I_G mode from the wear track of the GNP (f) nano lubricant was observed to be 0.87. Here, the existence of a lower D band intensity compared to that of the G band indicated the presence of less defects in the GNP [50]. Similarly, the I_2D/I_G mode from the same wear track was noted to be 0.58. Conversely, the I_G/I_2D was calculated to be 1.74. From the literature, this mode ratio having a magnitude of greater than one confirmed that there were multiple layers of GNP sheets deposited on the wear track, which aided in providing good lubrication performance [51]. These peaks validated the presence of deposited GNP (f) on the wear tracks that were responsible for showing significant anti-friction performance. Figure 12b explains the Raman spectra pertaining to the wear track of the 1.0 wt.% hBN (f) nano lubricant. One major peak was noted for the wear track of this nano lubricant at 1367 cm⁻¹. This peak established the existence of hBN additive on the worn track.

In the process of the tribological tests, the deposited GNP (f) and hBN (f) additives become deformed and act as cover to the interacting tribo-pair. These additives aid in sustaining the continuous tribo performance of the system and help improve the lubrication performance. The PO facilitated the establishment of tribo-film. This tribo-film, along with the deformed GNP (f) and hBN (f) additives, was responsible for the desirable lubrication performance shown by the PO-based nano lubricants. The deformed additives filled up the wear track of the interacting tribo-pair, aided in reducing the asperity contacts. Subsequently, they helped in boosting the lubrication performance of the overall system.

Figure 13 shows the SEM image and EDS spectrum of wear debris obtained from the 1.0 wt.% GNP (f) and hBN (f) nano lubricants. Figure 13a represents an SEM image from a section of the 1.0 wt.% GNP (f) nano lubricant obtained after ball-on-disk test. The image consisted of wear debris and GNP (f) additives. From the figure, the wear debris morphology can be classified as normal rubbing particles. Figure 13b represents the EDS spectrum pertaining to the observed wear debris. The majority of the elements noted here were from Al, O, and Si. A higher percentage of detection of the Al did confirm that the wear debris was indeed from the Al6061 disk. Figure 13c highlights the SEM image of a section of 1.0 wt.% hBN nano lubricant acquired after the ball-on-disk test. Here, a large size of wear debris was observed, which was much bigger than the wear debris generated for the GNP (f) nano lubricant. Figure 13d elucidates the EDS spectrum pertaining to this chunk of wear debris. In this spectrum, elements of Al and O were observed. However, a higher percentage was detected from the Al, which confirmed that they were from the Al6061 disk. The larger wear debris confirmed the fact that the hBN (f) nano lubricant demonstrated higher wear volume contrasted to the GNP (f) nano lubricant. This observation explained that the hBN (f) nano lubricant sample exhibited a not-so-desirable and only modest lubrication performance for the system.

Figure 14 highlights the friction and wear mechanism of this experimental study. While explaining the friction mechanism, for the first case pertaining to the PO, the tribo-pair is segregated by a thin film (Figure 14a). As a consequence, there will be more interactions between the tribo-pair leading to scratches. Here, the surface asperities will be damaged, thereby wearing out the softer tribo-pair and producing wear debris that might become entangled with the interacting pair. For the second case pertaining to the GNP (f) nano lubricants, the GNP nano additives deform under applied pressure (Figure 14b). The deformed GNP additives deposit on the wear track, as well as help in lowering shearing action, thereby lowering COF. The micelle-type structures from the functionalized GNP, and the fatty acid chains from the OA, also aid in lowering the COF. Moreover, the relatively smaller size of the GNP additives helped them enter the asperities more easily, and that is why almost all the concentrations of GNP nano lubricants exhibited a consistent and lower COF. As a result of all these factors, the GNP additives were useful for improving the lubrication performance of the PO.

For the third case pertaining to the hBN (f) nano lubricants, the functionalization process helped in reducing the agglomeration of the hBN additives (Figure 14c). The applied pressure deforms the hBN nano additives, then they were deposited on the worn track, thereby helping in reducing the shearing action. Moreover, the presence of micelle structures associated with the best-performing hBN additives from the functionalization process, and the fatty acid chains from the OA, help in improving the lubrication performance [52].

It can be realized that the total mechanisms involving deformation, deposition, and shearing from the functionalized GNP and hBN additives supported in improving the overall lubrication performance of the PO.

When studying the lubrication process pertaining to the PO, the film thickness is less (boundary lubrication). This will result in the generation of a higher amount of wear from the softer disk during the rubbing action. When the GNP (f) nano lubricants are studied, a small concentration of the GNP additives helps in lowering the wear volume between the interacting tribo-pair. On application of load, these small-sized GNP additives become mechanically deformed. This deformation helped in forming a layer that reduced the asperity contacts of the tribo-pair, thereby lowering further wear [53,54]. Increasing the concentration of the GNP additives had a not-so-significant impact on the increment in wear volume. This signifies that the GNP additives did not agglomerate and were more available at the asperity–asperity contacts resulting in a lower wear volume. When the hBN (f) nano lubricants were studied, it was noted that increasing their concentrations was not beneficial for reducing wear volume. Especially, very low concentration of hBN (f) nano lubricant exhibited a higher wear volume almost similar to that of the base PO lubricant. This can be reasoned by the fact that at very low concentrations, the hBN (f) could not develop a significant protective layer, and that is why the wear volume was relatively higher. However, a higher increase in concentrations of the hBN (f) nano lubricants caused an even more significant increase in the wear volume. The OA’s inherent acidic nature was responsible for the increase in the wear volume. Moreover, the larger size of the hBN (f) nano additives could not efficiently cover the asperities, thereby increasing the wear volume. The increment in wear volume could also be attributed to the antagonistic interactions between the PO, the OA, and the hBN nano additives. The antagonistic interactions and load applied did lead to the formation of a layer, but not a protective one. There was suppression of adhesion of the deformed functionalized hBN on the wear track, thereby not forming the protective layer [55]. As a consequence, the layer was not useful for lessening wear volume.

In summary, it could be inferred that the effect of concentration by the two different SLAs, for improving the lubrication performance of the PO, was very distinct. For multiple-layered-like GNP (f) additives aimed at enhancing the lubrication performance of the PO, the optimal concentration yielding the lowest COF and wear volume was 1.5 wt.% and 1.0 wt.%, respectively. However, for hexagonally structured hBN (f) additives, the optimal concentration providing the lowest COF and wear volume was 0.5 wt.% for both parameters.

Figure 15 illustrates the surface interaction mechanisms shown by the GNP (f) and hBN (f) nano lubricants. The surface interaction schematic prior to the conduction of the ball-on-disk tests is explained in Figure 15a. Figure 15b illustrates the schematic of the surface interaction study after the ball-on-disk tests. The lubrication process can be mainly explained in two ways. One, the OA aids in attaching its hydrophilic head with the GNP or hBN, thereby assisting in the functionalization process. Second, the hydrophobic tail of the OA attaches to the alkene and alkyne compounds from the PO. As a consequence, there will be the formation of two distinct layers. Also, because of the differences in polarity, the layers will remain segregated from each other. This segregation helps in slowing the sedimentation of GNP and hBN, thereby reducing their agglomeration in the PO. Additionally, there will be a weak van der Waals interaction present between the alkene and alkyne groups of the functionalized GNP and hBN, and from the PO. This weak interaction will aid the nanoparticles, the OA, and the PO to remain uniformly dispersed, thereby aiding in good lubrication performance. Moreover, one part of the functionalized GNP or hBN additive will bind with the Aluminum (Al) from the worn track by electrostatic interaction. The other part of the functionalized additive links with the Oxygen (O) (Figure 15). Moreover, on the application of load, during the ball-on-disk tests, the GNP (f) and hBN (f) additives helped in easy shearing, deformation, deposit, and adhering to the worn surface [56,57]. The adhered additives were observed from the Raman spectroscopy test in this research. Consequently, the deformed and adhered additives helped lower the asperity contacts of the interacting tribo-pair, supplemented the tribo-film, and, overall, improved the lubrication performance of the PO. All these interactions are important for the noteworthy improvements in the lubrication performance exhibited by the nano lubricants.

A previous study by the authors investigated the role of non-functionalized GNP and hBN additives incorporated into the PO for improved lubrication performance [17]. It was found that the non-functionalized additives enhanced the viscosity of the PO. From the present study of functionalized additives, it was noted that the functionalization process was beneficial in further enhancing the viscosity of the PO. Furthermore, the non-functionalized nano additives were observed to lose dispersion stability in the PO after 12 h of duration. On the contrary, the functionalized nano additives from the present study were observed to be relatively stable for up to 24 h. Interestingly, the functionalized nano lubricants were observed to exhibit similar or marginally better thermal degradation behavior compared to the non-functionalized lubricants. This explained the fact that the functionalization process on the nano additives did not hamper the degradation pattern of the PO.

Moreover, it was noticed that the best-performing non-functionalized GNP and hBN nano lubricants lowered COF by 41% and 17% in contrast to the PO. However, the functionalization process on the GNP and hBN additives further helped in lowering the COF by 53% and 63% relative to the PO. When the functionalization process was studied, it involved long chains of the OA wrapped around the surface of the additives. This led to the formation of micelles that induced repulsion amongst adjacent additives, thereby lowering the van der Waals interaction and sedimentation. As a result, the additives remained uniformly dispersed in the PO, and can be effectively more available at the contacting tribo-pair surfaces during rubbing [10]. In this way, the functionalized additives aided in reducing the COF. Additionally, non-functionalized GNP and hBN nano lubricants reduced wear volume by 563% and 65% in contrast to the PO. On the contrary, the functionalized GNP (f) and hBN (f) nano lubricants increased wear volume by 26.72% and 598.63%. It could be in general deduced that, relatively, the functionalized nano lubricants were more suitable for enhanced viscosity, dispersion stability, thermal degradation patterns, and friction reduction compared to the non-functionalized lubricants. However, the non-functionalized nano lubricants were more appropriate for reducing wear behavior.

It can be reasoned from this research that, comprehensively, the functionalized GNP and hBN nano lubricants lead to providing better lubrication properties by the amalgamation of homogenous dispersion, lowering agglomeration, and mechanical deformation, owing to the applied load, with the deformed additives being deposited on the wear track. These processes, overall, helped the functionalized nano lubricants exhibit relatively good lubrication performance.

5. Conclusions

In this research, a simple and cost-effective functionalization process for the GNP and the hBN additives was noted as being an appropriate technique to enhance the dispersion stability of the incorporated additives and thereby improve the lubrication performance of the PO. The tests and experiments carried out in this research made an effort to focus on a sustainable lubrication study from alternative sources of lubricants. The study explains the noteworthy improvements in the rheological, dispersion, thermal, and tribological behavior of the PO when incorporated with small amounts of cost-efficient functionalized GNP and hBN nano additives.

The significant conclusions observed from this study are as follows:

• The functionalization process was advantageous for enhancing the dispersion stability of the GNP and hBN additives. It was noted that the 1.0 wt.% hBN (f) nano lubricant started losing its dispersion stability after 8 h. However, the amount of the loss of dispersion stability was not significant. Also, the 1.0 GNP (f) nano lubricant remained homogenously dispersed for up to 24 h.
• The functionalization process was observed to have considerably enhanced the viscosity index of the PO.
• The incorporated GNP and hBN solid lubricant additives along with the oleic acid surfactant helped improve the thermal degradation resistance of the PO.
• This study highlighted that 1.0, 1.5, and 2.0 wt.% GNP (f) nano lubricants exhibited 20.0, 53.0, and 29.50% lower COF relative to the PO, respectively. However, the 0.5 GNP (f) nano lubricant was noted to increase the COF slightly by 1.58% compared to the PO. The 1.0 and 1.5 wt.% hBN (f) nano lubricants were observed to increase the COF by 2.38 and 3.96%, respectively, in contrast to the PO. However, the 0.5 and 2.0 wt.% hBN (f) nano lubricants were observed to lower the COF by 63.63, and 11.40%, respectively, in contrast to the PO. The 1.5 wt.% GNP (f) and 0.5 wt.% hBN (f) nano lubricants were noted to exhibit the lowest COF among all nano lubricants. Overall, the multi-layered GNP (f) and hBN (f) nano lubricants were observed to exhibit a synergistic behavior pertaining to the COF.
• This research highlighted that the 1.0, 1.5, and 2.0 wt.% GNP (f) nano lubricants increased wear volume by 26.72, 2.47, and 12.91%, respectively, compared to the PO. However, the 0.5 wt.% GNP nano lubricant was noted to lower wear volume by 3.16% compared to the PO. Moreover, the 0.5, 1.0, 1.5, and 2.0 wt.% hBN nano lubricants were observed to exhibit antagonistic behavior by increasing the wear volume by 50.08, 208.64, 354.11, and 589.63%, respectively, relative to the PO.
• The deposited GNP (f) solid lubricant additives were observed to be multi-layered, and with fewer defects. These aided in improving the lubrication performance of the PO.
• The wear debris study showed that the morphology from the GNP (f) nano lubricant was that of a smaller rubbing particle, whereas for the hBN (f) nano lubricant, it was that of a larger chunk particle. This explained the relatively better lubrication performance shown by the GNP (f) nano lubricants.
• The functionalization process on the nano additives was more suitable for enhancing viscosity, dispersion stability, thermal degradation, and anti-friction behavior pertaining to the PO.