Non-Polar Chain-Enabled Suspension of Carbon Nanoparticles in Base Oil

Abstract

The transition to electric vehicles (EVs) has introduced new challenges in lubrication, demanding innovative solutions to ensure consistent performance. One promising approach is the use of nanoparticle additives, which have the potential to improve lubrication performance significantly. However, achieving a stable suspension of these nanoparticles in lubricating oils remains a critical challenge, as suspension stability is essential for maintaining consistent performance and maximizing the benefits of these advanced additives. In this study, carbon nanoparticles (CNPs) were modified with dodecylamine (DDA) to achieve stable suspension in nonpolar fluids. The successful functionalization was confirmed by the FTIR results, which showed characteristic peaks of various bonding. The suspension stability tests demonstrated that DDA-CNPs remained suspended for over 60 days in the Polyalphaolefin (PAO) oil, whereas unmodified CNPs were sedimented within 3–7 days. The rheological behavior was measured under different shear rates and temperatures. Viscosity measurements indicated that DDA-CNPs maintained a lower value compared to base PAO. The lubricants’ friction coefficient (COF) was also determined under various speeds and loads. The addition of DDA-CNPs at a concentration of 0.05 wt.% resulted in a significant reduction in COF, with values decreasing by 26% compared to base PAO oil under a load of 1 N. Additionally, the COF for DDA-CNPs was consistently lower than that of PAO, with reductions ranging from 15% to 18% across all tested speeds. The Stribeck curve further highlighted the improved performance of DDA-CNPs across boundary, mixed, and hydrodynamic lubrication regimes. These findings suggest that DDA-CNPs significantly improve the lubrication performance of PAO oil, making them suitable for advanced lubrication applications in automotive and industrial systems.

1. Introduction

The tribological challenges in electric vehicles are gaining significant attention, with critical components like wheel bearings and drivetrains facing higher stresses, heavier loads, and cooling demands, necessitating advanced lubrication solutions [1,2]. Traditional lubricants, like Polyalphaolefin (PAO) oils, are favored for their excellent thermal stability and viscosity properties [3,4,5]. However, the continuous pursuit of improved efficiency and performance has driven research toward developing advanced lubricants enhanced with nanomaterial additives [5,6]. Among these, the incorporation of carbon-based particles—such as carbon quantum dots, nanotubes, graphene, and their derivatives—into lubricants has shown considerable promise in enhancing their tribological performance [7,8,9]. This enhancement is attributed to their small size, allowing them to insert narrow contact areas, facilitate interlaminar shearing, act as nano ball bearings, form tribofilms, and inhibit corrosion [10,11].

A significant challenge in utilizing nanoparticles as lubrication additives is achieving uniform suspension and preventing aggregation and sedimentation [12]. Poor suspension can lead to inconsistent lubrication, uneven wear protection, and reduced efficiency [13]. The frequent settling of particles necessitates regular re-suspension, complicating maintenance and increasing operational costs due to constant monitoring and agitation. In applications like driveline and transmission fluid, short suspension times can result in uneven lubrication, increased friction, and wear, potentially causing damage to engine and transmission components [10,12]. This instability can lead to more frequent maintenance and potential engine failures. Ensuring consistency in lubrication performance is vital for commercial applications, as short suspension periods make it difficult to guarantee that each batch of lubricant performs identically, potentially leading to variability in the protection and efficiency of engines and transmissions. Studies have shown that large agglomerates of WS₂ can only reach the outer edges of the contact area, limiting their effectiveness and resulting in higher friction values when present in the oil [13]. However, longer mixing times of oil with nanoparticles lead to smaller aggregates, which are more effective in reducing friction and providing better lubrication.

One of the efficient solutions to improve the suspension of nanoparticles in lubricants is through modification with long hydrocarbon chains. This method, known as steric stabilization, ensures that the nanoparticles remain well-suspended in the lubricant, enhancing its overall performance [12]. By strategically selecting surface functional groups, the affinity of carbon nanoparticles can be customized to achieve specific properties, making them dispersible in various non-polar and polar fluids. For instance, functionalizing carbon nanoparticles with Oleylamine has successfully enabled good suspension stability in PAO oil, with no precipitation observed even after standing for three months [14]. Another investigation found that polyethyleneimine–oleic acid-functionalized graphene oxide, when suspended in PAO4 oil, remained stable for a minimum of 96 h [15]. Similarly, modified reduced graphene oxide prepared by microwave-assisted ball milling with NaOH and hydrazine hydrate demonstrated that the nanoparticles remained stably suspended in PAO6 oil for at least 30 days without precipitation [16]. However, these approaches often require complex chemical reactions, and the long-term suspension stability under varying operational conditions remains a challenge, highlighting a need for further investigation.

The present study focuses on utilizing dodecylamine molecules to functionalize carbon nanoparticles (DDA-CNPs) to enhance the performance of PAO lubricants. Dodecylamine is a long-chain aliphatic amine characterized by its hydrophobic twelve-carbon alkane chain and reactive primary amine group [17]. Functionalizing carbon nanoparticles with DDA enhances suspension stability in oil and modifies the lubricant’s rheological and tribological behavior. A one-step procedure was investigated for the synthesizing of DDA-CNPs. The successful functionalization of CNPs with DDA was confirmed using FTIR spectra and EDX analysis. Additionally, a suspension stability test was carried out to validate the stability and longevity of the DDA-CNP suspension in PAO oil. Finally, rheological measurements and tribological tests were conducted to evaluate the effectiveness of the particle for lubrication.

2. Materials and Methods

2.1. Materials

Carbon nanoparticles (CNPs) were sourced from Nabors Industries. Dodecylamine (DDA) (purity ≥ 99%) was obtained from Sigma-Aldrich. Synthetic PAO oil (Batch number DCS-110315) was collected from Chevron Phillips Chemical Company (The Woodlands, TX, USA).

2.2. Sample Preparation

An adsorption reaction was performed to functionalize the CNPs, with a low temperature maintained throughout the process to prevent the oxidation of the particles. The synthesis method is demonstrated in Figure 1. Initially, 1 gm of DDA was added to 50 mL of isopropanol and stirred for 30 min to ensure complete dissolution. Subsequently, 0.25 g of CNPs was added to the solution and stirred continuously for 24 h. After the reaction, the mixture was centrifuged at 4000 RPM for 5 min to separate the unmodified DDA. During the centrifugation, the CNPs sedimented to the bottom, leaving the unreacted DDA in the supernatant solution. The sedimented particles were then washed with acetone to remove any unmodified DDA. Finally, the particles were dried at 50 °C for 6 h, yielding the DDA-modified CNPs (DDA-CNPs). Following the synthesis of the DDA-CNPs, they were suspended in PAO oil at a concentration of 0.05 wt.%. To achieve thorough mixing, the solution was stirred for 15 min at room temperature and subsequently subjected to bath ultrasonication (40 KHz) for 30 min.

2.3. Characterization Techniques

The DDA-CNPs and their performance in lubricating oil were characterized using various advanced techniques. Fourier Transform Infrared Spectroscopy (FTIR) was performed with a JASCO (Tokyo, Japan) FTIR spectrometer model FTIR-4600LE MidIR (Resolution: 0.7 cm⁻¹), equipped with a universal ATR mode accessory. The morphology of DDA-CNPs was observed using a JEOL (Tokyo, Japan) JCM-6000PLUS scanning electron microscope (SEM) with a resolution of around 15 nm. Viscosity measurements were conducted using an Anton Paar (Graz, Austria) MCR302e rheometer, featuring a high-precision air-bearing EC motor, normal force sensor, and optical encoder, with a torque range of 0.5 nN∙m to 230 mN∙m. The flow characteristics of DDA-CNPs in PAO oil across different shear rates (100 s⁻¹ and 1000 s⁻¹) and temperatures (25 °C, 40 °C, 80 °C, and 120 °C) were measured using a 50 mm parallel plate configuration. The friction coefficient (COF) was measured using an Anton Paar (Graz, Austria) TRB pin-on-disk tribometer (friction force resolution of 0.06 mN), under varying loads and speeds. The setup included an AISI 4130 low-carbon steel plate from NLMK (Farrell, PA, USA) paired with a 6.35 mm diameter AISI E52100 steel ball from RGP Balls (Milan, Italy).

3. Results and Discussions

3.1. Structural Analysis

CNPs are synthesized using a specialized combustion process involving gases subjected to high temperatures and pressures [18]. The particles exhibit a primary structure that repeats to form two-dimensional secondary sheets, which are similar to graphene [19,20,21]. The surface of these nanoparticles features functional groups such as hydroxyl, carboxyl, and ether groups, analogous to those found in graphene oxide [22,23]. These functional groups improve the reactivity of the CNPs, making them highly applicable in various fields.

The FTIR spectra of CNPs and DDA-CNPs (Figure 2) provide evidence of successful functionalization. The spectrum of the unmodified CNPs displays minimal peaks, reflecting the low concentration of surface functional groups. In contrast, the DDA-CNPs exhibit several distinctive peaks [24,25]. N-H stretching vibrations around 3000–3200 cm⁻¹ and C-H stretching vibrations between 2850 and 2960 cm⁻¹ indicate the presence of DDA in DDA-CNPs. The peak of N-H and O-H bonds in the 3000–3200 cm⁻¹ range may become less prominent or shift, indicating hydrogen bond formation between the hydroxyl groups and the amine group of DDA after functionalization. The peak at 1739 cm⁻¹ relates to the C=O stretching vibration of carboxyl groups. The C-N stretching vibrations observed around 1365 cm⁻¹ further confirm the successful incorporation of DDA. Additionally, the peak at 1217 cm⁻¹ corresponds to the stretching vibrations of epoxy groups. These spectral changes validate the effective modification process, as the new peaks correspond to the characteristic functional groups of DDA.

The SEM images in Figure 3 reveal distinct differences in the morphology of the unmodified CNPs and DDA-CNPs. Figure 3a shows the CNPs with noticeable aggregation, indicating a tendency to form large clusters in the absence of surface modification. Nanoparticles have a high surface area-to-volume ratio compared to bulk materials. This means that a larger proportion of their atoms or molecules are exposed on the surface, making them more available for reactions. The increased surface area enhances the probability of interactions with other molecules or reactants. In contrast to CNPs, the DDA-CNPs in Figure 3c display significantly fewer agglomerates, suggesting that functionalization with DDA reduces particle aggregation.

The EDX spectra provide further insight into the elemental composition. Figure 3b presents the EDX spectrum of the CNP sample, showing a dominant peak for carbon (C) confirming the presence of carbon-based nanoparticles. In contrast, the EDX spectrum of the DDA-CNP sample in Figure 3d shows an additional oxygen (O) and nitrogen (N) peak, indicating the successful incorporation of dodecylamine (DDA). The presence of nitrogen, absent in the raw CNP sample, supports the functionalization process, as nitrogen is a key component of the amine head in DDA. These findings align with the FTIR results, further validating the successful modification of CNPs with DDA.

3.2. Suspension Stability

In lubrication applications, uniform nanoparticle suspension is essential for consistent performance. Aggregation and sedimentation can result in uneven wear protection and compromised friction reduction. We observed that unmodified CNPs were sedimented within 3–7 days, as shown in Figure 4. Short-period suspension presents a significant challenge for the commercial application of nanoparticles as a lubrication additive. In contrast, the DDA-CNPs remained suspended for more than 60 days, with no sedimentation noted during this period. DDA molecules possess a hydrophobic dodecyl tail and a hydrophilic amine head [12,17]. When CNPs are modified with DDA, hydrophobic tails strongly interact with oil, while the amine heads attach to the CNPs. It is also possible that the slightly positive nature of the amine in DDA may interact with the negatively charged pi bonds of carbon nanoparticles or with OH and COOH groups. The hydrophobic tails of DDA could then align on the nanoparticle surface or extend into the oil, creating a more compatible interface, as both the oil and DDA contain alkane chains. This interaction reduces the tendency of the particles to aggregate and helps maintain a stable suspension in the oil medium. The long hydrophobic chains of DDA likely reduce the size of aggregates rather than preventing them entirely [10,12]. These chains create a steric barrier that limits the extent of aggregation, helping to maintain a more stable suspension over time, even though some degree of aggregation still occurs, as suggested by SEM analysis. So, the DDA-CNPs demonstrated excellent suspension stability, making them more viable for long-term use as additives in various fluids.

4. Rheological Properties

Viscosity is a critical property of lubricants that directly impacts their performance and efficiency in various applications. It measures a fluid’s resistance to flow, which affects the formation of a lubricating film between moving parts. Maintaining viscosity is essential for reducing friction and wear and ensuring optimal performance under varying thermal conditions [26].

To evaluate the performance of the lubricants, viscosity measurements were conducted at two shear rates, 100 s⁻¹ and 1000 s⁻¹, over a wide temperature range from 25 °C to 120 °C, as shown in Figure 5. As expected, the viscosity decreases with increasing temperature and shear rate due to the reduction in intermolecular forces and increased molecular motion. At an initial temperature of 25 °C, the viscosity of DDA-CNPs is lower compared to PAO. As the temperature increases, the viscosity values of both DDA-CNPs and PAO converge and overlap at specific temperatures: 63 °C for the shear rate of 100 s⁻¹ (Figure 5a) and 35 °C for the shear rate of 1000 s⁻¹ (Figure 5b). Notably, at higher temperatures above 100 °C, PAO shows a significant drop in viscosity for both shear rates, failing to maintain the trend. In contrast, DDA-CNPs maintain a more consistent viscosity trend even at elevated temperatures. The abrupt drop in viscosity for PAO at high temperatures can be ascribed to the increased kinetic energy of oil molecules. This heightened molecular motion reduces the intermolecular forces, allowing the oil to flow more easily and thus lowering its viscosity. Additionally, some oils exhibit shear thinning behavior, where their viscosity decreases with increasing the shear rate. High temperatures can exacerbate this effect by further reducing the intermolecular forces that contribute to viscosity. There is also the noticeable presence of Carreau–Yasuda shear thinning behavior [27] in PAO at higher temperatures at both shear rates. PAO, a synthetic hydrocarbon known as Polyalphaolefin, is created by polymerizing an alpha-olefin and hence can be modeled by the Carreau–Yasuda equations [28,29]. According to the model, the temporary or permanent shear thinning of the lubricant can occur upon reaching a critical condition dependent on the average molecular mass and molecular mass distribution of the polymer as well as the temperature, shear rate, and pressure, which is subject to [29]. Once a polymer reaches a critical strain rate that is more than its molecular relaxation time, its molecular network can rupture its linkages, causing a reduction in measured viscosity. The addition of CNPs results in nullifying or delaying this behavior by affecting the polymer’s branch length and content via the functionalization process, allowing the lubricant to maintain consistent properties at high temperatures and shear rates.

The viscosity was also measured for four different temperatures up to 10,000 1/s shear rate, as shown in Figure 6. Interestingly, the viscosity reduction was observed for all temperatures except 120 °C. According to Einstein’s equation for dilute suspensions of rigid spherical particles, the viscosity of a suspension increases with the volume fraction of the particles [30]. DDA-CNPs do not follow the trend. Their unique morphology and interaction with the base fluid led to a different impact on the fluid’s viscosity. Rather than increasing viscosity as predicted by Einstein’s equation, the addition of these materials can reduce viscosity under certain conditions. The layers of the DDA-CNP structure may align with the flow direction, creating a more streamlined path for fluid flow. This alignment reduces resistance and lowers the viscosity compared to the base fluid [26]. The surfaces can also act as nano-lubricants, reducing the effective viscosity by minimizing the frictional forces between fluid layers.

Reducing the viscosity of oil has several significant benefits, particularly in lubrication systems [26,31]. Lower-viscosity oils require less energy to pump through the lubrication system, which reduces the overall energy consumption of engines and machinery. This leads to improved fuel efficiency in automotive applications and lower operational costs in industrial settings. Lower-viscosity oils can also flow more freely, improving the heat transfer from critical components to oil. This enhanced heat dissipation helps maintain optimal operating temperatures, preventing overheating and reducing thermal stress on the components.

4.1. Tribological Performance

The friction coefficient (COF) is a critical parameter for evaluating the performance of lubricants under varying operating conditions. In this study, the frictional performance of DDA-CNPs was measured and compared with the base PAO oil. The COF values were recorded under an applied load of 5 N across different speeds ranging from 2 cm/s to 12 cm/s, as illustrated in Figure 7. The results indicate that with increasing speed, the COF values for both PAO and DDA-CNPs decreased up to 10 cm/s and then slightly increased. At low speeds, the lubrication regime is typically boundary or mixed lubrication, where direct contact between surfaces is more likely. At higher speeds, hydrodynamic lubrication becomes more dominant, with a fluid film effectively separating the surfaces. In all tested speeds, the COF for DDA-CNPs was consistently lower compared to PAO, showing a reduction in friction by 15% to 18%. This reduction demonstrates the superior lubricating properties of DDA-CNPs, making them more efficient in reducing friction across a range of operating speeds.

To fully understand the performance under different operating conditions, it is important to evaluate how the COF of lubricants varies with changing loads. The COF values were measured under different loads while keeping the speed constant at 10 cm/s with the results shown in Figure 8. At a lower load of 1 N, the COF value of PAO is higher than at other applied loads. However, after adding CNPs at a 0.05 wt.% concentration, the COF value decreased from 0.172 to 0.128, representing a 26% reduction in friction. As the load increased, the COF values for PAO decreased up to 7 N and then slightly increased. Similarly, the COF values for DDA-CNPs also followed this trend, showing a reduction in friction with increasing load.

To summarize the overall frictional performance, the Stribeck curves of the base oil (PAO) and the oil with added DDA-CNPs are plotted in Figure 9. In this figure, the x-axis represents the Sommerfeld grouping number (Speed × Viscosity/Load), which describes the testing conditions, and the y-axis represents the COF. The plots show the original data points—black squares for base PAO and blue circles for DDA-CNP-enhanced oil. The error bars indicate the standard deviation of the measurements. As seen in Figure 9, the lubricant with added DDA-CNPs outperforms the base PAO oil, exhibiting significantly lower friction across all lubrication regimes. The most notable improvement is observed in the boundary lubrication regime, with a 35–55% reduction in the COF, where metal-to-metal contact is prevalent. Additionally, the DDA-CNP-modified oil consistently exhibits a lower COF than the base PAO, with an approximately 25% reduction in the mixed lubrication regime and an around 15% reduction in the hydrodynamic regime. These results provide strong evidence of the enhanced lubrication performance offered by the DDA-CNP additive, particularly under boundary conditions, where friction is typically the highest.

4.2. Proposed Mechanism

The remarkable performance of nanoparticle-enhanced lubricants is primarily due to the distinctive properties of the suspended particles. Ensuring proper suspension within the lubricant is essential, as it allows the nanoparticles to be evenly distributed, preventing aggregation and sedimentation.

The functionalization of carbon nanoparticles (CNPs) with dodecylamine (DDA) involves a range of chemical interactions and bonding mechanisms that significantly enhance the stability and compatibility of the modified particles. Figure 10a provides a visual summary of these interactions, which occur between the oxygen-containing functional groups, such as hydroxyl (-OH), carboxyl (-COOH), and epoxide (-COC-) groups on the surface of CNPs and the amine group of DDA. These interactions are crucial for anchoring DDA molecules firmly onto the CNP surface, preventing agglomeration and improving the dispersibility of the nanoparticles. The hydroxyl groups (-OH) present on the basal planes and edges of the CNP surface play a significant role in hydrogen bonding. These hydroxyl groups interact with the nitrogen atoms in the primary amine group (-NH₂) of DDA, forming stable hydrogen bonds [32]. Another significant interaction occurs between the carboxyl groups (-COOH) on the CNP surface and the protonated form of the amine group in DDA. In an appropriate pH environment, the carboxyl groups ionize into carboxylate anions (COO⁻), which can form electrostatic bonds with the protonated amine group (NH₃⁺) in DDA [33]. Similarly, the epoxide groups (-COC-) on the CNP surface also provide reactive sites for further chemical bonding. The nucleophilic amine group of DDA can react with these epoxide groups through nucleophilic addition [32,33]. These multifaceted interactions—hydrogen bonding, electrostatic interactions, and nucleophilic addition—work synergistically to create a robust and stable interface between the DDA molecules and the CNPs. The long hydrocarbon chain of DDA adds steric hindrance, which further reduces particle–particle interactions, preventing aggregation and enhancing dispersion stability. As a result, DDA-functionalized CNPs exhibit superior suspension stability and compatibility across a variety of environments, making them highly effective as additives in lubrication fluids.

It can be hypothesized that when nanoparticles are well-suspended in lubricants, they may fill surface irregularities, potentially leading to a smoother contact area and reducing direct metal-to-metal contact, as demonstrated in Figure 10b. This even distribution could enable the nanoparticles to interact more effectively with the contact surfaces, possibly forming a consistent lubricating film that minimizes friction. Due to their small size and spherical shape, nanoparticles might act as tiny ball bearings, allowing them to roll between contact surfaces and potentially converting sliding friction into lower rolling friction [34]. This rolling mechanism reduces friction by converting sliding friction into rolling friction, which is typically lower. Additionally, the layered structure of certain nanoparticles, such as those found in graphene-based materials, may allow them to slide easily over one another, further reducing friction due to their low shear strength [21,35]. Moreover, the functional groups on DDA-CNPs interact with the surface material, forming a stable chemical layer that reduces adhesion and friction between surfaces [17]. The incorporation of DDA-CNPs into the lubricant also leads to a smoother surface finish over time [36]. Moreover, the earlier observed reduction in viscosity enables lower-viscosity fluids to flow more freely between contact surfaces, decreasing resistance and shear stress, which subsequently reduces friction. Future work will validate these hypotheses by observing wear protection and conducting further wear surface analyses to understand the mechanisms at play.

5. Conclusions

In this study, the functionalization of CNPs with DDA was explored to enhance their suspension stability and improve the lubricating performance of Polyalphaolefin (PAO) oils. The successful functionalization was confirmed by FTIR analysis, which identified characteristic peaks indicative of covalent bonding and interactions between DDA and the CNP surface. The DDA-CNPs resulted in significant improvements in lubrication performance:

• DDA-CNPs remained stably suspended in PAO oil for over 60 days, significantly improved compared to the 3–7-day suspension time of unmodified particles.
• Rheological measurements revealed that the DDA-CNPs resulted in better viscosity stability and a lower value across a range of temperatures and shear rates as compared to base PAO.
• Tribological tests showed a marked reduction in the friction coefficient with the addition of DDA-CNPs, highlighting their effectiveness in reducing friction by around 15–26% for different loads and speeds.

Overall, the superior performance of DDA-CNPs may be attributed to their ability to reduce sliding friction, enhance load-bearing capacity, and chemically interact with surface materials to create stable, low-friction interfaces. These findings suggest that DDA-CNPs significantly improve the lubrication performance of PAO oil, making them a promising candidate for advanced additives in lubrication fluids. Future work will focus on utilizing this enhanced fluid performance in electrified systems, addressing the unique tribological challenges in electric vehicles.