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Article

Mitigation of Electrical Discharge Damage in Electric Vehicle Bearings: Comparative Study of Multi-Walled Carbon Nanotubes and Alumina Nanoparticles in Lubricating Grease

by
Emmanuel R. Jonjo
1,
Islam Ali
1,2,
Tamer F. Megahed
3,4 and
Mohamed G. A. Nassef
1,2,*
1
Industrial and Manufacturing Engineering Department, Egypt-Japan University of Science and Technology, New Borg El Arab City, Alexandria 21934, Egypt
2
Production Engineering Department, Alexandria University, Alexandria 21544, Egypt
3
Electrical Power Engineering Department, Egypt-Japan University of Science and Technology (E-JUST), New Borg El Arab City, Alexandria 21934, Egypt
4
Electrical Engineering Department, Mansoura University, Mansoura 35516, Egypt
*
Author to whom correspondence should be addressed.
Vehicles 2025, 7(1), 19; https://doi.org/10.3390/vehicles7010019
Submission received: 3 January 2025 / Revised: 5 February 2025 / Accepted: 13 February 2025 / Published: 16 February 2025
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Abstract

:
The electrified environments encountered in electric vehicles (EVs) in terms of parasitic currents present significant challenges for the performance of EV bearings and their lubricants. This study investigates the effectiveness of various concentrations (0.1 wt.%, 0.2 wt.%, 0.3 wt.%, and 0.4 wt.%) of multi-walled carbon nanotubes (MWCNT) and alumina (Al2O3) as two different nanoparticles incorporated into lithium grease, specifically focusing on their ability to mitigate the bearing surface damage caused by varying magnitudes of bearing DC discharges. A specialized test rig was developed to evaluate the electrical discharge characteristics, vibration response, and extent of surface wear on bearings lubricated with both lithium grease without additives and when infused with each nano-additive. Microscopic examination was employed to qualitatively and quantitatively evaluate the surface degradation of each test bearing. The results of this study demonstrate that the addition of nano-additives into the lubricating grease of bearings subjected to electrical loads resulted in a reduction in electric discharge voltage thresholds and levels. This reflected on the mitigation of surface damage in terms of surface roughness and vibration amplitudes by up to 70.67% and 65.19% in the case of MWCNTs. In contrast, alumina nanoparticles yielded a reduction in vibration amplitude and surface wear by 44.89% and 37.5%, respectively.

1. Introduction

In the past decade, the transportation sector has witnessed a remarkable increase in the adoption of electric vehicle (EV) technologies across the globe with more than 14 million EVs registered in 2023 alone, which represents a 35% year-on increase from 2022 [1]. This growth is primarily driven by the urgent need to mitigate greenhouse gas (GHG) emissions, which has prompted many developed nations to implement policies aimed at reducing their carbon footprints [2]. For instance, the United States has the target of achieving carbon neutrality by 2030, while the European Union aims for a similar goal by 2050, and China by 2060 [3,4]. These policy initiatives are intrinsically linked to the growing promotion of electric vehicles, as they represent a viable solution to decrease reliance on fossil fuels and lower GHG emissions from the transportation sector. Furthermore, the inherent advantages of electric vehicles such as their reduced energy cost per mile, lower maintenance cost, and high reliability and efficiency coupled with these progressive policies are the driving force behind the continuing growth in demand for EV vehicles globally [5].
The performance efficiency and reliability of EVs, however, depend significantly on their electric motor-drive systems [6,7]. This dependency has led to intensified research initiatives focused on advancing motor-drive systems that offer greater efficiency, reliability, and durability. Among the persistent challenges faced in electric motor-drive systems, the failure of the bearings remains of utmost concern, accounting for more than 40% of breakdowns [6]. Such failures can lead to progressive deterioration in the operational efficiency of motors, as well as cause damage to other motor components, resulting in increased maintenance costs and, in severe cases, may culminate in the complete failure of the entire system.
One of the major causes of bearing failure in electric drive systems is the presence of electric currents flowing through the surfaces of the bearing raceways [8,9,10]. These currents are generated as byproducts of the Pulse Width Modulation (PWM) inverters, which are employed to convert direct current (DC) into alternating current (AC) [11,12]. Specifically, PWM inverters equipped with semiconductor electronic switching technologies have significantly accelerated switching processes, producing extremely high voltage gradients [13]. Due to differences in impedance between the connecting wires and the motor, overvoltage at the motor terminals is produced [14]. A portion of this voltage is induced on the motor shaft, generating what is known as a shaft voltage. When the magnitude of this shaft voltages exceeds the dielectric strength of the bearing lubricants, localized electrical discharges take place, producing temperatures exceeding the melting point of the bearing raceway material [15]. This compromises the integrity of bearing surfaces, causing damage such as micropits, frosting, and fluting. The degree of damage caused is dependent upon the current density, defined as the magnitude of the applied current divided by the Hertzian contact area and the duration of exposure to the electrical stress [16]. Several other key factors, such as motor size, motor speed, temperature, voltage levels, and the inverter switching frequency, have been identified as critical determinants affecting the nature and extent of degradation caused [8,17,18,19].
Numerous strategies have been proposed to mitigate the damaging effects of bearing current in electric motors, including (1) installing ceramic bearings to stop current flow across the bearing [20], (2) using a grounding brush to provide a low-impedance path [21], (3) electrostatic shielding to redirect the shaft voltage [22], and (4) employing novel inverter architectures to suppress the CMV at source [23], among others. Each solution has its own distinct limitations, such as high costs, implementation challenges, modifications to motor or inverter structures, and limited effectiveness in current mitigation. For example, a grounding brush addresses EDM currents, but they are less effective against other types of bearing currents. Ceramic bearings are expensive and may have poor heat dissipation properties. Additionally, they may redirect currents through alternative paths, potentially damaging other components.
Recent studies have demonstrated that the use of conductive grease can effectively mitigate electric damage to bearings [24,25,26]. Nevertheless, a limited number of lubricants possess the electrical conductivity capabilities required for applications in modern electric drives [27]. Therefore, there is a growing interest in understanding how lubricants behave under electrical loads, particularly in relation to bearing failure. The research by Li et al. [28] has shown that the magnitude of the voltage and the resulting type of wear induced in bearings are interrelated; specifically, at low voltages, adhesive wear becomes the predominant wear mechanism, while at high voltages, abrasive wear tends to dominate. Romanenko et al. [29] has suggested that electrostatic discharges in lubricating greases compromise their dielectric properties through the chemical degradation of both additives and thickeners present in the grease. This degradation process subsequently alters the rheological characteristics of the grease. These findings underscore the significant impact of electrical conditions on the tribological performance of lubricants. Understanding these interactions is crucial for the development of more effective lubricants that can withstand electrical current and enhance the reliability of motor-drive systems in EVs.
Conductive lubricants are created by incorporating conductive materials into the base lubricant [30]. Examples of these materials include nanomaterials such as carbon nanotubes [31], metal and metal oxides [32], and ionic liquids [33,34]. While ionic liquids generally exhibit higher conductivity, they are susceptible to forming harmful byproducts upon decomposition [35]. Additionally, when subjected to harsh environments, they are prone to tribo-corrosion [36]. Therefore, nano-additive-based conductive greases have garnered considerable attention due to their advantageous properties, such as enhanced thermal stability [37], improved wear resistance, and the ability to effectively reduce friction [38]. Christensen et al. [31] tested low-weight nano-additives of hydroxyl functionalized multi-wall carbon nanotubes to reduce the resistivity of electric conductive grease. However, the work did not provide insights into the impact of the nano-additives on the generated EDM voltages in rolling bearings or the corresponding raceways surface damage during operation under electric loads.
Despite a substantial body of research detailing the advantages of nano-lubricants in enhancing grease tribological properties, there remains a notable shortage in studies specifically examining the performance of nano-infused greases under electrified conditions [36]. Only a few studies have proposed that the incorporation of nanoparticles into bearing grease can enhance protection against bearing surface degradation arising from electric currents [39,40]. Bond et al. [39] investigated the efficacy of various grease formulations, with silver nano-additives, in mitigating electrical discharge machining (EDM) surface wear. However, the tests were limited to reciprocating rolling motion in a ball-on-disk tester. In contrast, Suzumura [40] employed carbon black as a thickener in several base oil formulations to enhance electric current discharge wear resistance in rolling bearings. Notwithstanding, there is still an essential need to investigate the performance and effectiveness of other types of nano-additive materials at different concentrations in controlling and mitigating bearing electric damage. Furthermore, researchers are yet to fully elucidate the influence of different nanoparticle characteristics on grease dielectric strength and EDM voltage margins when subjected to electrical stress. Addressing this research gap will not only contribute to the fundamental understanding of the tribological behavior of bearing grease under electrical loads but also pave the way for the development of advanced lubricants tailored for high-performance applications in electrified systems including EVs.
Therefore, this study aims to investigate the performance of various concentrations of two distinct nanomaterials, one that is carbonaceous (multi-walled carbon nanotubes) and the other of ceramic nature (alumina), on the electrical behavior of lithium-based grease in radially loaded rolling-element bearings subjected to electric current loads. The choice of MWCNT and Al2O3 nanoparticles is primarily motivated by their unique properties such as exceptional mechanical strength, a high aspect ratio, and outstanding thermal and electrical conductivity that contributes significantly to enhanced tribological performance when added to lubricating grease. These properties enable these nano-additives to effectively reduce friction and wear in lubricating applications. For instance, the work by Mohamed et al. [41] demonstrated that the addition of 1 wt.% of MWCNT in lithium grease (LG) resulted in a reduction in the coefficient of friction (COF) by 81.5% and a decrease in wear scar diameter (WSD) by 63%, underscoring the effectiveness of MWCNTs in minimizing friction and wear. A comparative study by Prasad et al. [37] between the performance of MWCNTs against other metal oxide nano-additives, zinc oxide (ZnO), and boric acid (nBA) in lithium grease found that a concentration of 0.75 wt.% of MWCNTs outperformed both metal oxide nano-additives in reducing COF and WSD. Additionally, it was shown that MWCNT reduced the average surface roughness (Ra) of the bearing raceways by 59% as compared to the base grease. In a similar direction, Kamel et al. [42], using a ball-on-disk testing machine, investigated the performance of MWCNT and Al2O3 in lithium–calcium grease. Their findings showed that 4 wt.% of MWCNT offered better wear reduction performance than Al2O3. Luo et al. [43] discovered that an optimal concentration of 0.1 wt.% of Al2O3 lowered the average COF and WSD of LG on a four-ball tribo-tester by 17.61% and 41.75%, respectively.
The extent to which a nano-additive positively influences the tribological performance of a lubricant is dependent on factors such as its shape, size, molecular structure, and chemical inertness. Kumar et al. [44] showed that smaller sizes of graphite perform better in enhancing the tribological performance of LG compared to larger-sized particles due to their ability to form a protective layer on the surfaces of the rolling element. Chebattina et al. [45], on the other hand, demonstrated that shorter-length multi-walled carbon nanotubes (MWCNTs) provide superior performance compared to their longer counterparts. This enhanced performance is attributed to the ability of the shorter-length nanoparticles to more easily slide between the rolling elements of the bearing, facilitating smoother motion and reducing friction.
In electrified applications, the primary attribute of nanoparticles that takes precedence is their electrical conductivity [35]. Depending on the type of nanoparticle, they can either enhance the insulating properties of lubricants or, conversely, decrease them. Both conditions offer improved protection against the electric current erosion of the bearing raceways. By improving the insulating properties of grease, a higher voltage threshold may be needed to cause electrical discharges, while decreasing the insulating property of the lubricant reduces or eliminates voltage accumulation across the lubricant. Investigations by Ge et al. [46] explored the efficacy of various carbon conductive additives in poly (ethylene glycol-ran-propylene glycol) monobutyl ether (PAG) base grease. The study compared single-walled carbon nanotubes (SWCNTs), MWCNTs, and carboxylate MWCNTs (CMWCNTs) against carbon black (CB). Utilizing a conductivity tester and pin-on-disk tests, the authors found that CNTs provide dual advantages of improved tribological performance and enhanced electrical conductivity when compared to carbon black. Guo et al. [47] demonstrated, in dry friction tests, that the electrical conductivity of composite aluminum oxide/copper (Al2O3/Cu) nanoparticles decreases, and the decrement is contingent upon the size of the nanoparticle. Additionally, it was shown that with the Al2O3/Cu composite, abrasive wear is the principal mechanism of failure instead of the formation of micropits.
While the tribological properties of MWCNTs and Al2O3 have been extensively documented in the literature, their electrical performance in electrified environments remains significantly underexplored. This gap in research is particularly pronounced when it comes to in situ bearing tests, where the real-world application of these materials is critical. Currently, there is a notable absence of studies that directly compare the performance of these two nano-additives under electrical discharge machining (EDM) conditions specifically for lubricating greases. This lack of comparative analysis highlights a crucial research gap, as understanding how MWCNTs and Al2O3 behave in such environments could provide valuable insights into optimizing lubricant formulations for enhanced performance and protection against electrical discharges in bearing applications.
To that end, this research examines how lithium grease, with and without different concentrations of these nanoparticles, influences the electrical modes of failure and their respective severity levels when a voltage is applied across the raceways of a rolling-element bearing (REB). A customized test set-up is developed to test rolling bearings lubricated with each grease blend at different voltage levels. The electric parameters of each test bearing are recorded and analyzed. A ball-on-disk tribo-tester was used to determine the coefficient of friction for each grease blend. Vibration measurements and analyses in time and frequency domains are also taken to reflect the vibration response of bearings lubricated with these different grease compositions. Analyses of the surface degradation of each test bearing are assessed using 3D laser microscopy, scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis.

2. Materials and Methods

2.1. PWM-Induced Common-Mode Voltages

The implementation of PWM techniques for controlling motor parameters has been shown to induce common-mode voltages (CMVs) at the motor terminals. This CMV, in conjunction with the parasitic capacitances inherent in the motor structure, results in a voltage being induced on the motor shaft. This shaft voltage subsequently flows across the bearings and, depending on the lubrication regime and the magnitude of the shaft voltage, various types of currents can be introduced across the bearing, including ohmic (or resistive), capacitive, or electrical discharge machining (EDM) currents [48]. Figure 1 shows the type of parasitic current that flows across a bearing, depending on the lubrication regime.
In the case of boundary lubrication, the rough asperities of the contacting surfaces puncture the thin lubricant film prohibiting voltage build-up, and the bearing contact in this instance behaves as an ohmic resistance as shown in Figure 1a [49]. Consequently, an ohmic or resistive current flows through the bearing in this state. Conversely, in full-film lubrication or hydrodynamic lubrication, the lubricant behaves as a dielectric material and the contact is modeled as a resistor in parallel with a variable capacitor as shown in Figure 1b. Depending on the magnitude of the applied voltage, two distinct scenarios arise. If the magnitude of the applied voltage is significantly less than the dielectric strength of the lubricant, a capacitive bearing current is induced. This capacitive current is generally considered to be unharmful to bearings, as its magnitudes are typically in the range of a few milliamperes [50]. However, if the magnitude of the applied voltage exceeds the breakdown voltage of the lubricant, electrical discharge occurs, leading to what is referred to as an electric discharge machining (EDM) bearing current [48]. An EDM current can result in significant wear and damage to the bearing surfaces.

2.2. Test Set-Up

A specialized set-up was designed and constructed to analyze the influence of nano-additives mixed with lithium grease on the surface degradation of rolling-element ball bearings subjected to electric current. The test rig comprises both mechanical and electrical components. The mechanical part of the test rig features the following components: (1) a 3 hp electric motor, (2) a coupling, (3) a metallic baseplate, (4) a rotating shaft, (5) two support bearings, (6) a test bearing mounted to the shaft within a specially designed housing, and (7) an applied radial load attached to the housing. The radial load served the dual purpose of acting as a load on the bearing and holding the housing and bearing outer race fixed while the shaft rotates with the inner ring of the bearing. The experiment test rig is adopted from the previous work of Nassef et al. [51], and the layout is shown in Figure 2.
The electrical part of the set-up comprises a signal generator, a dual-channel digital oscilloscope, a current probe, and electric wires. The signal generator is used to supply the voltage via the electric wires to induce a potential difference across the bearing. One wire terminal is connected to the bearing inner ring while the other terminal contacts the outer ring of the bearing, completing the circuit back to the generator. A dual-channel oscilloscope is utilized to measure and log both the applied voltage and current. The first channel was designated for current measurements, while the second channel recorded the voltage. The (RT-ZCO3) current probe is used to measure the current across the bearing. The electrical layout of the test set-up is shown in Figure 3. Table 1 details the specifications of the components of the experimental test set-up while the specifications of the test bearing, which is a single-row deep-groove ball bearing (type 6006-2Z-C3), are detailed in Table 2. This specific bearing model was selected to emulate the type of bearings used in EV motors that are subjected to parasitic currents [52].

2.3. Structural Characterization of the Nanomaterials

The MWCNT and Al2O3 nanomaterials utilized in these research experiments were procured from Nanogate Company, Cairo, Egypt. Material characterization was conducted to determine the size and morphology of the nanomaterials as well as to obtain a thorough understanding of the nanomaterials’ properties. A scanning electron microscope (SEM) (JSM-IT 200) as well as a transmission electron microscope (TEM) (JEOL JEM-2100, Tokyo, Japan) were utilized to investigate the structural characteristics of the nanomaterials.

2.4. Nano-Grease Preparation

A commercial multipurpose lithium grease (LG), produced by Schaeffler Technologies (Arcanol MULTI3), served as the lubricant for the test samples. This grease was selected due to its widespread application and favorable physical characteristics. The properties of the grease are detailed in Table 3 as provided by the manufacturer. To prepare the test samples, varying concentrations of both MWCNTs and alumina (Al2O3) were incorporated into the base grease at 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, and 0.4 wt.%. The selection of the nano-additive concentrations in this study was guided by previous studies [53,54,55] and considered the following key points. First, ensuring adequate particle dispersion stability was a critical factor. Proper dispersion allows nanoparticles to uniformly access contact surfaces, thereby enhancing their tribological performance. Higher concentrations of nano-additives often promote particle clustering and agglomeration, which may result in abrasive wear on bearing raceway surfaces. Second, the optimization of surface coverage played a crucial role. Low concentrations of nano-additives are generally sufficient to form a protective film between contact surfaces, which enhances friction reduction and wear resistance. Beyond certain thresholds, particle agglomeration reduces the effective surface area, thereby diminishing the intended tribological benefits. Finally, the impact of nanomaterial concentrations on grease viscosity was carefully considered. Elevated concentrations can significantly increase grease viscosity, impairing its pumpability and overall lubrication performance. This excessive viscosity may paradoxically elevate friction rather than reduce it. These considerations collectively guided the selection of optimal nano-additive concentrations in this research.
The specified amounts of nanomaterials were added to the grease and mechanically stirred at room temperature, utilizing a 150 W mechanical stirrer. The mechanical stirring technique was opted for due to its simplicity, efficiency, and ability to achieve short-term nanoparticle dispersion stability without inducing chemical reactions or altering the molecular properties of the base grease [56]. The mixing process was conducted for a duration of 30 min in each case to ensure a uniform dispersion of the nanoparticles within the grease. This was confirmed by the appearance of the grease blend in terms of color and smooth bulk texture.

2.5. Tribological Characteristics of Grease Blends

A ball-on-disk tribo-testing apparatus is used to evaluate the wear loss volume and coefficient of friction of all grease blends according to ASTM G99-23. A ball element (C40 steel) is used as the pin sample and is firmly attached to the holder with a radial load to press the ball against a rotating disk in the y axis. During each test, the COF is recorded over time for a 100 m sliding distance, 13.5 mm track radius, 0.5 kg radial load, and 100 mm/s linear sliding velocity. Then, for each grease sample, the average value and standard deviations for the COF values are determined.

2.6. Electrical Test

The electrical test was conducted in three distinct stages. In the initial stage, known as the run-in stage, each test bearing was operated continuously for 16 h. This process is performed to smoothen the bearing surface asperities in order to establish consistent surface conditions across all bearings. It also ensures the establishment of hydrodynamic lubrication conditions and ensures that voltage build-up is not hindered by contact between surface asperities. At the commencement of the run-in phase, a 100 N load was attached to the test bearing housing. The motor was operated at a constant speed of 1400 rpm throughout the experiment and all tests were conducted at ambient temperature without subjecting the bearing to any external heating source.
The second stage of the experiment involves the stepwise application of voltage to the bearing and subsequent data measurement and logging. A DC voltage in the range of 0–5 V is applied to the bearing while the current is set to limits of 0–3 A. These values represent the range of values that have been reported in the literature for DC applications [19]. Additionally, DC was applied due to its reported capacity to induce pitting damage without an elaborate power supply and measurement system configuration [57]. At the start of the second stage, an incremental voltage of magnitude 0.1 V is applied to the shaft in 5 min intervals until discharges are detected on the oscilloscope. This step ensures that breakdown occurs within the bearing. This step continues until the bearing electric circuit becomes resistive.
During the final stage of the experiment, the applied voltage is adjusted back to the highest recorded breakdown voltage, and the experiment is allowed to run continuously for an additional 6 h, marking the end of the experiment. Figure 4 shows the schematic illustration depicting all three stages of the experiment. The total time taken per experiment is approximately 24 h.

2.7. Vibration Test Procedure

At the end of each current discharge test, vibration measurements and analyses were carried out to examine the vibration behavior of bearings following exposure to electrified conditions. This test was undertaken to highlight the vibratory performance of the nano-grease blends investigated. To capture vibration signals, an accelerometer was attached to the bearing housing via a magnet which measured vibration in the radial vertical direction. The signals recorded by the accelerometer were directly transmitted to a data analyzer (COMMTEST VB5 vibration analyzer, Bently Nevada, Nevada, USA) in the time domain, which performed signal conditioning and processing to construct the signals in the frequency domain as well. The overall vibration value of the spectrum in terms of root mean square (RMS) value was chosen to evaluate the vibration characteristics as it reflects the overall magnitude of the vibration energy.

2.8. Surface Roughness Test

The investigation into the performance of nano-grease blends on the surface wear characteristics of the test bearing raceways was conducted using both qualitative and quantitative analysis. The severity of surface wear was analyzed using a 3D laser scanning microscope (KEYENCE VK-X100 SERIES, Mechelen, Belgium) in accordance with the JIS B0601:2001 (ISO 4287:1997) standard [58]. The mean surface roughness (Ra) was evaluated for each test case by taking surface roughness measurements in three perpendicular directions. Furthermore, a scanning electron microscope (JSM-IT 200) was used to validate the laser microscope results as well as to undertake EDX analysis on the bearing surfaces.

3. Results and Discussion

3.1. Results of the Nanomaterials’ Characterization

From the analysis of Figure 5a, the morphology of MWCNTs appears to be entangled and intertwined with one another, forming rope-like bundles. The TEM image shown in Figure 5b further highlights the distinctive curved hollow tube structures typical of MWCNTs. The average diameters of the MWCNTs are in the range of 10–15 nm with lengths up to 5 μm. Additionally, the MWCNT has high levels of purity up to 92 ± 2%.
Figure 6a presents the SEM micrograph of Al2O3 nanoparticles, clearly indicating their approximate spherical shapes with an average particle diameter in the range of 25–30 nm. Additionally, the agglomerated nature of these particles is evident in both the SEM and TEM micrographs. This shows a tendency of the nanoparticles to cluster together, which is seen consistently across both imaging techniques.

3.2. Results of Tribological Testing

The result for friction testing using a pin-on-disk tribo-tester on each grease blend showing the average coefficient of friction (COF) is presented in Figure 7. The LG sample exhibited the highest COF compared to grease blends containing nano-additives. With increasing concentrations of the nano-additives, a gradual decrease in the COF is observed. When compared to the LG, the nano-grease with MWCNT exhibited a reduction in the average COF values equivalent to 47.06%, 52.94%, 70.58%, and 76.47% for concentrations of 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, and 0.1 wt.%. Similarly, for Al2O3 grease blends, the recorded percentage reductions in the average COF were 23.53%, 35.29%, 47.60%, and 52.94% for the same concentrations as the MWCNT. These COF values agree with those reported in the literature [59].

3.3. Results of Electric Discharges’ Test

To investigate the impact of incorporating nanomaterials on voltage discharges in the test bearings, the voltage and current across each bearing was systematically recorded and analyzed. The bearings were subjected to a DC voltage incrementally from 0 to 5 V in 0.1 V steps, with each voltage increment maintained for approximately 5 min. The digital oscilloscope was used to capture both the electrical discharge currents and corresponding voltage signals during the capacitive phase and the breakdown phase of the bearing lubricant.
For the bearing lubricated with LG and operating at low-voltage amplitudes, no electric discharges were observed, which indicates a capacitive current state in the bearing electric circuit across the lubricant. Once the bearing voltage value exceeded 1.2 V, small spikes in the bearing current started to appear over the recorded time, which indicates reaching the dielectric strength of the bearing lubricant and causing the behavior of the lubricant to change into electric discharge machining (EDM) current. The severity of the EDM current continued to increase as the voltage amplitude gradually increased. At voltage amplitudes exceeding 3.5 V, significant surges in both voltage and current magnitudes were noted, accompanied by clear and more persistent discharge pulses. Figure 8 illustrates a portion of the voltage and current signals recorded for the bearing lubricated with LG subjected to a voltage of 4.5 V. Specifically, Figure 8a depicts the bearing voltage signal during a discharge event, while Figure 8b presents the corresponding current signal. Figure 8 illustrates the capacitive behavior of the lubricant, where the input voltage remains relatively constant until it suddenly drops to a significantly lower voltage level [60,61]. Discharges were observed to persistently occur until the voltage levels reached 4.5 V, beyond which no further EDM discharges were detected, and both the current and voltage measurements exhibited extreme fluctuations. It was confirmed that the current state at this stage is the resistive state.
Throughout the measurement process, fluctuations in voltage and current amplitudes were consistently observed. These variations are explained in terms of changes in lubricant film thickness when the bearing is subjected to DC. Cao-Romero-Gallegos et al. [62] determined that the passage of DC through lubricated steel interfaces results in dynamic changes in lubricant film thickness, which continuously alters the electric parameters in the contact zone.
The inclusion of MWCNT and Al2O3 nanoparticles into bearing grease lowered the applied voltage magnitudes at which discharges were observed to occur. Furthermore, the characteristics of the discharge pulses were notably different; specifically, the pulses were both lower in magnitude and less frequent when compared to those observed with LG. Additionally, these discharges occurred within a narrower range of applied voltage values. The experimental results for capacitive, EDM, and resistive cases of each test sample are shown in Figure 9, where M1-M4 and A1-A4 denote concentration levels of 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, and 0.4 wt.% for MWCNT and Al2O3. The reduction in the EDM discharge range as well as the magnitude of the discharge current pulses is attributed solely to the addition of nanoparticles to the grease, as all other parameters that influence electrical discharge in bearings, including load, speed, base grease, and temperature, were kept constant throughout the experiments.
This observation may be due to the increased conductivity of the grease, resulting from the addition of the nanoparticles. Other previous researchers have shown that the addition of nanoparticles such as silver and carbonaceous materials (including MWCNTs) into an insulating bearing grease increases its conductivity through the formation of electrical channels, which reduces the magnitude of arc discharges [39,40]. Furthermore, Prasad et al. [54] has suggested that adding nanoparticles, specifically MWCNTs, in LG decreases the lubricant film thickness by obstructing the flow of the lubricant into the contact zone. This thinner film may lead to current discharges at much lower applied voltages, albeit with a lower magnitude.
The influence of the volume concentration of the nano-additives on the electrical discharges was also investigated. For MWCNTs, increasing their concentration was observed to enhance the electrical conductivity of lithium grease (LG). This improvement can be attributed to the exceptional conductive properties of the carbonaceous nanomaterial. When dispersed within the grease matrix, MWCNTs facilitate the formation of conductive pathways, thereby reducing the grease’s electrical capacitance and improving its overall conductivity. This is manifested by two indicators. First, the charging time for LG with MWCNT in bearings under the electric test was found to be lower than that for bearings lubricated with LG only. The second indicator is that the corresponding electric discharge machining (EDM) voltage is reduced, which is reflected in less electric erosion on the bearing raceways.
In contrast, the behavior of Al2O3 nanoparticles was markedly different. At low concentrations of 0.1 wt.% and 0.2 wt.%, the lubricants exhibited a similar capacitive range to that of LG, suggesting that these concentrations do not significantly alter the electrical characteristics of the lubricant. However, at higher concentrations of 0.3 wt.% and 0.4 wt.%, a drop in the capacitive range is observed in comparison to the tested lower concentration values. Alumina in its bulk form (on the macroscale) is known as a dielectric material with high electrical resistivity. However, alumina nanoparticles show enhanced surface charge transfer properties compared to bulk forms due to the increased surface-to-volume ratio with the potential for surface charge polarization under certain conditions in its dispersed phase [63].

3.4. Results of Vibration Analysis

Radial vibrations emanating from the rolling elements of bearings are transmitted to the outer ring via the lubricant film. It follows that the lubricant film plays a crucial role in influencing the radial vibrations that the bearing experiences. Marau et al. [64] concluded that the addition of nanoparticles into bearing lubricants influences only the high-frequency vibration components. The high-frequency vibration components are associated with bearing surface roughness as opposed to the low-frequency components that are influenced by the bearing geometry [65]. Therefore, the high-frequency signals (0–1 kHz) in the radial vertical direction were measured for each test bearing and recorded in both time and frequency domains.
Figure 10 presents the vibration root mean square (RMS) acceleration amplitudes for tested bearing lubricated with the nano-grease blends. Notably, all tested bearings exhibited high-vibration RMS acceleration values, which can be ascribed to the surface degradations of bearing raceways induced by arc discharges.
Several key observations emerged from the results of the vibration analysis. Bearings lubricated solely with LG exhibited the highest recorded vibration levels, which indicates that substantial surface degradation occurred. In contrast, the bearings lubricated with grease containing MWCNT resulted in a significant reduction in vibration values compared to the LG. Specifically, as the weight percentage of MWCNT increased, a corresponding decrease in vibration levels was observed, with reductions quantified at 28.54%, 38.03%, 41.47%, and 65.19% for bearings lubricated with grease blends containing 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, and 0.4 wt.% of MWCNT, respectively. These results are in conformity with the results obtained for the tribological test, which showed that the grease containing MWCNT has a lower COF value than LG. Additionally, these results suggest that there is less wear on the bearing surfaces. It is known that the addition of MWCNTs into the grease facilitates the formation of a tribofilm layer on the bearing surfaces. Nassef et al. experimentally investigated the addition of different nanomaterials including MWCNTs, and they found that MWCNTs reduce surface wear by the formation of a tribofilm layer. This tribofilm layer serves as a protective barrier against wear and in this case, EDM discharges.
In the case of aluminum oxide (Al2O3) nanoadditives, the decrease in the vibration amplitude observed for volume concentrations of 0.1 wt.% and 0.2 wt.% is not substantial. The measured reduction in vibration amplitude was recorded at 7.82 and 8.06% for these concentrations, respectively. However, at weight concentrations of 0.3 wt.% and 0.4 wt.%, the reduction in vibration amplitudes was measured at 44.89% and 40.08% compared to the LG. The above results show that the addition of nano-additives into bearing grease decreases the surface degradation depending on the concentration and type of additive.

3.5. Results of Surface Wear Analysis

Following the completion of each test, the bearing was meticulously disassembled, and the grease was thoroughly cleaned from the surfaces. The cleaned surfaces were then examined under a 3D laser scanning microscope to qualitatively and quantitatively assess the extent of surface wear caused by bearing current. Figure 11a shows the results for bearings lubricated with LG. An analysis of the image reveals the presence of several microcraters, marked by red circles, which are indicative of high-energy electric discharge wear. This is in line with the findings of other works, which have shown that surface erosion on bearing raceways can occur when the potential difference applied exceeds the breakdown voltage of the lubricant [66,67,68]. Similarly, an examination of the surfaces of bearings lubricated with grease that contain MWCNT and Al2O3 shows the presence of microcraters particularly at lower concentrations of the nanoparticles, indicating that these concentrations are not optimal for mitigating EDM wear. However, it was observed that the severity of these microcraters is less pronounced compared to those found on bearings lubricated with LG, confirming that the addition of nanoparticles provides protection against discharge damage. Additionally, there were signs of ablated materials onto the bearing raceways. Figure 11b shows a bearing lubricated with 0.1 wt.% of Al2O3 nanoparticles, depicting the ablated materials on the bearing raceways. It is important to highlight that as the concentration of nano-additives in the lubricant increased, there was a corresponding decrease in discharge wear in all test cases. Figure 11, Figure 12 and Figure 13 show the laser microscope images of the surfaces for experiments conducted with grease samples of LG only, LG containing MWCNT, and LG blended with Al2O3, respectively.
Table 4 shows the surface roughness measurements of the bearing outer raceway for different concentrations of nano-additives. Initially, for the bearing lubricated with LG, the surface roughness value was recorded at 2.08 µm. In comparison, for experiments conducted with grease laden with nano-additives, the surface roughness measurements gradually decreased, with increasing additive concentration further confirming the finding that the nanoparticles enhance grease performance in mitigating EDM damages.
For the grease containing MWCNT, the average surface roughness values were measured at 0.78 µm, 0.75 µm, 0.64 µm, and 0.61 µm for concentrations of 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, and 0.4 wt.%, respectively. These values represent percentage reductions in the surface wear of the bearing raceway of up to 62.50%, 63.94%, 69.23%, and 70.67%. In comparison, for the bearings lubricated with grease containing Al2O3, the average surface roughness values were measured at 1.64 µm, 1.52 µm, 1.45 µm, and 1.30 µm for concentrations of 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, and 0.4 wt.%, respectively. These measurements correspond to reductions of 21.2%, 26.9%, 30.3%, and 37.5% when compared to the surface roughness values recorded for LG. Figure 14 illustrates the surface roughness measurements for all test cases.
The above observations indicate that nano-additives, particularly MWCNTs, significantly enhance the lubrication performance in electrified conditions by reducing surface roughness, which is a critical factor in the longevity and efficiency of bearing operations.
When well dispersed, MWCNTs form a robust network that supports and distributes the applied mechanical load while reducing surface interactions between raceways and balls in the bearing assembly. Hence, this action prevents localized stress concentration, which is critical for minimizing abrasive wear mechanisms compared to traditional greases [69,70]. The tubular geometry of MWCNTs manifested by the high aspect ratio allows for the formation of a stable lubricating film, which minimizes direct asperity contact during sliding [71], thus, it enhances wear resistance by acting as a physical barrier to surface deformation and material peeling.
Additionally, MWCNTs exhibit excellent friction-reducing characteristics due to their ability to roll and slide between contacting surfaces. The nanoscale tubular structure creates a ball-bearing effect, reducing the friction coefficient. This is also due to the alignment of MWCNTs along the sliding direction, providing a low-shear-strength interface.
Similarly, well-dispersed Al2O3 nanoparticles create a consistent microstructure that facilitates the formation of a protective tribofilm on contact surfaces. A previous study proved that lithium-based greases containing Al2O3 nanoparticles exhibit lower coefficients of friction (COF) and reduced wear scar diameters compared to base greases without nanoparticles [59]. Considering its high specific surface area and nanoscale diameters, the deposited alumina on the surface acts as a tribofilm, generating a mending effect that assists in repairing damaged surfaces and prevents direct asperity contacts.
From the SEM analysis of the alumina structure, Al2O3 nanoparticles can act as rolling elements between contact surfaces, similar to microscopic ball bearings [72]. This rolling mechanism reduces direct adhesion between surfaces, thereby decreasing friction and wear. The spherical shape also allows the nanoparticles to polish asperities on the contact surfaces, leading to a smoother interface and further reduction in friction.
To further explain the findings from the laser microscope analysis, SEM coupled with the energy-dispersive X-ray (EDX) analysis of the damaged raceway surface was undertaken at a resolution of 10 μm. Figure 15 presents the micrograph alongside the corresponding EDX spectrum for the bearing lubricated with lithium grease (LG). The SEM image in Figure 15a shows evidence of a localized melting area of the bearing raceway resulting from EDM discharges. This observation is confirmed by the EDX spectrum, which shows a high weight percentage (wt.%) of carbon (C) and oxygen (O), indicating the degradation of the organic compounds of the bearing grease. Other trace elements like phosphorus (P), manganese (Mn), calcium (Ca), and chromium (Cr) that are detected in the EDX spectrum are derived from the molten bearing material and grease additives. This complies with the remarkably high measured vibration RMS levels and laser microscope roughness values.
The micrographs and their corresponding EDX spectra for experiments conducted on bearings lubricated with nano-grease blended with MWCNT are shown in Figure 16a–d. The evaluation of the images reveals the presence of tiny microcraters at concentrations of 0.1 wt.% and 0.2 wt.%, which diminish with increasing concentration levels of the carbonaceous nano-additives to the grease. This further suggests that MWCNT mitigates the damaging effects of EDM discharges. An analysis of the EDX spectra indicates a substantial increase in the wt.% of carbon (C) and oxygen (O) across all test cases compared to the tests conducted with LG alone. This additional increase in carbon content is attributed to the incorporation of MWCNT into the lithium grease. Notably, as the weight percentages of MWCNT blended with the lithium grease increase, there is a corresponding increase in carbon deposition on the bearing surface. This deposited carbon forms a physical protective barrier between the rolling elements and the bearing raceway, which not only provides metal-to-metal separation but also absorbs any EDM discharges. Other research works in the literature confirm the behavior of MWCNT in forming a tribofilm layer to the surface of raceways during operation, which reduced both COF and wear volume in comparison with bearings lubricated with LG only [51].
MWCNTs exhibit synergistic effects with lithium grease conventional additives, such as molybdenum disulfide (MoS2), zinc dialkyldithiophosphate (ZDDP), and calcium sulfonates. For instance, it has been reported that the decomposition products of ZDDP interact with MWCNTs, forming a composite tribofilm with enhanced anti-wear properties [73]. Furthermore, the lamellar structure of MoS2 complements the rolling effect of MWCNTs, resulting in a lower friction coefficient [74,75].
When combined with grease EP additives such as ZnO, Al2O3 nanoparticles contribute to the formation of a robust hybrid protective layer on metal surfaces. This layer enhances the load-carrying capacity of the grease, providing superior protection under high-pressure conditions [72].
Similarly, the micrographs and EDX spectra for bearings lubricated with grease blended with Al2O3 are presented in Figure 17a–d. The SEM images show the presence of micropits and frosting bands of the raceway in all test cases [76]. As the concentration of Al2O3 increases, a progressive decrease in the degree of pitting is observed, which indicates that Al2O3 offers some protection against EDM discharges. Because of their minute sizes and high specific surface area, Al2O3 nanoparticles have been shown to have a mending effect on the bearing raceway surface by restoring worn-out portions [59]. An analysis of the EDX spectra confirms the presence of aluminum on the bearing surface at concentrations of 0.3 wt.% and 0.4 wt.%, while no aluminum was detected at the lower concentrations of 0.1 wt.% and 0.2 wt.%. This finding suggests that there is an optimal concentration at which Al2O3 becomes effective in mitigating EDM discharges.

4. Conclusions

In this study, test bearings lubricated with various concentrations of two different types of nano-additives in lithium grease were subjected to electric current, and the discharge behavior of the bearings was meticulously observed and documented. The vibration response of the bearings was subsequently assessed, and the type and extent of surface damage were quantified using scanning electron microscopy (SEM), EDX, and 3D laser microscopy techniques. The results from these comprehensive tests indicate that nano-additives play a pivotal role in mitigating electrical current damage in rolling-element bearings. The specific conclusions drawn from the outcomes of this research are as follows:
  • The addition of nanoparticles into bearing grease significantly reduces EDM discharge magnitudes compared to LG with the discharge magnitudes decreasing with increasing concentrations of nano-additives across all test cases.
  • The root mean square (RMS) vibration levels were also lower in bearings lubricated with nano-additives compared to those lubricated with LG. Specifically, MWCNT reduced the RMS acceleration levels by 65.19% at a volume concentration of 0.4 wt.%, while alumina nanoparticles resulted in a 44.89% reduction.
  • Surface analysis indicated increased wear in bearings lubricated with LG relative to those containing nano-additives, suggesting that the latter provide a protective effect against EDM discharges. MWCNTs demonstrated a remarkable 70.67% reduction in surface damage, whereas alumina nanoparticles exhibited a reduction of 37.5%.
  • The degree of protection afforded by the nano-additives was evident by the EDX analysis of the worn surface and proved to be contingent upon both the concentration level and the specific type of nano-additive employed. MWCNTs exhibited superior performance compared to alumina (Al2O3), especially at lower concentrations.

Author Contributions

Conceptualization, M.G.A.N. and T.F.M.; Methodology, E.R.J., I.A. and M.G.A.N.; Samples’ Preparation, E.R.J. and M.G.A.N.; Formal Analysis, M.G.A.N., E.R.J. and T.F.M.; Investigation, E.R.J., M.G.A.N. and T.F.M.; Resources, I.A. and M.G.A.N.; Data Curation, M.G.A.N. and E.R.J.; Writing—Original Draft Preparation, E.R.J. and M.G.A.N.; Writing—Review and Editing, I.A. and T.F.M.; Visualization, M.G.A.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Electric model of lubricant based on lubrication regimes.
Figure 1. Electric model of lubricant based on lubrication regimes.
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Figure 2. Experimental test layout.
Figure 2. Experimental test layout.
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Figure 3. Electrical layout of test equipment.
Figure 3. Electrical layout of test equipment.
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Figure 4. Schematic illustration of the experimental procedure to induce discharges in bearings.
Figure 4. Schematic illustration of the experimental procedure to induce discharges in bearings.
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Figure 5. Microstructure of the MWCNT as observed under (a) SEM and (b) TEM.
Figure 5. Microstructure of the MWCNT as observed under (a) SEM and (b) TEM.
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Figure 6. Microstructure of the Al2O3 as observed by (a) SEM and (b) TEM.
Figure 6. Microstructure of the Al2O3 as observed by (a) SEM and (b) TEM.
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Figure 7. Average COF for the tested greases.
Figure 7. Average COF for the tested greases.
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Figure 8. Typical discharge activity of LG in terms of (a) voltage and (b) current with time.
Figure 8. Typical discharge activity of LG in terms of (a) voltage and (b) current with time.
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Figure 9. Electrical behavior of tested lubricants.
Figure 9. Electrical behavior of tested lubricants.
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Figure 10. RMS vibration amplitudes for bearings lubricated with different concentrations of nano-additive greases.
Figure 10. RMS vibration amplitudes for bearings lubricated with different concentrations of nano-additive greases.
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Figure 11. D laser microscope images for the outer raceway surface of (a) a bearing lubricated with LG, and (b) a bearing lubricated with 0.1 wt.% of Al2O3.
Figure 11. D laser microscope images for the outer raceway surface of (a) a bearing lubricated with LG, and (b) a bearing lubricated with 0.1 wt.% of Al2O3.
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Figure 12. Laser microscope image of the outer race of bearings lubricated with (a) 0.1 wt.% of MWCNT, (b) 0.2 wt.% of MWCNT, (c) 0.3 wt.% of MWCNT, and (d) 0.4 wt.% of MWCNT.
Figure 12. Laser microscope image of the outer race of bearings lubricated with (a) 0.1 wt.% of MWCNT, (b) 0.2 wt.% of MWCNT, (c) 0.3 wt.% of MWCNT, and (d) 0.4 wt.% of MWCNT.
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Figure 13. Laser microscope image of bearings lubricated with (a) 0.1 wt.% of Al2O3, (b) 0.2 wt.% of Al2O3, (c) 0.3 wt.% of Al2O3, and (d) 0.4 wt.% of Al2O3.
Figure 13. Laser microscope image of bearings lubricated with (a) 0.1 wt.% of Al2O3, (b) 0.2 wt.% of Al2O3, (c) 0.3 wt.% of Al2O3, and (d) 0.4 wt.% of Al2O3.
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Figure 14. Average surface roughness for different concentrations of nano-additives.
Figure 14. Average surface roughness for different concentrations of nano-additives.
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Figure 15. (a) SEM micrograph of bearing lubricated with LG and (b) corresponding EDX spectrum.
Figure 15. (a) SEM micrograph of bearing lubricated with LG and (b) corresponding EDX spectrum.
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Figure 16. SEM micrographs and EDX spectra for bearings lubricated with MWCNT at concentrations of (a) 0.1 wt.%, (b) 0.2 wt.%, (c) 0.3 wt.%, and (d) 0.4 wt.%.
Figure 16. SEM micrographs and EDX spectra for bearings lubricated with MWCNT at concentrations of (a) 0.1 wt.%, (b) 0.2 wt.%, (c) 0.3 wt.%, and (d) 0.4 wt.%.
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Figure 17. SEM micrographs and EDX spectra for bearings lubricated with Al2O3 at concentrations of (a) 0.1 wt.%, (b) 0.2 wt.%, (c) 0.3 wt.%, and (d) 0.4 wt.%.
Figure 17. SEM micrographs and EDX spectra for bearings lubricated with Al2O3 at concentrations of (a) 0.1 wt.%, (b) 0.2 wt.%, (c) 0.3 wt.%, and (d) 0.4 wt.%.
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Table 1. Specifications of the components of the test set-up.
Table 1. Specifications of the components of the test set-up.
ComponentSpecification
Electric motorFELM (3.5 kW, and 1440 rpm)
ShaftSUS 420 stainless steel
BaseplateC45 carbon steel
Support bearingsNU1011M roller bearing (SKF)
Signal generator(Max voltage: 60 V; max current: 3.2 A) (Tektronix)
OscilloscopeTBS 1152B (150 MHz, 2 GS/s) (Tektronix)
Current probeRT-ZCO3 (Max current: 20 A) (Rhode and Schwarz)
Table 2. Specifications of the test bearings.
Table 2. Specifications of the test bearings.
DimensionsdDBr
30 mm55 mm13 mm1 mm
Race TypePlain
Weight (kg)0.116 kg
Dynamic Loading13.8 kN
Static Loading8.3 kN
Radial ClearanceC3
d = Inside diameter, D = outside diameter, B = bearing width, and r = race width.
Table 3. Properties of the base lithium grease.
Table 3. Properties of the base lithium grease.
CharacteristicsManufacturer Specifications
Base oilMineral oil
ThickenerLithium soap
Base oil viscosity @ 40 °C110 mm2/s
Base oil viscosity @ 100 °C12 mm2/s
Penetration220–250 (0.1 mm)
Drop point≥180 °C
Table 4. Surface roughness measurements for different nano-additive concentrations.
Table 4. Surface roughness measurements for different nano-additive concentrations.
Average Surface Roughness
LGMWCNTAl2O3
-0.1wt.%0.2wt.%0.3wt.%0.4wt.%0.1 wt.%0.2 wt.%0.3wt.%0.4wt%
2.080.780.750.640.611.641.521.451.30
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Jonjo, E.R.; Ali, I.; Megahed, T.F.; Nassef, M.G.A. Mitigation of Electrical Discharge Damage in Electric Vehicle Bearings: Comparative Study of Multi-Walled Carbon Nanotubes and Alumina Nanoparticles in Lubricating Grease. Vehicles 2025, 7, 19. https://doi.org/10.3390/vehicles7010019

AMA Style

Jonjo ER, Ali I, Megahed TF, Nassef MGA. Mitigation of Electrical Discharge Damage in Electric Vehicle Bearings: Comparative Study of Multi-Walled Carbon Nanotubes and Alumina Nanoparticles in Lubricating Grease. Vehicles. 2025; 7(1):19. https://doi.org/10.3390/vehicles7010019

Chicago/Turabian Style

Jonjo, Emmanuel R., Islam Ali, Tamer F. Megahed, and Mohamed G. A. Nassef. 2025. "Mitigation of Electrical Discharge Damage in Electric Vehicle Bearings: Comparative Study of Multi-Walled Carbon Nanotubes and Alumina Nanoparticles in Lubricating Grease" Vehicles 7, no. 1: 19. https://doi.org/10.3390/vehicles7010019

APA Style

Jonjo, E. R., Ali, I., Megahed, T. F., & Nassef, M. G. A. (2025). Mitigation of Electrical Discharge Damage in Electric Vehicle Bearings: Comparative Study of Multi-Walled Carbon Nanotubes and Alumina Nanoparticles in Lubricating Grease. Vehicles, 7(1), 19. https://doi.org/10.3390/vehicles7010019

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