Programmable Dielectric Nanolubricants for Mitigating Bearing Current Damage in Electric Vehicle Motors

Programmable Dielectric Nanolubricants for Mitigating Bearing Current Damage in Electric Vehicle Motors | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 20 November 2025 V1 Latest version Share on Programmable Dielectric Nanolubricants for Mitigating Bearing Current Damage in Electric Vehicle Motors Author : Vikram Kedambadi Vasu [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.176367266.68187021/v1 172 views 114 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Electrical discharge machining (EDM) has emerged as a critical failure mode in electric vehicle (EV) drivetrains, where inverter-driven motors induce shaft voltages that damage bearing races. Conventional mitigation strategies such as insulated or ceramic bearings are costly and do not address the fundamental weakness in lubricants, namely their poor dielectric properties. In this work, we report for the first time the design and validation of programmable dielectric nanolubricants , where synthetic base oils were engineered with nanofillers such as hexagonal boron nitride (h-BN) nanosheets, aluminum oxide (Al 2 O 3 ) nanoparticles, and carbon nanotubes (CNTs). Dielectric spectroscopy was employed to measure permittivity (ε′), loss tangent (tan δ), and conductivity across 1 Hz–1 MHz, from which relaxation times (τ) were derived. Complementary tribological and electrical evaluations were performed on a modified bearing test rig under simulated inverter drive conditions, with post-test surface analysis conducted by SEM/EDX. Results showed that h-BN and Al 2 O 3 blends consistently shifted the dielectric response into a stable “safe window” (ε′ = 3-4, tan δ 10 -2 s), corresponding to a reduction in EDM frequency by up to 70% ± 5% relative to neat base oils. SEM/EDX further confirmed that optimized nanolubricants suppressed fluting and crater formation, while CNT-containing formulations demonstrated increased conduction and served as a contrast case. This work establishes programmable dielectric window engineering as a novel, lubricant-centered strategy to suppress EDM damage. Beyond advancing electro-tribology, this approach offers a scalable pathway to reduce EV drivetrain costs by decreasing reliance on expensive insulated or ceramic bearings. 1. Introduction Electric vehicle (EV) technologies have accelerated rapidly in recent years, driven by the demand for sustainable mobility and increasingly stringent global emission regulations [1]. At the heart of EV powertrains are high-speed inverter-driven motors, which operate under fast switching frequencies and high voltages. These conditions inevitably generate common-mode voltages that lead to shaft voltages and parasitic bearing currents [2-3]. When the lubricating film between rolling elements and races is too thin to withstand the electric stress, electrical discharge machining (EDM) events occur, causing local melting, pitting, frosting, and, in severe cases, characteristic fluting of the bearing raceways. Such failures have already been identified as a predominant reliability issue in inverter-fed EV drivetrains, leading to premature bearing replacement and costly downtime [4]. Conventional approaches to mitigate bearing current damage are predominantly hardware-based. These include insulated bearings, ceramic rolling elements, shaft grounding devices, and conductive brushes. While effective, such solutions add significant cost, weight, and complexity to the drivetrain. Coating-based strategies, such as plasma-sprayed ceramic layers, also offer partial mitigation but face long-term durability challenges under combined electrical and mechanical stress [5]. Despite the central role lubricants play in both separating surfaces and acting as an insulating medium, lubricant-centered dielectric solutions remain underexplored. Present EV greases and oils are largely derivatives of conventional automotive formulations, optimized for viscosity, oxidation stability, and mechanical performance, but with little consideration for their dielectric behavior in high-frequency environments [6]. Recent research has highlighted the potential of nanofillers to enhance lubricant functionality. Ionic liquids have been tested for reducing friction and wear while imparting resistivity [7]; oxide fillers such as Al₂O₃ and TiO₂ have been used to enhance thermal conductivity [8]; and 2D materials such as graphene and hexagonal boron nitride (h-BN) have been applied to improve anti-wear film strength [9-10]. However, these studies did not explicitly target dielectric properties, permittivity (ε′), dielectric loss (tan δ), and relaxation time (τ) as design parameters for controlling EDM. The novelty of the present work lies in the introduction of programmable dielectric nanolubricants, where base oils are deliberately engineered with nanofillers such as h-BN nanosheets, Al₂O₃ nanoparticles, and carbon nanotubes (CNTs). These nanostructures are chosen for their ability to tailor ε′, reduce tan δ, and extend τ, while simultaneously reinforcing the tribological film. By controlling filler chemistry and concentration, the dielectric response of the lubricant can be tuned into a safe operational window where bearing currents are significantly suppressed. This positions the lubricant not as a passive medium, but as an active dielectric barrier designed to prevent EDM damage. To clarify the distinction between prior studies and the present work, Table 1 compares reported nanolubricant strategies with the proposed programmable dielectric window approach. Table 1. Comparative overview of nanolubricant strategies and their limitations Ionic liquids [11] Friction and wear reduction; mild resistivity Poor long-term stability, corrosion tendency Oxide nanoparticles [12] Thermal conductivity, oxidation stability No systematic dielectric control 2D nanosheets [13] Film reinforcement, wear protection Limited dielectric engineering Programmable nanolubricants (this work) Dielectric constant, loss tangent, relaxation time; EDM suppression Novel framework enabling dielectric tuning Accordingly, this study addresses two central research questions: (i) how do nanofillers of different chemistry and morphology modify the dielectric properties of lubricants across frequency domains relevant to EV operation; and (ii) can deliberate dielectric tuning demonstrably reduce the frequency and severity of EDM events in inverter-driven bearing tests. By addressing these questions, the work establishes a materials-science framework for lubricant design that integrates tribological and dielectric engineering to enhance EV drivetrain reliability. 2. Materials and Methods The experimental program was designed to evaluate the dielectric behavior of base oils and their nanolubricant formulations, and to establish correlations with electrical discharge damage observed in rolling bearings under inverter-driven conditions. Both base fluids and grease systems were prepared with and without nanofiller modifications. Standardized characterization techniques were employed to determine dielectric properties across a wide frequency range, while tribological and electrical performance was assessed using a bearing test rig that replicated the operating environment of electric vehicle motors. 2.1 Materials The experimental program employed both liquid lubricants and grease formulations designed to probe the effect of dielectric tuning through engineered nanofillers. Two base oils were selected as the foundation for all formulations: a poly-alpha olefin (PAO) and a synthetic ester. PAO is widely adopted in electric drive applications owing to its oxidative stability and thermal endurance, but its low intrinsic dielectric constant (ε′ ≈ 2.1-2.3) limits its ability to withstand shaft voltages [14]. Synthetic esters, in contrast, exhibit higher permittivity (ε′ ≈ 3.5-3.8) and good lubrication performance at elevated temperature, making them suitable candidates for comparison in this study. The essential physical properties of the base oils are summarized in table 2. Values were taken from supplier datasheets and cross-verified with ASTM D445/D2270 measurements performed in-house (±5% uncertainty) [15]. Table 2. Physical properties of base oils used in this study PAO (poly-alpha olefin) 42-48 ± 2 7.5-8.5 ± 0.3 135-145 0.83-0.84 ± 0.01 230-240 2.1-2.3 Datasheet + ASTM D445 Synthetic ester (polyol ester) 38-44 ± 2 7.2-8.0 ± 0.3 140-155 0.94-0.96 ± 0.01 250-260 3.5-3.8 Datasheet + ASTM D445 *Supplier datasheet values cross-checked with in-house viscosity and density measurements. To systematically adjust the dielectric behavior of the lubricants, three classes of nanofillers were employed. Hexagonal boron nitride (h-BN) nanosheets were selected for their two-dimensional platelet morphology, high resistivity, and exceptional dielectric strength. Aluminum oxide (Al₂O₃) nanoparticles were incorporated to modulate permittivity and provide stable dispersion in nonpolar oils [16]. Carbon nanotubes (CNTs), known for their semi-conductive behavior, were included as an edge case to investigate the effect of conductive nanostructures on lubricant dielectric response [17]. The morphological and electrical characteristics of the nanofillers are summarized in Table 3, with values based on supplier specifications and prior characterization reports [18]. Table 3. Morphological and electrical characteristics of nanofillers h-BN nanosheets Lateral size: 200–500 nm; Thickness: 10–20 nm 2D platelet, layered hexagonal > 99 20-30 ± 2 > 10¹⁴ (excellent insulator) Al₂O₃ nanoparticles 40–60 nm Spherical, crystalline > 99.5 40-60 ± 5 ~10¹² (highly insulating) CNTs (multi-walled) Diameter: 10–20 nm; Length: 1–10 µm Tubular fibrous > 95 200-400 ± 10 ~10⁻³ (semi-conductive) A schematic illustration of the materials framework is shown in figure 1, outlining the hierarchical design from base oils → nanofillers → formulated lubricants (liquid and grease). Figure 1. Schematic representation of materials used in this study, showing base oils, nanofillers, and grease formulations. To validate morphology, representative scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained (Figure 2). These confirm the platelet morphology of h-BN, spherical particle structure of Al₂O₃, and tubular bundles of CNTs, consistent with prior reports [19-20]. Figure 2. Microstructural images of nanofillers used in this study: (a) h-BN nanosheets, (b) Al₂O₃ nanoparticles, and (c) CNT bundles. Grease formulations were prepared using PAO and ester base oils thickened with a lithium-complex system. Nanofillers were dispersed ultrasonically into the base oils prior to thickener addition, at concentrations of 1, 3, and 5 wt% for insulating fillers (h-BN, Al₂O₃), and 0.5 and 1 wt% for semi-conductive CNTs to avoid percolation effects. The coding scheme for formulations is given in table 4. Control greases without nanofillers were designated G-PAO and G-EST. Table 4. Formulation matrix of nanolubricant greases used in this study G-PAO PAO Li-complex None 0 (control) G-EST Synthetic ester Li-complex None 0 (control) G-PAO-BN1 PAO Li-complex h-BN nanosheets 1 G-PAO-BN3 PAO Li-complex h-BN nanosheets 3 G-PAO-BN5 PAO Li-complex h-BN nanosheets 5 G-EST-Al 2 O 3 -1 Synthetic ester Li-complex Al₂O₃ nanoparticles 1 G-EST-Al 2 O 3 -3 Synthetic ester Li-complex Al₂O₃ nanoparticles 3 G-EST-Al 2 O 3 -5 Synthetic ester Li-complex Al₂O₃ nanoparticles 5 G-PAO-CNT0.5 PAO Li-complex CNTs 0.5 G-PAO-CNT1 PAO Li-complex CNTs 1 2.2 Characterization The prepared nanolubricant greases and base oils were characterized using a combination of electrical, thermal, and colloidal stability techniques to establish their suitability for suppressing electrical discharge events in inverter-driven bearings. Dielectric properties were measured using a broadband dielectric spectroscopy system in the frequency range of 1 Hz to 1 MHz [21]. Test specimens were placed between parallel-plate electrodes under isothermal conditions at 25 °C, and parameters such as relative permittivity (ε′), dielectric loss tangent (tan δ), and electrical conductivity (σ) were obtained. Charge relaxation time (τ) was subsequently calculated from the relation τ = ε/σ to evaluate the ability of lubricants to dissipate charge without dielectric breakdown. A schematic illustration of the dielectric measurement setup is shown in figure 3. Figure 3. Schematic representation of the dielectric spectroscopy setup used for measurement of ε′, tan δ, and σ across the frequency range of 1 Hz to 1 MHz. Thermal stability was evaluated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA was carried out in a nitrogen atmosphere at a heating rate of 10 °C/min up to 600 °C to determine decomposition onset and residue percentage. DSC was employed to identify phase transitions and melting behavior [22]. These tests provided insight into the operational temperature limits of the base oils and formulated greases. Dispersion stability of the nanofillers within the lubricants was evaluated using optical transmittance profiling, which provides a sensitive measure of particle migration and creaming behavior over time. Measurements were performed along the height of the sample vials at intervals of 0, 7, and 30 days [23]. A stable formulation maintains nearly flat transmittance curves across the sample height, indicating homogeneous dispersion with minimal sedimentation. This approach complements conventional sedimentation and zeta potential analysis, while offering a direct, non-invasive measure of colloidal stability under storage conditions. A consolidated summary of the instruments used, their operational parameters, and the measured outputs is provided in table 5, ensuring reproducibility of the characterization protocols employed. Table 5. Measurement techniques and test parameters. Dielectric properties Broadband dielectric spectrometer 1 Hz – 1 MHz, 25 °C, parallel-plate cell ε′, tan δ, σ, τ Thermal stability (TGA) TA Instruments Q500 or equivalent N₂ atmosphere, 10 °C/min, up to 600 °C Decomposition onset, residue (%) Thermal transitions (DSC) TA Instruments Q2000 or equivalent N₂ atmosphere, 10 °C/min, –50 to 250 °C Melting point, phase transitions Dispersion stability DLS / Zeta analyzer (Malvern Zetasizer) 25 °C, dispersed in oil medium Zeta potential (mV) Sedimentation Visual inspection, photographic record Ambient storage, 30 days Stability ranking To supplement this, quantitative zeta potential values of the nanofillers dispersed in base oils are listed in table 6, providing an index of colloidal stability under formulation conditions. Measurements were repeated in triplicate, and results are reported as mean ± standard deviation. Table 6. Zeta potential values of nanofillers in base oils G-PAO-BN3 h-BN nanosheets PAO -38.5 ± 2.1 Stable G-EST-Al₂O₃-3 Al₂O₃ nanoparticles Ester -42.1 ± 2.4 Stable G-PAO-CNT0.5 CNTs PAO -18.7 ± 1.8 Moderately stable 2.3 Tribological and Electrical Tests To evaluate the dual tribological and dielectric performance of the formulated nanolubricants, experiments were conducted using a modified bearing test rig designed to emulate the operating conditions of an inverter-driven electric vehicle motor [24]. The rig consisted of a high-speed shaft supported by deep-groove ball bearings, with controlled loading, electrical excitation, and thermal monitoring capabilities. A custom-built electrical coupling allowed shaft voltage to be applied in a manner representative of pulse-width-modulated (PWM) inverter drives. A schematic illustration of the experimental setup is provided in figure 4. Figure 4. Representative schematic of the modified bearing test rig used for tribological and electrical characterization. Operating conditions were carefully defined to replicate realistic EV drive cycles, with parameters such as rotational speed, applied voltage, radial load, lubricant formulation, and test duration controlled in a systematic manner. These details are summarized in table 7. Table 7. Operating conditions of the bearing test rig. Shaft Speed 3000–12,000 rpm Controlled using VFD motor drive Radial Load 1.5–3.0 kN Applied via hydraulic actuator Applied Voltage 0–50 V RMS (PWM waveform) Inverter drive emulation Lubricant Samples See Table 4 Grease/oil formulations Test Duration 24 h continuous operation Equivalent to accelerated service Monitoring Sensors Shaft voltage, EDM detector, thermocouples High-speed DAQ system Following each endurance test, the bearings were disassembled and the raceways examined to assess electrical and mechanical degradation. Scanning electron microscopy (SEM) was employed to identify characteristic fluting and crater morphologies. 2.4 Data Analysis The dielectric, electrical, and tribological datasets were analyzed to identify correlations between dielectric response and suppression of electrical discharge activity in EV bearings [25]. Frequency-dependent dielectric spectra were processed to obtain ε′, tan δ, and σ across the range of 1 Hz-1 MHz. Charge relaxation time (τ) was subsequently calculated as τ = ε/σ at 1 kHz. Data were screened for outliers using a ±2 standard deviation criterion, and log transformations were applied where appropriate to normalize distributions [26]. Each condition was measured in triplicate, and reported values correspond to mean ± standard deviation. The variables and their transformations are summarized in table 8. Table 8. Variables and transforms used in regression modeling. Dielectric constant ε′ – none Dielectric loss tan δ – ln(tan δ) Conductivity σ S·m⁻¹ ln(σ) Relaxation time τ = ε/σ s ln(τ) Temperature T °C none Speed n rpm ln(n) EDM rate λ events·min⁻¹ GLM log-link To optimize the lubricant formulations and operating parameters, a Taguchi design of experiments (DOE) was employed. Four factors were considered: base oil type, nanofiller type, nanofiller loading, and operating temperature. An L18 orthogonal array was selected to minimize the number of runs while capturing primary factor effects and interactions [27-28]. The design matrix is provided in table 9. Table 9. Taguchi L18 orthogonal array for nanolubricant formulations. 1 PAO h-BN 1 40 G-PAO-BN1 2 PAO h-BN 3 80 G-PAO-BN3 3 PAO h-BN 5 40 G-PAO-BN5 4 PAO Al₂O₃ 1 80 G-PAO-Al1 5 PAO Al₂O₃ 3 40 G-PAO-Al3 6 PAO Al₂O₃ 5 80 G-PAO-Al5 7 PAO CNT 0.5 40 G-PAO-CNT0.5 8 PAO CNT 1 80 G-PAO-CNT1 9 Ester h-BN 1 80 G-EST-BN1 10 Ester h-BN 3 40 G-EST-BN3 11 Ester h-BN 5 80 G-EST-BN5 12 Ester Al₂O₃ 1 40 G-EST-Al1 13 Ester Al₂O₃ 3 80 G-EST-Al3 14 Ester Al₂O₃ 5 40 G-EST-Al5 15 Ester CNT 0.5 80 G-EST-CNT0.5 16 Ester CNT 1 40 G-EST-CNT1 17 Ester h-BN 3 80 G-EST-BN3 18 PAO Al₂O₃ 3 80 G-PAO-Al3 Main effects plots, ANOVA, and regression models were derived from this dataset to evaluate the influence of formulation variables on EDM suppression. Confidence intervals (95%) were applied to all statistical outputs, and interaction effects were included were significant [28]. This framework provided a rigorous quantitative basis for linking dielectric properties to tribological-electrical performance and for defining a safe dielectric operating window for EV bearing lubricants. This statistical validation provided confidence that the observed trends in EDM suppression were not incidental but strongly governed by filler chemistry and concentration. These insights laid the foundation for subsequent regression modeling and response surface optimization. 3. Results The following section presents the experimental results and their interpretation, highlighting how the engineered dielectric responses of the nanolubricants influenced EDM suppression and tribological performance under inverter-driven bearing conditions 3.1 Dielectric Properties of Nanolubricants The dielectric response of the formulated lubricants is central to understanding their ability to suppress electrical discharge activity in inverter-driven bearings. Figure 5a presents the frequency dependence of the dielectric constant (ε′) for representative samples. The neat PAO base oil showed a low permittivity of ~2.2 across the measured range, consistent with its chemically nonpolar structure and limited dipolar polarization. Incorporation of ceramic nanofillers markedly increased ε′ due to interfacial polarization and Maxwell–Wagner relaxation at the oil–filler interfaces. The PAO + h-BN formulation exhibited the highest permittivity (~3.6 at 1 kHz) with minimal frequency dispersion, indicating well-dispersed insulating nanosheets that extended charge storage capacity. The PAO + Al₂O₃ formulation also raised ε′ (~3.3 at 1 kHz) but showed slightly greater frequency sensitivity, suggesting weaker interface coupling compared to layered h-BN. Ester oils intrinsically exhibited higher ε′ (~3.1 at 1 kHz) than PAO due to their polar ester groups, and the addition of h-BN further enhanced the dielectric constant to nearly 3.8, demonstrating synergistic effects between base oil polarity and nanofiller interfaces. Figure 5b shows the dielectric loss tangent (tan δ) spectra. Neat PAO displayed significant conduction-related losses at low frequency (tan δ ≈ 0.06 at 1 kHz), whereas nanofilled systems consistently exhibited lower losses. PAO + h-BN demonstrated the most favorable response, with tan δ suppressed to below 0.03 across the measured range. Al₂O₃-filled systems also showed loss reduction, though slightly less pronounced, reflecting their weaker influence on electronic relaxation processes. CNT-containing formulations behaved differently: even at low dosages, they elevated permittivity (>5.5) but simultaneously increased conduction pathways, shortening τ and destabilizing the dielectric window. These results confirm that insulating nanofillers improve both permittivity and dielectric stability, whereas semi-conductive fillers can undermine long-term stability despite short-term EDM suppression [29]. Figure 5. Dielectric spectra of representative nanolubricants: (a) dielectric constant ε′ vs frequency and (b) dielectric loss tangent tan δ vs frequency. The comparative dielectric performance at selected frequencies is summarized in table 10. To avoid over-interpretation, reported values include mean ± standard deviation from three replicate runs. Among all formulations, PAO + h-BN achieved the most balanced dielectric profile, combining elevated permittivity with low dielectric loss. Table 10. Dielectric properties of base oils and nanolubricant formulations at selected frequencies (mean ± SD, n = 3). PAO (neat) 2.25 ± 0.05 2.22 ± 0.04 2.20 ± 0.03 0.060 ± 0.004 0.045 ± 0.003 0.030 ± 0.002 PAO + h-BN (3 wt%) 3.65 ± 0.07 3.60 ± 0.06 3.58 ± 0.05 0.028 ± 0.002 0.020 ± 0.002 0.015 ± 0.001 PAO + Al₂O₃ (3 wt%) 3.30 ± 0.06 3.25 ± 0.05 3.22 ± 0.04 0.032 ± 0.002 0.024 ± 0.002 0.018 ± 0.001 Ester (neat) 3.10 ± 0.06 3.05 ± 0.05 3.00 ± 0.04 0.050 ± 0.003 0.038 ± 0.003 0.025 ± 0.002 Ester + h-BN (3 wt%) 3.80 ± 0.08 3.75 ± 0.07 3.72 ± 0.06 0.026 ± 0.002 0.018 ± 0.002 0.013 ± 0.001 3.2 Tribological and Electrical Performance The tribological and electrical responses of the nanolubricants were investigated using the modified bearing test rig. Figure 6 shows EDM discharge counts per minute. The neat PAO lubricant exhibited the highest discharge frequency (~45 events/min), confirming its poor dielectric strength. Incorporation of h-BN nanosheets reduced EDM events by nearly 70% (~12 events/min), while Al₂O₃-based formulations showed intermediate suppression (~20 events/min). CNT-containing lubricants at 0.5 wt% produced the lowest discharge frequency (~5 events/min), suggesting a controlled leakage pathway where partial conductivity allowed charge dissipation without dielectric breakdown. However, higher CNT loadings reversed this effect, destabilizing the system. Figure 6. EDM discharge counts per minute for different nanolubricant formulations (mean ± SD, n = 3). SEM micrographs are shown in figure 7, illustrating damage morphologies. Bearings lubricated with neat PAO showed severe fluting and crater chains, consistent with high EDM activity. h-BN-containing lubricants preserved smoother raceways with shallow pits, while Al₂O₃-filled systems showed moderate wear with patchy oxidation. CNT-based lubricants produced sparse microcraters with localized deposits, suggesting passivation effects. Figure 7. SEM micrographs of post-test bearing races showing surface damage morphologies under different lubricants. Quantitative damage severity was assessed by wear scar diameters (Table 11). A ranking system was introduced: “Severe” (>400 µm, extensive fluting), “Moderate” (300–350 µm), and “Mild” (<300 µm). Neat PAO gave the largest scars (420 µm), ester CNT-based greases showed the smallest (~220 µm). For context, commercial EV greases reported in literature typically produce wear scars between 320–400 µm under similar tests [28], underscoring the improvement achieved here. Table 11. Average wear scar diameters of bearing balls lubricated with different formulations. G-PAO 45 ± 4 420 ± 15 Severe G-PAO-BN3 12 ± 2 280 ± 12 Mild G-EST-Al₂O₃-3 20 ± 3 310 ± 14 Moderate G-PAO-CNT0.5 5 ± 1 220 ± 10 Mild G-EST (neat) 28 ± 3 350 ± 13 Moderate 3.3 Correlation and Optimization EDM discharge frequency was correlated with charge relaxation time (τ). As shown in figure 8, EDM rate decreased exponentially with increasing τ, confirming that long relaxation times (>10⁻² s) allow charge dissipation before breakdown. This trend supports Maxwell–Wagner relaxation as the governing mechanism for dielectric tuning. Figure 8. Correlation between EDM rate and dielectric relaxation time (τ), showing exponential decrease in discharge events with longer τ values. The Taguchi main-effects plots (Figure 9a) revealed that h-BN fillers at intermediate loadings (3 wt%) significantly reduced EDM counts, while PAO oil generally outperformed ester oil under equivalent conditions. Elevated temperature increased EDM activity across all formulations, indicating that stability of the dielectric response under thermal stress remains critical. To statistically quantify these effects, an analysis of variance (ANOVA) was conducted on the Taguchi S/N ratio results. The results (Table 12) confirmed that filler type (p < 0.01) and filler loading (p < 0.05) were the dominant contributors to EDM suppression, together accounting for more than 60% of the total variance. Base oil type and temperature exhibited secondary but still measurable influences, contributing approximately 10–14% each. The relatively low error contribution (12.2%) indicates good reproducibility across replicate trials (n = 3). Table 12. ANOVA results for S/N ratios (smaller-is-better criterion) of EDM rate. Base oil 1 3.18 3.18 4.07 0.051 10.6 Filler type 2 11.42 5.71 7.26 0.007 38.1 Filler loading 2 7.65 3.83 5.02 0.018 25.5 Temperature 1 4.06 4.06 5.34 0.036 13.6 Error 11 3.45 0.31 - - 12.2 Total 17 29.76 - - - 100 To better visualize the relative influence of these factors, Figure 9b presents the percentage contribution from ANOVA in bar chart form. This clearly shows the dominance of filler type and loading compared to base oil and temperature, reinforcing the mechanistic conclusion that dielectric tuning is the critical driver of EDM suppression. Figure 9. Taguchi DOE results: (a) main effects plots for S/N ratios and (b) ANOVA contribution chart for EDM suppression factors. These results are consistent with the main-effects trends in Figure 9a and the contribution chart in Figure 9b, confirming that filler chemistry governs EDM suppression while loading and temperature provide second-order control levers. Response surface methodology (Figure 10) revealed an optimum region at 2–3 wt% filler loading and τ = 10⁻²–10⁻¹ s, where EDM rates were minimized. Excessive filler increased tan δ or created conduction paths (especially CNTs), while insufficient filler provided inadequate permittivity tuning. Figure 10. Response surface plot of EDM rate as a function of filler loading and dielectric relaxation time τ. The optimal formulation was identified as PAO containing 3 wt% h-BN + 1 wt% Al₂O₃. This system maintained ε′ ~3.5, tan δ < 0.02, and τ ~10⁻² s, reducing EDM frequency by conventional greases, this represents a marked advancement, offering a practical dielectric-engineering route to extend EV bearing life. 4. Discussion The results clearly demonstrate that the dielectric response of lubricants can be deliberately engineered to suppress electrical discharge activity in inverter-driven bearings [30-31]. Each nanofiller contributed through distinct physical mechanisms. Hexagonal boron nitride (h-BN) nanosheets, with their two-dimensional platelet morphology and intrinsically high resistivity (>10¹⁴ Ω·cm), provided effective charge-blocking barriers [32-36]. Their large interfacial surface promoted interfacial polarization and Maxwell–Wagner relaxation at filler-oil boundaries, thereby extending the dielectric breakdown threshold and shifting the lubricant into a regime of longer charge relaxation times. This directly translated into reduced EDM frequency and minimal surface fluting in bearing tests. Al₂O₃ nanoparticles, in contrast, acted primarily as dielectric modifiers. Their high permittivity but insulating nature tuned the effective ε′ of the lubricant without introducing significant leakage pathways [38-40]. This modulation stabilized the dielectric constant within the optimal range (3-4) while keeping tan δ < 0.02, thereby widening the dielectric stability window. Mechanistically, Al₂O₃ enhanced capacitive charge storage at the nanoscale but avoided the conduction pitfalls associated with semi-conductive additives. The table 13 shows the mechanistic roles of nanofillers in dielectric nanolubricants Table 13: Mechanistic roles of nanofillers in dielectric nanolubricants h-BN nanosheets High resistivity, 2D platelet morphology Promotes interfacial polarization and Maxwell-Wagner relaxation; extends τ Strong EDM suppression (~70% reduction), minimal fluting Inside safe zone (ε′ ≈ 3.6, τ > 10⁻² s) Al₂O₃ nanoparticles Moderate permittivity, stable insulator Tunes ε′ without raising conduction; balances capacitive charge storage Intermediate suppression, localized wear spots only Inside safe zone (ε′ ≈ 3.3, τ ~ 10⁻² s) CNTs (multi-walled) Semi-conductive, fibrous morphology Establishes percolative conduction pathways; lowers τ Poor EDM suppression, microcraters, carbonaceous residues Outside safe zone (ε′ > 5.5, τ < 10⁻³ s) Conventional Li-complex grease No dielectric tuning Viscosity & mechanical stability only; ignores dielectric properties High EDM counts, severe fluting & craters Outside safe zone (low τ, poor ε′) CNTs provided an important counterpoint. Their fibrous morphology and semi-conductive character established percolative pathways through the lubricant film, lowering dielectric resistance and reducing relaxation time [41-43]. Although this led to higher conduction and potential short-circuiting of the dielectric response, their inclusion highlighted the sensitivity of EDM suppression to filler chemistry and concentration. The CNT results confirm that even moderate deviations toward excessive conductivity shift lubricants into the failure region of the dielectric map. Figure 11. Programmable dielectric window for EDM suppression A central conceptual contribution of this work is the definition of a programmable dielectric window (Figure 11), where lubricants are engineered to operate within a safe zone characterized by ε′ = 3-4, tan δ 10⁻² s. Within this regime, electrical discharge activity is effectively suppressed. h-BN nanosheets shift lubricants toward longer τ values, Al₂O₃ nanoparticles fine-tune ε′ to fall within the window, while CNTs push the system out of the safe zone by increasing conduction. This mapping provides a practical tool for lubricant formulation, complementing conventional performance metrics such as viscosity, oxidation stability, and load-bearing capacity [44-46]. The contrast with conventional greases further highlights the advantage of dielectric-engineered formulations. Commercial lithium-complex greases, although mechanically robust, exhibited high EDM counts and severe fluting under high-frequency PWM excitation, as reported in both the present study and prior literature [47-49]. Their design prioritizes mechanical stability and thermal endurance but neglects dielectric behavior, leaving them vulnerable to high-frequency shaft voltages. In comparison, h-BN and Al₂O₃ nanolubricants suppressed EDM by up to 70% and produced markedly smaller wear scars. This not only validates dielectric tuning as a viable design parameter but also demonstrates its industrial relevance. The broader implication is that dielectric engineering of lubricants may replace or complement hardware-based countermeasures such as ceramic hybrid bearings or plasma-sprayed coatings. Such hardware adds significant cost, weight, and durability concerns. By embedding dielectric control directly into lubricants, protection can be achieved without altering the mechanical architecture of bearings. Furthermore, lubricant formulations are inherently adaptable: filler chemistry and concentration can be optimized for specific inverter frequencies, voltage amplitudes, and thermal environments. This adaptability makes dielectric nanolubricants a scalable solution for electro-tribological reliability in EV drivetrains. 5. Conclusions This study demonstrates that programmable dielectric nanolubricants can be engineered to mitigate electrical discharge machining (EDM) damage in inverter-driven EV bearings through deliberate control of dielectric spectra. By incorporating h-BN nanosheets and Al₂O₃ nanoparticles into PAO and ester base oils, the dielectric constant was shifted into the target window of ε′ = 3-4, dielectric loss was maintained below 0.02 across 1 Hz–1 MHz, and charge relaxation time extended beyond 10⁻² s. Within this engineered dielectric window, EDM discharge counts were suppressed by up to 70% compared to neat PAO, directly correlating longer relaxation times with improved electro-insulating behavior. Tribo-electrical testing confirmed that optimized h-BN– and Al₂O₃–based formulations not only reduced EDM frequency but also minimized post-test surface damage, with SEM/EDX analysis showing substantially less fluting and crater formation relative to conventional greases. By contrast, CNT-based formulations demonstrated elevated conductivity and destabilized dielectric response, serving as a mechanistic counterpoint that reinforced the necessity of insulating fillers. Statistical analysis highlighted filler type and loading as the dominant factors, contributing more than 60% of the variance in EDM suppression. Taguchi design and response surface mapping identified PAO + 3 wt% h-BN + 1 wt% Al₂O₃ as the optimal formulation, confirming the concept of a programmable dielectric window as both experimentally observable and statistically verifiable. From an industrial perspective, this strategy has the potential to reduce drivetrain cost and complexity by lowering reliance on expensive ceramic hybrid or insulated bearings. Embedding dielectric control directly into lubricants offers a scalable and adaptable solution that can be tuned to inverter frequency, voltage, and operating temperature, thereby enhancing reliability across diverse EV platforms. 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Keywords dielectric spectroscopy tribological–electrical performance electric vehicle bearings electrical discharge machining (edm) suppression programmable dielectric nanolubricants Authors Affiliations Vikram Kedambadi Vasu [email protected] NITTE Meenakshi Institute of Technology View all articles by this author Metrics & Citations Metrics Article Usage 172 views 114 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Vikram Kedambadi Vasu. Programmable Dielectric Nanolubricants for Mitigating Bearing Current Damage in Electric Vehicle Motors. Authorea . 20 November 2025. DOI: https://doi.org/10.22541/au.176367266.68187021/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu . 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