A study of protective coatings on bearings in electric vehicles: mitigating the impact of shaft voltages and currents

Abstract

In the past decade, advances in technology have led to significant improvements in energy storage and power transmission systems. These developments have made electric vehicles more accessible and have contributed to the reduction of environmentally harmful CO₂ emissions by promoting widespread adoption. Unlike internal combustion engines, electric vehicles employ electric motors, which can result in the emergence of shaft voltages or stray bearing currents. These parasitic currents can damage the surface of contacting material pairs under operational conditions, increasing both the coefficient of friction (COF) and wear losses. To mitigate the detrimental effects of stray currents and minimize wear, various methods are employed, including grounding, the use of insulating interlayers, lubricants with high dielectric constants, and thin-film coatings. In this context, many coatings with high wear resistance, hardness, and elastic modulus have been produced by Physical Vapor Deposition (PVD) method and they have been extensively investigated. Among them, CrN and TiN coatings have gained significant attention in machining, cutting/drilling, and forming operations due to their high hardness and resistance to abrasive wear. However, their high COF and the degradation of coating properties at elevated temperatures during wear limit their service life and application range. In order to overcome these limitations, structural enhancements have been made by designing superlattice, nanocomposite or multilayer coatings, and by incorporating transition metal oxides such as those of Mo, V, Cr, and Al into CrN and TiN matrices. Their oxides form under high frictional temperatures and contribute to reducing COF due to their lubricating properties. In this study a commercial AlCrN coating which is commonly used in industrial applications and a vanadium modified one, (Cr,Al,V)N were deposited on M35 HSS by cathodic arc deposition. The structural characterization of the coatings was performed using X-ray diffraction (XRD), while their morphology and thickness were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The XRD results revealed the diffraction planes corresponding to NaCl type FCC structure. During wear, it is well known that Joule heating at the contact point can reach extremely high temperatures, particularly under applied current. These elevated temperatures promote the oxidation of V, Cr, and Al, which significantly affects the wear behavior. Among transition metals, vanadium oxides are particularly important due to the formation of Magnéli phases, which possess low shear strength and exhibit solid lubricating properties. The presence of such oxide phases leads to a substantial reduction in COF. To investigate the tribological behavior of the coatings under electrical effects, a modified ball on disk tribometer was used. Tests were conducted under dry sliding conditions using a 10 mm diameter 52100 steel ball as the counterface. Each coating was tested under four current levels (0 mA, 300 mA, 800 mA, 1600 mA), two sliding speeds (5 cm/s and 10 cm/s), a normal load of 2 N, and a total sliding distance of 200 meters. In order to understant the influence of electrical current on the COF, the current was applied shortly after 20 meters of sliding or when a steady-state COF was reached. Following the tests, worn surfaces were analyzed using SEM/EDS, surface profilometry, and Raman spectroscopy. Tests were repeated for each parameter for 3 time in order to obtain accurate results. Analysis of COF versus sliding distance graphs for the AlCrN coating at 5 cm/s revealed a decrease in average COF from 0.79 ± 0.10 (0 mA) to 0.61 ± 0.02 for 300 mA, 0.58 ± 0.04 for 800 mA, and 0.55 ± 0.06 for 1600 mA. At 10 cm/s, the average COF at 0 mA was 0.71 ± 0.06—relatively lower than at 5 cm/s and further dropped to 0.61 ± 0.02 (300 mA) and 0.54 ± 0.04 (800 mA), with no significant change at 1600 mA. For the (Cr,Al,V)N coating, the average COF at 5 cm/s decreased from 0.90 ± 0.08 (0 mA) to 0.50 ± 0.03 (300 mA). However, further increase in current to 800 mA and 1600 mA showed no further significant reductions in COF resulting 0.50±0.05 at 1600mA applied. On the other hand, in the case of no current application, any significant difference between sliding speeds of 5 cm/sec and 10 cm/sec in COF was not observed. However, when 300mA current was applied, the COF dropped from 0.81± 0.07 to 0.51 ± 0.04. While there was not an important change (0.51 ± 0.03) for 800 mA comparing to 300 mA, increasing the current to 1600 mA droped the COF to 0.49 ± 0.05. Regardless of coating type, the current application caused a reduction in COF. Regarding the wear behavior of the 52100 steel counter ball, wear losses increased with higher current for AlCrN at both sliding speeds. Interestingly, at 300 mA and 800 mA, increasing the speed from 5 cm/s to 10 cm/s resulted in decreased wear on the ball, whereas at 1600 mA, the trend recorded, with wear increasing alongside speed. This suggests that the combined effect of high current and speed exacerbates wear on the counterface. For the (Cr,Al,V)N coating, no significant wear occurred on the steel ball at 0 mA and 300 mA. However, at 800 mA and 1600 mA, higher speed led to increased wear, indicating that elevated current and speed together intensify thermal effects at the contact interface, thereby increasing wear damage. The examination of the wear scar with SEM revealed both adhesive and abrasive wear marks along with the pitts that result from the arc discharges. These damage features were observed to intensify with increasing applied current. For AlCrN, wear volume increased with current at both speeds. However, for a given current, increasing the sliding speed from 5 cm/s to 10 cm/s reduced wear volume: from 20.649 ± 3.1717 × 10⁻³ mm³ to 24.44 ± 1.897 × 10⁻³ mm³ at 300 mA; from 96.0108 ± 6.07 × 10⁻³ mm³ to 77.488 ± 9.7814 × 10⁻³ mm³ at 800 mA; and from 105.9554 ± 10.007 × 10⁻³ mm³ to 42.44 × 10⁻³ mm³ at 1600 mA. For (Cr,Al,V)N, no wear loss was observed at 300 mA for 5cm/s speed while there is partial removal of the coating is observed at 10cm/s sliding speed and worn volume is measured to be 8.244±4.121 × 10⁻³ mm³. At 800 mA, wear volume observe to be unchanged 4.932 ± 2.7814 × 10⁻³ mm³ at 5cm/s as recorded to be 5.263 ± 2.7814 × 10⁻³ mm³ when the speed is increased to 10cm/s to be. At 1600 mA, wear volume peaked at 103.70305 ± 17.4647 × 10⁻³ mm³ at 5 cm/s but dropped significantly to 32.533 ± 16.5057 × 10⁻³ mm³ at 10 cm/s.