The global automotive industry is undergoing an unprecedented transformation, with electrification becoming an irreversible trend. According to the International Energy Agency (IEA), global new energy vehicle (NEV) sales exceeded 17 million units in 2025, with a market penetration rate surpassing 20%. Compared to conventional internal combustion engine vehicles, NEVs feature fundamentally different powertrain systems—traction motors replace engines, single-speed reducers replace multi-gear transmissions, and regenerative braking is integrated into the braking system. These changes present new challenges for lubrication technology: motor bearings must withstand ultra-high speeds of 15,000–20,000 rpm, reducer gears endure instantaneous high-torque impacts, and braking systems alternate frequently between high temperatures and energy recovery. Molybdenum disulfide (MoS₂), a layered-structure solid lubricant with a low friction coefficient (0.02–0.06), high-temperature resistance (up to 1100°C in vacuum), and excellent chemical inertness, demonstrates broad application prospects in the critical friction pairs of new energy vehicles. This article systematically analyzes MoS₂ application solutions in NEV motors, reducers, and braking systems, referencing GB/T 23274-2009 and ASTM D5707 standards.

Unique Lubrication Challenges in New Energy Vehicles

The power transmission system of NEVs differs fundamentally from that of conventional vehicles, presenting the following lubrication characteristics:

GB/T 34014-2017 specifies that traction motor bearings are typically designed for rated speeds of 10,000–16,000 rpm, with some high-performance motors (such as the Porsche Taycan rear motor) reaching maximum speeds of 16,000 rpm. At these speeds, bearing DN values (bore diameter in mm × rotational speed in rpm) can reach 800,000–1,200,000, far exceeding conventional vehicle wheel hub bearings (approximately 200,000–300,000). Under high DN conditions, grease shear stability, base oil viscosity, and additive performance face severe challenges.

MoS₂ in Traction Motor Bearings

### Lubrication Failure Mechanisms in High-Speed Bearings

Traction motor bearings operating at ultra-high speeds face the following primary lubrication failure risks:

**Grease Shear Thinning**: High rotational speeds subject grease to intense mechanical shear, destroying the thickener fiber structure and causing rapid base oil separation. According to ASTM D5707 (four-ball shear stability test), conventional lithium-based greases exhibit cone penetration changes of 50–80 units after 10,000 shear cycles, whereas composite lithium greases with 3–5% MoS₂ can limit this change to within 30 units. MoS₂ layered particles act as "flow modifiers" in the shear field, slowing the destruction rate of the thickener network.

**Electrical Erosion (E-charging)**: PWM inverter-powered traction motors experience shaft voltage issues. When shaft voltage exceeds the bearing oil film breakdown threshold (typically 5–15 V), discharge corrosion occurs. MoS₂ possesses semiconductor properties (band gap approximately 1.8 eV), with a volume resistivity in the range of 10²–10⁴ Ω·cm, intermediate between insulators and conductors. Adding appropriate amounts of MoS₂ (1–3%) to grease can modulate the electrical conductivity, allowing shaft currents to discharge uniformly through the lubricating film and preventing localized discharge-induced bearing raceway pitting.

### Motor Bearing Lubrication Solutions

For traction motor bearings operating at 10,000–15,000 rpm, a PAO-based complex lithium grease containing 3% MoS₂ is recommended. The advantages of this solution include:

- PAO base oil has a high viscosity index (>140), maintaining stable viscosity characteristics across the motor operating temperature range (-40°C to 150°C)

- MoS₂ particles (D50 approximately 2 μm) fill micro-asperity gaps between bearing rolling elements and raceways, reducing boundary friction

- Polyurea thickeners offer superior thermal oxidation stability compared to lithium soaps, achieving service life exceeding 5000 hours at 150°C continuous operation

For high-performance motors operating at 15,000–20,000 rpm, a polyurea grease containing 5% MoS₂ with ester oil base is recommended, combined with insulated bearings (ceramic balls or PVD-coated outer rings) to form a dual protection system of "material insulation + lubrication electrostatic dissipation."

MoS₂ in Reducer Gear Applications

### Contact Stress in High Torque Density Gears

NEV single-speed reducers typically feature reduction ratios of 8–12, with output torque ranging from 300–600 N·m (compact EVs) to 1000–2000 N·m (high-performance EVs). Since multi-gear transmissions are eliminated, reducer gears must withstand torque transmission across the full operating range, with tooth surface contact stresses reaching 1500–2500 MPa, placing them in typical boundary and mixed lubrication regimes.

According to DIN 3990 and ISO 6336 standards for cylindrical gear load capacity calculations, tooth surface contact fatigue strength is closely related to lubrication conditions. Under boundary lubrication conditions where the elastohydrodynamic lubrication film thickness ratio λ<1, actual tooth surface contact occurs at micro-asperity peaks, with local flash temperatures reaching 300–500°C. The mechanisms of MoS₂ action under these conditions include:

**Transfer Film Lubrication**: Under the combined action of high contact pressure and relative sliding on tooth surfaces, MoS₂ particles undergo interlayer shear and transfer to the tooth surface, forming a transfer film with thickness of 0.1–1 μm. The interlayer shear strength of this transfer film is only 0.49–0.83 MPa, far lower than the shear strength of steel-steel dry friction (approximately 160 MPa), effectively reducing the tooth surface friction coefficient.

**Extreme Pressure Anti-Wear Performance**: Under heavy-load startup and rapid acceleration conditions, oil film thickness between tooth surfaces may decrease to the nanometer scale. With a Mohs hardness of only 1.0–1.5, significantly lower than gear steel surface hardness (HRC 58–62), MoS₂ acts as a "soft filler" that preferentially fills micro-pits and scratches on tooth surfaces, preventing the expansion of micro-pitting. ASTM D2783 (four-ball extreme pressure test) data indicate that gear oil containing 5% MoS₂ can increase the weld load (PD value) from 1200 N to 1800 N, a 50% improvement.

MoS₂ in Braking Systems

### Regenerative and Friction Braking Coupling

NEVs are generally equipped with regenerative braking systems that can recover 30–50% of braking energy in urban driving conditions. This significantly reduces the usage frequency of conventional friction brakes—statistics show that friction braking accounts for only 20–30% of total braking demand in urban conditions. The reduced braking frequency introduces two lubrication-related issues:

**Brake Rotor Corrosion**: Brake disc surfaces are prone to iron oxide rust layer formation in humid environments, leading to brake judder and noise. MoS₂-based brake lubricating grease (used for caliper guide pins and piston seal grooves) exhibits excellent water resistance and chemical stability. Its hydrophobic characteristics stem from the low electronegativity of sulfur atoms, which do not readily form hydrogen bonds with water molecules.

**Brake NVH Optimization**: Brake noise (squeal) is a critical evaluation indicator in NEV NVH (noise, vibration, and harshness) assessments. The application of MoS₂ in brake pad friction materials can reduce stick-slip vibrations during braking. The layered structure of MoS₂ provides a stable low-shear layer at the friction interface, changing the friction coefficient-speed curve slope from negative to positive, thereby eliminating the self-excited vibration conditions that cause brake squeal.

Cold Start and Thermal Management

### Lubrication Performance in Extreme Cold Environments

NEVs face severe cold-start challenges during northern winters. GB/T 18386-2017 specifies that the range degradation rate under low-temperature (-7°C) conditions should not exceed 30%. Low temperatures not only affect battery discharge performance but also impose higher demands on lubrication systems:

- Traction motor bearings at -30°C startup exhibit grease starting torque 5–8 times higher than at room temperature

- Reducer gear oil viscosity at -40°C can reach 100 times that at room temperature, causing dramatically increased churning losses

- Braking systems on icy roads require extended braking distances, demanding stable friction coefficients from brake materials at low temperatures

The lubrication performance of MoS₂ under extremely low temperatures is unaffected by base oil viscosity changes—its layered shear mechanism is an intrinsic solid-state property independent of temperature. At liquid nitrogen temperature (-180°C), MoS₂ maintains a friction coefficient in the range of 0.03–0.05 (data source: NASA Glenn Research Center low-temperature tribology tests). Therefore, adding MoS₂ to low-temperature greases can partially compensate for friction losses caused by increased base oil viscosity at low temperatures.

### Battery Thermal Management and MoS₂ Thermal Interface Materials

Power battery pack thermal management is critical for vehicle performance and safety. MoS₂ itself possesses moderate thermal conductivity (approximately 35–40 W/(m·K)) with low interlayer thermal resistance. By filling MoS₂ nanosheets (thickness <10 nm) into silicone rubber-based thermal interface materials, thermal conductivity can be enhanced while maintaining electrical insulation. Experimental data show that silicone rubber composites with 20 wt% MoS₂ nanosheets can increase in-plane thermal conductivity from 0.2 W/(m·K) to 1.8 W/(m·K), suitable for thermal pads between battery modules and liquid cooling plates.

Engineering Application Solutions and Standard Compliance

Based on the above analysis, typical MoS₂ application solutions for critical friction pairs in new energy vehicles are summarized as follows:

Regarding standard compliance, all MoS₂ materials used in the above solutions should meet the technical requirements of GB/T 23274-2009: MoS₂ content ≥98%, iron content ≤0.02%, moisture ≤0.5%, pH value 6–8. For brake friction materials, coarse-grade MoS₂ with D50 particle size of 5–15 μm is recommended for optimal tribological performance; for bearing greases, fine-grade MoS₂ with D50 particle size of 1–3 μm is recommended to ensure dispersion stability in grease.

Market Outlook

According to Frost & Sullivan forecasts, the global new energy vehicle grease market will grow from USD 1.2 billion in 2025 to USD 2.8 billion in 2030, representing a compound annual growth rate of 18.5%. As a high-performance solid lubricant additive, MoS₂ possesses the following competitive advantages in this rapidly growing market:

1. **Technical Adaptability**: MoS₂'s ultra-high-speed lubrication performance, adjustable electrical conductivity, and extreme pressure anti-wear properties are highly aligned with NEV technical requirements

2. **Environmental Compliance**: The NEV industry chain imposes strict material environmental requirements; MoS₂ contains no heavy metals or PFAS, complying with EU REACH regulations and RoHS directives

3. **Domestic Substitution Opportunity**: China accounts for over 60% of global NEV production, while high-end lubricant additives have long relied on imports. The large-scale supply of domestic high-purity MoS₂ (purity ≥99%) provides critical material support for the localization of NEV lubricants

Conclusion

The electrification transformation of the automotive industry has set new requirements for lubrication technology that exceed those of conventional internal combustion engine vehicles: ultra-high-speed bearing lubrication, high-torque-density gear protection, brake NVH optimization, and cold-start reliability. MoS₂, with its low interlayer shear characteristics, adjustable electrical properties, and wide-temperature stability, demonstrates systematic technical advantages in the four critical systems of traction motor bearings, reducer gears, braking systems, and battery thermal management. Test data under the GB/T 23274-2009 standard framework indicate that motor bearing greases with 3–5% MoS₂ can improve shear stability by over 40%; ASTM D2783 extreme pressure tests prove that MoS₂ can increase gear oil weld load by 50%; and brake friction materials with 5–10% MoS₂ can achieve 3–5 dB noise reduction. As global NEV penetration continues to rise, MoS₂ solid lubrication technology will play an increasingly important role in lubrication solutions for the electrification era.