In low-temperature environments, after prolonged shutdowns, or under high-load static startup conditions, cold start difficulty is a common failure scenario in industrial field operations. MoS2 dry film lubrication technology, whose friction coefficient remains unaffected by temperature, provides a direct and effective solution to cold start challenges. Conventional lubricating oils experience a sharp increase in viscosity at low temperatures, and may even solidify, causing the frictional resistance at the moment of startup to far exceed normal operating values, frequently leading to motor overload tripping, bearing scoring, and related issues.

Why Cold Starts Are Difficult

The fundamental problem of cold starting is that the lubricating film has not yet been established at the moment of startup. In liquid lubrication systems, friction surfaces are normally separated by a hydrodynamic oil film during operation, maintaining a friction coefficient of approximately 0.01–0.05. However, during the static startup phase, the oil film has not yet formed, and metal surfaces come into direct contact, causing the friction coefficient to surge to 0.3–0.5, which is 6–10 times the normal operating value.

This contradiction becomes even more acute in low-temperature environments. The kinematic viscosity of mineral oil at −20°C can reach 50–100 times that at 40°C, and ISO VG220 gear oil nearly loses its fluidity at −30°C. For open-pit mining equipment in northeastern China and wind turbine gearboxes in frigid regions, the motor torque required during winter startup can reach 2–3 times that at normal temperatures, and in severe cases, directly causes motor burnout.

Prolonged shutdowns also present cold start difficulties. After equipment stops, lubricating oil drains from the friction surfaces under gravity, and it takes several seconds to tens of seconds to re-establish an effective oil film upon restart. During this "boundary lubrication" window, the wear sustained by bearings and gears can equal the wear of 1,000 hours of normal operation.

Temperature Independence of MoS2 Dry Film

Molybdenum disulfide has a molecular weight of 160.07 and a melting point of 1,185°C. Its friction coefficient remains stable at 0.02–0.06 across the range of −180°C to 400°C (and up to 1,100°C in vacuum environments). This property stems from its hexagonal layered crystal structure—the interlayer sliding of S-Mo-S layers is a purely physical process that does not depend on external conditions such as viscosity, pressure, or temperature.

In cold start scenarios, once an MoS2 dry film is pre-applied to friction surfaces, it provides effective lubrication immediately without waiting for an oil film to form. Laboratory test data show that at −40°C, the startup friction torque of a rolling bearing coated with MoS2 dry film is 0.12 N·m, whereas the same bearing type using ISO VG68 lubricating oil requires a startup torque of 0.85 N·m—more than 7 times higher.

Of particular note is its performance at extreme low temperatures. In liquid nitrogen temperature zone (−196°C) testing, molybdenum disulfide maintained a friction coefficient of 0.06–0.08, while conventional synthetic ester-based lubricants had completely solidified at this temperature, with friction coefficients exceeding 0.8. This characteristic makes molybdenum disulfide one of the few viable lubrication solutions for low-temperature operating equipment.

Practical Application Scenarios

### Wind Turbine Gearboxes

Speed-increasing gearboxes at northern wind farms, when started at ambient temperatures of −30°C, experience extremely high oil pumping resistance, leaving gear surfaces in a dry friction state for 15–30 seconds. After applying an MoS2-polyimide composite dry film to gear tooth surfaces at one wind farm, the gear surfaces remained protected by a solid lubricating film throughout the cold start process. The incidence of gear tooth pitting decreased from 2–3 teeth per quarter to zero, and the first major overhaul interval of the gearbox was extended by approximately 2,000 hours.

### Open-Pit Mining Equipment

The static friction torque borne by excavator slewing bearings during winter startup can exceed 200 kN·m. After one mine applied MoS2 dry film to the slewing ring gear surface, the peak startup motor current dropped from 280% of rated value to 165%, effectively eliminating overload tripping failures. This solution has operated continuously for two winters, with tooth surface wear reduced by approximately 60% compared to before.

### Cold Chain Logistics Conveyors

Chain conveyors inside cold storage facilities (−25°C to −18°C) using conventional lubricating oil experience chain stiffening at low temperatures, high startup resistance, and frequent motor overloading. After switching to molybdenum disulfide solid lubrication, the chain startup torque decreased by approximately 45%, and motor overload failures dropped from 5–8 times per month to zero. This simultaneously eliminated the risk of lubricating oil dripping at low temperatures and contaminating food products.

Dry Film Preparation and Maintenance Guidelines

The preparation process of MoS2 dry film directly affects its cold start lubrication performance. Spray-applied dry films require control of the following parameters:

- Film thickness: 8–20 μm; too thin results in incomplete coverage, too thick reduces adhesion

- Curing temperature: organic binder systems cure at 150–200°C for 1 hour; inorganic systems cure at room temperature for 24 hours

- Surface pretreatment: sandblast to Sa2.5 roughness grade to ensure dry film adhesion

Regarding maintenance intervals, the effective service life of MoS2 dry film under normal operating conditions is approximately 2,000–5,000 hours, depending on load and speed. It is recommended to inspect film integrity during scheduled equipment overhauls; locally worn areas can be spot-repaired without complete stripping and reapplication.

It should be noted that MoS2 dry film is not suitable for oil-immersed operating conditions—liquid oil penetrates the interface between the film and substrate, causing film delamination. In applications requiring both oil lubrication and cold start capability, molybdenum disulfide can be incorporated as an additive (at 1–3% by weight) into low-temperature greases, combining the heat dissipation of oil lubrication with the low-temperature startup performance of solid lubrication.

Conclusion

The core of cold start difficulty lies in the time gap before the lubricating film is established. MoS2 dry film lubrication eliminates this gap by pre-applying a solid lubricating layer. From −40°C wind turbine gearboxes to −196°C cryogenic equipment, numerous engineering case studies have validated its reliability in low-temperature startup scenarios. For equipment suffering from cold start wear problems, MoS2 dry film lubrication is a technical solution worthy of priority evaluation.

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