Under heavy load, shock, and boundary lubrication conditions, the oil film strength of ordinary grease is often insufficient to isolate direct metal-to-metal contact, leading to scuffing, galling, and premature wear. Molybdenum disulfide (MoS₂), added to grease as a solid lubricant, forms a transfer film with extremely low shear strength on metal surfaces, fundamentally altering the lubrication mechanism at the friction interface. This article systematically compares the performance of MoS₂ grease and ordinary grease based on measured data from four-ball extreme pressure testing (SH/T 0202, equivalent to ASTM D2596) and Timken wear testing.

Fundamental Differences in Tribological Mechanisms

The lubrication mechanism of ordinary grease relies on hydrodynamic or elastohydrodynamic lubrication (EHL) oil films formed by the base oil on friction surfaces. Under low-speed, high-load, or frequent start-stop conditions, the oil film thickness is insufficient to fully separate the two surfaces, leaving the friction pair in boundary lubrication. At this point, metallic asperities make direct contact, with contact stresses reaching 1,000–2,500 MPa, causing a sharp increase in friction and material transfer.

The lubrication mechanism of MoS₂ grease is fundamentally different. MoS₂ possesses a hexagonal S-Mo-S layered crystal structure — covalent S-Mo bonds within layers are strong, while van der Waals S-S bonds between layers are only 1/100 the strength. Under shear forces, interlayer slip occurs, and MoS₂ particles form a continuous transfer film 0.05–0.5 μm thick on metal surfaces. The friction coefficient of this transfer film is only 0.02–0.08, significantly lower than the 0.12–0.20 exhibited by ordinary grease under boundary lubrication. The transfer film possesses a self-renewal capability — worn film is continuously replenished by MoS₂ particles in the grease, forming a dynamic self-healing system.

More critically, the shear strength of the MoS₂ transfer film decreases as load increases. This property enables MoS₂ grease to exhibit unexpectedly superior extreme pressure performance under high load — the higher the load, the more the transfer film orients parallel to the friction surface, facilitating interlayer slip.

Extreme Pressure Performance: Four-Ball Test Comparison

The four-ball extreme pressure test (SH/T 0202 / ASTM D2596) is the standard method for evaluating the load-carrying capacity of lubricating greases. Using identical lithium base grease, a comparison was conducted between blank grease and MoS₂-containing grease with 5% molybdenum disulfide (D50=3–5 μm, purity ≥98.5%).

The PD value jumping from 1,570N to 4,905N indicates that MoS₂ grease can withstand over three times the ultimate load without metal sintering. The engineering significance is profound: in low-speed heavy-duty gearboxes, construction machinery pin joints, and mining equipment bearings, MoS₂ grease substantially reduces the risk of tooth surface scuffing and galling caused by impact loads.

The PB value represents the maximum load at which the lubricating film still prevents metal-to-metal contact. The increase from 373N to 726N (+94.6%) demonstrates that the MoS₂ transfer film maintains effective lubrication at approximately 730N contact load, while the oil film of ordinary grease fails near 370N.

Wear scar diameter (WSD) reflects the anti-wear capability of grease under sustained load. The WSD of MoS₂ grease decreased from 0.69mm to 0.47mm, a 31.9% reduction. Under long-duration friction at 3,600 cycles per hour, this difference translates to a wear rate reduction of approximately one-third.

Anti-Wear Performance: Timken Test and Industrial Cases

The Timken OK load test (ASTM D2509) further validates the anti-wear advantages of MoS₂. In the ring-block contact configuration, grease containing 5% physically beneficiated MoS₂ (aspect ratio 20:1–50:1) achieved OK values of 220–280N, compared to approximately 150–180N for base grease — an improvement of roughly 50–80%.

Industrial application cases corroborate laboratory data. A construction machinery manufacturer conducted a 500-hour comparative test on loader pin joint lubrication, using lithium grease with 3% MoS₂ versus ordinary lithium grease. Pin wear measurement results showed: the MoS₂ grease group had an average pin clearance increase of 0.12mm, while the ordinary grease group showed 0.28mm — a 57% wear reduction. SEM surface morphology analysis revealed smooth surfaces with uniform wear marks in the MoS₂ grease group, while the ordinary grease group exhibited distinct furrowing and material spalling.

In long-term wind turbine yaw bearing applications, composite lithium grease with 4% MoS₂, after 36 months of operation, showed a median bearing raceway wear depth of 15 μm, compared to 32 μm for bearings using ordinary composite lithium grease — a 53% reduction.

High-Temperature Stability and Oxidation Resistance

High-temperature performance of grease is an important consideration in industrial lubrication. Thermogravimetric analysis (TGA) data shows that PAO base grease has a 5% weight loss temperature of 273°C, which increases to 284°C (+11°C) with MoS₂ addition. While the increase may appear modest, this 11°C improvement indicates that the MoS₂ transfer film retards the oxidative decomposition of the base oil at elevated temperatures.

The copper strip corrosion test (GB/T 7326 / ASTM D4048) is a key indicator of grease-metal compatibility. MoS₂ grease tested at 100°C for 24 hours showed a copper strip rating of 1a–1b (slight tarnish), equivalent to ordinary lithium grease. This indicates that MoS₂ does not accelerate metal corrosion under normal operating temperatures — provided the MoS₂ used has low acid value (physically beneficiated products: acid value ≤0.3 mg KOH/g) and is free from residual acid ions and chloride ions (Cl⁻ <50 ppm) that may be present in chemically produced materials.

Selection and Dosage Recommendations

When selecting molybdenum disulfide grease, MoS₂ addition level and particle size are critical parameters.

The recommended addition level is 2–5% by weight. A 2–3% addition is suitable for medium-load bearings and gears under normal conditions; 4–5% is appropriate for heavy load, shock, or predominantly boundary lubrication conditions. Addition levels exceeding 5% may result in excessive grease consistency and reduced low-temperature pumpability.

For particle size selection, fine powder with D50=3–5 μm (approximately 2,500 mesh) is suitable for most industrial lubrication needs, offering good dispersion at moderate cost. Ultrafine powder with D50=1–2 μm is recommended for precision bearings and high-speed applications to achieve a more uniform transfer film.

MoS₂ purity should be ≥98.5%, with iron content ≤0.25% and acid insolubles ≤0.50%. High acid values and impurities may affect the oxidation stability and long-term storage stability of the grease.

The base grease type should be chemically compatible with MoS₂. Lithium and lithium complex greases are the most commonly used MoS₂ carriers in industry; polyurea greases perform better under high-temperature (>120°C) conditions; calcium sulfonate complex greases are suitable for wet conditions with water washout.

Tags: molybdenum disulfide MoS2 grease extreme pressure anti-wear four-ball test solid lubricant friction coefficient lithium grease EP additive