MoS2 grease, formulated by adding MoS2 solid lubricant into base grease, demonstrates significantly superior anti-wear performance under boundary lubrication and extreme pressure conditions compared to ordinary grease. Four-ball tester data shows that lithium grease with 3% molybdenum disulfide increases the sintering load (PD value) from 1960N to 3087N and reduces the wear scar diameter from 0.72mm to 0.45mm. For equipment operating under heavy loads, impact forces, or frequent start-stop cycles, selecting grease containing molybdenum disulfide can directly extend the wear life of bearings and gears.

Test Methods and Parameters

Laboratory evaluation of extreme pressure and anti-wear performance primarily employs the four-ball friction and wear tester, following ASTM D2596 (extreme pressure properties) and ASTM D2266 (anti-wear properties). Test conditions include: room temperature, rotational speed of 1450r/min, and test balls of 12.7mm diameter GCr15 bearing steel.

The samples used in this comparison are: Group A — commercial ordinary lithium grease (base grease without solid lubricant), Group B — the same lithium grease with 3% molybdenum disulfide powder (purity 99%, D50=3μm), and Group C — the same lithium grease with 5% molybdenum disulfide powder.

Extreme Pressure Performance: PD and PB Values

Extreme pressure performance is measured by PB value (last non-seizure load) and PD value (weld load). The PB value reflects the maximum load the lubricating film can withstand without seizure on the ball surface, while the PD value represents the load at which the lubricating film completely fails and the balls weld together.

Group B's PB value increased 25% over Group A, and PD value increased 57.5%. Group C (5% addition) achieved a PD value of 3920N, double that of ordinary grease. The mean Hertz load ZMZ rose from 28.5 to 51.7, an 81.4% increase, indicating that molybdenum disulfide effectively suppresses direct metal-to-metal contact under heavy loads.

The layered structure of MoS2 is the fundamental reason for improved extreme pressure performance. In MoS2 crystals, S-Mo-S trilayers are stacked via van der Waals forces, with an interlayer shear strength of only 0.08MPa. Under frictional stress, the layers slide along the interlayer direction, forming a transfer film on the metal surface. When the base grease film ruptures, the MoS2 transfer film continues to provide solid lubrication, preventing metal welding and sintering.

Anti-Wear Performance: Wear Scar Diameter

Anti-wear performance is evaluated by wear scar diameter (WSD), tested at 392N load, 60 minutes, 1450r/min. Smaller wear scars indicate better wear control on the ball surface.

Group A's wear scar diameter was 0.72mm, Group B decreased to 0.45mm (37.5% reduction), and Group C to 0.41mm (43.1% reduction over Group A). Notably, the improvement from 3% to 5% addition yielded only an 8.9% reduction in wear scar diameter, showing diminishing marginal returns. In practical applications, 3% addition already meets most anti-wear requirements; higher additions above 5% show reduced cost-effectiveness and may affect grease consistency and pumpability.

Wear scar morphology analysis shows Group A had obvious plowing grooves and adhesive tearing at the scar edges, with surface roughness Ra=0.38μm. Group B showed significantly shallower grooves, surface roughness Ra=0.21μm, with a uniform dark gray MoS2 transfer film covering the wear area. XPS analysis confirmed the transfer film contained mixed phases of MoO3 and MoS2, where MoO3 is an oxidation product of MoS2 during friction that still provides certain lubricating properties.

Friction Coefficient Variation

Real-time friction coefficient monitoring during four-ball tests showed Group A started at 0.10-0.12 but spiked above 0.35 when load increased to 1568N, indicating lubricating film rupture. Group B maintained a stable friction coefficient of 0.06-0.09 throughout the entire load gradient, even at 2450N. Group C achieved lower values (0.05-0.07), though the difference from Group B was not significant.

Selection Recommendations for Practical Conditions

Based on the measured data, the benefits of adding molybdenum disulfide to grease are concentrated in extreme pressure conditions. For applications with contact stress below 1000MPa under normal conditions, ordinary lithium grease is sufficient and the marginal benefit of adding MoS2 is limited. However, MoS2-containing grease provides irreplaceable advantages in the following scenarios: mining crushers with frequent impact loads, steel mill continuous casters under heavy low-speed conditions, and universal joints and CV joints with frequent start-stop cycles. For these applications, lithium or polyurea grease with 3% MoS2 (D50=1.2-3μm) is recommended, balancing anti-wear performance and cost control.

Tags: MoS2 grease, molybdenum disulfide grease, extreme pressure anti-wear, four-ball test, weld load, wear scar diameter, friction coefficient, heavy load lubrication, lithium grease, solid lubricant