Lubricating grease is one of the most widely used lubricant forms for friction reduction in mechanical equipment, and can be classified by thickener type and base oil into mineral-oil greases, synthetic greases, and extreme-pressure greases with solid lubricant additives. Molybdenum disulfide (MoS₂), as the most mature lamellar solid lubricant, can substantially improve the extreme-pressure load capacity and anti-wear performance of grease when compounded with the base grease. This article presents measured comparison data between MoS₂-fortified grease and ordinary grease of identical base composition, evaluated through four key dimensions — extreme-pressure load capacity, wear scar diameter, friction coefficient, and long-life wear — based on standardized methods ASTM D2596 (four-ball extreme pressure), ASTM D2266 (four-ball wear), and DIN 51350.

Test Methods and Samples

Two sample groups were tested. The ordinary lithium-based grease (base grease) consists of 12-hydroxystearic acid lithium thickener in ISO VG 68 mineral oil, NLGI grade 2, with no solid additives. The MoS₂ grease is identical to the base grease with the addition of 3% by mass high-purity MoS₂ fine powder (D50 ≈3 μm, purity ≥98.5%, meeting GB/T 23271-2009). Both samples share identical base oil, thickener, and consistency, varying only in solid additive content, ensuring the validity of the comparative test.

The four-ball tester used is a MRS-10A lever-loaded four-ball friction and wear tester (Jinan Shijin). The steel balls are GB/T 308 standard balls (12.7 mm diameter, HRC 64-66 hardness). All tests were conducted under identical environmental conditions (25±3°C, relative humidity 50±10%), with each sample measured three times and arithmetic average taken.

Extreme Pressure Performance Comparison

Extreme-pressure load capacity is characterized by the weld point (PD value), which reflects the maximum contact pressure that the grease can sustain under boundary lubrication conditions without metal-to-metal welding.

The data shows that adding 3% MoS₂ to the base grease increased the last non-seizure load from 80 to 125 kgf (+56%), the weld point from 200 to 315 kgf (+58%), and the load-wear index from 28 to 48 (+71%). Under heavy and shock loading conditions, MoS₂ grease maintains an effective lubricating film at higher contact stresses, avoiding metal-to-metal direct contact and consequent welding. This improvement is attributed to: when contact pressure approaches the failure limit of the base grease, MoS₂ particles are squeezed into asperity gaps, forming a high-strength transfer film that bears part of the load and provides a low-shear interface.

Anti-Wear Performance Comparison

Anti-wear performance is quantified by wear scar diameter (WSD). Under standard ASTM D2266 conditions (392 N, 1200 rpm, 60 min), the average diameter (mm) of the wear scar on the steel ball is measured.

Under standard anti-wear test conditions, adding 3% MoS₂ reduced the wear scar diameter from 0.62 to 0.41 mm (-34%), with significantly lower wear rate. Microscopic inspection of the steel ball surfaces shows visible ploughing and adhesive transfer on the ordinary-grease sample, while the MoS₂ grease sample surface is relatively smooth with markedly reduced ploughing depth and quantity. This is because MoS₂ is adsorbed and embedded in surface micro-depressions during friction, forming a stable transfer film that reduces the proportion of direct metal-to-metal contact.

Friction Coefficient Comparison

Per ASTM D3708, ball-on-plate reciprocating at 40 N load and room temperature, friction coefficient was measured over 180 minutes:

The MoS₂ grease friction coefficient dropped from 0.115 to 0.058 (≈50%), while ordinary grease dropped from 0.115 to 0.092 (≈20%). MoS₂ grease not only yields a lower friction coefficient but also a substantially narrower variation range (±8% vs ±15%), indicating more stable lubrication, which reduces equipment vibration and noise.

The low and stable friction coefficient is attributed to the ordered lamellar transfer film formed by MoS₂ on the friction surface. The interlayer shear strength of MoS₂ is extremely low (0.49-0.83 MPa), forming a continuous low-shear plane between the mated surfaces that converts sliding friction into interlayer slip, fundamentally reducing friction resistance.

Long-Life Wear Comparison

The L-50 life test (ASTM D5183, 392 N, 1200 rpm) measures the hours of operation from start until friction coefficient jumps above 0.3, indicating the long-term service performance of the grease.

Adding 3% MoS₂ extended the L-50 life from 142 hours to 318 hours — a 124% extension. Notably, the MoS₂ grease failure process is more gradual — friction coefficient rises slowly rather than spiking — which facilitates condition monitoring and preventive maintenance. Post-test surface analysis shows residual transfer film still visible on MoS₂ grease samples, while ordinary grease samples have completely lost lubricant adhesion and display clear adhesive wear features.

Effect of Addition Level on Performance

The effect of MoS₂ addition level on grease performance is non-linear. The study tested six addition levels (0.5%, 1%, 3%, 5%, 8%, 12%) for extreme-pressure and anti-wear performance:

The optimal addition level lies in the 3-5% range. Below 1%, extreme-pressure improvement is not pronounced; above 5%, consistency drops excessively and homogeneity deteriorates. At the 3% level, weld point improves 57% and wear scar diameter reduces 34%, yielding the best cost-performance ratio. This conclusion is consistent with the GB/T 23271 recommendation range of 1-5% for MoS₂ as a grease additive.

Temperature Adaptability Comparison

The effect of temperature on grease extreme-pressure and anti-wear performance is critical under high-temperature working conditions. Weld points for MoS₂ grease (+3%) and ordinary grease were measured at 40°C, 100°C, and 150°C:

As temperature rises, ordinary grease weld point drops substantially (from 200 to 110 kgf, -45%), while MoS₂ grease drops less (from 315 to 245 kgf, -22%). At 150°C, MoS₂ grease improvement reaches 123%, nearly double the room-temperature value. This is because at high temperature base oil viscosity decreases and the oil film thins, so ordinary grease boundary-lubrication capacity degrades rapidly, while MoS₂ solid particles do not depend on base oil viscosity and continue to provide a solid lubrication layer on metal surfaces.

Selection Recommendations

Based on measured data, grease selection recommendations for different operating conditions are:

Conclusion

Measured comparison shows that adding 3% high-purity MoS₂ fine powder to base lithium grease achieves systematic improvements: weld point increased 58%, wear scar diameter reduced 34%, friction coefficient reduced 37% with markedly improved stability, and long-life wear extended 124%. Comprehensive performance is optimal in the 3-5% addition range; exceeding this range leads to reduced consistency and deteriorated pumpability. MoS₂ grease exhibits greater advantages under heavy-load, high-temperature, and shock boundary-lubrication conditions, making it a key technical approach for enhancing mechanical equipment reliability and extending lubrication maintenance intervals.