The particle size of molybdenum disulfide (MoS2) is a critical parameter determining its lubrication performance and application effectiveness. Different particle sizes of MoS2 powder exhibit significant differences in friction coefficient, dispersion stability, load-bearing capacity, and film formation characteristics, directly affecting the final product performance in grease additives, powder metallurgy, plastic modification, and coatings. According to GB/T 23274-2009 standard, industrial-grade MoS2 is classified by particle size into four grades: coarse powder (D50>10um), medium powder (D50 3-10um), fine powder (D50 1-3um), and ultrafine powder (D50<1um). Purchasers should select particle size specifications according to specific application conditions.

Particle Size Classification and Detection Methods

MoS2 particle size is typically characterized by the D50 value (median diameter), which corresponds to the particle size at 50% of the cumulative volume distribution. A complete particle size distribution also requires reference to D10 (fine end) and D90 (coarse end) values. Detection methods primarily employ laser diffraction (ISO 13320) and sedimentation methods. Laser diffraction covers a measurement range of 0.1-2000um, applicable to all particle size ranges; the sedimentation method based on Stokes' law offers higher precision for particles in the 1-100um range.

Typical parameters for the four particle size grades are as follows:

- Coarse powder: D50=10-30um, D90<50um, specific surface area 1-3 m2/g

- Medium powder: D50=3-10um, D90<20um, specific surface area 3-8 m2/g

- Fine powder: D50=1-3um, D90<8um, specific surface area 8-15 m2/g

- Ultrafine powder: D50=0.5-1.5um, D90<3um, specific surface area 15-30 m2/g

Tribological Performance Differences Across Particle Size Grades

The effect of particle size on MoS2 friction coefficient does not follow a linear decreasing relationship. Four-ball test data from Sandia National Laboratories (USA) demonstrates that MoS2 with D50 in the 2-5um range achieves the lowest friction coefficient of 0.04-0.06; when particle size increases above 10um, the friction coefficient rises to 0.08-0.10; while ultrafine powder (D50<1um) shows a rebound in friction coefficient to 0.07-0.09 due to particle agglomeration tendency. This is because medium and fine powder particles can form a uniform transfer film at the friction interface, while coarse powder particles are too large to fully spread, and ultrafine powder agglomerates due to high surface energy, degrading film continuity.

In terms of load-bearing capacity, coarse powder performs better under heavy-load conditions. At contact stresses exceeding 1500 MPa, coarse powder particles can withstand greater loads without crushing, with sintering load (PD value) approximately 20% higher than ultrafine powder. This relates to the packing density and structural integrity of the particles — coarser particles retain more complete MoS2 crystal layered structures, and interlayer shear can effectively function under high pressure.

Particle Size Selection Recommendations for Different Applications

**Grease additives**: D50=2-5um medium or fine powder is recommended. This particle size range provides optimal dispersion uniformity in grease, the lowest friction coefficient, and does not clog grease fittings or filters. Lithium-based grease with 3% MoS2 of this particle size shows a four-ball test wear scar diameter reduction from 0.58mm to 0.42mm, a 27.6% decrease.

**Powder metallurgy self-lubricating bearings**: D50=5-15um medium or coarse powder is recommended. The PM process requires MoS2 powder to be mixed with iron and copper powders for compaction and sintering. Medium-coarse powder offers good flowability and appropriate apparent density, forming uniformly distributed lubricating phases within the alloy matrix after sintering. Ultrafine powder tends to segregate during mixing, resulting in uneven lubricating phase distribution.

**Plastic modification**: D50=1-3um fine powder is recommended. Plastic modification requires MoS2 to be uniformly dispersed in the matrix to improve wear resistance and dimensional stability. Fine powder can be thoroughly dispersed in engineering plastics such as PA and POM through twin-screw extruders. POM composites with 15% fine MoS2 powder show friction coefficient reduction from 0.42 to 0.12, with wear rate decreased by 82%.

**Dry film lubrication coatings**: D50=1-3um fine or ultrafine powder is recommended. When dry film coatings (such as MoS2 + phenolic resin) are sprayed onto metal surfaces, fine and ultrafine powder can maintain suspension stability in solvents, and the coated surface roughness Ra<0.8um meets the surface finish requirements of precision mechanical components.

Balancing Particle Size and Dispersion Stability

Although ultrafine powder has a larger specific surface area and more active sites, it exhibits strong agglomeration tendency in non-polar media. Measured data shows that ultrafine powder with D50=0.5um has a sedimentation rate exceeding 60% after 24 hours in mineral oil, while fine powder with D50=3um shows only 15% sedimentation. Therefore, when selecting ultrafine powder, surface modification (such as silane coupling agent treatment) or dispersants must be used, otherwise the actual dispersed particle size may be far larger than the nominal value.

From an economic perspective, the production cost of ultrafine powder is typically 3-5 times that of medium powder, and is not a necessary choice under non-extreme conditions. It is recommended that purchasers clearly specify D50, D10, D90, and specific surface area requirements in technical specifications, and comprehensively determine particle size specifications based on load, speed, temperature, and media conditions of the application scenario.

Tags: MoS2 particle size 粒径選択 particle size selection coarse fine ultrafine powder grease additive D50 D90 specific surface area powder metallurgy plastic modification dry film lubrication