The lubrication performance of molybdenum disulfide (MoS₂) depends not only on chemical purity but is also significantly influenced by particle size distribution and dispersion stability. In industrial applications, MoS₂ powders of different particle sizes exhibit marked differences in friction coefficient, load-bearing capacity, and film-forming characteristics. Selecting the appropriate particle size and ensuring uniform dispersion in the lubricating medium are critical to maximizing the lubricating efficacy of MoS₂. Studies show that MoS₂ with a particle size of 1–10 μm delivers optimal overall performance in greases, while nano-scale (<100 nm) products offer unique advantages in thin-film lubrication.

Effect of Particle Size on Tribological Performance

The lubrication mechanism of molybdenum disulfide relies on interlayer sliding, and particle size directly determines the effective load-bearing area and the degree of friction film coverage. Laser particle size analyzer (D50 value) test results show that MoS₂ powder with a D50 in the 2–5 μm range performs best in friction tests, with the friction coefficient stably maintained at 0.03–0.04.

The effect of particle size on film-forming characteristics can be verified through the four-ball test (ASTM D2596). Test data on the wear scar diameter of lithium-based grease with 3% MoS₂ added under a 196 N load show that: the sample with D50 of 3 μm has a wear scar diameter of approximately 0.45 mm, the sample with D50 of 10 μm approximately 0.58 mm, and the sample with D50 of 30 μm reaches 0.72 mm. MoS₂ with smaller particle sizes more readily forms a continuous, dense transfer film on the friction surface, effectively preventing direct metal-to-metal contact.

However, smaller particle size is not always better. When D50 falls below 0.5 μm, the specific surface area of MoS₂ increases dramatically, and the elevated surface energy causes particles to agglomerate more easily, making uniform dispersion in grease more difficult and potentially increasing the actual friction coefficient to 0.06–0.08. Furthermore, ultrafine particles are easily squeezed out of the contact zone under load, failing to form an effective load-bearing film.

Dispersion Stability and Sedimentation Control

Molybdenum disulfide has a density of 4.8–5.0 g/cm³, far exceeding that of mineral oil (approximately 0.85 g/cm³) and synthetic oil (0.9–1.1 g/cm³), leading to sedimentation issues in liquid lubricating media. The Stokes' sedimentation velocity formula indicates that particle settling velocity is proportional to the square of the particle size; the settling velocity of a 10 μm particle is approximately 100 times that of a 1 μm particle.

In greases, the fibrous network structure of the thickener can retard the settling of MoS₂ particles to a certain extent. However, in low-consistency greases (NLGI Grade 0 or 00) or lubricating oils, the sedimentation problem is more pronounced. Actual storage test data show that after ISO VG 220 base oil containing 5% MoS₂ (D50=5 μm) is left standing for 7 days, the MoS₂ concentration at the bottom can reach 3–5 times that at the top, requiring thorough stirring before use.

Common methods for improving dispersion stability include: adding dispersants (such as petroleum sulfonates, sorbitan oleate, and other surfactants), controlling the particle size distribution range, and surface-modifying MoS₂ particles. Nano-scale MoS₂, due to its extremely high surface energy, typically requires grafting with organic molecules or compositing with inorganic oxides to achieve stable suspension in the medium.

Particle Size Selection for Different Application Scenarios

| Application Area | Recommended D50 (μm) | Addition Amount | Key Considerations |

|---------|-------------|--------|---------|

| Grease | 2–8 | 1–10% | Balance of load-bearing and dispersion |

| Powder Metallurgy | 5–15 | 0.5–3% | Uniform mixing with metal powder |

| Dry Film Coating | 1–5 | 20–40% | Adhesion and coverage |

| Plastic Modification | 3–10 | 5–20% | Dispersion and mechanical properties |

| Nano Lubrication Film | 0.05–0.1 | 0.1–1% | Ultra-low friction research |

For physical-process molybdenum disulfide products with purity ≥99%, since the production process does not involve acid leaching, the particle morphology is intact, the crystal structure is undamaged, and the particle size distribution is more concentrated (typical D50 in the 3–8 μm range), resulting in better dispersion in lubricating media compared to acid-leached products. Physical-process products have Fe content ≤0.02% and MoO₃ content ≤0.15%, with fewer impurity particles, making them less prone to abrasive wear on friction surfaces.

Testing Methods and Quality Control Standards

Particle size testing typically employs laser diffraction (ISO 13320), with three characteristic values—D50, D10, and D90—reported simultaneously. Dispersion stability can be evaluated through centrifugal acceleration testing: after centrifuging at 3000 rpm for 30 minutes, the MoS₂ concentration ratio between the upper and lower layers is measured; the closer the ratio is to 1.0, the better the dispersion.

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