WS2 (Tungsten Disulfide) is increasingly the preferred solid lubricant additive for premium grease formulations, replacing or supplementing MoS2 in applications demanding lower friction, better oxidation stability, and PFAS-free performance. However, adding WS2 to grease correctly requires understanding particle size requirements, concentration effects, and dispersion stability — getting any of these wrong results in poor performance, filter clogging, or rapid sedimentation. This guide is based on Powderful Solutions' formulation experience with WS2 grades from 0.4 µm to 12.5 µm in greases ranging from NLGI-1 to NLGI-3.
WS2 functions as a physical Extreme Pressure (EP), Anti-Wear (AW), and Anti-Friction (AF) additive — its lamellar platelet structure shears under load between metal surfaces, providing a persistent solid lubricant film that remains effective even when the base grease film has collapsed under extreme pressure or temperature. Unlike chemical EP additives (sulfur-phosphorus types), WS2 does not corrode yellow metals or degrade at high temperature, making it compatible with copper, brass, and bronze components.
Key measurable benefits of WS2 in grease formulations:
| Application | WS2 wt% in finished grease | Notes |
|---|---|---|
| Light-duty bearing grease | 1–2 wt% | General purpose, maintenance cost reduction |
| Standard industrial grease | 2–3 wt% | Most common treat rate — good EP/AW performance |
| Heavy-duty EP grease | 3–5 wt% | Mining, construction, extreme load applications |
| High-performance specialty | 5–10 wt% | Aerospace, high-temp, extended drain greases |
| Dry film / NLGI-00 | 10–25 wt% | Specialized dry or semi-fluid applications |
Note: Treat rates above 5 wt% typically show diminishing returns in standard bearing greases. Higher concentrations are warranted only for extreme load or high-temperature applications.
Particle size is the single most critical variable in WS2 grease performance. Using the wrong particle size will result in either filter clogging (too large) or poor tribofilm coverage (poorly dispersed).
D90 0.4 µm (sub-micron) — recommended for: All oil and grease additive applications. Particles smaller than 1 µm pass through standard grease filters (25 µm nominal), remain suspended without clogging, and provide maximum surface area per gram of WS2 for tribofilm formation. Solidex W004 (dry powder) and Lubricore W850 (dispersion) use this grade.
D90 2.5 µm — recommended for: Open gear lubricants, heavy-duty NLGI-00 grease, and applications where larger particle coverage is needed on rough surfaces. Not suitable for filtered grease systems or precision bearings.
D90 12.5 µm — for: Dry film coatings, spray lubricants, and applications where the particles are applied directly to surfaces without subsequent filtration.
There are two methods to incorporate WS2 into grease: using a pre-dispersed concentrate (Lubricore W850) or adding dry powder (Solidex W004) directly to the base grease. Each has distinct advantages:
Lubricore W850 contains 50 wt% WS2 in PAO-6 synthetic oil, surface-functionalized for long-term colloidal stability. Blend directly into base grease at 2–10% by weight (2% letdown gives 1% WS2; 10% gives 5% WS2 in finished grease). No special equipment needed — standard grease blending equipment at 60°C with moderate agitation is sufficient. Particle size is pre-optimized at D90 0.4 µm. Shelf life: 24 months with re-agitation before use.
Adding dry WS2 powder requires: (1) pre-wetting the powder in a small amount of base oil (1:1 by weight) using high-shear (Silverson or similar), (2) passing through a triple roll mill or high-pressure homogenizer to break agglomerates to sub-micron size, (3) blending the resulting WS2 paste into base grease at temperature (60–80°C). Skipping the milling step results in visible agglomerates, poor performance, and filter clogging.
WS2 particles in grease are denser than the base grease matrix (WS2 density: 7.5 g/cm³ vs grease density: ~0.9 g/cm³). Without stabilization, particles will sediment during storage. Prevention measures:
| Parameter | WS2 | MoS2 |
|---|---|---|
| COF in grease | 0.03–0.05 | 0.05–0.10 |
| Max temp (air) | 650°C | 400°C |
| Humidity stability | Excellent | Moderate (forms H₂SO₄ in humid air) |
| Color | Dark grey | Black |
| Cost | Higher | Lower |
| Best use | Premium, high-temp, ambient | Cost-sensitive, vacuum/inert |
A: If using Lubricore W850 (pre-dispersed), standard grease blending equipment (paddle mixer, Z-blade) at 60°C is sufficient. If using dry Solidex W004 powder, you need a high-shear mixer (Silverson, IKA Turrax, or equivalent) and preferably a triple roll mill or bead mill for agglomerate breakdown.
A: Visual inspection after blending (no visible grey specks), grease fineness (Hegman) gauge test, and microscopy are the standard QC methods. A well-dispersed WS2 grease will have uniform grey color with no visible particle agglomerates at 40× magnification.
A: Yes — WS2 is chemically inert and compatible with all standard grease thickener types. Polyurea greases with WS2 are particularly effective for high-temperature bearing applications.
A: WS2 itself does not degrade. Shelf life is governed by the base grease's oxidation stability — typically 24–36 months for properly stored grease. Re-homogenize before use after extended storage.