Solid lubricants
Solid lubricants consist of extremely small, plate-like particles that can easily slide against one another. Due to their layered crystal-lattice structure, they can effectively separate metallic sliding surfaces.
In these crystal structures, the bonds within the individual layers are strong, whereas the bonds between the layers are considerably weaker.
As a result, these materials can easily move parallel to the layers while withstanding very high loads perpendicular to them. This produces the excellent sliding properties and high load-carrying capacity of solid lubricants.
During sliding motion, the solid-lubricant platelets become embedded in the surface irregularities of the sliding materials and form a dry lubricating film through the build-up of layers. This prevents the sliding surfaces from coming into direct contact.
Forces acting perpendicular to the surface align the randomly oriented solid-lubricant platelets within the lubrication gap parallel to the surface and compress them. This creates a continuous solid-lubricant layer that, due to its compressibility, partially absorbs the applied pressure and distributes it over a larger area.
Correspondingly high counterpressures acting parallel to the surface create a damping effect within the lubrication gap. This interaction prevents the upper metal layers from hardening and inhibits the formation of hardened, deformed or mechanically worked zones—the so-called “flaking” in areas near the edges of the plain bearing.
Applications
Solid lubricants are used wherever hydrodynamic lubrication cannot be achieved and where impact loads would cause conventional lubricating films to fail.
The combined solid lubricants developed by SL Gleitlagertechnik GmbH meet these requirements in every respect.
In addition to 99.9% pure, chemically neutral and corrosion-resistant graphite, molybdenum disulfide (MoS₂), tungsten disulfide (WS₂) and polytetrafluoroethylene (PTFE) are used as standard inorganic lubricants.
Optimum lubricant performance is achieved by combining components with complementary beneficial properties and by overlapping the solid-lubricant pockets in the direction of sliding, thereby ensuring an even distribution of the lubricant.
Operating temperatures in air:
- Graphite (99.9% pure): up to 430°C
- Graphite and PTFE: up to 250°C
Special lubricants and materials are also available upon request.
Although the plain bearings are self-lubricating and do not require additional lubrication, they can be used together with other lubricants, such as oil or grease, or with lubrication systems, provided that these do not contain other solid lubricants or substances with abrasive properties.
In such cases, we recommend mentioning the intended lubricant combination when submitting an enquiry to ensure that no adverse interactions occur between the lubricants.
Solid Lubricants with a Layered Crystal-Lattice Structure
⁺ The coefficient of friction of pure graphite in water—including seawater—other liquids, humid air, steam or under ideal tribological conditions may decrease to as little as 0.03.
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| Property | Graphite (99.9% pure) | Molybdenum disulfide | Tungsten disulfide | Polytetrafluoroethylene |
|---|---|---|---|---|
| Chemical formula | C | MoS₂ | WS₂ | PTFE |
| Operating temperature range (°C) | −120 to 600 | −100 to 400 | −180 to 600 | −200 to 260 |
| Coefficient of friction (μ) | 0.10⁺ to 0.18 | 0.04 to 0.09 | 0.08 to 0.18 | 0.04 to 0.09 |
| Suitability in air | Very good/good | Very good | Very good | Good |
| Suitability in water | Very good | — | Good | Good |
| Suitability in vacuum | Less suitable | Good | Very good | Very good |
| Resistance to seawater corrosion | Very good | Less suitable | Less suitable | Good |
| Resistance to chemicals | Very good | Good | Good | Good |
| Resistance to radiation | Very good/good | — | Good | Good |
| Crystal structure | Hexagonal | Hexagonal | Hexagonal | — |
| Colour | Grey-black | Grey-black | Grey-black | Transparent/white |
| Oxidation/decomposition products | CO, CO₂ | MoO₃, SO₂ | WO₃, SO₂ | C₂F₄ |
