A solid lubricant is a dry film — PTFE, molybdenum disulphide, graphite or a bonded coating — that shears between the rolling element and the raceway instead of an oil film. It is used where oil cannot survive or is not allowed: high vacuum, cryogenic temperatures, cleanrooms, radiation, and temperatures above the limit of any grease.
PTFE has the lowest friction and the widest chemical resistance, and works from cryogenic temperatures up to about 260 °C. MoS2 is better under load and in vacuum, but needs some humidity to perform well in air. Graphite needs humidity and is unsuitable for dry or vacuum duty. Choosing the wrong one is the most common failure in dry-lubricated bearings.
A bonded film has a defined thickness and wears away. Life is estimated from the film thickness, the contact pressure and the number of revolutions — and it is usually much shorter than a grease-lubricated bearing. For a vacuum application that is acceptable; for a continuously running machine it is not.
| Materials | PTFE, MoS2, graphite, bonded coatings |
|---|---|
| Temperature | −200 °C to +350 °C depending on material |
| Vacuum | Suitable — no vapour pressure |
| Life | Finite — the film is consumed, not replenished |
| Speed | Low — poor heat removal |
| Load | Moderate; MoS2 the best under load |
| Cleanroom | Suitable — no particle generation from grease |
| Typical use | Semiconductor, aerospace, cryogenics, kilns |
| Material | Temperature | Vacuum | Load | Needs humidity | Typical use |
|---|---|---|---|---|---|
| PTFE | −200 to +260 °C | Yes | Low–moderate | No | Cleanroom, chemical, cryogenic |
| MoS2 | −180 to +350 °C | Yes | High | Slightly | Aerospace, high load |
| Graphite | −200 to +500 °C | No | Moderate | Yes | Furnaces, high temperature |
| Bonded coating | −70 to +250 °C | Yes | Low | No | One-shot duty, instruments |
Only with a low-vapour-pressure grease such as a PFPE or a specially formulated vacuum grease. Standard grease outgasses and contaminates the vacuum chamber.
It is a wear calculation, not a fatigue one: film thickness divided by wear rate. Typically far shorter than a grease-lubricated bearing, so dry bearings are sized for the mission, not for L10.
For load and vacuum, usually yes. For low friction, chemical resistance and cleanroom use, PTFE is better.
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