In the mechanical seal business a lubricant is defined as any fluid that will maintain a film thickness of one micron or more at its operating temperature and load. We use this number because seal faces are lapped flat to one micron (3 helium light bands), or less.
Cold water is a lubricant, hot water is not. Many solvents are not lubricants, but most liquids are.
Seal faces receive part of their lubrication from the graphite in the carbon- graphite seal face leaching out and depositing on the lapped hard face.
- Moisture must be present for this to occur, so hot, dry running applications present some sealing problems unless a special carbon is provided that contains an organic material that will release the graphite.
Many lubricants cause corrosion problems so their use is limited in bearing lubrication.