Lubricant

A substance placed between touching surfaces to reduce friction, wear, or heat. Lubricants may be liquids, solids, gases, or semi-solids.

Hydrodynamic lubrication: A lubrication regime in which motion draws a fluid film between surfaces, keeping them apart under load. This regime explains how moving parts can ride on a lubricant film without direct contact.
Lubricating oil: A liquid lubricant formulated to reduce friction and wear in machinery or other moving contacts. Oils are the most familiar lubricant class and can also carry heat and contaminants away.
Engine oil: A lubricant that reduces friction and protects surfaces inside an internal-combustion engine. It must lubricate moving engine parts while resisting heat, oxidation, and contamination.
Water-based lubricant: A lubricant whose principal lubricating fluid is water or an aqueous solution. It contrasts with oil-based lubricants in viscosity, cooling capacity, and corrosion-control needs.
Friction modifier: A lubricant additive that lowers friction, especially under boundary or mixed lubrication conditions. These additives can improve efficiency when the lubricant’s bulk viscosity alone cannot prevent friction.
Boundary lubrication: A lubrication regime in which thin molecular films limit friction when surfaces remain in close contact. It matters when speed or lubricant supply is too low to maintain a separating fluid film.
Grease (lubricant): A semi-solid lubricant made by thickening a base oil, usually with a soap or other thickener. Grease stays in place where a free-flowing oil would drain or leak.
Gear oil: A lubricant formulated for gears, especially where high contact pressures and sliding occur. Its viscosity and additives protect gear teeth against scuffing and wear.
Silicone lubricant: A lubricant based on silicone fluids or compounds, often chosen for water resistance and temperature stability. Its chemistry offers properties unlike those of conventional mineral and synthetic oils.
Anti-wear additive: A lubricant additive that forms protective surface films to reduce material loss under contact. It protects components when surfaces approach closely despite the presence of lubricant.

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