Lubricants may be solid, such as graphite, molybdenum disulphide, poly-tetrafluoroethylene and talc; or gaseous, commonly air; but the principal lubricants are liquids such as mineral oil, or the semi-solid greases formed from liquids by the use of thickening agents. 24.1.1 Main Regimes of Lubrication HYDRODYNAMIC LUBRICATION (HL) A viscous fluid film between two solid surfaces moving in very clos