In simple terms a grease is a thickened oil, the thickener acts as a sponge to hold the oil in place. Under pressure the 'sponge' releases the oil film to lubricate the contacts.
Oils come in many different types, both mineral and synthetic, with different properties depending on their chemical make up. In a lubrication application the oil creates a sacrificial layer or film which shears and isolates the moving surfaces. Within oils different additives can be used to enhance their properties including their viscosity, viscosity index, oxidation resistance and corrosion protection.
A grease is based on a mineral or synthetic oil, thickened with a metallic soap or sometimes an absorbent particle, clay, silica or PTFE and the additives are either in the base oil or held within the matrix. Solid additives are also sometimes included to re-inforce the boundary lubrication properties of the material under extreme pressure.
A Dry Film Lubricant is either a solid lubricant that is added to a coating which is bonded to the surface like a ‘low friction paint’ or, a layer of un-bonded solid lubricant applied between surfaces to act as a sacrificial layer or film.
Aerosol or spray lubricants are one of the above oils, greases or dry film lubricants (or a mixture in some cases), suspended in a carrier fluid or solvent with the addition of a propellant to push it out of the can. Depending upon the nozzle and pressure, the product can be a fine, atomised mist or a steady stream of fluid.