Dissemination of IT for the Promotion of Materials Science (DoITPoMS)


Lubrication - introduction and types of lubricants

Lubrication is the process of reducing friction between touching surfaces moving relative to each other by introducing a lubricant between the surfaces, which is a material with a lower shear strength than the surfaces.

Lubricants do not necessarily completely prevent asperities, but they reduce their number and weaken their junctions. So lubrication also reduces the rate of sliding wear.

μ for many dry engineering materials is rarely below 0.5 and in most cases is significantly higher. Such high values would lead to large frictional forces and hence energy losses (and almost certainly high wear rates). With lubrication μ can be very low (ยป 0.001), which is why lubricants are widely used.

Good lubricants have high pour points (the lowest temperature at which an oil will flow), high viscosity indices (see later) and good resistance to oxidation.

Types of Lubricant:

Mineral Oils:

Commercial mineral oils are based on several different hydrocarbon species with mean molecular weights between 300 and 600. Examples are pariffinic oils, which have a predominance of paraffin-like species, i.e., long-chained hydrocarbons with either straight or branched chains, as shown schematically below.


Synthetic Oils:

These have fewer impurities than mineral oils, but are significantly more expensive. They are used when relatively high or low temperatures or loads are to be experienced in service, or if low flammability is essential. Examples are synthetic hydrocarbon oils (SHCs) and silicones (below).


Solid Lubricants:


These can be used at higher temperatures. They have a layered structure with weak intermolecular forces between layers, allowing them to slide easily relative to each other (low shear strength) thus giving lubricant properties. To work best the layers should be oriented parallel to the surface and in the direction that the motion will occur, so that on movement the layers can slide over each other easily. In the adjacent diagram the crystal structures of two common solid lubricants are shown: (a) graphite and (b) molybdenum disulphide.

Solid lubricants can be used to produce ‘self-lubricating’ systems which do not need an external source of lubrication during the lifetime of the system. Also, they are particularly useful in vacuum technology and space applications, because they do not evaporate away.


The most important property of an oil for lubricating purposes is its viscosity. Viscosity provides a measure of the resistance of a fluid to shearing flow.

Click here for a recap of viscosity.