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


Lubrication - additives

Additives to oils:

Additives either prolong the life of a lubricant or increase its viscosity index (VI), preventing the oil from becoming too thin at high temperatures or too greasy at low temperatures. Example include:

  • Viscosity-index improvers:  oil-soluble long-chain polymers which increase the VI by decreasing the viscosity at low temperatures (pour-point depressants - prevent the oil from becoming too viscous at lower temperatures) or increasing viscosity at high temperatures.
  • Extreme pressure (EP) additives: react with the sliding surfaces under the service conditions giving compounds with low shear strength which behave as thin lubricating films, partially separating asperities and preventing them from welding together. They usually contain sulphur or chlorine to facilitate the chemical reactions. An example is zinc dialkyl dithiophosphate (ZDDP). EP additives give boundary lubricating properties.
  • Boundary lubricants (e.g. stearic acid, C17H35COOH): Polar end-groups on the hydrocarbon chain bond to the surfaces, providing layers of lubricant molecules which slightly reduce direct contact between asperities on the surfaces. The lubricant film is very thin, so there is significant asperity formation, but the asperity junctions are weakened compared to normal.
  • Fig

Other additives are detergents, antioxidants and dispersants. Detergents clean and neutralize oil impurities which would normally cause deposits. Antioxidants prevent oils from oxidising. Dispersants prevent contaminants from aggregating into larger groups that hinder the flow of the oil.     

Regimes of lubrication

The Stribeck curve: the variation in the coefficient of friction with the dimensionless quantity η U/W for a lubricated bearing. Here, η is viscosity (dimensions of ML-1T-1), U is peripheral speed (dimensions of LT-1), of the bearing and W the load (per unit width) (dimensions of MLT-2/L = MT-2), carried by the bearing (after Hutchings and Shipway, 2nd ed., p. 90). A nice commentary on Stribeck’s work can be found in B. Jacobson, ‘The Stribeck memorial lecture’, Tribology International 36, 781-789 (2003).

Hydrodynamic lubricationIn this regime the surfaces are separated by a fluid film usually thick in comparison with the heights of asperities. The normal load is supported by the pressure within the film. This pressure is generated hydrodynamically.

Elasto-hydrodynamic lubrication / ‘Mixed’ lubrication - Here, the separation of the surfaces is very low – for example, the load on a bearing has increased to bring it into this regime of behaviour because of local line or point contact. In steel components such as gears local pressures can be several GPa. Elastic deformation of bearing surfaces occurs and the oil film behaves almost like a solid – to a good approximation the viscosity of the oil, η, increases exponentially with the pressure, P. As η U/Wfalls, the lubricant film begins to break down and a sharp rise in friction ensues because of direct asperity interaction. Therefore, this is also a regime referred to as being of ‘mixed’ or partial lubrication.

Boundary lubrication - This occurs at very low sliding speeds and/or high contact pressures. Steric forces between polar molecules either present naturally (e.g., in castor oil) or deliberately added to the oil, such as stearic acid or EP additives, prevent or limit contact between asperities of the two surfaces.

Under the most favourable circumstances μ can be very low (e.g., » 0.001), so lubrication is certainly beneficial in reducing wear of materials.