Tribological characterisation of lubricants

The basic function of a lubricant is to reduce friction and wear between two surfaces in relative motion. This is usually achieved by forming a load-bearing fluid film at the contact interface. However, in many applications, the role played by lubricants is far more complex. For instance, in addition to the basic tasks mentioned above, lubricant used in an automotive engine is expected to dissipate heat, wear and foreign particles away from the mating interface, prevent oxidation and corrosion of the metal parts, maintain stability at high temperature, provide effective sealing, to name a few.

All these requirements, however, cannot be fulfilled by the base oil alone. Therefore, specific chemical components known as additives are added to it to address these explicit requirements. While each of the additives is expected to fulfill its designated function, it is important that any interactions between the additives themselves do not have a negative impact on the overall performance of the lubricant.

The demand for increased efficiency and the need to conform to the environmental norms and regulations are two factors driving the lubricant industry to improve their products. This however is easier said than done as lubricants are generally comprised of numerous chemical components which coexist in a delicate balance. With almost every minor tweak in the formulation of a lubricant, there is a fair chance of casting a significant influence on its friction and wear behaviour. This influence can either be positive or negative and therefore, oil formulations are thoroughly investigated before they find their way onto the market.

In an application report produced by Anton Paar, tribological tests were carried out to investigate the friction and wear performance of three engine oils with slightly varying composition. The results show marked differences in the initial running-in processes of the three oils.