Lubricity is the measure of the reduction in friction of a lubricant. It is due to chemical action between the lubricant or some components of it and the rubbing surfaces in the physical conditions there, due to which a tenatious layer develops, which is able to support the load that the bulk lubricant alone by virtue of its viscous resistance could not. The study of lubrication and mechanism wear is called tribology.
Measurement of lubricityLubricity of a material cannot be directly measured, so tests are performed to quantify a lubricant's performance. This is done by determining how much wear is caused to a surface by a given friction-inducing object in a given amount of time. Other factors such as surface size, temperature, and pressure are also specified. The greater the wear scar the worse the lubricity. For this reason lubricity is also termed a substance's anti-wear property.
Examples of test setups include "Ball-on-cylinder" and "Ball-on-three-discs" tests.
Lubricity in diesel engines
In a modern diesel engine, the fuel is part of the engine lubrication process. Diesel fuel naturally has sulfur compounds in it that gives it good lubricity properties, but because of regulations in many countries (such as the US and the EU) this sulfur must be removed. Thus, the reformulated diesel fuel has a lower lubricity and requires lubricity improving additives to prevent excessive engine wear.