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What is a friction modifier and how does it work?

One of the most important features of a lubricant is the reduction of friction between two surfaces. Besides base oil, additives are added to perform this function. Friction modifiers are used to increase the lubricity. But what are friction modifiers and how do they work?

Friction modifiers are oil soluble chemicals used as additives in lubricants for internal combustion engines and transmissions. Friction modifiers are initially used for applications like transmission fluids, power steering, limited slip gear oils, and tractor fluids. Their main purpose is to smoothen the motion in moving parts to prevent the wear on metal surfaces. They can also be called boundary lubrication additives. This is mainly due to the fact that they can prevent direct contact of the solid surfaces reducing friction and wear. This in turn helps improve the fuel economy of the engine.

A friction modifier molecule consists of two parts; namely a polar end (head) and an oil-soluble end (tail). The head attaches itself to the metal surface to create a cushion for the metal surface against another metal surface. The tails stand up like a roll. These molecules hold up when cushioned surfaces come in light contact with each other. This forms a thick boundary film that is softer than metal surfaces.

Getting into detail with this, we come across anti-wear (AW) and extreme pressure (EP) additives. Both of the additives work by developing a durable film on the metal surface when there is a reaction. AW additives should be used when there are low loads and high speeds to reduce the rate of continuous wear. Commonly used AW additives in the lubricants are zinc dialkyldithiophosphates (ZDDP). But when the load is very heavy and is run at high temperature and lower speeds, EP additives must be used to prevent the damage of the engine. Commonly used ones are molybdenum disulfide.

As vehicle emission regulations are getting more and more challenging, restrictions are likely to be placed on engine oil additives. Although modern engine oils are heavily reliant on ZDDP to provide anti-wear, anti-oxidation and anti-corrosion, they are a likely target for the emission control as they contain phosphorous, sulphur, and zinc. The impact of these elements are already studied by the emission control and they are not necessarily positive. This will most likely lead to innovations such as the reduction of ZDDP in engine oils to make sure the performance of the engine is not harmed and use of the friction modifier is still used through other alternative additives.