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The purpose of a lubricant is to reduce the amount of friction between two surfaces. In some cases, the base oil in the oil or grease may not have enough lubricity to perform this function sufficiently. The component metallurgy may also require special chemistry. For example, with worm gears, traditional extreme-pressure or anti-wear additives often are too chemically aggressive for the softer yellow metals. In this situation, friction modifiers are added to increase the oil’s lubricity.
Conversely, in an automatic transmission, one fluid is used to provide lubrication, hydraulic power transfer and many other functions. The clutches and friction bands within the transmission need friction to function properly. In this instance, friction modifiers are required to smooth the transition from one speed to another. Otherwise, the clutches and bands would “chatter,” causing damage and an irritating condition for the driver.
A number of compounds are used to modify a lubricant’s coefficient of friction. These are known collectively as friction modifiers. They are designed to change the amount of energy needed to cause two surfaces to move past one anoth er. The different types of friction modifiers are shown in the table above.
The purpose of a friction modifier varies based on the application. In a combustion engine, the goal is to lower the amount of friction, thereby gaining fuel economy. In clutches, automatic transmissions and industrial applications, the aim is not simply to control friction in order to maximize efficiency but to reduce slippage. To a degree, this seems a bit counterintuitive, since a lubricant’s objective is to reduce friction and wear. However, there are many situations in which a certain amount of traction friction is required for equipment to operate properly. The friction modifiers used in these applications are not intended to increase or decrease friction but to act differently under specific shear conditions. This essentially smooths the transition from a dynamic condition to a static condition, such as during a gear change in a transmission or the engagement of a clutch.
Friction modifiers in engine oils can be traced back to 1915. They were used first in differentials, wet-clutch applications and transmissions. Their use escalated considerably in the late 1970s. Fuel economy became very important during the oil embargo at that time, which triggered a significant shift in the automotive industry. This led to the introduction of friction modifiers in passenger car motor oils.
The vast majority of friction modifiers in use today are designed to reduce friction or increase lubricity for better fuel economy. Recently, the U.S. government increased fuel economy standards with the goal of raising the Corporate Average Fuel Economy (CAFE) to 54.5 miles per gallon. This number, which is double the current standard, is for gasoline engines only, but there is a similar push for diesel engines as well. One way to achieve this goal would be to reduce the viscosity of the engine oils in use. The challenge is lowering the viscosity while maintaining a sufficient lubricant film to reduce wear and friction.
Great strides have been made in engineering to lower the friction generated in an engine. This has increased fuel economy. There have also been many advances in lubricant technology, including in the development of friction modifiers.
Friction modifiers are most efficient under boundary conditions or where metal-to-metal contact occurs. Organic friction modifiers have long, soluble chains and a polar head. The polar head attaches to the metal surfaces. The soluble chains line up beside each other much like fibers in a carpet. The polar heads may be comprised of phosphoric or phosphonic acids, amines, amides or carboxylic acids. The soluble chains form dense mono layers or thick, reacted viscous layers. These layers shear easily and create a relatively slippery surface.
Mechanical types of friction modifiers form layers of platelets that align with one another, providing a reduction in friction. The most common of these is molybdenum dithiocarbamate (MoDTC). These additives reduce friction by forming nano-sized single sheets dispersed in either a carbon or pyrite matrix. These nano-sized sheets are oriented in layers and slide against one another, reducing the generated friction.
Organic molybdenum compounds have been shown to work well in conjunction with zinc dialkyldithiophosphate (ZDDP). Used in engine oils for the better part of 80 years, ZDDP has been one of the most successful additives developed for oils. It has many functions, such as serving as an antioxidant, corrosion inhibitor and anti-wear additive. These additives also have a polar head and an oil-soluble tail structure. They form relatively thick, sacrificial boundary films that are much softer than steel or iron surfaces.
It should be noted that not only do the polar heads of friction modifiers need to be able to attach to ferrous metals, but they also must be able to attach to the zinc layers that will be present due to the ZDDP. These thick films formed by ZDDP are dependent on temperature and consist primarily of zinc, orthophosphate and polyphosphate glass, with an increasing proportion of polyphosphate chains closer to the surface.
As fuel economy standards become more stringent, more will be required of engine oils. While the technology for friction modifiers continues to evolve, the most effective way to improve fuel economy or energy consumption is to lower the viscosity of the lubricant. However, you can only go so far before losing the hydrodynamic film and operating in either mixed-film lubrication or boundary lubrication. It is in these two lubrication regimes that the use of friction modifiers becomes critical for reducing friction.
|51%||of lubrication professionals use lubricants in which friction modifiers have been added, according to a recent poll at MachineryLubrication.com|