Around the world, industry, consumers and governments are calling for better fuel efficiency in their vehicles and machines. In response, the lubricant industry is returning to its roots and looking for ways to reduce friction.
Included in that work is the search for new types of friction modifiers, including cutting-edge substances such as nanoparticles. According to a British academic, though, the industry should not forget about older technologies such as organic or molybdenum-based friction modifiers. In a presentation to Januarys International Colloquium Tribology in Ostfildern, Germany, Hugh Spikes said that formulators still have room to improve their understanding of such materials. If they do, he added, they might find ways to extract more performance from those substances.
A paramount concern in the modern world is to reduce energy consumption, said Spikes, a professor of lubrication who also heads the tribology research group at Imperial College London. One important way of doing this is to improve the energy efficiency of machines. This can be achieved by reducing friction.
The First Additives
Based on current tribology theory, most machines experience at least three types of friction, each requiring different types of lubrication. In the fluid film regime, the lubricant serves as a buffer to prevent contact between moving components. In the boundary regime, chemical additives in the lubricant help protect surfaces that do come into direct contact. The mixed regime occurs during the transition phase between the other two. Spikes said it is important to address all three regimes.
Most base stocks impart lubricity that helps reduce friction in the fluid film regime. Friction modifiers, however, work at the other end of the spectrum. Conventionally, friction modifiers are additives that reduce friction in boundary lubrication conditions, he said.
According to Spikes, the first friction modifier – in fact, the first lubricant additive – was oleic acid, which was first used around 1920. Oleic acid is found in plant and animal oils, and it was found that mixing it with mineral oil improved the oils friction and wear properties.
In the 1930s and 40s, lubricant suppliers began using additional types of additives to enhance properties such as pour point, oxidation stability, dispersancy, antiwear, corrosion inhibition and viscosity index, among others. In the 70s, a second type of friction modifier debuted: oil-soluble molybdenum-based compounds that were able to improve efficiency in engines and transmissions.
Then, around the turn of this century, a third category came into use – functionalized adsorbing polymers, which had been found to improve boundary lubrication. Today, Spikes said, researchers are studying another potential category, dispersed nanoparticles, which show some promise to effectively reduce friction in a variety of applications.
Lessons Yet Unlearned
Proponents of nanotechnology have high hopes that it will enable big strides in lubricant performance, but Spikes suggested that it is still worthwhile to continue researching older technologies – partly because they are not completely understood. For example, he said, since the 1980s, some have believed that organic friction modifiers such as oleic acid have contributed to the oiliness of lubricants modifiers by forming layers a single molecule thick on component surfaces. Others contended that they formed much thicker films that were possibly in liquid crystal or micellar form.
In cooperation with Castrol, members of Imperial Colleges tribology research group conducted several experiments to try to learn more about the workings of these substances. Initially, they focused on the differences between saturated and unsaturated molecular chains.
First they used a high-frequency reciprocating rig to test oils with varying concentrations of several types of organic friction modifiers. They found that the additives needed to be in concentrations of at least 350 parts per million to be effective. These tests also showed that a saturated acid such as stearic acid was slightly more effective in reducing friction than an unsaturated acid such as oleic acid – though not enough to make a difference.
Next the researchers tested the same samples on a tribometer, a rig that exerts pressure on a lubricated disk turning at slow speeds. This time the stearic acid performed significantly better than its oleic counterpart. Then the group tried categorizing the substances in a different way. It ran similar tests on alkyl sulfonates with linear and branched molecular chains. In this case, the additive with a linear structure performed better at slow speeds than the branched version. The same test was run comparing ZDDP, a popular antiwear agent with a linear molecular structure, with ZDP, a branched member of the same chemical family. Again the linear molecule achieved lower friction.
Spikes said this may indicate that the performance of these friction modifiers is affected by their molecular shape and how those molecules collect on surfaces.
Linearity may be more important than saturation, he said.
The Imperial College-Castrol team also noted that friction readings were higher on the first turn of their test rigs than afterward. Spikes said its unlikely that this has anything to do with chemical interaction by organic friction modifiers – and that their mechanism of performing is likely not chemical. Instead, it suggests that a pass of the contacting surface is required to iron the molecules into place, another sign that their mechanism could be mechanical.
There are also things that remain unknown about the workings of molybdenum-based friction modifiers, Spikes said. Researchers do know that these substances help form very thin layers of molybdenum disulfide (MoS2) crystallites that help protect high spots of rough surfaces rubbing against each other. They also know that it requires two solid surfaces rubbing together to activate the formation of those crystallites and that minimum temperatures and concentrations of the additives are also necessary.
But, Spikes added, We still dont know precisely how the reaction to form MoS2 is triggered.
In some ways, moly-based substances are excellent at reducing friction, but they also have limitations.
The main practical problem with molybdenum-based friction modifiers is their longevity, Spikes said. They are powerful antioxidants and tend to get used up rapidly in engine oils. However, their useful life can be extended by other antioxi-dants, and there is not so much of a problem when using them in transmission fluids.
Conducting studies like these allows formulators to better understand how different friction modifiers work and under what conditions they perform best, Spikes said. Then they can make more effective use of existing technologies. Spikes drew a number of conclusions from the work by Imperial College and Castrol, including:
At high concentrations, most organic friction modifiers perform better than oleyl varieties, such as oleic acid, especially at low speeds.
Liquid cell Atomic Force Microscopy can be an effective tool when studying behavior of organic friction modifiers.
Saturated and linear organic friction modifiers reduce friction as relative speeds of rubbing surfaces slow, but oleyl modifiers do not.
Spikes made it clear that he believes further research is needed.
There is still a lot to learn about organic friction modifiers, he said. Even after nearly a century of use.