Temperature greatly affects friction properties because as it rises, the friction coefficient decreases, and smoothness increases gradually. Additionally, the wear rates increase significantly due to the cumulative friction heat, further damaging the appliances. To improve the life of appliances, scientists from the University of Leicester are understanding superlubricity for energy efficiency. They discovered that lowering surface temperature reduces friction by dealing with the effects of random vibrations on surface atoms.
A team of scientists from the University of Leicester has been actively researching ways to make surfaces even more slippery. It is a breakthrough in the understanding of superlubricity, where two surfaces exhibit minimal to almost non-existent friction while sliding against each other. The concept of superlubricity is linked to very smooth surfaces at the molecular level, like graphene. It has only been seen in lab conditions where these surfaces are created at very small scales.
Professor Nikolai Brilliantov from the University of Leicester and an international team of scientists has discovered that random vibrations of surface atoms create synchronic fluctuations on the surfaces of objects, leading to friction. Vibrations persist at all temperatures above absolute zero, and their strength diminishes as the temperature drops. This implies that reducing the surface temperature can effectively reduce the impact of friction.
Leicester School of Computing and Mathematical Sciences Professor Brilliantov said, “Such a dramatic difference with common friction is intriguing and needs explanation. There are other surprising features of superlubricity, such as the unusual dependence of friction force on the sliding velocity, on temperature, and contact area. All these dependencies are opposite to those predicted by the traditional Amontons-Coulomb laws. Explaining the enigmatic behavior of superlubricity will help to control ultralow friction, which can open the breathtaking horizons of its industrial applications.”
Breakthroughs in Friction
Superlubricity is a remarkable phenomenon that is vastly different from traditional friction. In this state, friction can be smaller, and it is not influenced by the weight of an object. This means that increasing the weight of an object from grams to tens of kilograms will not change the level of friction force.
The potential applications of this technology for reducing friction in machines and mechanisms are very promising. It can reduce friction up to 1000 to 10,000 times compared to conventional methods. It’s a well-known fact that heavier objects experience greater resistance to sliding compared to lighter ones, a principle known as the Amontons-Coulomb friction law, which was established over 300 years ago.
Experimental Setup and Measurements
In order to explore the principles of superlubricity, researchers established contact between two molecularly smooth surfaces. Using lateral force microscopy, they measured the friction force by sliding a tip on a substrate. They both were coated with a layer of graphene. Also, they conducted comprehensive numerical experiments using Molecular Dynamic simulations to develop a realistic model of the phenomenon. It is essential that the surfaces remain incommensurate. This will guarantee that the peaks of one surface do not align up with the valleys of the other surface.
When the surfaces have a non-zero temperature, friction occurs due to surface corrugations caused by thermal fluctuations. Scientists have shown that friction is caused by synchronised bending of two surfaces in close contact, creating thermal fluctuations. The temperature of the surfaces directly affects the amplitude of these synchronic fluctuations, with higher temperatures resulting in larger amplitudes. The size of contact area affects surface fluctuations, and impedes motion.
After understanding superlubricity for energy efficiency, Professor Brilliantov added, “We have been able to explain the atomistic mechanism of the enigmatic independence of friction force on the weight of a body and formulated new friction laws for superlubricity. These laws, although being in sharp contrast with the Amontons-Coulomb laws, describe this phenomenon rather well. Once molecular smooth-surface layers are produced on the scale of millimeters or centimeters, all moving, rotating, oscillating contacts in machines and mechanisms will be covered with such surface layers. It will drastically decrease energy consumption worldwide. To further decrease energy consumption, the largest contacts will be possibly kept at low temperatures.”