Superlubricity, also known as zero friction, is the phenomenon wherein friction vanishes or very nearly vanishes. This phenomenon has started a number of researches from scholars and engineers from all over the world. In fact, superlubricity has become a new study under the facets of tribology. Recently, this field experienced its peak as more solid and liquid materials have been created that exhibit zero friction. New methods for achieving the phenomenon has surfaced as well. Amongst liquids, very good zero friction characteristics have been observed in a new set of liquid substances using a newly engineered mechanism. Among solid materials, molecular dynamics simulation (MDS) experiments have revealed more superlubricity characteristics.
Zero friction phenomenon happens when two surfaces, particularly crystalline, slide over each other with minimal resistance. The effect was first noted in 1991 and verified thirteen years later using two graphite surfaces. The nature of zero friction in graphite is dependent on the orientation of its atoms. Graphite is known to have hexagonal atoms which can be compared in appearance to egg crates. Two graphite surfaces are said to be “in registry” every sixty degrees. This is where friction is at its peak. When the same surfaces are “out of registry”, the friction is greatly reduced. The amount of zero friction observed depends on how far the graphite surfaces are from in-registry orientation.
Superlubricity is often termed with other extreme conditions like superconductivity and superfluidity. It was in 2012 that zero friction was observed in graphite structures at the microscale. This was the result of shearing a square graphite mesa some microns across and then taking note of the self-retraction that occurred. The phenomenon was also described for layers of nickel. The observation was achievable even in ambient conditions and thus catapulted zero friction from purely academics to possible technological applications.
Liquid Zero Friction
The mixture of acid and polyhydroxy alcohol exhibits zero friction. This acid solution has been found having superlubricity properties between a glass plate and silicon nitride. A break-in method was utilized to achieve the zero friction phenomenon. The experiment revealed that zero friction and the existence of hydrogen ions are directly related. Thus, the researchers suggested that zero friction might be more achievable using phosphoric acid and water which could generate a network of hydrogen bonds. The result is an acid solution with added polyhydroxy alcohol.
There is a question of whether superlubricity can be achieved using oil-based lubricants. This was answered by the breaking-in acid solution to tribo-surfaces; the result is silicone oil exhibiting the zero friction phenomenon. It was shown in experiments that the friction coefficient of silicone oil is reduced to one-thirtieth of its original value after the running-in with acid.
Solid Zero Friction
More solids have been found in superlubricity state in recent years. Scientists have found a direct measurement of sliding friction between graphene and graphene, and between graphene and hexagonal boron nitride. The experiment, utilizing graphene-coated microsphere (GMS) probe, was done with contact under a pressure of 1 billion Pascals. The method, to be specific, was a chemical vapor deposition technique using a chemical vapor without any metal catalyst. With this, a 0.003 friction coefficient was achieved – a number considered extremely for the given material. What’s more promising is that all was done with only 51% of relative humidity. The perceived roughness of the contact surface due to the random orientation of the graphene molecules are said to be the primary reason why the zero friction was achieved. Such asperity is considered sustainable on the said material and can even be further achieved by different layers in two dimensions.
Zero Friction Hybrid Material
In 2015, scientists from the Argonne National Laboratory used a supercomputer to identify and improve a method for reducing friction to almost zero. The process was utilized in the development of a hybrid material that exhibited zero friction at the macroscale for the very first time. Ten million atoms in a humid environment and 1.2 million atoms in a dry environment were successfully simulated with the procedure. The technology invented by the team is about to achieve a patent. The research team expects the zero friction hybrid material will find applications in mechanical rotating seals in both nano and micro-electromechanical systems. More commonly, due to friction inhibiting their performance, computer hard drives and wind turbine gears might find the hybrid material useful in the nearest future.
Outlook and future work
Zero friction was first observed at the nanoscale and in environments at high vacuum. However, scientists are learning more about microscale possibilities which may lead to practical applications of the technology. This is what researchers at Tsinghua University are aiming for.
Motorisa Hirano was one of the original discoverers of superlubricity in the 1990’s. Even he cannot hide his excitement on the future of zero friction. He believes that microscale development could lead to using superlubricity as a lubricant that could be very useful in mechanical engineering, particularly power management.
In the experiments by the Tsinghua University researchers, pyrolytic graphite — a type of graphite created under high temperature — was utilized. Using lithography, the researchers made square columns – or mesas – of graphite up to 20 µm wide and up to 400 nm in height. They then transferred these mesas to a scanning electron microscope or an optical microscope and, with a tungsten probe, sheared them into flakes, which they could rotate into different orientations.
The researchers found that the flakes aligned symmetrically with respect to the underlying mesa stayed still. Poking it with the tungsten probe has no observable effects. But when the researchers changed the orientation of the same flakes and poked them using the tungsten probe, the flakes returned to their original, lowest energy position. The researchers say that this is only possible because of the extremely low friction achieved.
So what would be the greatest significance of the discovery? The researchers say that what they did was to prove that achieving zero friction is easier than originally thought. Their method showed that creating microscale superlubricity is indeed reproducible. This could also mean that there is a lot of possible practical applications of the technology especially in the creation of Nano-machines.