#superlubricity

Electronic friction can be tuned and switched off

Researchers in China have isolated the effects of electronic friction, showing for the first time how the subtle drag force it imparts at sliding interfaces can be controlled. They demonstrate that it can be tuned by applying a voltage, or switched off entirely simply by applying mechanical pressure. The results, published in Physical Review X, could inform new designs that allow engineers to fine-tune the drag forces materials experience as they slide over each other. Friction's hidden electronic side In engineering, friction causes materials to wear and degrade over time, and also causes useful energy to be wasted as heat. While this problem can be mitigated through lubricants and smoother surfaces, friction can also arise from deeper, more subtle effects.

Physicists achieve near-zero friction on macroscopic scales

For the first time, physicists in China have virtually eliminated the friction felt between two surfaces at scales visible to the naked eye. In demonstrating "structural superlubricity," the team, led by Quanshui Zheng at Tsinghua University, have resolved a long-standing debate surrounding the possibility of the effect. Published in Physical Review Letters, the result could potentially lead to promising new advances in engineering. What structural superlubricity promises When two objects slide over each other, any roughness on their surfaces will almost inevitably resist the motion, creating the force of friction. Yet in 2004, physicists showed that friction can be virtually eliminated between two graphite surfaces, simply by rotating their respective molecular structures.

Revolution in friction: A way to make super-smooth materials

Scientists from the Faculty of Physics and Applied Informatics at the University of Lodz have published an article on friction in the journal Small. Their research on "bismuth islands" moving on the surface of graphite confirmed the existence of a totally new form of so-called superlubricity—a friction-free contact between two solid bodies. This discovery could revolutionize the way we design nanoscale machines, and even vehicles, in the future. By understanding these processes, we can create devices that can operate much more efficiently, saving on energy and resources. Scientists led by Dr. Hab. Paweł Kowalczyk, associate professor at the University of Lodz, have discovered a new phenomenon related to the disappearance of friction—superlubricity. This special phenomenon was observed at the contact of two solid materials, bismuth and graphite.

While many of us are treading carefully to avoid a slip in the frosty weather, scientists led by the University of Leicester have been investigating how to make surfaces even slippier! They have solved a conundrum in the principles of superlubricity -- a state in which two surfaces experience little to almost vanishing friction when sliding across one another. They have published their conclusions in a paper for the journal Physical Review Letters. Superlubricity is associated with molecular smooth surfaces such as graphene and has only been observed in the laboratory environment where these surfaces are synthesized at nano and micron scales. It looks very promising for technological applications where it could potentially reduce friction up to 1000 -- 10000 times, as compared to conventional friction in machines and mechanisms.

Superlattice films developed to achieve macroscale superlubricity

A research group led by Prof. Wang Liping at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. Li Qunyang at Tsinghua University, developed a universal superlattice film that can be applied to mechanical parts to achieve long-term macroscale superlubricity with self-rejuvenation. The study was published in Cell Reports Physical Science.

It is estimated that about one-third of global energy is consumed to get rid of friction, which is quite common in nature. Reducing friction to an ultra-low level or even a superlubricious state with a friction coefficient on the order of 10-3 can exert significant impacts on human life. However, achieving robust macroscale superlubricity between dry solid surfaces remains a challenge, which has been realized mainly at nanometer scales with lamellar materials in previous studies.

To tackle this challenge, the researchers designed a universal molybdenum disulfide (MoS2) /ceramic superlattice architecture, which enables long-term (close to 300,000 sliding cycles) macroscale superlubricity in mild vacuum after a short running-in period in air.

Humidity may be the key to super-lubricity 'switch'

Sometimes friction is good, such as the friction between a road and a car's tires to prevent the vehicle from skidding. But sometimes friction is bad—if you did not put oil in that very same car, there would be so much friction in the bearings of the engine that the car could not operate.

A material state known as super-lubricity, where friction between two contacting surfaces nearly vanishes, is a phenomenon that materials researchers have studied for years due to the potential for reducing the energy cost and wear and tear on devices, two major drawbacks of friction. However, there are times when friction is needed within the same device, and the ability to turn super-lubricity on and off would be a boon for multiple practical engineering applications.

Seong Kim, distinguished professor of chemical engineering and associate head of the Department of Chemical Engineering at Penn State, and Zhe Chen, a researcher with the State Key Laboratory of Fluid Power and Mechatronic Systems and the Department of Mechanical Engineering at Zhejiang University, proposed in a study in Applied Materials Today that this super-lubricity switch may be found in humidity. Specifically, water vapor and vapor in phenol, which is a family of organic compounds.

Microscale superlubricity could pave way for future improved electromechanical devices

Lubricity measures the reduction in mechanical friction and wear by a lubricant. These are the main causes of component failure and energy loss in mechanical and electromechanical systems. For example, one-third of the fuel-based energy in vehicles is expended in overcoming friction. So superlubricity—the state of ultra-low friction and wear—holds great promise for the reduction of frictional wear in mechanical and automatic devices.

A new joint Tel Aviv University/Tsinghua University study finds that robust structural superlubricity can be achieved between dissimilar, microscale-layered materials under high external loads and ambient conditions. The researchers found that microscale interfaces between graphite and hexagonal boron nitride exhibit ultra-low friction and wear. This is an important milestone for future technological applications in space, automotive, electronics and medical industries.

The research is the product of a collaboration between Prof. Oded Hod and Prof. Michael Urbakh of TAU's School of Chemistry; and Prof. Ming Ma and Prof. Quanshui Zheng of Tsinghua University's Department of Mechanical Engineering and their colleagues. It was conducted under the auspices of the joint TAU-Tsinghua collaborative XIN Center and was published in Nature Materials on July 30.

Research team discovers 'rubber material' that could lead to scratch-proof paint for car

Led by Dr Elton Santos from the University's School of Mathematics and Physics, an international team of researchers have found superlubricity in a few layers of graphene - a concept where friction vanishes or very nearly vanishes. The experts also found that a few layers of hexagonal boron nitride (h-BN) are as strong as diamond but are more flexible, cheaper and lighter.

The findings, which have been reported in Nature Communications, reveal that the h-BN layers form the strongest thin insulator available globally and the unique qualities of the material could be used to create flexible and almost unbreakable smart devices, as well as scratch-proof paint for cars.

Dr Santos explains: "We have all at some point in life stepped on a slippery surface where we have to steady our balance so that we don't fall. In most cases, liquid such as water or oil is the cause and this slippery state is what we describe as superlubricity - there is basically no friction on a surface.

"In graphene, this superlubricity state comes from atomic orbitals that compose carbon atoms. Normally, to generate friction some orbitals must overlap and heat, or some energy, must be released. Surprisingly, our research shows that graphene does not require this process, it just spontaneously slides on top of other layers but does not release heat. This means that graphene, which is 300 times stronger than steel, becomes mechanically weaker and can easily break."