A characterization technique used to study the structural and chemical properties of a material.

Hybridization is the combining or mixing up of atomic orbitals (an expected region of electron density around an atom) to form new hybrid orbitals that have geometries suitable to form bonds. Electrons can be found in s, p, d, and f orbitals. When an s orbital combines with three p orbitals, it results in four sp³ hybridized orbitals. Similarly, the combination of an s orbital with two p orbitals gives rise to three sp² hybrid orbitals.

Learn more about hybrid orbitals with these videos from Khan Academy.

The building block of graphite, which is used in pencil tips. Graphene is a one-atom-thick layer of carbon atoms arranged in a hexagonal lattice (with atoms arranged at the corners of a hexagon). The thickness of graphene is a million times less than that of a single human hair. Graphene is the world's first 2D material and the 2010 Nobel Prize in Physics was awarded for its discovery.

 Source: Wikimedia

A crystal is a 3D periodic array of atoms. Materials with regularly ordered arrays of components are termed crystalline materials.

Carbon nanotubes (CNTs) are rolled up cylinders of graphene sheets with diameters in the nanoscale. Based on geometry, they are termed as either single-walled (formed by rolling a single sheet of graphene) or multiwalled (multiple sheets of graphene) carbon nanotubes.

In a crystal, the atoms are arranged in a regular repeated pattern in a 3D lattice. A lattice is defined as the set of points representing these atomic positions.

Fullerene is the zero-dimensional form of graphitic carbon. The carbon atoms in fullerenes are arranged in closed shells.

Optical microscopes are instruments which use visible light and a system of lenses to produce magnified images of small objects.

A covalent bond is formed by the sharing of electrons between atoms. In the case of graphene, each carbon atom forms covalent bonds with three neighboring atoms of hexagons in a plane, with atoms placed in corners of the hexagon. This type of in-plane bonding is called intralayer covalent bonding.

There are some limitations for the present observed near-zero friction sliding. A relatively poor tribological performance is found in humid test environments. The water molecules present at the interface is found to prevent the scrolling of graphene layers. Moreover, the defects present on graphene sheets act as catalytic sites further hampering the near-zero friction sliding in a humid environment.

Quasicrystals are materials with perfect long-range order, but with no 3D translational periodicity of crystals.

The mechanism of superlubricity at the sliding interfaces of materials is studied. Dienwiebel and co-researchers have investigated the superlubricity of graphite using a home-built frictional force microscope. They have also investigated the role of incommensurability in the origin of the ultralow friction of graphite.

The authors have deposited a few drops of graphene in a colloidal liquid state on a SiO₂ substrate to give it a non-uniform coverage of graphene upon evaporating the solution. Simultaneously, they also deposited a solution containing nanodiamonds to get nanodiamond particles on the SiO₂ surface.

The authors have designed and executed experiments to calculate the coefficient of friction. The combination of nanomaterials they used were found to be successful in achieving super-low friction values in a dry environment.

This study reports the tribological properties of graphene-lubricated 440C steel. At medium loads and under dry nitrogen environments, graphene is found to maximize its performance as a solid lubricant.

This study details the first experimental evidence of reproducible superlubricity at the microscale under ambient conditions. The self retraction of graphite mesas upon shearing is described as a direct evidence of ultra-low friction between the incommensurate surfaces. The variation in superlubric conditions with contact area is also described. A similar observation is made in the current study as well.

The authors demonstrate an experimental set up for finding friction between multi-walled nanotube layers using a nanomanipulator and insitu TEM imaging. Ultra-low friction is observed between the core and outer nanotube layers in a dry environment, which is similar to the superlubricity conditions in the present study.

The authors report bridging the gap between macroscale and nanoscale laws of friction. A linear relationship was found to exist between the friction force and contact area at both regimes. In the present study, authors also observed a similar phenomenon.

This study reports measurements of friction as a function of commensurability of the contacting surfaces using ultra-high vacuum scanning tunneling microscopy. Authors have used atomically clean surfaces to experimentally check the superlubric conditions. They found a match between experimental results and theoretical predictions.

Researchers aim at achieving the lowest possible friction and wear in mechanical systems. The area of superlubricity gained momentum in the last few decades following the discovery of novel 2D materials.

Please watch the TED talk describing superlubricity and its potential future applications by the lead author here:

https://www.youtube.com/watch?v=ml1Rj6_W3eY

The mismatch of the lattices at the sliding interfaces prevents interlocking of atoms, resulting in near-zero friction. In the present case, a perfectly incommensurate surface along with reduced contact area benefited from the graphene scroll formation and contributed to superlubricity.

The mechanism of superlubricity in the present system is explained. Authors found that nanodiamonds play a key role in the scroll formation and hence in facilitating the near-zero friction state.

Size plays a major role in superlubricity and many researchers have investigated the dependence of size of materials in friction reduction. Liu and co-workers reported one such size-dependent study of superlubricity on graphite. They have detailed the probability percentages of the superlubric state for different contact areas and also the mechanism of superlubricity (self retraction) of graphite layers.

The failure of moving mechanical assemblies due to friction and wear is a major concern in today's world. Different types of lubricants are employed to minimize friction. Though this helps to improve the lifetime of mechanical systems, researchers are on a quest to find a perfect lubricant to achieve the lowest friction and wear possible with no harmful additives or chemicals.

Research in superlubricity is gaining interest due to its potential applications in mechanical systems to reduce friction- and wear-related failures. Various researchers proposed different materials to achieve near-zero friction. Even though these materials show some promise, most materials are perfectly crystalline (which is not the case in real life) and the effect occurs mostly at the nanoscale regime.

A high-purity graphite material with a high degree of preferred crystallographic orientation.

A perfect crystal is an idealization and in real materials, atom arrangements do not follow perfect crystalline patterns. Crystal defects can be due to missing atoms, introduction of an impurity, broken crystal patterns along fault lines, or the joining of distinct crystal planes.

The authors have designed and performed superlubricity experiments by sliding DLC-coated stainless steel balls against a graphene surface. However, after analyzing the initial test results, they needed to modify the design by also incorporating nanodiamonds into the system. They anticipated that the nanodiamonds can act as nano-ball bearings, thereby enhancing the mechanical strength of graphene and contributing to superlubricity.

The Nature of Science and the Next Generation Science Standards:

NS3-Scientific knowledge is open to revision in light of new evidence.

Diamondlike carbon (DLC) film is a widely researched material to combat friction and wear in a number of applications, owing to its impressive tribological properties and low deposition costs. The tribological properties of DLC films sliding against each other are investigated and they are found to display super-low friction under specific conditions.

Wear is the progressive loss of materials from contacting surfaces relative in motion. The wear process results in the generation of debris—or particles—of various size, shape, color distributions, and chemical composition.

An imaging technique capable of generating high-resolution (nanometer-scale) images of a sample.

The structural analysis of graphene-wrapped nanodiamonds by electron energy-loss spectra (EELS) reported in this study is analogous to the previously reported structure of detonated nanodiamonds by other researchers.

Researchers use supercomputing techniques to replicate complex experimental conditions, in order to envision large-scale systems at the atomic/molecular regions.

To read more details about the simulation techniques used in this study, please visit:

https://www.greencarcongress.com/2015/07/20150722-mira.html

This study compares the nanotribological properties of atomically thin sheets of 2D materials such as graphene, molybdenum disulfide, boron nitride, and niobium diselenide using friction force microscopy. The dependence of friction on the number of atomic sheets and substrate effect are also reported.

This book provides an overview of the theory and practice of tribology, that is, the science of interacting surfaces in motion.

Refers to the components in a friction system. Here, the DLC-coated ball and the graphene-plus-nanodiamonds constitute the tribopair.

A nanomaterial structure with a spiral-wrapped geometry. Think of a scroll, or a roll of paper, on the nanoscale.

Molecular dynamics is a computer simulation method that allows for prediction of the time evolution of a system of interacting particles such as atoms and molecules.

This review outlines the synthesis, characterization, and applications of DLC films. The mechanism of friction and wear behaviors of DLC films and the conditions to achieve superlubricity are reported.

Authors have conducted theoretical simulation to investigate the sliding behavior of an ensemble of graphene sheets to elucidate the macroscale scrolling phenomena. To explore the mesoscopic friction behavior, number density (number of particles per unit volume) of the patches as a function of the coefficient of friction and time is calculated by grouping the friction coefficients collected over the ensemble of graphene patches.

To understand the transition of friction from the nanoscale to the macroscopic superlubric condition observed in experiments, authors have simulated a mesoscopic scenario. They have created and analyzed an ensemble (assembly of systems) of graphene patches and nanodiamonds between the DLC and graphene substrate subjected to sliding friction.

To understand the role of defects in superlubricity, the authors performed computer simulations by introducing double vacancies and Stone-Wales defects on graphene sheets. Studies were conducted in both dry and humid environments.

Upon observing experimentally the effect of humidity on friction conditions, authors have extended their studies. They have performed computer simulations to further analyze the interaction between water molecules and graphene in the DLC-nanodiamond-graphene system in a humid environment.

The application of macroscopic laws of friction to the nanoscale region remained controversial. Mo and his team used molecular dynamics simulations to study friction laws in dry nanoscale contacts. They have investigated the effect of the number of atoms in chemical contact in friction and reported on the relationship between contact area and friction force in the nanoregime.

The minimum energy needed to overcome the attractive forces between atoms at the sliding interface is referred to as energertic barriers.

This force occurs due to the out of plane interaction between adjacent graphene layers.

A general term used to describe the intermolecular forces of attraction between molecules.

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An unsatisfied valence of an immobilized atom.

NGSS-Science and engineering practices

Practice 2: Developing and using models

https://ngss.nsta.org/PracticesFull.aspx

Nitrogen gas with no water vapor.

In order to understand the wear properties of the graphene-nanodiamond compound after the sliding experiments, the authors performed electron microscopy studies which can reveal the structure of the material in the wear debris.

Superlubricity experiments at any sliding interface should follow certain local load and sliding velocity conditions. Thermal effects are also dominant in frictional energy dissipation during sliding. Authors have tested a wide range of experimental conditions and shown that their system exhibits stable superlubricity under the tested conditions.

Raman spectroscopy is a chemical analysis technique capable of probing the chemical structure, crystallinity, and molecular interactions of materials.

Refers to the material's resistance to failure by fracture or deformation when a load is applied.

Materials with perfect atomic structures and long periodicity are traditionally sought for superlubricity. Carbon compounds such as carbon nanotubes, graphite, and graphene are investigated for this purpose. One concern is the difficulty in attaining sustainable superlubricity at the engineering scale.

Misaligned or misfit.

Next Generation Science Standards Disciplinary Core Ideas:

ESS3.D Global Climate Change

Though the magnitudes of human impacts are greater than they have ever been, so too are human abilities to model, predict, and manage current and future impacts.

Relating to the large-scale, likely visible to the human eye.

Refers to dimensions less than 100 nanometers (where one nanometer is one billionth of a meter).

This region is an intermediate between atomic and macroscopic scales.

The ratio of the frictional resistance force to the normal force. When two objects are in contact, the force which presses the surfaces together is called normal force.

A carbon material that exhibits many of the desirable properties of the diamond material.

The state of near-zero friction at the sliding interfaces of two contacting solid surfaces is called superlubricity. In superlubric regimes, the extent of physical and/or chemical interactions is extremely small and hence the surfaces can slide over one another without causing much friction.

AP Physics 1

Dynamics: Essential Knowledge 3.C.4

Contact forces result from the interaction of one object touching another object and they arise from interatomic electrical forces. These forces include tension, friction, normal, spring, and buoyant. (See page 23)

The authors defined contact area as the area of graphene atoms which are in the range of chemical interactions from the DLC tip atoms. The normalized contact area is defined as the contact area at any time (t) with respect to the initial contact area at time t=0 (when the graphene patches are fully expanded).

This study investigates theoretically, the dependence of superlubricity on interlayer distance between graphene sheets and atomic defects on graphene for both commensurate and incommensurate configurations.

This earlier study by the current authors identifies and reports the potential of graphene layers in reducing friction and wear at the tribological interface of steel in air.

Common Core State Standards English Language Arts-Literacy

RST.11-12.8

Evaluate the hypotheses, data, analysis, and conclusions in a science or technical text, verifying the data when possible and corroborating or challenging conclusions with other sources of information.

The mechanism of friction reduction at the macroscale is found to be the same as that at the nanoscale. At the macroscale, nanoscrolls at different orientations are observed while an assembly of graphene patches at different initial configurations slide over the nanodiamonds. Over time, this reduced contact area leads to ultralow friction sliding at the large scale.

In order to elucidate the mechanism of graphene nanoscroll formation and the origin of superlubric state, the authors have conducted computer simulation studies.

To investigate the effect of environmental conditions on nanoscale friction and superlubricity, the authors have conducted experiments in humid air in place of dry nitrogen.

An anisotropic crystal is one which has different properties in different orientations.

The smallest building block of a crystal is called unit cell. The repetition of these identical structural units in space constitute the crystal lattice.

A vacancy defect wherein two of the atoms are missing from the lattice structure is termed as double vacancy. Sometimes, dangling bonds arising from the missing of atoms in the hexagonal structure of graphene leads to its straining and formation of pentagons and heptagons. These types of defects are called Stone-Wales defects.

In a crystal, the atoms are arranged in a regular order and this property of the crystal is called its periodicity.

Diamond particles in the nanoscale dimensions are called nanodiamonds.