Solid lubricant

Also called dry lubricant

Everyday term

In plain words

A powder or that makes surfaces slide over each other easily without oil. Layered crystals such as graphite, MoS₂, WS₂ and hBN are natural lubricants: their layers are strongly bonded inside but only weakly stuck to each other, so they shear apart and coat the rubbing surfaces with slippery sheets. They work where oil cannot – in a vacuum, in space or at high temperatures – and are among the oldest uses of layered materials.

Going deeper

Layered crystals are bonded strongly within each layer and weakly between layers, so their sheets slide off and coat rubbing surfaces – a lubricant that needs no oil and works in vacuum and heat. When two layers are rotated out of register their atoms never lock together, and the friction between them nearly vanishes.

Why layers slide

In graphite, MoS₂ or hBN each layer is held together by strong bonds, while the layers stick to each other only by weak . Rubbed between two surfaces, a layered powder or coating shears between its layers and spreads into a thin film of lying flat, so the surfaces slide on the easy planes of the crystal rather than grinding against each other. Friction coefficients of a few hundredths are typical, a tenth of what dry metal on metal gives.

The details differ. Graphite lubricates well only when water or other molecules are adsorbed between the flakes and fails in vacuum, where MoS₂ is at its best; MoS₂ in turn oxidises slowly in humid air. hBN survives high temperatures in air, which suits it for metal forming and moulds.

Where they are used

Solid lubricants work where oils fail. In space, oils evaporate or creep away, so MoS₂ films coat bearings, gears and hinges of satellites and instruments. At high temperatures oils burn, and in clean environments they contaminate. Layered powders also go into oils and greases as additives, where flakes or nanoparticles of MoS₂ and WS₂ reduce wear under heavy load, and into cosmetics and coatings for their slip. These are among the oldest and largest uses of layered materials, long before anyone isolated a .

Friction that almost disappears

When two crystalline layers lie in register, their atoms nest into each other and sliding means climbing over them. Rotated out of register, the two lattices are : every atom sees a different neighbourhood, the bumps cancel out and friction can almost vanish – structural superlubricity. It was measured in 2004 between a graphite flake and graphite as the flake was rotated: friction was high only near the aligned angles, every 60°, and nearly zero in between. Making it work over large areas and under load is the open challenge.

Atomically thin sheets add a surprise: the friction felt by a tip sliding over graphene or MoS₂ rises as the sheet gets thinner, because a single layer puckers up in front of the tip, unless it is held flat by a strongly bonding .

For specialists

Lamellar solids whose weak interlayer bonding allows easy shear: graphite (which needs adsorbed water or vapour to lubricate well), MoS₂ and WS₂ (best in vacuum and dry gas, oxidising in humid air), and hBN (stable to high temperature in air). Used as burnished or sputtered films, bonded coatings and additives in oils and greases, they reach friction coefficients of a few hundredths, and in space mechanisms MoS₂ is a standard. At the nanoscale, incommensurate contact between layers can make friction nearly vanish – structural superlubricity, seen when graphite flakes are rotated out of register and in MoS₂ – while friction on atomically thin sheets rises as they get thinner because the sheet puckers ahead of the tip.

Where this comes from

Superlubricity of molybdenum disulphide Martin et al. · Physical Review B 48, 10583 (1993)
Superlubricity of graphite Dienwiebel et al. · Physical Review Letters 92, 126101 (2004)
Frictional characteristics of atomically thin sheets Lee et al. · Science 328, 76 (2010)

Composites and coatings

The biggest market for 2D materials so far: flakes of graphene or other layered materials mixed in small amounts into plastics, rubber, concrete, paints or battery electrodes, to make them stiffer, tougher, more conductive or less permeable. Sold by the tonne, these products use cheap flakes made in bulk rather than perfect single layers. Whether a few percent of flakes really helps depends on how well they are spread out, how they line up and how firmly they hold on to the material around them.

Bulk uses in which exfoliated or chemically derived flakes – graphene nanoplatelets, reduced graphene oxide, hBN, MXenes, clays – are dispersed in a matrix or applied as a film. In polymer composites a few weight percent can raise stiffness and toughness, cross the percolation threshold for electrical conduction – as low as about 0.1 percent by volume for thin, wide flakes – and raise thermal conductivity; aligned flakes in barrier coatings lengthen the diffusion path of water, oxygen and ions and slow corrosion; flakes used as conductive additives replace part of the carbon black in battery electrodes; MXene films give strong electromagnetic shielding. Performance depends on flake size and thickness, dispersion without restacking, alignment and bonding to the matrix, and on powders whose properties vary widely between suppliers.

Full explanation

Commensurate and incommensurate

Whether two repeating patterns fit each other. If one pattern repeats after exactly a whole number of steps of the other – every three atoms, say – the two are commensurate, and together they repeat again after a short distance. If the ratio is not a simple fraction, they are incommensurate and never line up exactly. The question comes up whenever a crystal grows a second pattern of its own – a charge density wave, a magnetic spiral – and whenever two different layers are stacked, as in a moiré.

A modulation is commensurate when its wavevector is a rational fraction of a reciprocal lattice vector, so that crystal and modulation share a finite supercell, and incommensurate when the ratio is irrational, leaving no exact common period. The charge density wave of 1T-TaS₂ passes from incommensurate (below about 550 K) to nearly commensurate – commensurate domains separated by discommensurations – and locks into the commensurate √13 × √13 Star-of-David phase below about 180 K; RTe₃ keeps an incommensurate wave, and NiI₂ an incommensurate spin spiral. For two lattices – graphene on hBN, twisted bilayers – the same competition between elastic energy and interlayer adhesion decides whether the layers stretch into commensurate domains or keep their own lattices.

Full explanation

Substrate

The base a thin film or flake sits on – often a polished slice of silicon, glass or sapphire. For a crystal one atom thick, the substrate is not just a table: its bumps, stray charges and vibrations reach right into the sheet and change how it conducts and glows. That is why the same material can behave differently on two substrates, and why the flattest, cleanest one – boron nitride – gives the best results.

The material beneath a 2D layer. It acts on the layer through surface roughness, charged impurities, surface optical phonons, dielectric screening, strain and charge transfer, and during growth it sets orientation through epitaxy. Replacing SiO₂ with hexagonal boron nitride, which is atomically flat and nearly free of dangling bonds and charge traps, improved the mobility and charge homogeneity of graphene by close to an order of magnitude.

Full explanation