Superlubricity Enabled by Load‐Driven Redistribution of Electrons

Abstract

How to reduce friction is a key issue of tribology and plays an important role in solving a number of problems of physics and engineering, e.g., the durability of the nano-friction generator. [2] The well-known and widely used Amontons-Coulomb friction law stipulates that the tangential frictional force is proportional to the normal compressive force. [3] With the main advantage of being simple, this phenomenological macroscopic law fails to be applicable in the field of nano-tribology. [4] Indeed, nano-friction is affected by a number of factors such as the deformations of the components in contact and the degree of atomic mismatches at the contacting interfaces. [5,6] In recent years, a promising fundamental approach to quantitatively understanding micro-or nano-friction consists in determining the redistribution of electrons due to pressure and sliding. [7][8][9] This approach has been successfully used to reveal the relationship between adsorption and static friction and the one between in-plane strain and friction. [10,11] But, to the best of our knowledge, it has not yet been elaborated and applied to establish a direct and quantitative relationship between friction and electrons redistribution driven by the normal load.Some pioneering works have shown that quasi-zero friction or superlubricity is feasible. [12][13][14][15] At present, there are two main strategies for achieving frictionless sliding between both solids in contact: structural superlubricity (SSL) and pressuredriven superlubricity. [12][13][14] SSL occurs only if two contacting surfaces that slide one on the other are incommensurable. [16] It was theoretically predicted and shown to be achievable for microscopic phenomena and even for engineering design. [14,17] Nevertheless, it is very difficult to prepare and keep both ultra-clean surfaces in incommensurate contact. [18] Thus, it is urgent to seek an alternative solution to avoid the harsh incommensurate contact required for SSL. Fortunately, a few works have indicated that the external load may regulate friction to superlubricity even in a commensurate contact. [12,19,20] Initially, pressure-and tension-actuated frictionless sliding was expected to be realized at a gas-solid interface (rare gas layers and metal surfaces), [12,19] but the structural instability of rare gas layer limited further experimental explorations. Recently, the pressure-induced friction collapse rekindles hope for load-driven superlubricity at 2D solid-solid interfaces. [13,20] However, the exploration of load-driven superlubricity has fallen into another dilemma: the critical load of ≈300 GPa required for superlubricity is so demanding that it is experimentally unrealistic. [13] By first-principles calculations, it is shown that the friction at solid-solid interfaces between 2D nanomaterials (TDNMs), such as h-BN and graphene, can be reduced nearly to zero even if the normal load is smaller than 5 GPa. The quantitative analysis of interfacial charge density demonstrates a detailed process in which the pressure-dr...

Long-Term Stable Superlubricity Coatings Enabled by the Interaction between the Polydimethylsiloxane Brush and Silicone Oil

“…5,6 In recent decades, the realm of exploring superlubricity in both solids and liquids has garnered significant interest in the fields of fundamental science and engineering, thus, witnessing significant strides. For solid lubrication materials, such as MoS 2 , 7 graphene, 8 hexagonal boron nitride, 9 and diamond-like amorphous carbon coatings, 10 the realization of superlubrication required some strict conditions such as the incommensurate contact state, the protection of inert gases, or an ultrahigh vacuum condition, which limited its application. 11−13 To date, liquid superlubrication systems can be divided into water-and acid-based solutions, hydrated materials, ionic liquids, 2D materials as lubricant additives, and oil-based lubrication materials.…”

“…Since it can decrease resistance and save energy, materials with superlubricity are of tremendous importance from the perspective of saving energy and protecting the environment. , In recent decades, the realm of exploring superlubricity in both solids and liquids has garnered significant interest in the fields of fundamental science and engineering, thus, witnessing significant strides. For solid lubrication materials, such as MoS 2 , graphene, hexagonal boron nitride, and diamond-like amorphous carbon coatings, the realization of superlubrication required some strict conditions such as the incommensurate contact state, the protection of inert gases, or an ultrahigh vacuum condition, which limited its application. – …”

Strain‐Driven Superlubricity of Graphene/Graphene in Commensurate Contact

“…Friction and wear are two main modes of energy dissipation and component failure in various mechanical systems, [ 1 ] especially in micro‐nano devices under strong confinement and extreme external conditions where liquid lubricants fail. [ 2 ] Superlubricity, [ 3,4 ] a slipping state with ultra‐low friction, which is desirable for various applications that involve the sliding contact between two components, [ 5 ] is thus paramount to tribology and surface science. To achieve superlubricity for two solids in sliding contact, the ratio between the lateral and normal stiffnesses [ 6,7 ] is optimized, an external field actuation is applied, [ 8 ] or the contact is rendered and kept incommensurate, [ 9 ] so as to eliminate the potential barriers.…”

Experimental Decoding and Tuning Electronic Friction of Si Nanotip Sliding on Graphene

“…Theoretically, the electric-field-controlled charge depletion or accumulation of semiconductor and metallic materials would fundamentally lead to the change in charge density distribution across the device surface. Recent researches have revealed a strong correlation between charge redistribution and interfacial adhesion and friction. − Cheng et al and Song et al have proposed quantitative models that describe the impact of load/electric-field-induced charge redistribution and current-induced electron transfer on the sliding barrier, respectively. Sun et al have utilized density functional theory (DFT) calculations to establish a universal connection between sliding energy landscapes and the evolution of charge redistribution along sliding pathways.…”

“…To uncover the physical mechanism underlying the relationship between electric properties and friction, we further calculated the charge density redistribution ( ρ diff ( x , y , z )) between Si(111) and graphene/ h -BN during sliding. Given that friction essentially arises from electromagnetic interaction, and charge density redistribution is closely associated with interfacial friction. ,− It is observed that the ρ diff ( x , y , z ) between Si(111) and graphene/ h -BN changes significantly upon applied electric field, companying with the transfer of charge from the graphene into Si(111) layer (Figure S8a). To quantify this effect of electric field on the charge density distribution, the evolution of the charge density redistribution (Δ ρ diff ) was assessed using the method proposed by Sun et al (see Section 10 of the SI for details), who has reported a linear relationship (Δ E = k Δ ρ diff ) between Δ ρ diff and Δ E on the van der Waals (vdW) joints (e.g., MoS 2 /MoS 2 , graphene/graphene), metallic joint (Cu/Cu), ionic joint (NaCl/NaCl), and covalent joint systems without external electric field.…”