Atomic scale mechanisms of friction reduction and wear protection by graphene (original) (raw)
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Nano-scale friction and wear of polymer coated with graphene
Background: Friction and wear of polymers at the nano scale is a challenging problem due to the complex viscoelastic properties and structure. Using molecular-dynamics simulations, we investigate how a graphene sheet on top of a semicrystalline polymer (PVA) affects the friction and wear.Results: Our setup is meant to resemble an AFM experiment with a silicon tip. We have used two different graphene sheets: an unstrained, flat sheet, and one that has been crumpled before being deposited on the polymer.Conclusion: The graphene protects the top layer of the polymer from wear and reduces the friction. The unstrained flat graphene is stiffer, and we find that it constrains the polymer chains and reduces the indentation depth.
Nanoscale friction and wear of a polymer coated with graphene
Beilstein Journal of Nanotechnology
Friction and wear of polymers at the nanoscale is a challenging problem due to the complex viscoelastic properties and structure. Using molecular dynamics simulations, we investigate how a graphene sheet on top of the semicrystalline polymer polyvinyl alcohol affects the friction and wear. Our setup is meant to resemble an AFM experiment with a silicon tip. We have used two different graphene sheets, namely an unstrained, flat sheet, and one that has been crumpled before being deposited on the polymer. The graphene protects the top layer of the polymer from wear and reduces the friction. The unstrained flat graphene is stiffer, and we find that it constrains the polymer chains and reduces the indentation depth.
Effect of surface morphology on friction of graphene on various substrates
Nanoscale, 2013
The friction of graphene on various substrates, such as SiO 2 , h-BN, bulk-like graphene, and mica, was investigated to characterize the adhesion level between graphene and the underlying surface. The friction of graphene on SiO 2 decreased with increasing thickness and converged around the pentalayers due to incomplete contact between the two surfaces. However, the friction of graphene on an atomically flat substrate, such as h-BN or bulk-like graphene, was low and comparable to that of bulklike graphene. In contrast, the friction of graphene folded onto bulk-like graphene was indistinguishable from that of mono-layer graphene on SiO 2 despite the ultra-smoothness of bulk-like graphene. The characterization of the graphene's roughness before and after folding showed that the corrugation of graphene induced by SiO 2 morphology was preserved even after it was folded onto an atomically flat substrate. In addition, graphene deposited on mica, when folded, preserved the same corrugation level as before the folding event. Our friction measurements revealed that graphene, once exfoliated from the bulk crystal, tends to maintain its corrugation level even after it is folded onto an atomically flat substrate and that ultra-flatness in both graphene and the substrate is required to achieve the intimate contact necessary for strong adhesion.
Friction and Adhesion of Different Structural Defects of Graphene
ACS Applied Materials & Interfaces, 2018
Graphene structural defects, namely edges, step-edges and wrinkles are susceptible to severe mechanical deformation and stresses under frictional operations. Applied forces cause deformation by folding, buckling, bending and tearing the defective sites of graphene, which lead to a remarkable decline in normal load and friction bearing capacity. In this work, we experimentally quantified the maximal normal and friction forces corresponding to the damage thresholds of the different investigated defects as well as their pull-out (adhesion) forces. Horizontal wrinkles (with respect to the basal plane, i.e. folded) sustained the highest normal load, up to 317 nN, during sliding, whereas for vertical (i.e. standing collapsed) wrinkles, step-edges and edges, the load bearing capacities are up to 113 nN, 74 nN and 63±5 nN, respectively. The related deformation mechanisms were also experimentally investigated by varying the normal load up to the initiation of the damage from the investigated defects and extended with the numerical results from Molecular Dynamics and Finite Element Method simulations.
Extraordinary Macroscale Wear Resistance of One Atom Thick Graphene Layer
Advanced Functional Materials, 2014
During the last few years, graphene's unusual friction and wear properties have been demonstrated at nano to micro scales but its industrial tribological potential has not been fully realized. The macroscopic wear resistance of one atom thick graphene coating is reported by subjecting it to pin-on-disc type wear testing against most commonly used steel against steel tribo-pair. It is shown that when tested in hydrogen, a single layer of graphene on steel can last for 6400 sliding cycles, while few-layer graphene (3-4 layers) lasts for 47 000 cycles. Furthermore, these graphene layers are shown to completely cease wear despite the severe sliding conditions including high contact pressures (≈0.5 GPa) observed typically in macroscale wear tests. The computational simulations show that the extraordinary wear performance originates from hydrogen passivation of the dangling bonds in a ruptured graphene, leading to signifi cant stability and longer lifetime of the graphene protection layer. Also, the electronic properties of these graphene sheets are theoretically evaluated and the improved wear resistance is demonstrated to preserve the electronic properties of graphene and to have signifi cant potential for fl exible electronics. The fi ndings demonstrate that tuning the atomistic scale chemical interactions holds the promise of realizing extraordinary tribological properties of monolayer graphene coatings.
Nanoscale friction properties of graphene and graphene oxide
Diamond and Related Materials, 2014
Achieving superlow friction and wear at the micro/nano-scales through the uses of solid and liquid lubricants may allow superior performance and long-lasting operations in a range of micromechanical system including micro-electro mechanical systems (MEMS). Previous studies have indicated that conventional solid lubricants such as highly ordered pyrolitic graphite (HOPG) can only afford low friction in humid environments at micro/macro scales; however, HOPG is not suitable for practical micro-scale applications. In this study, we explored the nano-scale frictional properties of multi-layered graphene films as a potential solid lubricant for such applications. Atomic force microscopy (AFM) measurements have revealed that for high-purity multilayered graphene (7-9 layers), the friction force is significantly lower than what can be achieved by the use of HOPG, regardless of the counterpart AFM tip material. We have demonstrated that the quality and purity of multilayered graphene plays an important role in reducing lateral forces, while oxidation of graphene results in dramatically increased friction values. Also, for the first time, we demonstrated the possibility of achieving ultralow friction for CVD grown single layer graphene on silicon dioxide. This confirms that the deposition process insures a stronger adhesion to substrate and hence enables superior tribological performance than the previously reported mechanical exfoliation processes. 1. Introduction.
Effect of elastic deformation on frictional properties of few-layer graphene
Physical Review B, 2012
We describe the results of Brownian dynamics (BD) simulations of an atomic force microscope (AFM) tip scanned on locally suspended few-layer graphene. The effects of surface compliance and sample relaxation are directly related to the observed friction force. We demonstrate that the experimentally observed reduction of friction with an increasing number of graphene layers in case of a narrow scanning tip can be a result of decreased sample deformation energy due to increased local contact stiffness under the scanning tip. Simulations with varying scan rates indicate that surface relaxation at a given temperature can affect the frictional characteristics of atomically thin sheets in a manner not explained by conventional thermally activated models.
Advanced Materials Interfaces, 2015
The ultrathin fi lm of solid lubricants, which can reduce the friction and extend the life of the underlying surface, shows immense potential for M/NEMS applications. Graphene, a 2D honeycomb lattice of sp 2 hybrid carbons, has attracted a large amount of interest for fundamental studies to application perspective because of its remarkable electronic, optical, thermal, and mechanical properties. [ 4-6 ] Recently, graphene has been established as an ultrathin solid lubricant exhibiting notable friction-reducing and antiwear properties. [ 7,8 ] The nano-and microscale tribological properties of single/few-layer graphene on solid substrates and chemically derived graphene nanosheets as an additive to liquid lubricants revealed the potential of graphene for lubricant applications. [ 7-13 ] Friction force microscopy and simulation studies demonstrated that friction, adhesion and wear characteristics are controlled by the adhesion between the graphene and the underlying substrate, the interaction between the atomic force microscope (AFM) tip and the graphene, the roughness and presence of wrinkles/folds in the graphene, the number of lamellae in the graphene thin fi lm, the sliding behavior/ direction, the morphology and mechanical properties of the underlying substrate, and the presence of defects and chemical functionalities in the graphene thin fi lm, etc. [ 7,14-21 ] The ultrasmooth morphology of graphene and underlying substrate is very crucial to achieving conformal contact for low friction. [ 14 ] Sumant and co-workers demonstrated the wear-resistivity of graphene for a steel tribo-pair under hydrogen atmosphere. At a contact pressure of ≈0.5 GPa, a single layer of graphene showed wear-resistivity for 6400 cycles, while few-layer graphene could last for 47 000 cycles under the hydrogen atmosphere. The extraordinary antiwear properties of graphene was attributed to the stabilization of dangling bonds in the ruptured graphene by hydrogen. [ 22 ] Considering the remarkable friction-reducing and antiwear properties, graphene can be a long-lasting solution to M/ NEMS applications. However, the deposition of graphene thin fi lm on the silicon substrate by epitaxial growth, chemical Few-layer graphene oxide (GO) is assembled on the silicon surface by a self-assembly approach via covalent interaction using 3-aminopropyltrimethoxysilane (APTMS) as a bifunctional chemical linker. X-ray photoelectron spectroscopy results suggest chemical interactions between oxygen functionalities of GO and amino group of APTMS thin fi lm. The oxygen functionalities of GO thin fi lm are eliminated by vacuum ultraviolet (VUV) photon exposure. Topographic images reveal effi cient grafting of GO on the silicon and suggest the presence of few layers in the GO thin fi lm along with wrinkles and folds. Microtribological properties of VUV-reduced GO (rGO) thin fi lm are probed under the mean contact pressure of 0.3-0.6 GPa. The rGO thin fi lm exhibits low and steady friction (0.12-0.15) compared to that of bare silicon (0.6). The rGO thin fi lm could survive for ≈37 000 laps at 100 mN load, revealing its remarkable wear-resistivity. Microscopic images and carbon mapping reveal the deposition of delaminated graphene lamellae on the counter steel ball surface. The low friction and excellent wear-resistivity of rGO thin fi lm are collectively attributed to low-resistance to shear between the neighboring lamellae of rGO, full coverage and strong interaction of rGO thin fi lm with silicon, and deposition of delaminated graphene lamellae on the counter steel surface.