Yuan-Zhong Hu

Vanishing stick–slip friction in few-layer graphenes: the thickness effect

We report the thickness dependence of intrinsic friction in few-layer graphenes, adopting molecular dynamics simulations. The friction force drops dramatically with decreasing number of layers and finally approaches zero with two or three layers. The results, which are robust over a wide range of temperature, shear velocity, and pressure are quantitatively explained by a theoretical model with regard to lateral stiffness, slip length, and maximum lateral force, which could provide a new conceptual framework for understanding stick-slip friction. The results reveal the crucial role of the dimensional effect in nanoscale friction, and could be helpful in the design of graphene-based nanodevices.

A Computer Thermal Model of Mixed Lubrication in Point Contacts

This paper presents a transient thermal model for mixed lubrication problems in point contacts. The model deterministically calculates pressure and surface temperature by simultaneously solving a system of equations that govern the lubrication, contact and thermal behaviors of a point contact interface. The pressure distribution on the entire computation domain is obtained through solving a unified Reynolds equation system without identifying hydrodynamic or asperity contact regions. The point heat source integration method is applied to determine the temperature distributions on contact surfaces. The interactions between pressure and temperature are considered through incorporating viscosity-temperature and density-temperature relations in the Reynolds equation, then solving the equation system iteratively. With the successful implementation of an FFT-based algorithm (DC-FFT) for calculation of surface deformation and temperature rise, the numerical analysis of lubricated contact problems, which used to involve a great deal of computation, can be performed in acceptable time. The model enables us to simulate various lubrication conditions, from full film elastohydrodynamic lubrication (EHL) to boundary lubrication, for a better understanding of the effect of surface roughness. Numerical examples are analyzed and the results show that the present model can be used to predict pressure and surface temperature over a wide range of lubrication conditions, and that the solution methods are computationally efficient and robust.

Partial Slip Contact Analysis on Three-Dimensional Elastic Layered Half Space

An elastic contact model for three-dimensional layered or coated materials under coupled normal and tangential loads, with consideration of partial slip effects, has been developed in this paper. The response functions for calculating the displacements and stresses were determined in the frequency domain by using the Papkovich–Neuber potentials. The partial slip contact problem was solved by a numerical procedure based on the conjugate Gradient method and fast Fourier transform technique. The contact pressure, surface shear tractions, stick ratios, rigid body displacements, and subsurface stresses are analyzed under different conditions with variations in the material properties and coating thickness. Results show that stiffer coatings tend to decrease the stick ratios and the rigid ball tangential displacements in comparison to those with compliant coatings under the same contact conditions. For stiffer coatings, the values of the von Mises stress and compressive surface stress increase and the positions of maximum von Mises stress move up to the surface; meanwhile, the distributions of the compressive stress become asymmetric due to the action of the tangential load. DOI: 10.1115/1.4001011

A shear localization mechanism for lubricity of amorphous carbon materials

Amorphous carbon is one of the most lubricious materials known, but the mechanism is not well understood. It is counterintuitive that such a strong covalent solid could exhibit exceptional lubricity. A prevailing view is that lubricity of amorphous carbon results from chemical passivation of dangling bonds on surfaces. Here we show instead that lubricity arises from shear induced strain localization, which, instead of homogeneous deformation, dominates the shearing process. Shear localization is characterized by covalent bond reorientation, phase transformation and structural ordering preferentially in a localized region, namely tribolayer, resulting in shear weakening. We further demonstrate an anomalous pressure induced transition from stick-slip friction to continuous sliding with ultralow friction, due to gradual clustering and layering of graphitic sheets in the tribolayer. The proposed shear localization mechanism sheds light on the mechanism of superlubricity, and would enrich our understanding of lubrication mechanism of a wide variety of amorphous materials.

Simulations and Measurements of Sliding Friction Between Rough Surfaces in Point Contacts: From EHL to Boundary Lubrication

ing surfaces with 3D roughness in point contacts. The numerical approach is developed on the basis of the deterministic solutions of mixed lubrication, which is able to predict the locations where the asperity contacts occur, and the pressure distribution over both lubrication and contact areas. If the friction coefficients over the contacting asperities have been determined, total friction force between the surfaces can be calculated by summing up the two components, i.e., the boundary friction contributed by contacting asperities and the shear stress in hydrodynamic regions. The frictions from asperity contact were determined in terms of a limiting shear stress or shear strength of boundary films while the fluid shear stress in the lubrication areas was calculated using different rheology models for the lubricant, in order to find which one would be more reliable in predicting fluid tractions. The simulations covered the entire lubrication, regime, including full-film Elastohydrodynamic Lubrication (EHL), mixed lubrication, and boundary lubrication. The results, when being plotted as a function of sliding velocity, give a Stribeck-type friction curve. This provides an opportunity to study friction change during the transition of lubrication conditions and to compare friction performance on different rough surfaces, which is of great value in engineering practice. Experiments were conducted on a commercial test device—universal material tester (UMT) to measure friction at a fixed load but different sliding velocities in reciprocal or rotary motions. The results also give rise to the Stribeck friction curves for different rough surfaces, which are to be compared with the results from simulations. The samples were prepared with typical machined surfaces in different roughness heights and textures, and in point contacts with steel ball. Results show that there is a general agreement between the experiments and simulations. It is found that surface features, such as roughness amplitude and patterns, may have a significant effect on the critical speed of transition from hydrodynamic to mixed lubrication. In the regime of mixed lubrication, rougher samples would give rise to a higher friction if the operation conditions are the same. DOI: 10.1115/1.2736432

Superlubricity of two-dimensional fluorographene/MoS2 heterostructure: a first-principles study

The atomic-scale friction of the fluorographene (FG)/MoS2 heterostructure is investigated using first-principles calculations. Due to the intrinsic lattice mismatch and formation of periodic Moiré patterns, the potential energy surface of the FG/MoS2 heterostructure is ultrasmooth and the interlayer shear strength is reduced by nearly two orders of magnitude, compared with both FG/FG and MoS2/MoS2 bilayers, entering the superlubricity regime. The size dependency of superlubricity is revealed as being based on the relationship between the emergence of Moiré patterns and the lattice mismatch ratio for heterostructures.

Effect of impact angle and substrate roughness on growth of diamondlike carbon films

Molecular dynamics simulations are performed to study the growth of diamondlike carbon films. The effect of impact angles on deposited film structures is quantitatively studied, the result of which shows that the transverse migration of incident atoms facilitates the film relaxation. Atomic-scale behaviors of the incident atoms are analyzed to give a clear picture of the phenomenon, through which a model concerning the transverse-migration-induced film relaxation is brought forward to elucidate the process of film relaxation. The effects of surface roughness of the substrate on the film growth process are also investigated. The evolution of microstructure and surface morphology of the film exhibits different characteristics in different stages of the deposition process. In the initial stage, the film shows a preferred growth at the valley, which results in smoothening of the film. In the later stage, the film shows a homogeneous growth mode. The film smoothening is attributed to the transverse migration o...

Tribochemical Mechanism of Amorphous Silica Asperities in Aqueous Environment: A Reactive Molecular Dynamics Study

Reactive molecular dynamics (ReaxFF) simulations are used to explore the atomic-level tribochemical mechanism of amorphous silica (a-SiO2) in a nanoscale, single-asperity contact in an aqueous environment. These sliding simulations are performed in both a phosphoric acid solution and in pure water under different normal pressures. The results show that tribochemical processes have profound consequences on tribological performance. Water molecules could help avoid direct adhesive interaction between a-SiO2 surfaces in pure water under low normal load. However, formation and rupture of interfacial siloxane bonds are obviously observed under higher normal load. In phosphoric acid solution, polymerization of phosphoric acid molecules occurs, yielding oligomers under lower load, and tribochemical reactions between the molecules and the sliding surfaces could enhance wear under higher load. The bridging oxygen atoms in silica play an important role in the formation of interfacial covalent bonds, and hydrogen is found to have a weakening effect on these bonds, resulting in the rupture during shear-related loading. This work sheds light on tribochemical reactions as a mechanism for lubrication and wear in water-based or other tribological systems.

Formation of linear carbon chains during the initial stage of nanostructured carbon film growth

The initial stage of nanostructured carbon film growth is investigated by molecular dynamics simulations. The carbon film exhibits amorphous structures with linear chains and cyclic rings on the surface at low incident energies. The structural transformations from linear chains to cyclic rings and to atom networks are observed during the growth process, which is explained in terms of system stability. The atomic adsorption behavior is analyzed through the calculation of the surface potential field. The formation of linear chain structure is due to the predominance of inhomogeneous adsorption of incident atoms on the surface and preferential growth at the tip of the chain. The formation of nanostructures on the surface is argued to be the initial nucleation process of amorphous carbon films.The initial stage of nanostructured carbon film growth is investigated by molecular dynamics simulations. The carbon film exhibits amorphous structures with linear chains and cyclic rings on the surface at low incident energies. The structural transformations from linear chains to cyclic rings and to atom networks are observed during the growth process, which is explained in terms of system stability. The atomic adsorption behavior is analyzed through the calculation of the surface potential field. The formation of linear chain structure is due to the predominance of inhomogeneous adsorption of incident atoms on the surface and preferential growth at the tip of the chain. The formation of nanostructures on the surface is argued to be the initial nucleation process of amorphous carbon films.

Effects of Surface Roughness on Sliding Friction in Lubricated-Point Contacts: Experimental and Numerical Studies

The objective of the present work is to investigate experimentally and numerically the influences of surface roughness, produced by typical machining processes, on friction performances in lubricated-point contacts. Prior to the full experimental investigation, a series of tests had been conducted to examine the experimental errors, resulting from repeated tests on the same specimen but at different tracks, with different amounts of lubricant supply, or after the sample reinstallation. Then, the effects of amplitude and texture of surface roughness on friction behavior are investigated in rotational and reciprocal-mode tests, respectively. The measured friction, averaged over the repeated tests and plotted as a function of sliding speed, shows Stribeck-type curves, which manifest the transition from full-film, mixed, to boundary lubrication. Results show that the roughness amplitude imposes a strong influence on the magnificence of friction and the route of lubrication transition. It is also observed that transverse roughness would give rise to a smaller friction coefficient than the longitudinal one under the same operating conditions. Moreover, the deterministic numerical solution of mixed lubrication has been extended to evaluate friction between rough surfaces over a wide range of lubrication regimes. The numerical simulation results are compared and agree very well with experiments. DOI: 10.1115/1.2768081