Kinetics and dynamics of frictional stick-slip in mesoscopic boundary lubrication

The kinetics and dynamics of frictional stick-slip motion of a slider of size extending from mesoscopic upward is analyzed within the framework of a multi-contact, earthquake-like model. The microscopic contacts are characterized by a distribution of static thresholds for individual breaking. The condition for an overall elastic instability leading to stick-slip sliding are derived and details of the slip motion are studied theoretically. The crucial model parameters emerging from this analysis include the delay time for each micro-contact to reform after breaking, the strength of elastic interaction between the contacts, the elasticity of contacts and of the slider, and the distribution of static thresholds for their breaking. The dynamics is also studied with the help of a scaling procedure. As a prototype application, we adopt parameters appropriate to describe recent surface force apparatus (SFA) boundary lubrication experiments. Despite suggestions of extremely large lubricant viscosities, the experimental data are shown to be fully compatible with ordinary, bulk-like viscosity values once the multi-contact aspects are taken into account.     

Application of Two-Dimensional Frenkel-Kontorova Model to Nanotribology

The dry friction force between two contacting surface layers is studied. The upper layer is arranged on a two-dimensional square lattice and driven by an external driving force. The lower layer is approximated by a two-dimensional periodic substrate potential. This model, usually called two-dimensional Frenkel-Kontorova model, is applied to study the friction forces in this paper. The behaviors of different substrate potential strongly affect the static frictionforce. It is found that the system has strong anisotropic character. The possibility to obtain superlubricity is suggested.     

Friction and normal forces of model friction modifier additives in simulations of boundary lubrication

ABSTRACT

Interaction forces between solid surfaces are often mitigated by adsorbed molecules that control normal and friction forces at nanoscale separations. Molecular dynamics simulations were conducted of opposing semi-ordered monolayers of united-atom chains on sliding surfaces to relate friction and normal forces to imposed sliding velocity and inter-surface separation. Practical examples include adsorbed friction-modifier molecules in automatic transmission fluids. Friction scenarios in the simulations had zero, one, or two fluid layers trapped between adsorbed monolayers. Sliding friction forces increased with sliding velocity at each stable separation. Lower normal forces were obtained than in most previous nanotribology molecular simulations and were relatively independent of sliding speed. Distinguishing average frictional force from its fluctuations showed the importance of system size. Uniform velocities were obtained in the sliding direction across each adsorbed film, with a gradient across the gap containing trapped fluid. The calculated friction stress was consistent with measurements reported using a surface forces apparatus, indicating that drag between an adsorbed layer and trapped fluid can account sufficiently for sliding friction in friction modifier systems. An example is shown in which changes in molecular organisation parallel to the surface led to a large change in normal force but no change in friction force.     

Atomistic simulations of the lubricative mechanism of a nano-alkane lubricating film between two layers of Cu-Zn alloy

We describe simulations of lubrication by a hexadecane molecular lubricating film during the shearing process of a Cu-Zn alloy performed using the atomistic method. The results indicate that with increasing Zn contents, the interface slip between the alloy wall and the lubricating film first decreases and then increases, according to variations of the radius distribution function (RDF), while the interface slip reaches its lowest value of 0.12 during the shearing of CuZn30 alloy. We also discuss the relationship between interface roughness and the lubricating film. During film lubrication, the interface's roughness effectively inhibits interfacial slip. For the convex contact model, the presence of the hexadecane lubricating film reduces the interfacial contact pressure from 11.9 GPa to 8.7 GPa and the friction coefficient from 0.81 to 0.52.     

Nanotribology of Mo–Se–C films

Transition metal dichalcogenides with a layered structure are well known for their self-lubricating properties, particularly in a vacuum or dry atmosphere. The macrotribological properties of these films have been studied extensively. However, the tribological behaviour of these films in the nanonewton load range has hardly been reported. Study of tribological properties with load in the nanonewton range is required for applications related to microelectromechanical systems or nanoelectromechanical systems. In view of the above, the hardness, surface force, friction force, etc. of Mo–Se–C films were investigated at an applied load in the nanonewton range using a nanoindenter and atomic force microscopy. The effect of carbon content, applied load and scanning speed on the friction coefficient was determined. Data pertaining to topography, lateral force and pull-off force of various surfaces are illustrated. The observed nanotribological behaviour of these films is analysed in the light of their nanohardness. The results indicate that the friction force of all the films is very low and in general dependent on surface force. However, a film having the highest carbon content exhibits the maximum friction force. With increasing carbon content of the films tested, the hardness increases and wear decreases. The above results pertain to investigations under ambient conditions.     

Anisotropic character of atoms in a two-dimensional Frenkel–Kontorova model

The dynamics of a certain density of interacting atoms arranged on a two-dimensional square lattice, which is made to slide over a two-dimensional periodic substrate potential with also the quare lattice symmetry, in the presence of dissipation, by an externally applied driving force, is studied. By rotating the misfit angle θ, the dynamical behaviour displays two different tribological regimes: one is smooth, the other becomes intermittent. We comment both on the nature of the atomic dynamics in the locked-to-sliding transition, and on the dynamical states displayed during the atom motion at different values of the driving force. In tribological applications, we also investigate how the main model parameters such as the stiffness strength and the magnitude of the adhesive force affect the static friction of the system. In particular, our simulation indicates that the superlubricity will appear.     

Nanomechanical and nanotribological characterization of multilayer self-lubricating Ti/MoS2/Si/MoS2 nanocoating on aluminium-silicon substrate

Ti/MoS₂/Si/MoS₂ multilayer coating was fabricated by a pulse laser deposition method from a titanium, molybdenum disulphide, and silicon targets, and the coating was deposited in layers on aluminium-silicon substrates, at room temperature. The structural analysis and surface morphology of multilayer Ti/MoS₂/Si/MoS₂ coating were analysed using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy integrated with energy dispersive X-ray spectroscopy. Nanomechanical tests were performed on Ti/MoS₂/Si/MoS₂ coating at small loads of 2000 to 6000 μN to study the effect of load on hardness and Young's modulus. Nanoscratch and nanowear tests were conducted on Ti/MoS₂/Si/MoS₂ coating at a low load of 1000 to 5000 μN and 100 to 500 μN, respectively, to study deformation and failure behaviours of coating/substrate system and also their nanotribological properties. The results show that hardness and Young's modulus of Ti/MoS₂/Si/MoS₂ coating decrease with increase in load. A low friction coefficient of 0.09 to 0.16 was observed, which proves that the Ti/MoS₂/Si/MoS₂ coating possesses self-lubricating property. The wear rate of Ti/MoS₂/Si/MoS₂ coating increases 3.3 × 10⁻¹⁰ to 7.8 × 10⁻¹⁰ mm³/Nm with increase in load. Ti/MoS₂/Si/MoS₂ multilayer coating shows a smooth wear track with no cracks and debris on the surface, which attributed plastic flow of softer coating material around the wear track.     

机械剥离折叠石墨烯粘附与纳米摩擦性质

本文用原子力显微镜研究了空气和氮气两种不同气氛环境下的机械剥离石墨烯粘附力,发现氮气环境下的粘附力更小,且石墨烯边缘的粘附力比内部区域大.在氮气环境下探究了折叠石墨烯粘附力与层数的关系及其摩擦性能,结果表明粘附力与折叠石墨烯层数无明显关系,折叠石墨烯各区域的摩擦性能都远超二氧化硅基底,且单层、单层上折叠、双层以及双层上折叠区域的摩擦系数依次降低,分别为0.049,0.031,0.023和0.021,摩擦力也依次降低,折叠处由于层与层之间的结合力弱于相同层数的石墨烯,摩擦性能有所降低,但未发现粘附力与摩擦力之间的明显关系.在采用尖针和球针测量粘附力时,测量历史不会对后续粘附力产生明显影响.对空气环境下出现的新鲜折叠石墨烯的研究表明新鲜折叠石墨烯的折叠区域摩擦力较未折叠区域显著增大.