Nanotribology of SiP nanosheets: Effect of thickness and sliding velocity

Two-dimensional compounds combining group IV A element and group V A element were determined to integrate the advantages of the two groups.As a typical 2D group IV–V material,SiP has been widely used in photodetection and photocatalysis due to its high carrier mobility,appropriate bandgap,high thermal stability,and low interlayer cleavage energy.However,its adhesion and friction properties have not been extensively grasped.Here,large-size and high-quality SiP crystals were obtained by using the flux method.SiP nanosheets were prepared by using mechanical exfoliation.The layer-dependent and velocity-dependent nanotribological properties of SiP nanosheets were systematically investigated.The results indicate the friction force of SiP nanosheets decreases with the increase in layer number and reaches saturation after five layers.The coefficient of friction of multilayer SiP is 0.018.The mean friction force,frictional vibrations,and the friction strengthening effect can be affected by sliding velocity.Specially,the mean friction force increases with the logarithm of sliding velocity at nm/s scale,which is dominated by atomic stick-slip.The influence of frequency on frictional vibration is greater than speed due to the different influences on the change in contact quality.The friction strengthening saturation distance increases with the increase in speed for thick SiP nanosheets.These results provide an approach for manipulating the nanofriction properties of SiP and serve as a theoretical basis for the application of SiP in solid lubrication and microelectromechanical systems.

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.

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.

Non-Local and Memory Character of Frictional Energy Dissipation on Atomic Scale

The traditional description of atomic-scale friction, as investigated in Friction force microscopy, in terms of mechanical stick-slip instabilities appears so successful that it obscures the actual mechanisms of frictional energy dissipation. More sophisticated theoretical approach, which takes into account damping explicitly, reveals the existence of some hidden, unexplained problems, like the universal nearly-critical damping and unexpectedly high value of the dissipation rate. In this paper, we combine analysis in the framework of nonequilibrium statistical mechanics with simple atomistic modeling to show that the hidden problems of atomic scale friction find their origin in the nontrivial character of energy dissipation that is non-local and dominated by memory effects, which have not been addressed before in the context of dry, atomic-scale friction.

Boundary lubrication by adsorption film

A complete understanding of the mechanism of boundary lubrication is a goal that scientists have been striving to achieve over the past century.Although this complicated process has been far from fully revealed,a general picture and its influencing factors have been elucidated,not only at the macroscopic scale but also at the nanoscale,which is sufficiently clear to provide effective instructions for a lubrication design in engineering and even to efficiently control the boundary lubrication properties.Herein,we provide a review on the main advances,especially the breakthroughs in uncovering the mysterious but useful process of boundary lubrication by adsorption film.Despite the existence of an enormous amount of knowledge,albeit unsystematic,acquired in this area,in the present review,an effort was made to clarify the mainline of leading perspectives and methodologies in revealing the fundamental problems inherent to boundary lubrication.The main content of this review includes the formation of boundary film,the effects of boundary film on the adhesion and friction of rough surfaces,the behavior of adsorption film in boundary lubrication,boundary lubrication at the nanoscale,and the active control of boundary lubrication,generally sequenced based on the real history of our understanding of this process over the past century,incorporated by related modern concepts and prospects.

Atomistics of superlubricity

Friction is a phenomenon observed ubiquitously in daily life,yet its nature is complicated.Friction between rough surfaces is considered to arise primarily because of macroscopic roughness.In contrast,interatomic forces dominate between clean and smooth surfaces.“Superlubricity”,where friction effectively becomes zero,occurs when the ratio of lattice parameters in the pair of surfaces becomes an irrational number.Superlubricity has been found to exist in a limited number of systems,but is a very important phenomenon both in industry and in mechanical engineering.New atomistic research on friction is under way,with the aim of refining theoretical models that consider interactions between atoms beyond mean field theory and experiments using ultrahigh vacuum non-contact atomic force microscopy.Such research is expected to help clarify the nature of microscopic friction,reveal the onset conditions of friction and superlubricity as well as the stability of superlubricity,discover new superlubric systems,and lead to new applications.

Running-in process of Si-SiO_(x)/SiO_(2)pair at nanoscale-Sharp drops in friction and wear rate during initial cycles

Using an atomic force microscope,the running-in process of a single crystalline silicon wafer coated with native oxide layer(Si-SiO_(x))against a SiO_(2)microsphere was investigated under various normal loads and displacement amplitudes in ambient air.As the number of sliding cycles increased,both the friction force Ft of the Si-SiO_(x)/SiO_(2)pair and the wear rate of the silicon surface showed sharp drops during the initial 50 cycles and then leveled off in the remaining cycles.The sharp drop in Ft appeared to be induced mainly by the reduction of adhesion-related interfacial force between the Si-SiO_(x)/SiO_(2)pair.During the running-in process,the contact area of the Si-SiO_(x)/SiO_(2)pair might become hydrophobic due to removal of the hydrophilic oxide layer on the silicon surface and the surface change of the SiO_(2)tip,which caused the reduction of friction force and the wear rate of the Si-SiO_(x)/SiO_(2)pair.A phenomenological model is proposed to explain the running-in process of the Si-SiO_(x)/SiO_(2)pair in ambient air.The results may help us understand the mechanism of the running-in process of the Si-SiO_(x)/SiO_(2)pair at nanoscale and reduce wear failure in dynamic microelectromechanical systems(MEMS).

Nanotribological studies using nanoparticle manipulation: Principles and application to structural lubricity

The term“structural lubricity”denotes a fundamental concept where the friction between two atomically flat surfaces is reduced due to lattice mismatch at the interface.Under favorable circumstances,its effect may cause a contact to experience ultra-low friction,which is why it is also referred to as“superlubricity”.While the basic principle is intriguingly simple,the experimental analysis of structural lubricity has been challenging.One of the main reasons for this predicament is that the tool most frequently used in nanotribology,the friction force microscope,is not well suited to analyse the friction of extended nanocontacts.To overcome this deficiency,substantial efforts have been directed in recent years towards establishing nanoparticle manipulation techniques,where the friction of nanoparticles sliding on a substrate is measured,as an alternative approach to nanotribological research.By choosing appropriate nanoparticles and substrates,interfaces exhibiting the characteristics needed for the occurrence of structural lubricity can be created.As a consequence,nanoparticle manipulation experiments such as in this review represent a unique opportunity to study the physical conditions and processes necessary to establish structural lubricity,thereby opening a path to exploit this effect in technological applications.

The nature of the frictional force at the macro-, micro-, and nano-scales

Nowadays it is accepted that the friction force is a combined effect arising from various phenomena:adhesive forces,capillary forces,contact elasticity,topography,surface chemistry,and generation of a third body,etc.Any of them can dominate depending on the experimental force and length scales of the study.Typical forces in macro-tribology are in the Newtons,while are reduced to milli-/micro-Newtons,and nano-Newtons in micro-and nano-tribology,respectively.In this paper,experimental friction results from fullerene like CNx films and single-crystal Si at the three scales will be discussed.The complex and broad variety of processes and phenomena connected with the dry friction coefficient at the macro-,micro-,and nano-scale point of view will be highlighted.

How linear molecules resist to shear:the origin of nanoscale friction

Friction force measurements between smooth surfaces across two layers of linear alkanes over five decades of speeds are presented. A maximum friction dissipation is observed at a characteristic speed. The behaviour is described by a new approach: the formation and destruction of molecular bridges between confined alkane layers. The bridges interdigitated between the layers exhibit a thermally activated resistance to shear.An analytical model involving activation barriers accounts for the overall behaviour of the forces over four decades of speed. This first simple semi-quantitative description sheds new light on the subtle mechanisms of friction at the nanoscale level and shows how the molecular length influences the tribological properties of the liquid.

Effect of Replacing Vanadium by Niobium and Iron on the Tribological Behavior of HIPed Titanium Alloys

This study aims to examine the effect of replacing vanadium by niobium and iron on the tribological behavior of hot-isostatic-pressed titanium alloy （Ti-6Al-4V） biomaterial, using a ball-on-disk-type oscillating tribometer, under wet conditions using physiological solution in accordance with the ISO7148 standards. The tests were carried out under a normal load of 6 N, with an AISI 52100 grade steel ball as a counter face. The morphological changes and structural evolution of the nanoparticle powders using different milling times （2, 6, 12 and 18 h） were studied. The morphological characterization indicated that the particle and crystallite size continuously decrease with increasing milling time to reach the lowest value of 4 nm at 18-h milling. The friction coefficient and wear rate were lower in the samples milled at 18 h （0.226, 0.297 and 0.423; and 0.66 × 10-2, 0.87 × 10-2 and 1.51 × 10-2 μm3 N-1 i, tm-1） for Ti-6Al-4Fe, Ti-6A1-7Nb and Ti-6Al-4V, respectively. This improvement in friction and wear resistance is attributed to the grain refinement at 18-h milling. The Ti-6Al-4Fe samples showed good tribological performance for all milling times.

A friction force microscope employing laser beam deflectionfor force detection

Nanotribology is now one of the most active research areas of tribology.Frictionforce microscopy(FFM),based on atomic force microscopy(AFM),is a method forstudying the feature of micro-friction of material surface with high resolution.So theFFM has received more attention and has been widely used in the area of nanotribology sinceit was developed.The latest results in the area of nanotribology are mostly obtained byusing AFM or FFM,which are the relationship between micro-scale friction force andslope of surface roughness,cyclicity of friction force on atomic scale,features ofmicro-wear,etc.However,there are many problems on how to employ AFM

Chemical effects on the sliding friction of Ag and Au(111)

We have investigated the sliding friction behavior of metallic couples with different enthalpy of mixing or reaction by friction force microscopy. Comparing the friction behavior of miscible and immiscible couples we find that in the first case friction is governed by adhesion while the shear strength is low(τ = 3–6 MPa). In the latter case of immiscible couples, adhesion is found to be low and the shear strength is large(τ≈ 70 MPa). Statistical analysis of atomic stick-slip images recorded on an Au(111) surface with tips of different affinities with gold allows for a deeper understanding of our results. The periodicity of atomic stick-slip images corresponds to the interatomic distance of gold for immiscible counter-bodies. In contrast, for a reactive couple the periodicity of atomic stick-slip significantly differs from the gold interatomic distance and may correspond to the structural length of an ordered intermediate phase at the tip-surface interface.

Preparation and nanotribological behavior of PAH/graphite oxide molecular deposition film

Polyallylamine hydrochloride/graphite oxides ultra thin film (coded as PAH/GO ultra thin film) was prepared by molecular deposition (MD) in laboratory. The resulting PAH/GO ultra thin film was heated to convert the electrostatic force towards covalent bond so as to increase the bonding strength of the film. The structure and nanotribological properties of the film were analyzed by means of atomic force microscopy and ultraviolet-visible spectroscopy. It was found that the PAH/GO ultra thin film had a much smaller friction force than the glass substrate and the friction force was dependent on the hardness and appearance of the glass substrate.

Construction and nanotribological study of a glassy covalent organic network on surface

Unraveling the nature of complex condensed matter systems is of paramount importance in a variety of fields such as pharmacology and materials science.Here we report the synthesis,by the dynamic covalent chemistry(DCC),of a robust,continuous,and low-defect glassy covalent organic network(GCON).The direct imaging of the molecular structure clearly shows the amorphous nature of GCONs,which consists with the competing(nano)crystallite model,not Zachariasen continuous random networks(Z-CRN).Remarkably,the microscopic friction properties were measured on GCONs by atomic force microscopy(AFM),and the GCONs showed lower friction force in comparison with crystalline covalent organic frameworks(COFs).