Load Induced Microstructure Evolution and Friction in an Organic Monolayer Self-assembled on a Silicon Substrate

In sliding of organic self-assembled monolayer against a probe the friction force is generally found to vary linearly with normal load. Here, lateral force microscopy is used to track the physical changes at the interface brought about when an octadecyltrichlorosilane monolayer, self-assembled on a silicon wafer, is slid against a Si₃N₄ tip in the 0–30 nN load range. Regarding a morphologically heterogeneous monolayer domain to be made up of tiles of characteristic friction forces, each tile is in a unique physical state; the variation of area fraction (in a scan area) of each tile is tracked as a function of normal load. The area averaged friction force at a load is obtained by summing the fractional forces of constituent friction tiles. The friction force obtained thus, is found to vary linearly with normal tip load. It is observed that this force is dominated by the low-friction crystalline tiles at low loads and by the high friction more amorphous tiles at high loads. This suggests that for a self- assembled monolayer the load governance of friction as implied by the Amontons Law may be attributed to the physical changes that are brought about at the interface by changing the normal load.     

Formation and Oxidation of Linear Carbon Chains and Their Role in the Wear of Carbon Materials

The atomic-scale processes taking place during the sliding of diamond and diamond-like carbon surfaces are investigated using classical molecular dynamics simulations. During the initial sliding stage, diamond surfaces undergo an amorphization process, while an sp ³ to sp ² conversion takes place in tetrahedral amorphous carbon (ta-C) and amorphous hydrocarbon (a-C:H) surface layers. Upon separation of the sliding samples, the interface fails. A rather smooth failure occurs for a-C:H, where the hydrogen atoms present in the bulk passivate the chemically active carbon dangling bonds. Conversely, sp-hybridized carbon chains are observed to form on diamond and ta-C surfaces. These carbynoid structures are known to undergo a fast degradation process when in contact with oxygen. Using quantum-accurate density functional theory simulations, we present a possible mechanism for the oxygen-induced degradation of the carbon chains, leading to oxidative wear of the sp phase on diamond and ta-C surfaces upon exposure to air. Oxygen molecules chemisorb on C–C bonds of the chains, triggering the cleavage of the chains through concerted O–O and C–C bond-breaking reactions. A similar reaction caused by adsorption of water molecules on the carbon chains is ruled out on energetic grounds. Further O₂ adsorption causes the progressive shortening of the resulting, O-terminated, chain fragments through the same O–O and C–C bond breaking mechanism accompanied by the formation of CO₂ molecules.     

Periodicities in the properties associated with the friction of model self-assembled monolayers

The classical molecular dynamics simulations presented here examine the periodicities associated with the sliding of a diamond counterface across a monolayer of hydrocarbon chains that are covalently bound to a diamond substrate. Periodicities observed in a number of system quantities are a result of the tight packing of the monolayer and the commensurate structure of the diamond counterface. The packing and commensurability of the system force synchronized motion of the chains during sliding contact. This implies that the size of the simulations for this special case can be reduced so that the simulations can be conducted with sliding speeds and time durations that may bridge the gap between theory and experiment.     

Large friction anisotropy of a polydiacetylene monolayer

Friction force microscopy measurements of a polydiacetylene monolayer film reveal a 300% friction anisotropy that is correlated with the film structure. The film consists of a monolayer of the red form of N‐(2‐ethanol)‐10,12‐pentacosadiynamide, prepared on a Langmuir trough and deposited on a mica substrate. As confirmed by atomic force microscopy and fluorescence microscopy, the monolayer consists of domains of linearly oriented conjugated backbones with pendant hydrocarbon side chains above and below the backbones. Maximum friction occurs when the sliding direction is perpendicular to the backbones. We propose that this effect is due to anisotropic film stiffness, which is a result of anisotropic side chain packing and/or anisotropic stiffness of the backbone itself. Friction anisotropy is therefore a sensitive, optically‐independent indicator of polymer backbone direction and monolayer structural properties. This revised version was published online in September 2006 with corrections to the Cover Date.     

Adhesion and friction properties of hydrogenated amorphous carbon films measured by atomic force microscopy

Atomic force microscopy has been used to measure adhesion and friction forces at the interface between an oxidized metal probe tip and amorphous carbon films of varying hydrogen contents (12.3–39.0 atomic percent hydrogen). The interface of an oxide surface and a hard carbon coating models the unlubricated head-disk interface of current hard disk products. Adhesion forces normalized by the radius of curvature of the contacting tip range from 1.09 to 8.53 N/m. Coefficients of friction values, measured as the slope of the friction versus load plot, range from 0.33 to 0.87. A trend of increasing adhesion forces and coefficients of friction is observed for increasing hydrogen content in the films. We attribute the increase in adhesion and friction to increases in the surface free energy of the carbon films with the incorporation of hydrogen.     

Wear of a single asperity using Lateral Force Microscopy

This report describes an observation of alternating transitions between linear (Amontons) and non-linear friction-load behavior during Lateral Force Microscope experiments using a silicon tip sliding on a quartz surface. Initially, a transition from linear to non-linear behavior was attributed to nanoscale ‘running-in’ of the tip to form a single contact junction at the interface. Once this had occurred, a non-linear relationship between friction and applied load was observed during a number of loading and unloading cycles. For higher compressive loads, a further transition to a more linear friction-load behavior was attributed to nanoscale wear in the contact zone. Notably, when applied load was reduced below this ‘high-load’ transition point, the same non-linear friction-load behavior was again observed, but with a larger (friction per load) magnitude than seen previously. This cycle was repeated five times in these experiments, and each time, switching between non-linear and linear friction-load behavior occurred, along with a progressive increase in friction (per load) each time load was reduced below the transition point. The progressive increase in friction is attributed to an increased area of contact, caused by nanoscale wear at higher applied loads. An increase in tip size was confirmed by tip profiling before and after experiment. By progressively wearing the asperity at higher loads, the (interfacial or true) contact area, A, between the surfaces could be progressively increased, and as a result, a progressive increase in interfacial sliding friction, F _f , was obtained at lower loads (according to F _f = τA).     

Friction,Wear, and Aging of an Alkoxy-monolayer Boundary Lubricant on Silicon

We study the friction, wear, and aging of a model boundary lubricant, an alkoxy monolayer covalently bonded to a Si(111) surface, using an interfacial force microscope with a spherical diamond probe. The robust alkoxy bond creates a film that effectively lubricates and prevents wear of Si at stresses comparable to those found in microelectromechanical systems devices. Sliding on the monolayer over 50 nm produced friction approximately three times greater than that of sliding over molecular length scales (∼2 nm); this is attributed to deformation dynamics of the experiment. By repeated scanning over the same location, we observed wear on a level that reduces the friction by thinning and/or reordering the monolayer film.     

Generation of defects in model lubricant monolayers and their contribution to energy dissipation in friction

The structural, mechanical (friction) and spectroscopic properties of model lubricant films made of self-assembled and Langmuir–Blodgett monolayers on quartz, mica and gold have been investigated with atomic force microscopy, the surface forces apparatus and sum-frequency generation. In these films, the molecules tend to form densely packed structures, with the alkane chains mostly vertical and parallel to each other. The SFG results suggest that under moderate pressures of a few tens of MPa, the methyl end group of the alkane chains is rotated to accommodate a terminal gauche distortion. The molecule, however, retains its upright close-packed structure with a lattice periodicity when ordered, which can be resolved by AFM. At pressures above 0.1 GPa, changes in the form of collective molecular tilts take place that lower the height of the monolayer. Only certain angles of tilt are allowed that are explained by the interlocking of methylene units in neighboring chains. The discrete angular tilts are accompanied by increases in friction. A model based on the van der Waals attractive energy between chains is used to explain the stability of the films and to estimate the cohesive energy changes during tilt and, from that, the increases in friction force.     

Friction characteristics of nanoscale sliding contacts between multi-asperity tips and textured surfaces

Nanoscale sliding contacts of smooth surfaces or between a single asperity and a smooth surface have been widely investigated by molecular dynamics simulations, while there are few studies on the sliding contacts between two rough surfaces. Actually, the friction of two rough surfaces considering interactions between more asperities should be more realistic. By using multiscale method, friction characteristics of two dimensional nanoscale sliding contacts between rigid multi-asperity tips and elastic textured surfaces are investigated. Four nanoscale textured surfaces with different texture shapes are designed, and six multi-asperity tips composed of cylindrical asperities with different radii are used to slide on the textured surfaces. Friction forces are compared for different tips, and effects of the asperity radii on the friction characteristics are investigated. Average friction forces for all the cases are listed and compared, and effects of texture shapes of the textured surfaces are discussed. The results show that textured surface II has a better structure to reduce friction forces. The multi-asperity tips composed of asperities with R=20r0 （r0=0.227 7 nm） or R=30r0 get higher friction forces compared with other cases, and more atoms of the textured surfaces are taken away by these two tips, which are harmful to reduce friction or wear. For the case of R=10ro, friction forces are also high due to large contact areas, but the sliding processes are stable and few atoms are taken away by the tip. The proposed research considers interactions between more asperities to make the model approach to the real sliding contact problems. The results will help to vary or even control friction characteristics by textured surfaces, or provide references to the design of textured surfaces.     

Observation of proportionality between friction and contact area at the nanometer scale

The nanotribological properties of a hydrogen‐terminated diamond(111)/tungsten‐carbide interface have been studied using ultra‐high vacuum atomic force microscopy. Both friction and local contact conductance were measured as a function of applied load. The contact conductance experiments provide a direct and independent way of determining the contact area between the conductive tungsten‐carbide AFM tip and the doped diamond sample. We demonstrate that the friction force is directly proportional to the real area of contact at the nanometer‐scale. Furthermore, the relation between the contact area and load for this extremely hard heterocontact is found to be in excellent agreement with the Derjaguin–Müller–Toporov continuum mechanics model. This revised version was published online in September 2006 with corrections to the Cover Date.     

基于有限元法分析超精密切削中的摩擦问题

基于更新的拉格朗日公式,建立了热-机械耦合的平面应变大变形正交切削模型.根据此模型,对金刚石车削过程中的摩擦问题进行了仿真研究.对两种摩擦模型仿真所得的切削力与实验数据进行了比较,验证了前刀面上的摩擦状态应是粘结-滑移摩擦同时存在,并研究了刀具摩擦系数各向异性对超精密切削中切屑变形、切削力、剪切角的影响.     

Adhesion and friction studies of a nano-textured surface produced by spin coating of colloidal silica nanoparticle solution

This paper presents the results of adhesion and friction studies on a nano-textured surface. The nano-textures were produced by spin coating colloidal silica nanoparticle solution on a flat silicon substrate. Surface morphology was characterized by environmental scanning electron microscopy (ESEM) and scanning probe microscopy (SPM). Adhesion and friction studies were conducted using a TriboIndenter employing diamond tips with 5 μm and 100 μm nominal radii of curvature. The results show that the adhesion forces and coefficients of friction of the nano-textured surface measured by the 100 μm tip were reduced up to 98 and 88%, respectively, compared to those of a baseline silicon oxide film surface.     

Ni nanodot-patterned surfaces for adhesion and friction reduction

Surface nano-patterning with Ni nanodot arrays was investigated for adhesion and friction reduction of contacting interfaces. Self-assembled anodized aluminum oxide (AAO) templates in conjunction with thermal evaporation was used to fabricate nano-patterned surfaces with ordered Ni nanodot arrays on Si substrates. Surface morphology of the Ni nanodot-patterned surfaces (NDPSs) was characterized by scanning electron microscopy (SEM). Adhesion and friction studies on a Ni NDPS and a baseline smooth Si(100) surface were conducted using a TriboIndenter employing a diamond tip with 100 μm nominal radius of curvature. The results show that the ordered Ni nanodot-patterning reduced the adhesion forces and coefficients of friction up to 92 and 83%, respectively, compared to those of the smooth silicon surface. Surprisingly, the nanoscale multi-asperity contact between the diamond tip and inhomogeneous Ni NDPSs under low loads follows a continuum contact mechanics model.     

High-speed tribology of PFPEs with different functional groups and molecular weights coated on DLC

In this article, we study the friction and wear durability of perfluoropolyether (PFPE) with different functional groups and molecular weights (MW) for a range of disk rotational speeds (500–7200 rpm or 1.2–17.33 m/s). A 4 mm diameter silicon nitride ball under a normal load of 4 g was employed as slider against PFPE lubricated diamond like carbon (DLC) film on magnetic hard disk. The coefficient of friction increases with increasing speeds, to certain extent, but it decreases for the higher speeds. At very high speeds, the fluctuations in the coefficient of friction of low MW PFPEs were larger than those of high MW PFPEs. The optical microscope image of the ball after sliding showed that evaporation might have occurred more easily in low MW than in high MW when sliding speed was increased due to the frictional heat generated at the interface. The wear lives of Z-lube (carboxyl group at both ends) and Z-dol are significantly higher than AS1 (alkoxy silano group at both ends) at low speed (1.2 m/s). In comparison to low MW PFPEs, high MW PFPEs show better wear durability at higher rotational speeds.     

Influence of humidity on microtribology of vertically aligned carbon nanotube film

The aim of this study is to probe the influence of water vapor environment on the microtribological properties of a forestlike vertically aligned carbon nanotube (VACNT) film, deposited on a silicon (001) substrate by chemical vapor deposition. Tribological experiments were performed using a gold tip under relative humidity varying from 0 to 100%. Very low adhesion forces and high friction coefficients of 0.6–1.3 resulted. The adhesion and friction forces were independent of humidity, due probably to the high hydrophobicity of VACNT. These tribological characteristics were compared to those of a diamond like carbon (DLC) sample.     

Effects of Tail Group and Chain Length on the Tribological Behaviors of Self-Assembled Dual-Layer Films in Atmosphere and in Vacuum

In the present study, three kinds of self-assembled dual-layer films with various tail groups and chain length were prepared by adsorption of different carboxylic acids (stearic acid, STA; propionic acid, PPA; and phenylacetic acid, PAA) to the top of 3-aminopropyltriethoxysilane (APS) film on silicon surface. Using an atomic force microscopy, the films were found to reveal smaller adhesion and friction forces in vacuum than in atmosphere. Due to the effect of the adsorbed water layer on the samples, the more hydrophilic film exhibited the larger difference between the friction forces in vacuum and in atmosphere. For the dual-layer films either in atmosphere or in vacuum, the densely packed long chains can lead to lower friction than the poor-packed short chains, and the tail phenyl groups may induce higher friction than the methyl groups. In the initial stage of nanowear process by a diamond tip, a series of hillocks were observed on silicon surface along the scratching line. It was found that all the films can effectively enhance the antiwear ability of silicon surface and the self-assembled dual-layer film terminated by long chains (STA/APS) or –C₆H₅ groups (PAA/APS) performed much better than that terminated by short chains. Finally, the microwear abilities of the films were examined on a universal micro-tribometer. With the increase in normal load from 50 to 200 mN, the wear life varied for different films and good antiwear performances were also assigned to STA/APS and PAA/APS. This work can be indicative in the application of self-assembled films in the micro/nanoelectromechanical systems.     

Transmission Electron Microscopy Analysis of Mo–W–S–Se Film Sliding Contact Obtained by Using Focused Ion Beam Microscope and In Situ Microtribometer

To better understand the fundamentals of solid lubrication, microstructural analyses on the wear scar surface and contact interface of Mo–W–S–Se composite films produced by pulsed laser deposition were completed. Focused ion beam (FIB), transmission electron microscopy (TEM), and X-ray energy dispersive spectroscopy were employed to study the cross-sectional microstructure and chemistry of wear scars. In particular, a novel microtribometer was built for in situ tribological measurements within a FIB microscope. The sliding tip was welded in contact to the wear scar surface on the film under load by re-deposition of sputtering materials from the FIB cut of the tip. Using this technique, cross-sectional TEM specimens were prepared precisely at the contact point without tip/film separation. Here, the in situ FIB microtribometer is critically important for retaining the microstructure of lubricant films as formed at the sliding contact interface between the tip and film without separation. It provides the unique ability to stop sliding, section the contact, and reveal microstructural changes to that contact without disrupting the sliding interface. The cross-sectional TEM measurements were performed on the sliding contact interface for both the regions in contact and just past contact, and both the reorientation and recrystallization of lubricant films were revealed.     

A numeric investigation of friction behaviors along tool/chip interface in nanometric machining of a single crystal copper structure

The interaction between the tool rake face and the chip is critical to chip morphology, cutting forces, surface quality, and other phenomena in machining. A large body of existing literature on nanometric machining or nano-scratching only considers the overall friction behavior by simply regarding the total force along tool movement direction as the friction force, which is not suitable for describing the intriguing friction phenomena along the tool/chip interface. In this study, the molecular dynamic (MD) simulation is used to model the nanometric machining process of single crystal copper with diamond tools. The effects of three factors, namely, tool rake angle, depth of cut, and tool travel distance, are considered. The simulation results reveal that the normal force and friction force distributions along tool/chip interface for all cases investigated are similarly shaped. It is found that the normal force consistently increases along the entire tool/chip interface when a more negative rake angle tool is used. However, the friction force increases as the rake angle becomes more negative only in the contact area close to the tool tip, and it reverses the trend in the middle of tool/chip interface. Meanwhile, the increase of depth of cut overall increases the normal force along the tool/chip interface, but the friction force does not necessarily increase. Also, the progress of tool into the work material does not change the patterns of normal force, friction force, or friction coefficient distributions to a great extent. More importantly, it is discovered that the traditional sliding model with a constant friction coefficient can be used to approximate the later section of friction distributions. However, no friction model for traditional machining is appropriate to describe the first section of friction distributions obtained from the MD simulation.     

Exploring the “friction modifier” phenomenon: nanorheology of n‐alkane chains with polar terminus dissolved in n‐alkane solvent

Dilute solutions of two polar end‐functionalized linear alkanes (1‐hexadecylamine and palmitic acid), each dissolved in tetradecane, were confined between two mica surfaces and investigated using a surface forces apparatus modified to study shear nanorheology. These two solutions showed similar nanorheological properties that differed from those observed for pure n‐alkanes. In static measurements, a “hard wall”, rather than an oscillatory force, was observed as a function of film thickness. The polar alkane component formed a weakly adsorbed single layer at each mica surface, disrupting the layered structures found in neat n‐tetradecane. In dynamic experiments at low shear amplitude, the storage modulus G' exceeded the loss modulus G" at low frequencies; above some characteristic frequencies G' increased such that g' ≈ G", indicating significantly more energy loss through viscous modes at higher frequency. When the amplitude was varied at fixed frequency, no stick–slip was observed and the limiting value of the shear stress at high effective shear rate was an order of magnitude less than for unfunctionalized n‐alkanes at similar loads. Together, these results show that the addition of a small amount of polar alkane component, by disrupting the layered structures that would have been formed in the neat n‐alkane, is effective in suppressing static friction and reducing kinetic friction in the boundary lubrication regime. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluations of tribological characteristics of PFPE lubricants on DLC surfaces of magnetic disks

This article presents measurements of adhesion and friction of perfluoropolyether (PFPE) lubricant films dip-coated on magnetic disks covered with diamond-like carbon (DLC) film. We have developed a custom-built pin-on-disk type micro-tribotester to perform the tribological measurements. The adhesion tests were performed by pulling down/up a 1.5-mm-diameter glass ball on a stationary disk surface, and the friction tests were carried out by sliding the glass ball on a rotating disk surface without changing head-disk interface conditions from the adhesion tests. Experiments were performed for the different kinds of 2- and 6-nm-thick PFPE lubricants (polar: Zdol4000 and Zdol2000; nonpolar: Z03) under lightly loaded and slow sliding conditions to minimize disturbance against the molecular layered structure. The adhesive forces were found to decrease with increasing film thickness in the order of Z03 > Zdol2000 > Zdol4000 (decreasing rate), which closely corresponds to the order of monolayer thickness, and then to saturate to almost the same calculated values. As for the friction forces of 2-nm-thick films, Zdol2000 featured extraordinarily large friction in comparison with Zdol4000 and Z03, while Zdol4000 was slightly larger than Z03. The largest friction of Zdol2000 reveals that the 2-nm-thick Zdol2000 formed a monolayer that served as an immobile layer. With the increase in film thickness, the friction force of Zdol2000 decreased, indicating that extra lubricant molecules served as a mobile layer, while that of Z03 remained unchanged as the lowest value. By extrapolating the loading force versus friction force relationship into a negative loading force region, it is found that the friction force of Z03 tended to zero at zero net load (loading force plus adhesion force), while those for Zdol2000 and Zdol4000 exhibited finite values, indicating formation of an immobile layer, which shows similar characteristics to those of adhesive rubber material. The dewetted surface is found to feature violently changing friction force only at the first stage of sliding, and it then becomes stable after several sliding passes.