Nanotribology: an UHV-SFM study on thin films of AgBr(001)

We performed scanning force microscopy (SFM) in ultrahigh vacuum (UHV) on AgBr thin films which were in situ deposited on NaCl(001) substrates. The morphology of the initial growth stage and the nanotribological properties of these thin films are characterized and discussed. The lateral (frictional) forces are measured as a function of normal load. The local friction coefficients are extracted by means of the two-dimensional histogram technique. In the low load regime, friction coefficients of 0.33 ± 0.07 and <0.03 are found between probing SiO _x tip and AgBr and NaCl, respectively. The two-dimensional histogram reveals the transition from the force regime of wearless friction to the initial stage of wear on this thin film system. High-resolution SFM images of AgBr(001) are presented which reveal the atomic-scale periodicity of an unreconstructed AgBr(001) surface. The stick-slip nature of the frictional force is demonstrated.     

Friction force microscopy investigations of potassium halide surfaces in ultrahigh vacuum: structure,friction and surface modification

Friction force microscopy measurements on the vacuum‐cleaved (001) surfaces of KF, KCl and KBr have been carried out. All surfaces exhibit atomically flat terraces with monatomic steps aligned preferentially along low‐index lattice directions. Stick‐slip lateral forces with the lattice periodicity are observed on all surfaces. Tip‐sample contact creates higher friction domains on the terraces of all three materials. The structure, topography and degree of friction force contrast of these domains is material dependent. The dependence of friction upon load generally does not coincide with the behavior expected for an elastic contact. We propose that the observed domains result from surface structural changes created by low load tip‐sample contact on these relatively soft materials. This revised version was published online in June 2006 with corrections to the Cover Date.     

Friction force microscopy in ultrahigh vacuum: an atomic-scale study on KBr(001)

We present an atomic-scale study on friction performed by a bidirectional atomic force microscope operated in ultrahigh vacuum. Experiments on surfaces of in situ cleaved KBr crystals are presented. On a m scale the cleavage structure with monoatomic steps of 3.5 ± 0.3 Å is revealed. On the atomically flat terraces, atomic-scale resolution is achieved. The resolved square lattice shows a periodicity of 4.7 Å and corrugations of 0.3–0.7 Å and exhibits the cubic symmetry of KBr(001). The lateral (frictional) force map shows all characteristics of the stick-slip movement of the probing tip. From analysis of the friction loops, the kinetic friction force was determined as a function of load. For a load regime of -4 to 10 nN, lateral force corrugations ranging from 1 to 5 nN were found. A comparison with a novel theoretical model is discussed qualitatively.     

Nanotribological Properties of Fluorinated, Hydrogenated, and Oxidized Graphenes

Recently, the tribological properties of graphene have been intensively examined for potential applications in micro- and nano-mechanical graphene-based devices. Here, we report that the tribological properties can be easily altered via simple chemical modifications of the graphene surface. Friction force microscopy measurements show that hydrogenated, fluorinated, and oxidized graphenes exhibit, 2-, 6-, and 7-fold enhanced nanoscale friction on their surfaces, respectively, compared to pristine graphene. The measured nanoscale friction should be associated with the adhesive and elastic properties of the chemically modified graphenes. Density-functional theory calculations suggest that, while the adhesive properties of chemically modified graphenes are marginally reduced down to ~30 %, the out-of-plane elastic properties are drastically increased up to 800 %. Based on these findings, we propose that nanoscale friction on graphene surfaces is characteristically different from that on conventional solid surfaces; stiffer graphene exhibits higher friction, whereas a stiffer three-dimensional solid generally exhibits lower friction. The unusual friction mechanics of graphene is attributed to the intrinsic mechanical anisotropy of graphene, which is inherently stiff in plane, but remarkably flexible out of plane. The out-of-plane flexibility can be modulated up to an order of magnitude by chemical treatment of the graphene surface. The correlation between the measured nanoscale friction and the calculated out-of-plane flexibility suggests that the frictional energy in graphene is mainly dissipated through the out-of-plane vibrations, or the flexural phonons of graphene.     

不同石墨烯添加量下MoS2基复合涂层的摩擦磨损及耐腐蚀性能

为改善MoS2基固体润滑涂层的摩擦磨损性能和耐蚀性能,制备了不同石墨烯（GE）添加量的MoS2复合涂层,利用HSR-2M摩擦磨损试验机测试了复合涂层的摩擦磨损性能,并分析了其磨损机理,通过极化曲线、交流阻抗谱（EIS）研究了涂层在3.5%NaCl溶液中的电化学腐蚀行为。试验结果表明,0.8-GE/MoS2复合涂层的摩擦磨损和耐腐蚀性能最优,其平均摩擦因数和磨损率分别为0.232和2.379×10-13 m3/(N·m),较未添加石墨烯的MoS2涂层分别降低了49.56%和43%,腐蚀速率（1.96×10-8 A/cm2）较纯MoS2涂层（5.54×10-6 A/cm2）降低了近2个数量级。石墨烯的二维片状结构具有良好的自润滑性能,在涂层中均匀分布时能有效阻隔腐蚀介质的渗透,因此,石墨烯的添加提高了MoS2基复合涂层的摩擦学性能和耐腐蚀性能,石墨烯的最优添加量为0.8%（质量分数）。     

Enhanced Mechanical and Tribological Properties of Graphene-Reinforced TiAl Matrix Composites

This study investigated the mechanical properties and dry-sliding friction and wear behaviors of graphene-reinforced TiAl matrix composites in expectation of providing valuable information for the application of graphene. The results suggested that the incorporation of graphene apparently improved the microhardness, fracture toughness, and tribological properties of the composites. For the composite with 3?wt% graphene, the microhardness increased by 129%, the fracture toughness increased by 149%, the friction coefficient decreased by 37% and the wear rate decreased by 78%. Also, the microstructural analyses of the worn surfaces indicated that three types of graphene-rich films, with different percentages of coverage, were generated on the worn surfaces under various wear conditions. An evolution mechanism of the films as a function of wear conditions was proposed, and the corresponding variation of friction and wear behavior was also discussed.     

Frictional Behavior of Carbon Film Embedded with Controlling-Sized Graphene Nanocrystallites

Graphene nanocrystallites embedded in amorphous carbon matrix can bring excellent tribological, electrical and magnetical properties to the carbon films. But too large size of graphene nanocrystallite would lead to degradation of the tribological performance. So it is necessary to clarify the dependence of frictional behavior of the carbon film on graphene nanocrystallite size. In order to control the size, different electron irradiation densities were introduced during film growth in the electron cyclotron resonance plasma sputtering process. Frictional tests on the films were carried out with a Pin-on-Disk tribometer. The evolution of graphene nanocrystallite size along with electron irradiation density was examined by transmission electron microscopy and Raman spectroscopy. The results showed that the graphene nanocrystallite size increased with increasing of the electron irradiation density. The film with a graphene nanocrystallite size of 1.09 nm exhibited a low friction coefficient of 0.03 and a long wear life. When nanocrystallite size increased, the friction coefficient increased and the wear life decreased. Observation on transfer film revealed that the nanocrystallite in transfer film grew larger when initial size was 1.09 nm, and changed smaller when initial size was 1.67 nm. The results suggested that embedded graphene nanocrystallite played an important role in the formation of transfer film, the initial size of graphene nanocrystallite strongly affected the frictional behavior of the film, and the graphene nanocrystallite needed to be controlled under a certain size in order to keep the good tribological performance.     

Isotope- and Thickness-Dependent Friction of Water Layers Intercalated Between Graphene and Mica

The lubricating properties of water have been discussed extensively for millennia. Water films can exhibit wearless high friction in the form of cold ice, or act as lubricants in skating and skiing when a liquid. At the fundamental level, friction is the result of a balance between the rate of energy generation by phonon excitation during sliding and drainage of the energy from the interface by coupling with bulk atoms. Using atomic force microscopy, we found that when H₂O intercalates between graphene and mica, it increases the friction between the tip and the substrate, dependent on the thickness of the water and graphene layers, while the magnitude of the increase in friction was reduced by D₂O intercalation. With the help of first-principles density functional theory calculations, we explain this unexpected behavior by the increased spectral range of the vibration modes of graphene caused by water, and by better overlap of the graphene vibration modes with mica phonons, which favors more efficient energy dissipation. The larger increase in friction with H₂O versus D₂O shows that the high-frequency vibration modes of the water molecules play a very important role in the transfer of the vibrational energy of the graphene to the phonon bath of the substrate.     

类金刚石薄膜摩擦机理及其摩擦学性能影响因素的研究现状

类金刚石薄膜(DLC)具有十分优异的减摩耐磨性能,是一种极具发展潜力的固体润滑材料。但其摩擦学性能受到很多因素的影响,这些因素主要可以分为两大类:固有因素和外在因素。在不同的固有因素和外界因素影响下DLC薄膜的摩擦学性能会产生较大差异,这大大制约了人们对其摩擦学行为及摩擦机理的认识,限制了其应用范围的扩展。总结了目前有关DLC薄膜摩擦机理的三种理论,即转移膜理论、滑行界面石墨化理论和化学吸附钝化悬键理论,并在此基础上概括分析了各固有因素和外界因素对DLC薄膜摩擦学性能的影响及其机理,提出未来可以从基础理论和相关技术两方面对DLC薄膜的摩擦学性能展开深入研究。     

Charge Distribution View: Large Difference in Friction Performance Between Graphene and Hydrogenated Graphene Systems

Density functional theory calculations including dispersion correction (DFT-D2) were used to investigate the relationship between charge distribution and nanofriction characteristics of graphene-based material systems. In our calculations, the single-side-hydrogenated graphene (SSHGraphene) system exhibits lower coefficient of friction, whereas the graphane system exhibits larger one compared with graphene system. These results are attributed to the adjustments of interfacial charge distribution that are induced by different hydrogen passivations. The charge distribution is smooth along the sliding direction for the SSHGraphene sheet, which yields a small potential barrier. Corrugation of the charge distribution in graphane system is much steeper than that in graphene system, which leads to a larger potential barrier. Comparative investigations reveal that the interfacial charge distributions determine the nanofriction performance, which may be helpful for friction modulation and design of new controlling lubricant material.     

石墨烯/MoS2复合涂层多环境下的摩擦学性能

以MoS2作为润滑剂,以石墨烯(GE)作为润滑添加剂,采用喷涂法在GCr15钢样片表面制备不同含量的GE/MoS2复合涂层.利用HSR-2M型高速往复式摩擦磨损试验机测试涂层在干摩擦及海水环境中的摩擦磨损性能,并分析了磨痕形貌及磨损机制.结果表明:添加适量石墨烯可明显改善MoS2涂层的摩擦磨损性能,且海水环境中涂层的摩...     

Tribological studies of epoxy and its composite coatings on steel in dry and lubricated sliding

Industrial lubricants are invariably used with additives (with high sulfur and phosphorous contents) for tribological performance enhancement. However, these additives are environmentally very harmful. Hence, there is an urgent need to find alternate solutions for enhancing the tribological performance of lubricants and components without the use of harmful additives. The objective of this work is to investigate the feasibility of using polymer composite coatings in enhancing the tribological properties of steel surfaces in dry and base oil lubricated conditions. Pure epoxy and its composite (with 10?wt-% of graphene or graphite powder) films were coated onto steel substrates and tested under dry and base oil lubricated conditions. Friction and wear experiments were conducted on a ball on cylinder tribometer between polymer/composite coated cylindrical steel surface (shaft) and an uncoated steel ball as the counterface. Tests were conducted at various normal loads and speeds. In dry condition at 3 N load and 0.63?m s^??1 sliding speed, the wear life of epoxy was increased by five times and coefficient of friction was nearly the same (0.18) on inclusion of graphene nanoparticle. In lubricated case, epoxy/graphene composite coating performed eight times and more than five times better than pure epoxy and epoxy/graphite respectively.     

Study on Tribological Performance of NiAl Matrix Self-Lubricating Composites Containing Graphene at Different Loads

This study aimed to explore the possibility of improving the tribological performance of NiAl matrix composites by graphene addition. Friction and wear experiments of as-prepared specimens were conducted under different conditions using a pin-on-disk wear testing machine. NiAl matrix composites containing graphene showed satisfactory performance in friction coefficient and wear resistance compared to NiAl matrix composites without graphene. For the active effect of graphene, the friction coefficient and wear rate of NiAl matrix composites were maintained at relatively lower values. The beneficial antifriction and antiwear effects of graphene gradually failed when the applied load was above 8 N. Graphene in NiAl matrix composites played an active role in the formation of a friction layer, which was beneficial to the lower friction coefficient and wear rate. In light of this research, graphene plays an active role in reducing the friction coefficient and wear rate. Hence, graphene has great potential in applications as an effective solid lubricant to promote tribological behavior.     

Theoretical Study of Superlow Friction Between Two Single-Side Hydrogenated Graphene Sheets

First-principles calculations within density functional theory were performed to predict the friction properties between two single-side hydrogenated graphene sheets at the atomic scale. In this study, the coefficients of the friction along two sliding paths were calculated under normal loads ranging from 1 to 9?nN. The calculated results show a general increase of the coefficient of friction with increasing normal load, and the coefficients of friction for this system exhibit an isotropic feature. Coefficients of friction in the range of 0.01?C0.05 were predicted. Compared with clean graphene, the coefficients of friction decreased greatly due to electrons accumulating between the carbon and attached hydrogen atoms, leading to a significant decrease in the potential energy of the single-side hydrogenated graphene. This study suggests that atomic-scale friction may be controlled by adjusting the electronic structure.     

Multiple Slips in Atomic-Scale Friction: An Indicator for the Lateral Contact Damping

The occurrence of multiple jumps in 2D atomic-scale friction measurements is used to quantify the viscous damping accompanying the stick–slip motion of a sharp tip in contact with a NaCl(001) surface. Multiple slips are observed without apparent wear for normal forces between 13 and 91 nN. For scans parallel to [100] directions, the tip jumps between minima of the substrate corrugation potential in a zigzag fashion. An algorithm is applied to determine histograms of lateral force jumps which characterize multiple slips. The same algorithm is used to classify multiple slips occurring in calculated lateral force maps. Comparisons between simulations and experiments indicate that the nanometer-sized contact is underdamped at intermediate loads (13–26 nN) and becomes slightly overdamped at higher loads. The proposed procedure is a novel way to estimate the lateral contact damping which plays an important role in the interpretation of measurements of the velocity and temperature dependence of friction, of slip duration, and of the reduction of friction by applied perpendicular or parallel oscillations.     

Nanotribological characterization of molecularly thick lubricant films for applications to MEMS/NEMS by AFM

Molecularly thick perfluoropolyether (PFPE) films are considered to be good protective films for micro/nanoelectromechanical systems (MEMS/NEMS) to reduce stiction, friction, and improve their durability. Understanding the nanotribological performance and mechanisms of these films are quite important for efficient lubrication for MEMS/NEMS devices. These devices are used in various operating environments and their effect on friction, adhesion and durability needs to be clarified. For this purpose, mobile and chemically bonded PFPE films were deposited by dip coating technique. The friction and adhesion properties of these films were characterized by atomic force microscopy (AFM). The effect of rest time, velocity, relative humidity, and temperature on nanotribological properties of these films was studied. Durability of these films was also measured by repeated cycling tests. The adhesion, friction mechanisms of PFPE at molecular scale, and the mechanisms of the effect of operating environment and durability are subject of this paper. This study found that adsorption of water, formation of meniscus and its change during sliding, viscosity, and surface chemistry properties play a big role on the friction, adhesion, and durability of the lubricant films.     

石墨烯作为轧钢机轴承润滑脂添加剂的研究*

为改善轧钢机轴承用润滑脂的性能,采用不同质量分数的石墨烯对润滑脂进行了改性,测定各润滑脂样品的锥入度和滴点,使用四球摩擦试验机研究石墨烯对润滑脂摩擦学性能的影响,使用扫描电子显微镜、白光干涉仪和拉曼光谱仪等分析石墨烯在润滑脂中的减摩抗磨机制。结果表明:石墨烯作为添加剂能提高润滑脂的滴点和改善润滑脂的极压性能以及减摩抗磨性能。当石墨烯质量分数为0.2%时,对润滑脂极压性能的提升效果最好,表现为烧结负荷和综合磨损值最大,较基础脂分别提高了29.0%和24.0%;当石墨烯质量分数为0.3%时,对润滑脂减摩抗磨性能的提升效果最好,摩擦因数和磨斑直径较基础脂润滑时分别下降了22.4%和13.0%,磨损体积减少了43.0%,且最大无卡咬负荷提高了21.2%。石墨烯在摩擦过程中,吸附在摩擦表面,形成保护薄膜阻止了摩擦副材料的直接接触,减少了磨损,同时提高了润滑脂的承载能力。     

电化学反应过程中黄铜表面的原位显微拉曼光谱分析

利用显微共焦拉曼光谱仪在线分析了在以十二烷基硫酸钠水溶液为电解液、石墨为辅助电极时电化学反应过程中黄铜表面化学成分的变化，进而分析了外加电压引起摩擦因数变化的机理。结果表明：外加电压对摩擦因数的控制主要通过对表面有机离子吸附膜的影响来实现；黄铜表面存在有机离子吸附或反应而形成的边界润滑膜时摩擦因数较低；黄铜表面不存在有机离子润滑膜而存在较多的氢氧根离子时摩擦因数较高；溶液的搅拌对电控摩擦的恢复有较重要的作用。     

Friction and Wear on Single-Layer Epitaxial Graphene in Multi-Asperity Contacts

Friction and wear of single layers of graphene have been studied at the micrometer scale. Epitaxial graphene grown by thermal decomposition on SiC-6H(0001) is found to have an initial friction coefficient of 0.02, significantly lower than graphite under the same experimental conditions. During reciprocal sliding the graphene layer is damaged. The evolving friction coefficient of 0.08 for the carbon-rich interface layer terminating the SiC layer is still lower than that of graphite and five times lower than that of the hydrogen-etched SiC substrate. Micrometer-sized patches within the sliding track retain the low friction coefficient of graphene even after hundred sliding cycles.     

Amorphous hydrogenated carbon films for tribological applications I. Development of moisture insensitive films having reduced compressive stress

Although earlier investigations on the tribological behaviour of amcrphous hydrogenated carbon (AHC) films in sliding contact with steel showed encouraging results, four open issues were identified. They were: (a) dependence of friction and wear on humidity (i.e., the friction coefficient and the wear increased with humidity), (b) limitations on film thickness (i.e., films greater than 2 μm thick delaminated due to large compressive stress), (c) deposition of films on substrates other than silicon and (d) lubricant compatibility (i.e., formation of lubricant-derived antiwear films on AHC film surfaces). Steps were taken to address some of these open issues by incorporating silicon in AHC films. Friction and wear tests were conducted on AHC films containing various amounts of silicon. Incorporation of silicon in AHC films rendered the friction coefficients and the wear of a steel counterface insensitive to moisture. Silicon incorporation in AHC films also significantly reduced compressive stress. This allowed deposition of 10 μm thick films. These effects were achieved without any compromise with the friction coefficient and the film wear if the amount of silicon in the film was kept within a certain concentration range. In addition, silicon-containing AHC films were thermally more stable than silicon-free films. Experiments conducted with two lubricants resulted in significantly lower wear of the silicon-free AHC films than that obtained for unlubricated sliding. Similar friction coefficients were obtained for AHC film/steel and steel/steel combinations in lubricated sliding.