A predictive analytical friction model from basic theories of interfaces, contacts and dislocations

Friction between crystalline bodies is described in a model that unifies elements of dislocation drag, contact mechanics, and interface theory. An analytic expression for the friction force between solids suggests that dislocation drag accounts for many of the observed phenomena related to solid–solid sliding. Included in this approach are strong arguments for agreement with friction dependence on temperature, velocity, orientation, and more general materials selection effects. It is shown that calculations of friction coefficients for sliding contacts are in good agreement with available experimental values reported from ultrahigh vacuum experiments. Extensions of this model include solutions for common types of dislocation barriers or defects. The effects of third-body solid lubricants, superplasticity, superconductivity, the Aubry transition, and supersonic dislocation motion are all discussed in the framework of dislocation-mediated friction.     

Influence of ethyl acetate and alkyl phosphate adsorption on the frictional properties of VC(100)

This report presents ultrahigh vacuum measurements of the frictional properties of the non-polar (100) surface of vanadium carbide (VC) as a function of the room temperature uptake and reaction of ethyl acetate, triethyl phosphate, and trimethyl phosphate. Atomic force microscopy, employing a silicon nitride probe tip, has been used to determine the changes in friction and interfacial adhesion as a function of adsorbate uptake. Changes in surface morphology have been monitored with scanning tunneling microscopy while the composition of the surface species formed through the reaction of these adsorbates with the VC surface has been determined by X-ray photoelectron spectroscopy. Adsorption and reaction of ethyl acetate leads to an increase in friction with little change in interfacial adhesion. The adsorption and reaction of triethyl phosphate and trimethyl phosphate have no influence on either the friction or adhesion properties of VC. The observed results are discussed in terms of the surface chemical composition, the extent of surface coverage, and the molecular details of the adsorbed species.     

Explanation for the mechanical strength of amyloid fibrils

The presence of “proteinaceous β-sheet rich fibrillar structures” and amyloidogenic material, has been alluded to extensively in the literature, in association with natural materials exhibiting superior mechanical strength per unit volume. Here we provide a clear experimental demonstration and explanation for why individual amyloid quaternary structures themselves have beneficial mechanical characteristics.     

On the current status of quartz crystal microbalance studies of superconductivity-dependent sliding friction

Recent observations of superconductivity-dependent friction in nitrogen films adsorbed on lead substrates have inspired a number of theoretical and experimental efforts to understand the origins of this phenomenon. The status of these efforts is discussed, focussing on the role of electronic contributions to friction.     

Friction properties of triazine-containing hybrid composites

This paper presents micro and nanofriction studies on thin silica/aminosilica-triazine hybrid composite coatings on silicon substrate. The silica/aminosilica coatings were made on silicon using the dip-coating method and then modified by alkoxy- and aryloxy-triazine derivatives. Formation of hybrid coatings permits a greater flexibility of their resulting properties. Microfriction tests were carried out with a sliding ceramic ball (load range of 10–80 mN) on the composite flat surface, using a tribometer ball-on-sample system. Sliding speed was 25 mm/min at room temperature. Nanofriction was measured using the Friction Force Microscopy. It is shown that surface modification of silica/aminosilica layers by triazine derivatives, to form inorganic–organic hybrid coatings, improves friction properties. All tested triazine derivatives, particularly long alkoxy- substituted s-triazines, cause a decrease in the friction coefficients in microfriction tests, in comparison to a non-modified silica/aminosilica thin film. The same effect was also observed for some tested triazine derivatives at nanofriction measurements.     

Surface topography and properties of frictional contacts

The influence of surface topography on contacting solids is considered. The rough surface model is suggested and is used for the calculation of some tribological contact characteristics. A rough surface is modelled by a set of asperities of regular shape (wedge, cone, cylindrical, spherical segment), of differing height. A simple height distribution function and asperity shape function are used. These functions may be integrated analytically in further calculations.The surface model is used for calculation of one of the main contact parameters - real contact pressure (or real contact area) and other principal contact parameters, such as deformation, number of contact spots, average spot area, average distance between contact spots and intercontact gap.It is shown how the above parameters may be used for the calculation of such operational contact characteristics as friction coefficient, wear rate and electrical and thermal resistance.     

Dispersion Stability and Tribological Characteristics of TiO2/SiO2 Nanocomposite-Enriched Biobased Lubricant

The stable dispersion of nano-additives is highly desirable for the effective lubrication performance of nanolubricants. The compatibility of base oil with selected nano-additives is required for uniform and stable dispersion. This research evaluated the dispersion stability and tribological characteristics of nano-TiO₂/SiO₂ (average particle size 50 nm) as an additive in a biobased lubricant. The wear protection and friction reducing characteristics of the formulations were evaluated by four-ball extreme pressure tests and piston ring–cylinder liner sliding tests. Surface analysis tools, including scanning electron microscopy, energy-dispersive X-ray spectroscopy, and atomic force microscopy, were used to characterize the worn surfaces. Results showed that the nanolubricants demonstrated appreciable dispersion capability in the absence of a surfactant and an improvement in load-carrying capacity, antiwear behavior, and friction reduction capability.     

The effects of methane-debris molecules on the frictional properties of diamond

Molecular dynamics simulations have been used to examine the friction between two surfaces placed in sliding contact. These simulations show that the presence of methane-debris molecules squeezed between two hydrogen-terminated diamond (111) surfaces significantly reduces the friction compared to the same two surfaces in the absence of debris. These findings are in agreement with macroscopic experiments that have examined the friction of diamond on diamond in the presence of debris. In addition, the friction coefficients are significantly lower compared to the same system with methyl groups chemisorbed to one of the diamond surfaces.     

Experimental and theoretical evaluation of friction at contacting magnetic storage slider–disk interfaces

To understand better the friction force and wear processes at contacting slider–disk interfaces, we have developed an experimental method for measuring and a theoretical method for calculating the friction force. For this study, a slider with a 1500 μm² contact pad located at the recording head is burnished against a relatively rough disk (～12 Å rms), which ensures smooth sliding. In the experimental method, the friction force is measured as the disk is spun-down to bring the slider–disk interface into an increasing degree of contact. A modified air bearing code is used to determine the experimental normal contact force for each friction measurement. In the theoretical method, the friction force and other relevant interfacial forces are calculated using an improved sub-boundary lubrication (ISBL) rough surface model. The friction force calculation in this model is based on the force needed to induce yielding of the individual disk asperities contacting the flat surface of the contact pad without any assumption of the coefficient of friction. Good agreement is found between the measured and theoretical friction vs. normal contact force curves, indicating that the model is capturing the essential origins of friction at this interface. The model also provides valuable insights into how wear particles may be generated at this contacting slider–disk interface.     

Evaluation of frictional properties of tribomaterials under sparse lubrication

Frictional properties of typical tribomaterials are evaluated under sparse lubrication conditions where lubricating oil is supplied but its amount is far from sufficient even for boundary lubrication. These conditions are provided by employing a mist oiling system which can continuously supply oil at constant, extremely low and quantitatively controlled rates. First, basic data on the properties of the oil mist are given and a quantitative determination of oil supply onto a sliding surface is described. Then frictional properties evaluated under stepwise increasing loads are presented for iron/steel, surface modification, ceramics and self-lubricating materials, and their characteristic behaviour is discussed.     

Test procedure for rapid assessment of frictional properties of engine oils at elevated temperatures

A fifteen-minute test sequence has been developed for screening and ranking base oils and friction modifiers. The tests are run on a high-frequency friction machine developed at the authors' company and based on the Mills-Cameron design. The procedure can distinguish clearly and repeatably between the behaviour of different friction modifiers and with a degree of discrimination which is better than has been achieved in engine mechanical loss tests     

Prediction of the frictional coefficient between a hard protuberance and a soft metal surface

Sliding contact is investigated between a soft metal surface (duralumin) and a hard protuberance ball (steel). The frictional coefficient in this case does not obey Amonton's law of friction, and it increases with an increase of normal load. The experimental value of the frictional coefficient agrees well with the calculated one. In the calculation, the variation of work-hardened depth corresponding to normal force is taken into consideration. It is confirmed that the normal force dependence of the frictional coefficient is generated from the contact pressure determined by the work-hardened depth corresponding to normal force, and from the ploughing effect.     

Adhesion of cleaned nanoscopic metal contacts

Nanoscopic metal contact in the context used in this paper means atomic force microscope (AFM) tips in contact with cleaned, flat, smooth surfaces in ultrahigh vacuum. Such idealised contact conditions are of more and more relevance for technical friction systems despite the fact that for the latter the loads and the velocities are usually higher, the materials used are impure (e. g. alloys) and contaminants or lubricants are present between the sliding surfaces. Sliding contacts in micro‐system technology, which is unequivocally a key technology of the future, will presumably be compared with such idealised conditions. Another important reason for the investigation of nanoscopic contacts is the strong interest in the understanding of the elementary processes and mechanisms causing friction. The investigation of friction systems with a reduced number of influencing parameters offers a chance to gain insight into elementary processes and to compare the results with theoretical models based on physical principles, such as molecular dynamics simulation.     

Observation of controlled,electrochemically induced friction force modulations in the nano-Newton range

The local chemical properties of the contacting asperities in a real tribosystem are of crucial importance for the resulting macroscopic tribo-behavior. Thus, the lateral forces acting on the tip of a standalone scanning force and friction microscope have been investigated as a function of controlled surface chemistry, realized by potentiostatic control of the sample. The results obtained show a clear dependence of nanoscale friction behavior upon changes in the electrochemical state of the system.     

单晶硅纳米力学性能的测试

对材料纳米力学性能测试手段进行了研究,着重分析了纳米压痕技术的原理和方法.结合纳米压痕技术,采用尖端四面体Vickers型单晶金刚石压头对单晶硅(100)晶面进行了纳米压痕实验测试.实验发现,在载荷为1 000 mN时,晶体硅出现了明显的裂纹和脆性断裂;而在载荷低于80 mN的情况下,晶体硅则表现出延性特性.此外,在不同载荷条件下对晶体硅的硬度进行了实验测试,测试结果发现,不同载荷条件下晶体硅的硬度测量值存在较大的差异,认为导致这种差异的原因在于压痕区域晶体硅所受压力不同,使得晶体硅内部结构发生了改变,较为准确的单晶硅的硬度测量值为15.7 GPa.     

Influence of protein conformation on frictional properties of poly (vinyl alcohol) hydrogel for artificial cartilage

Poly (vinyl alcohol) (PVA) hydrogel is one of the anticipated materials for artificial cartilage. In our previous studies, wear of PVA hydrogel depended on content of proteins in lubricants. The secondary structures of bovine serum albumin (BSA) and human gamma globulin (HGG) were investigated in circular dichroism spectroscopy to clarify the influence of the proteins on frictional properties. BSA and HGG were mainly composed of the α-helix and the β-sheet, respectively. BSA containing the α-helix structure showed low friction compared to HGG composed of the β-sheet structure in mixed or boundary lubrication mode. The α-helix structure forms low shear layer because the α-helix structure is easily released from surfaces and low cohesive strength. HGG forms uniform adsorption layer, but showed higher friction than BSA in the rubbing with single protein. In the repeated rubbing with changing of lubricants from HGG to BSA, however, the final friction was reduced, because an optimum layered structure of proteins was formed. Hence, layered structure of proteins appears to play an important role to protect rubbing surfaces and to reduce friction. In heat treatment tests, heat-induced BSA showed very low friction because of reduction of the α-helix structure. Heat-induced HGG did not show large differences from native HGG, but could not bring low friction with heat-induced BSA. Thus it was shown that the protein conformation has effective influences on friction.     

Tribological properties of bio-mimetic nano-patterned polymeric surfaces on silicon wafer

Nano-patterns made of poly(methyl methacrylate) (PMMA) were fabricated on silicon wafer using a capillarity-directed soft lithographic technique. Patterns with three different aspect ratios were investigated for their adhesion and friction properties at nano-scale and for friction at micro-scale. The patterned samples exhibited superior tribological properties, at both these scales when compared to those of flat PMMA thin films.     

单自由度金属橡胶隔振系统干摩擦阻尼的识别

小波变换方法在非线性识别领域得到了越来越多的应用。针对金属橡胶单自由度干摩擦阻尼隔振系统的非线性阻尼识别问题进行了研究。采用了常数变异的思想,基于K-B算法,对金属橡胶干摩擦阻尼系统进行了阻尼系数的识别。结果表明,运用小波分析方法求解非线性阻尼系数是可行的。     

In situ studies of the lubricant chemistry and frictional properties of perfluoropolyalkyl ethers at a sliding contact

In situ analyses of lubricated sliding contacts were performed by interfacing an ultraviolet Raman spectrometer to a ball-on-flat tribotester. The sliding contact was simulated by rotating a sapphire window that is transparent to ultraviolet radiation against a stationary ball. Various loads were transmitted to the contact center through the ball. A branched perfluoropolyaklyl ether (Krytox 479) and two linear perfluoropolyalkyl ethers (Fomblin 491 and Fomblin 497) have been studied under various loads at a 10 cm/s sliding speed. Krytox and a Fomblin of lower viscosity, Fomblin 497, decomposed to amorphous carbon upon sliding on a chrome steel ball but no amorphous carbon was detected from Fomblin 491. The amount of amorphous carbon at the contact area during sliding was a balance of formation and removal rates. It is postulated that surface activity of the chrome steel ball was the main cause for the lubricant degradation. The lubricant degradation at the chrome steel/sapphire interface was found to slightly increase the kinetic coefficient of friction at the contact center. However, catastrophic scuffing was not observed.     

The hydrogen-induced modification of the properties of the metal surface coated with oil and lubricant

The change of elastic and damping properties of the subsurface layer of iron and steel due to the coating with mineral oil and lubricants has been found in internal friction measurements. The effects were not observed for oil coated ceramics but did occur for iron and steel charged with hydrogen. In the case of coated metals, the formation of new internal friction peaks, increase in yield strain and increase in density of the slip lines of stretched foils were accompanied by the hydrogen uptake, was similar to the case of hydrogen charged metals. The origin of the phenomena is to be the hydrogen evolution from oil and lubricants at their catalytic decomposition, the hydrogen entry into the metal and the hydrogen-dislocation interaction within the subsurface layer of a metal. The hydrogen-induced modification of this layer and the possibility of hydrogen-induced softening or hardening should be taken into account in consideration of the tribological contacts.