The Effect of Adhesion on the Rheological and Frictional Behavior of a Confined Lubricant Film

We investigated the rheological and frictional behavior of a model system of lubricated, atomically-smooth, solid surfaces at zero and negative external normal load. The measurements were performed with a surface forces apparatus modified for oscillatory shear. For low deflection amplitudes, and negative loads up to the point when the surfaces jumped apart, the confined liquid layer (0.7 ± 0.2 nm perfluorinated heptaglyme) showed a highly elastic behavior independent of load. In the sliding regime at large amplitudes, the behavior was mostly dissipative but also independent of normal load. The force necessary to separate the surfaces was not affected by any sliding conditions. However, the friction force showed a very pronounced decrease as a consequence of sliding at large amplitudes. Thus, for our system, friction and adhesion are decoupled. We propose a mechanism of in-plane rearrangements of the molecules and explain the shear-induced reduction of friction by the formation of shear-bands.     

Nano-scale friction of polystyrene in air and in vacuum

Using atomic force microscopy (AFM), we measured friction between an AFM tip and a polystyrene surface at 25 °C, as a function of the sliding velocity and the applied normal load, both in air and under vacuum conditions. The objective was to analyze the influence of humidity on the frictional behavior of polystyrene. Our experimental results as a function of sliding velocity revealed a logarithmic increase of the friction force in air whereas a logarithmic decrease of this force is found in vacuum. Our comparative results unveil that two different dissipation mechanisms are dominating the frictional behavior of polystyrene in air and in vacuum. We propose a tentative explanation.     

Relationship between adhesion and friction forces

The Surface Forces Apparatus technique was used to measure the normal (perpendicular) and lateral forces between variously prepared surfaces under both dry and lubricated conditions. 'Normal' forces include the force vs distance functions, F(D), for surfaces separated by thin liquid films as well as the adhesion forces and energies, γ, for two surfaces in adhesional contact. 'Lateral' forces include the static and kinetic friction forces F of the surfaces as they slide past each other at a given separation, D. The results show that very thin liquid films confined between two solid surfaces can sustain both normal and shear forces or stresses. The results further indicate that the normal force, F(D) or γ, may be directly related to the static friction force, F_s, and simple equations are proposed that relate these forces (by 'static' friction force is meant the lateral force that must be applied to initiate motion, but not necessarily to maintain this motion). In contrast, the kinetic friction force, F_k, which is the force that must be continually applied to maintain motion at a given velocity, was found to be related, not to the equilibrium or reversible interaction but to the dissipative or irreversible part of the adhesion or interaction energy during a loading-unloading cycle. There is a high degree of correlation in the way that normal forces and friction forces are affected by changes in applied load or pressure, sliding velocity, loading-unloading rates and temperature. These systematic correlations can be conveniently represented by non-equilibrium 'adhesion' and 'friction' phase diagrams.     

基于纵向力的轮胎模型及参数分析

通过垂直负荷分布修正和滑动摩擦因数参数化,建立轮胎纵向力模型,利用其对轮胎纵向力特性进行分析,并研究各因素对轮胎纵向力的影响。结果表明,随着静摩擦因数的增大,接地胎面完全滑移的滑移率增大,滑移力（Fsx）和附着力（Fax）增大;纵向刚度主要影响峰值前的纵向合力,随着轮胎纵向刚度和接地印痕长度的增大,接地胎面完全滑移的滑移率减小,Fax减小,Fsx增大。     

滑动速率对La2O3–MoSi2与SiC摩擦副的高温摩擦磨损行为的影响

在XP-5高温摩擦磨损试验机上考察了La2O3–MoSi2与SiC摩擦副在1 000℃、30 N载荷以及不同滑动速率下的摩擦磨损行为。利用扫描电子显微镜和X射线衍射仪分析了La2O3–MoSi2复合材料和SiC的磨损表面形貌与相组成。结果表明:La2O3–MoSi2与SiC摩擦副的摩擦因数随滑动速率的增加而减小,在滑动速率为0.084 m/s时,La2O3–MoSi2复合材料磨损率最大;0.126 m/s时磨损率最小。其磨损机理除氧化磨损之外,还表现为黏着磨损、研磨和疲劳点蚀。SiC的磨损率随滑动速率的增加而减小,始终表现为磨损质量增加,这归因于氧化质量增加大于磨损质量损失。     

The determination of the coefficient of dynamic friction of organic powder compacts on steel

A practical experimental model system has been successfully used to study the frictional response of organic powder compacts sliding across a polished steel plate, representative of the die bore of a production compaction system. This system offers a controlled approach to the study of frictional phenomena occurring during compaction and facilitates a more detailed investigation into the fundamental mechanisms of friction than a simple resolution of forces within a punch and die apparatus.For acetylsalicyclic acid sliding on steel, the dynamic friction coefficient was found to be dependent on the displacement and, to a lesser extent, the initial normal load, whereas for PTFE on steel, the dynamic friction coefficient was independent of displacement and load and estimated at 0.09. Thus, soft organic materials exhibit very different frictional characteristics to those of brittle materials. These differences reflect the differing importance and magnitude of the three frictional components, adhesion, shear and ploughing.     

Friction and wear properties of polyimide matrix composites reinforced with short basalt fibers

Short basalt fiber (BF) reinforced polyimide (PI) composites were fabricated by means of compression‐molding technique. The friction and wear properties of the resulting composites sliding against GCr15 steel were investigated on a model ring‐on‐block test rig under dry sliding conditions. The morphologies of the worn surfaces and the transfer films that formed on the counterpart steel rings were analyzed by means of scanning electron microscopy. The influence of the short BF content, load, and sliding speed on the tribological behavior of the PI composites was examined. Experimental results revealed that the low incorporation of BFs could improve the tribological behavior of the PI composites remarkably. The friction coefficient and wear rate decreased with increases in the sliding speed and load, respectively. The transfer film that formed on the counterpart surface during the friction process made contributions to reducing the friction coefficient and wear rate of the BF‐reinforced PI composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

High-Load Friction Behavior of a Hinge Bearing Based on a Carbon/Silicon Carbide Composite

Two-dimensional carbon fiber-reinforced silicon carbide matrix (C/SiC) composites used for hinge bearing were prepared by chemical vapor infiltration. The testing and results of unlubricated friction behavior of hinge bearing under high-load transmitting motion was investigated. The effects of load on friction behavior between different sliding couple were analyzed. Finally, worn surfaces and debris were observed by scanning electron microscopy to study the wear mechanism. A constant friction coefficient between self-mated C/SiC composites of 0.68 was obtained on increasing load up to about 5800 N. Excellent wear resistance and load-carrying ability was demonstrated by low wear and especially small deformation.     

Three‐body abrasive wear behavior of short jute fiber reinforced epoxy composites

In present investigation, the three‐body abrasive wear behavior of short jute fiber reinforced epoxy composites was studied. The effect of various parameters such as fiber loading, sliding velocity, normal load, and abrasive size on the abrasive wear rate of composite has been analyzed. Abrasive wear study has been carried out using a dry sand/rubber wheel abrasion tester. The abrasive wear and friction characteristics of these composites are analyzed successfully using Taguchi orthogonal array and analysis of variance. The experimental study reveals that sliding velocity, fiber loading, and abrasive size have greater influence on the specific wear rate of the composites. The results show that the specific wear rate of the composites decreases with the increase in sliding velocity whereas, with the increase in normal load the specific wear rate increases. The study also revealed that the coefficient of friction of the composites increases up to a certain value than decreases with the increase in normal load as well as sliding velocity. The worn surfaces of the abraded specimens were examined using SEM to understand the mechanism involved in material removal. POLYM. COMPOS., 270–278, 2016. © 2014 Society of Plastics Engineers     

Sliding Friction Phenomena of a Ball-on-Disc Contact under Different Lubricating Conditions

Experiments were carried out to measure the friction of a ball-on-disc contact in a simple sliding experiment under different lubricating conditions using a Universal Micro Materials Tester (UMT-2). The measurements were also taken for the direct contact between the ball and the disc. The sliding speed was between 5.0 × 10^–6 m/s and 1.0 × 10^–2m/s. The load of the contact ranged between 1 N and 40 N giving the nominal maximum Hertzian contact pressures of 0.59 GPa to 2.03 GPa. It was found that when the sliding speed was below 1.0 × 10^–5m/s, the contact is in boundary lubrication and the measured friction coefficients for all the lubricants could be considered as independent of the sliding speed. It is suggested that the boundary film-contact interfacial slippage in the loaded zone of the asperity contact between the two surfaces is responsible for this friction phenomenon and the friction coefficient is determined by the boundary film-contact interfacial shear strength. Under the same load condition, the boundary film-contact interfacial shear strength for the 68 mechanical oil was found to be the highest; that for the 32 mechanical oil was higher than that for the PB2400 oil; while that for the purified and deionized water was the lowest and significantly lower than that for the other three lubricants. The friction coefficients measured for the direct contact between the ball and the disc were found to be close to those for water lubrication. When the sliding speed was above 1.0 × 10 ^–5m/s, the friction phenomena, taking into account sliding speed variation, was found to be quite different for the different lubricants. For the applied loads, the measured friction coefficients can be considered as independent of the load. From this phenomenon, it is suggested that the asperity contacts formed between the coupled surfaces in the experiment are almost in full plastic deformation mode.     

A tribological investigation of SLM fabricated TC4 titanium alloy with carburization pre-treatment

The tribological properties of SLM fabricated TC4 discs (before and after carburization) sliding against ZrO₂ ball at different loads (0.98–4.94 N) under bovine serum lubricated conditions were investigated by using a Ball-On-Disc (BOD) tribo-meter. In comparison, the friction and wear behaviors of the forging fabricated TC4 disc against a ZrO₂ ball were performed under the same lubricated condition, and the results represented the similar trends with the SLM fabricated TC4 (SLM -TC4). Meanwhile, the friction coefficients of SLM-TC4 and SLM-C-TC4 disc (SLM fabricated TC4 after carburization) against ZrO₂ ball both decreased with the increase of the load. Combined with the worn surface analysis and theoretical analysis, it can be concluded that the friction force of the sliding pairs mainly originated from the resistance of deformation and shear stress, and the friction coefficients are linearly correlated to negative one-third power of the normal load. Furthermore, a hard TiC layer was observed on the surface of SLM-C-TC4 disc, which may provide a certain anti-wear capacity between the sliding pairs because of its effective resistance to deformation and shear stress. Therefore, the SLM-C-TC4/ZrO₂ bearing couple showed significantly lower wear than that of the SLM-TC4/ZrO₂ bearing couple and forged TC4/ZrO₂ sliding pair. So, SLM-C-TC4 may be more suitable for joint prosthesis design.     

Friction of a C/SiC Composite Bearing in Air and in Combustion Environments

A bearing used for the load-carrying hinge was manufactured based on a carbon fiber-reinforced silicon carbide matrix (C/SiC) composite with a SiC coating. Chemical vapor infiltration was applied in the process of the composites. Experiments were conducted by an experimental simulation method that would facilitate testing under coupling stress in air and in combustion environments. Modeling analysis deduced the relationship of applied load, normal contact force, and friction force in the hinge bearing. Friction behavior was comparatively investigated up to 10 kN at room temperature in air and at 1800°C in combustion environments. Worn surfaces were observed by scanning electron microscopy, and surface roughness was measured by atomic force microscopy. Reliable thermal load-carrying ability and stable friction performance of the hinge bearing are demonstrated under this testing framework. The oxide products in the combustion environment play an important role in reducing friction by providing a reaction layer to moderate the contact stresses.     

Tribological investigation of nanocrystalline diamond films at low load under different tribosystems

The mechanical and frictional properties of hydrogen- and oxygen-terminated nanocrystalline diamond films (NCD) grown by hot-filament chemical vapor deposition (HFCVD) have been investigated in the present work.The structure and morphology of the NCD films have been characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman-effect spectroscopy. In addition, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) have been used to investigate the surface chemical groups on the NCD surface. Mechanical and frictional properties are determined using atomic force microscopy (AFM), nano-indentation, nano-scratching and micro-tribometer. The friction behavior of these films in the load range of 25 to 200 mN under reciprocating sliding conditions, using steel counter-body material has been thoroughly studied.It is noted that these films are highly crystalline with nanometer size grains and contain a very high fraction of sp³ carbon bonds. They exhibit high hardness and high elastic modulus. The friction coefficient of the film is lower under unidirectional scratch with diamond indenter than the friction coefficient under low load reciprocating sliding against steel ball. Transfer of the film from the counter-body, oxidation of transfer film and mixing of transfer film with carbonaceous layer on the worn surfaces are responsible for such behavior. Although, the friction responses of H-terminated and O-terminated films are similar under unidirectional scratch with diamond indenter, the friction coefficient of O-terminated film is always higher than the friction coefficient of H-terminated film under reciprocating sliding condition against steel counter-body material.     

混杂填料增强聚四氟乙烯复合材料的摩擦学性能研究

采用MM-200型摩擦磨损试验机对纳米SiC、MoS2和石墨填充聚四氟乙烯(PTFE)复合材料在干摩擦条件下与45#钢对摩时的摩擦磨损性能进行了研究,探讨了MoS2、石墨及纳米SiC的协同效应。认为纳米SiC的加入大大提高了复合材料的承载能力,石墨、MoS2的加入减少PTFE复合材料的摩擦因数。利用扫描电子显微镜(SEM)对PTFE复合材料的摩擦面进行了观察。结果表明:实验中5%nano-SiC和3%MoS2填充PTFE复合材料的摩擦磨损性能最好,且在高载荷下的摩擦磨损性能尤为突出,具有一定的应用价值。     

The Friction and Wear Behaviors of Vegetable Oil-Based Waxes,Natural Beeswax,and Petroleum Paraffin Wax

Quantitative analyses on the coefficient of friction of common coating waxes are necessary and essential for designing systems for coating, conveying, packaging operations, transporting, and storing of papers and paperboards, while analyses on wear behavior can be helpful for predicting performance durability of the coating surface. In this study, we investigated the friction and wear behaviors of six waxes including four commercial waxes and two soybean oil-based wax developed in our lab for bulk corrugated coating. The effect of normal load, sliding velocity, and environmental temperature was evaluated. The friction coefficient of different waxes varies with sliding conditions. Higher normal load, sliding velocity, and environmental temperature resulted in significantly greater wear loss. Crystalline morphology and crystallinity of waxes were affected by the environmental temperature, and they correlate to the variations in friction coefficient and wear loss of these materials.     

Adhesion and friction studies on silicon dioxide nanoparticle-textured surfaces prepared by the spin-coating process

Silicon dioxide nanoparticle-textured surfaces were prepared by the spin-coating process. The adhesion and friction properties of the nanoparticle-textured surfaces were investigated using an atomic force microscope colloidal probe. Experimental results revealed that the nanoparticle-textured surfaces can significantly reduce adhesive and friction forces compared with a flat surface. The main reason for this phenomenon was that the nanotexture can reduce contact area between the sample surface and the colloidal probe. The relationships between surface root mean square (RMS) roughness, packing density, and spinning rate were also discussed. The effects of surface RMS roughness and packing density on the adhesion and friction behaviors of the nanotextured surfaces were investigated. The adhesive and friction forces of the nanoparticle-textured surfaces decreased with increasing packing density. The friction forces of the nanoparticle-textured surfaces increased with increasing applied load and sliding velocity. This approach should be applied to new developments in nanosystems to reduce adhesive and friction forces between contact pairs.     

Friction coefficient and wear resistance of a modified polypropylene impregnated with different oils

The aim of the study was to determine the changes in the sliding friction coefficient and wear mechanism of PP impregnated by oils with respect to the unmodified PP under dry sliding conditions. The study showed that the impregnation of the PP clearly influenced the wear rate of PP and its friction coefficient. During the test, the wear behavior of the unmodified and impregnated polypropylenes was investigated using ASTM G77-98 standard wear test equipment in which the specimens were worn by counter sample made by steel 100Cr6. The recording program of the test system enabled the visualization and registration of the following parameters: specimen rotation and load value, linear wear of specimen, friction coefficient, and temperature of specimen and environment. The wear of the specimens was evaluated by weight loss and their wear mechanisms were investigated using scanning electron microscopy. The predominant wear mechanism between a polymer and steel was adhesion. However, at the high sliding velocity, when a higher temperature was generated, the predominant mechanism of wear was thermal wear. Impregnation of PPs moved the thermal wear into the higher sliding velocity values and caused better resistance of the PPs to melting and plastic flow. The most important finding of this investigation was that impregnation of PPs improved their tribological properties at a low value of load represented by the sliding velocity.     

Atomistic insight into the lubrication of glycerol aqueous solution: The role of the solid interface-induced microstructure of fluid molecules

Molecular dynamics simulations are performed to investigate the solid surface-induced microstructure and friction coefficient of glycerol aqueous solutions with different water contents confined in graphene and FeO nanoslits. Results show that the friction coefficient of glycerol aqueous solutions confined in both nanoslits presents similar nonlinear variation tendencies with increasing water content, but their lowest value and the corresponding water contents differ. Distinctive microstructures of the near-surface liquid layer induced by surfaces with different hydrophilicity are responsible for their difference in lubrication. The sliding primarily occurs at the solid–liquid interface for the hydrophobic graphene nanoslit owing to almost the same velocity difference in fluid molecules. By contrast, the sliding mainly occurs at the liquid–liquid interface for the hydrophilic FeO nanoslit because of the large velocity difference in fluid molecules. The weaker the interaction force at the sliding position, the lower the friction coefficient.     

A synergistic effect of nano‐Tio2 and graphite on the tribological performance of epoxy matrix composites

The influence of incorporated 300 nm TiO₂ (4 vol %), graphite (7 vol %), or combination of both fillers on the tribological performance of an epoxy resin was studied under various sliding load (10–40 N) and velocity conditions (0.2–3.0 m/s). Mechanical measurements indicated that the incorporation of TiO₂ significantly enhanced the flexural and impact strength of the neat epoxy and the graphite including epoxy. Tribological tests were conducted with a cylinder‐on‐flat testing rig. The incorporation of nano‐TiO₂ significantly improved the wear resistance of the neat epoxy under mild sliding conditions; however, this effect was markedly diminished under severe sliding conditions (high velocity and normal load). Nano‐TiO₂ reduced the coefficient of friction only under severe sliding conditions. Graphite showed a beneficial effect in reducing the wear rate and the coefficient of friction of the neat epoxy. Compared to the nano‐TiO₂‐filled epoxy, the graphite‐filled epoxy showed more stable wear performance with the variation of the sliding conditions, especially the normal load. A synergistic effect was found for the combination of nano‐TiO₂ and graphite, which led to the lowest wear rate and coefficient of friction under the whole investigated conditions. The synergistic effect was attributed to the effective transfer films formed on sliding pair surfaces and the reinforcing effect of the nanoparticles. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2391–2400, 2006     

Effect of Aluminum Filler on Friction and Wear Characteristics of Glass Epoxy Composites

An experimental study was carried out to investigate the dry sliding friction and wear characteristics of woven glass epoxy composites filled with Al particulates sliding against steel using a pin-on-disc tribometer. The glass fiber weight fraction was kept constant at 60 wt% and Al wt% varied as 0, 5, 10, and 15%. The composite was fabricated by a hand lay-up technique followed by light compression molding. Friction and wear behavior under dry sliding condition are presented as a function of sliding speed varying between 1–5 m/s and normal load ranging between 10–40 N. Friction characteristics of composites depend strongly on a combination of filler content, sliding speed and load. Wear loss increases with both sliding speed and load. Incorporation of a smaller amount of Al filler reduces wear loss compared to un-filled glass epoxy composites. An attempt has also been made to observe the distribution of fiber and Al particles in the composite, and to correlate the wear behavior using Scanning Electron Microscopy (SEM) observations.