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The friction behavior of clay particles plays an essential role in controlling its mechanical properties, but remains unclear in microscale. As one of the major clay particles, the nanoscale friction properties of hydrated montmorillonite have been studied using steered molecular dynamics (SMD) simulation method, considering the coupling effect of water content of 0 ∼ 30 % and normal load of 1 atm ∼ 10 GPa. The SMD friction along the y -direction has been performed to investigate the frictional mechanism of hydrated montmorillonite and its structural properties of interlayer water. The evolution of average friction load with water content, friction coefficients, and the distribution of interlayer water molecules were obtained. Furthermore, the SMD pulling along the z -direction was conducted to study the effect of interlayer water molecules on the interaction between montmorillonite layers. All simulation results showed that more energy and a higher friction load were required to slide at a higher normal load. The friction coefficient of 0.058 ∼ 0.17 and internal friction angle of 3.3°∼9.6° for hydrated montmorillonite agreed well with previous studies in microscale, showing the weak friction properties. The evolution of friction coefficient with water content was different at different normal loads, where the friction coefficient of 5 % hydrated montmorillonite was the highest at 3 ∼ 10 GPa. Moreover, the increasing normal load could change the distribution of interlayer water molecules. [ABSTRACT FROM AUTHOR]

To clarify the unconscious response actions to a friction stimulus applied to a finger, ten participants performed a friction test using their right index finger and a specimen on which a self-assembled monolayer was striped to intentionally change the shear strength. Drastic changes were observed in both normal load and shear strength when a finger slid across the striped surface. Because the rate of change under normal load was negligible as the sliding speed increased, the response action to the friction stimulus appeared to depend on sliding speed. • A self-assembled monolayer was striped to change the shear strength. • A negative correlation between the differences in normal load and shear strength. • The shear strength remained constant as the sliding speed increased. • The change rate of the normal load approached zero as the sliding speed increased. • The response to the stimulus depended on the speed at which the finger slid. [ABSTRACT FROM AUTHOR]

Amorphous carbon–containing CrAlCN coating was deposited on Cr12MoV steel by cathodic arc ion plating. The effect of amorphous carbon on the tribological performance of obtained coating under the different normal loads was investigated using a ball–on–disc tester, and the lubrication and wear mechanisms of amorphous carbon were discussed in detail. The results show that the CrAlCN coating is composed of AlN, CrN and CrC phases with the amorphous carbon. The average coefficient of friction of CrAlCN coatings under the normal loads of 2, 4 and 6 N are 0.458, 0.586, and 0.666, respectively, where the amorphous carbon plays a self–lubrication role in the friction process. The wear degree is deteriorated with the increase of normal load, and the wear mechanism is changed from abrasive wear to adhesive wear. • The amorphous carbon–containing CrAlCN coating presents the high nanohardness and elastic modulus • The CrAlCN coating improves the friction reduction and wear resistance performances. • The amorphous C in the CrAlCN coating plays the roles of friction reduction and wear resistance. • The wear mechanism of CrAlCN coating is changed with the increase of normal load. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Relationship between material removal mode and scratching direction is investigated. • A theoretical model is developed to relate machining depth with normal load. • An optimal machining direction is selected based on the quality of the grooves. Abstract The material removal state, which is closely related to machining quality, is an important factor in any machining process. During the machining using a non-rotational symmetric cutting tool, scratching direction manifests significant influence on the material removal state. In the present study, the relationship between material removal mode and scratching direction was investigated during the nanoscratching process on single crystal copper using an atomic force microscopy (AFM) tip. Considering the installation angle of the cantilever and the height of pile-up, a theoretical model was developed to relate machining depth with normal load for the machining of a single groove. Experimental results revealed that the theoretical normal load model was more suitable when materials were expelled in constant chip formation with small pile-ups. In addition, based on the scratching tests in different directions, the critical included cutting angle determined whether materials were expelled in chip formation or not. The height of pile-up was also influenced by the included cutting angle during the scratching in face-forward direction; however, during the machining in edge-forward direction, the distance between the cutting swept faces formed by the lateral and front edges determined the height of pile-up. Finally, an optimal machining direction is selected based on the quality of the machined grooves, which can be used in the potential application. [ABSTRACT FROM AUTHOR]

Composite materials are wildly applied in the engineering and a lot of them behave as orthotropic. An accurate and efficient method for the orthotropic plate will contribute to the accurate design and quality promotion of this kind of structure. In this paper, the 2D Green's function for the orthotropic plate loaded by a normal line force is derived at first. By virtue of the 2D general solution of orthotropic material, a series of new harmonic functions which contain undetermined constants are constructed. Inserting the harmonic function into the general solution, the corresponding elastic fields in the orthotropic plate are obtained. These undetermined constants can be determined by the boundary conditions and the equilibrium equations. Then, based on the superposition principle and the obtained Green's function, the solutions for the arbitrary distributed normal loads can be obtained. At last, numerical analysis is presented to discuss the accuracy and efficiency of this method and the stress distribution in an orthotropic plate loaded by the normal loads are studied. [ABSTRACT FROM AUTHOR]

Electrostatic charging resulted by sliding contacts of particles and wall is ubiquitous in gas-solid systems. However, the mechanism of sliding electrification involved the initial charge is still unclear. In the current paper, to accurately obtain triboelectric charge, the initial charge and the final charge were measured. A quantitative comparison of the experimental results and the value of theoretical model based on the condenser model is provided. And the results suggest the final charge of the grain is linear with the initial charge for a short sliding distance, whereas the experimental data become scattered for a long sliding length. The longer sliding length generation was found to generate more triboelectric charge, and the influences of the normal load on the triboelectric charge density were analyzed by considering the contact area. It provides a comprehensive perspective that the triboelectrification is fundamentally described as a synergistic effect of the sliding contact area and the real contact area. [ABSTRACT FROM AUTHOR]

The influences of contact time, normal load, piezo velocity, and measurement number of times on the adhesion force between two silicon surfaces were studied with an atomic force microscope (AFM) at low humidity (17–15%). Results show that the adhesion force is time-dependent and increases logarithmically with contact time until saturation is reached, which is related with the growing size of a water bridge between them. The contact time plays a dominant role among these parameters. The adhesion forces with different normal loads and piezo velocities can be quantitatively obtained just by figuring out the length of contact time, provided that the contact time dependence is known. The time-dependent adhesion force with repeated contacts at one location usually increases first sharply and then slowly with measurement number of times until saturation is reached, which is in accordance with the contact time dependence. The behavior of the adhesion force with repeated contacts can be adjusted by the lengths of contact time and non-contact time. These results may help facilitate the anti-adhesion design of silicon-based microscale systems working under low humidity. [ABSTRACT FROM AUTHOR]

The effects of applied loads and abrasive sizes on the wear of a 2.9% carbon and 19.9% chromium martensitic white cast iron were investigated. Abrasive wear tests using a pin test on silica and garnet papers (ASTM G132) were carried out using three different abrasive average sizes: 67 μm, 149 μm and 177 μm and two applied normal load: 5 N and 10 N. A modified rubber wheel abrasion test (ASTM G65) was carried out using three abrasive average sizes - 90 μm, 115 μm and 170 μm quartz sand - and three applied normal loads: 44 N, 80 N and 130 N. The abrasive shape and the wear surface of the specimens were examined using a scanning electron microscope (SEM). The worn surfaces topography was obtained with a Taylor Hobson 3-D surface analyser. In the pin test with silica paper, a change in the magnitude of the volume loss was observed as a result of increasing the applied load from 5 N to 10 N. In the rubber wheel abrasion tests the mass loss increased with the increase of particle size until a critical particle size was reached. In general, the mass loss increased with the increase of the applied load and the abrasive size. For the lower load and the smallest abrasive size wear grooves were observed only in the martensitic matrix while the carbides remained intact. With higher loads and larger abrasives, scratching was observed also in the carbides. A wear map is proposed with a ‘wear rate’ parameter developed in this work plotted against abrasive sizes. [ABSTRACT FROM AUTHOR]

High stability and movement compatibility under friction is vital to guarantee long-term use of implants in the human body. In this study, the negligible wear of the poly (vinylphosphonic acid) (PVPA)-modified Ti6Al4V/polytetrafluoroethylene (PTFE) interface in phosphate-buffered saline (PBS, PH = 7.2) were confirmed. The depth of scratches on the PVPA-modified Ti6Al4V was no more than 20 nm, while there was slight wear on the PTFE ball, as indicated by the radius of curvature being almost the same as the initial value after sliding for 2 h. In addition, it should be noted that the novel velocity and load-independent tribological behaviors are exciting. The superlubricity of the PVPA-modified Ti6Al4V can be maintained over a wide velocity range, from 0.3 mm/s to 48 mm/s, even under a high pressure of 63.49 MPa, confirming its potential application in implants. The sufficient thickness of the firm PVPA coatings with specific lubricating state and the compatibility of the two tribo-pairs in this tribological system primarily account for the novel tribological stability. This study provides insights into the tribological mechanisms of the high-stability polymer coatings. [ABSTRACT FROM AUTHOR]

The purpose of this study is to investigate the effect of normal load, electric current and sliding speed on tribological performance of electrical contact interface. Sliding electrical contact tests of H65 material are carried out by using a customized ball-on-flat tribometer. Results show that with the increase of normal load, both the friction coefficient and contact voltage drop show a downward trend. Too low or too high load will not cause the generation of friction-induce vibration (FIV). Although increasing the normal load appropriately can improve the wear behavior of the interface, it will intensify wear if the normal load is overly high. Larger electric current will increase the friction coefficient, contact voltage drop and surface temperature, but reduce the contact resistance. A certain value of electric current will cause the generation of FIV. Larger electric current will destroy the surface lubricating film of interface and intensify wear. With the increase of sliding speed, the friction coefficient firstly increases and then decreases, while the contact voltage drop shows the opposite trend. Strong FIV will be generated at a certain value of sliding speed. Larger sliding speed will cause the formation of a dense film layer, which plays a good lubrication effect. • The electrical sliding tribological properties of H65 material are studied. • Effect of normal load, electric current and speed are studied systematically. • Friction-induced vibration during sliding electrical contact is analyzed. • Different wear mechanisms for electrical sliding wear of H65 are presented. [ABSTRACT FROM AUTHOR]

Water-solubility alcohols are reported to exhibit excellent lubricity at both low and high normal loads during sliding, and the lubrication mechanisms for water-solubility alcohols are well known until now. However, the lubricity of water-insolubility alcohols is rarely reported and its lubrication mechanisms were still not well understood so far. Here, we first investigate the lubricity of cyclohexanol between self-mated YG8 balls. The experimental and computational results reveal that cyclohexene and its derivative, and tungsten oxide is initially indeed formed during sliding, then, the polymerization is induced by friction-induced effect between cyclohexene and its derivative, revealing that the formation of polymers can effectively stabilize friction coefficient, and the formation of tungsten oxide plays an important role in enhancing wear resistance. Besides, for 196 N, the formation of carbon quantum dots makes friction coefficient reduce with increasing sliding time compared to that of 98 N. The new find will be potentially applied in machinery and equipment. [ABSTRACT FROM AUTHOR]

Micro-crystalline diamond (MCD) films are deposited on reactive sintering SiC substrates by the bias enhanced hot filament chemical vapor deposition (BE-HFCVD) method, respectively using the methane, acetone, methanol and ethanol as the carbon source. Two sets of standard tribotests are conducted, adopting Si 3 N 4 balls as the counterpart balls, respectively with the purpose of clarifying differences among tribological properties of different MCD films, and studying detailed effects of the carbon source C , normal load F n and sliding velocity v based on orthogonal analyses. It is clarified that the methane-MCD film presents the lowest growth rate, the highest film quality, the highest hardness and the best adhesion, in consequence, it also performs the best tribological properties, including the lowest coefficient of friction ( COF ) and wear rate I d , while the opposite is the methanol-MCD film. Under a normal load F n of 7 N and at a sliding velocity v of 0.4183 m/s, for the methane-MCD film, the maximum COF ( MCOF ) is 0.524, the average COF during the relatively steady-state regime ( ACOF ) is 0.144, and the I d is about 1.016 × 10 −7 mm 3 /N m; and for the methanol-MCD film, the MCOF is 0.667, the ACOF is 0.151, and the I d is 1.448 × 10 −7 mm 3 /N m. Moreover, the MCOF , ACOF , I d and the wear rate of the Si 3 N 4 ball I b will all increase with the F n , while the v only has significant effect on the ACOF , which shows a monotone increasing trend with the v . [ABSTRACT FROM AUTHOR]

This study focused on the normal load dependence of the friction coefficient for the sliding friction of a rubber material with a rough surface. A developed friction tester was used to visualize the real contact regions distributed within the transparent contact interface between poly-dimethyl siloxane (PDMS) and glass surfaces. Based on experimental results, an adhesion friction model was developed to explain the normal load dependence of the friction coefficient. This model provides a simple technique that can roughly but easily estimate the real contact area and shear stress without in situ observation of the contact interface. [ABSTRACT FROM AUTHOR]

A fundamental study of wear transition regimes was carried out for a pin-on-disk sliding couple, involving titanium and steel. The sliding speed was varied from 0.38 to 1.5 m s −1 and the normal load from 10 to 50 N. Wear mapping approaches have been undertaken to represent the transitions in wear modes and wear mechanisms regimes, as a function of applied normal loads and sliding speeds and for both pin and disc separately on the basis of experimental results. Dry sliding wear behaviour of steel was characterized by tribo-oxidative wear with high material transfer from the titanium. In contrast, adhesive wear was more prevalent for the titanium and oxidative wear mechanisms led to formation of non-protective films on the surface. [ABSTRACT FROM AUTHOR]

An in-situ tire test rig was developed for field research on tire tractive and maneuverability performances. The Single Wheel Tester (SWT) was mounted on a tractor and a tested wheel was driven by a hydromotor, along a frame of 3 m length. In the SWT, four load cells were utilized to measure longitudinal and lateral forces, input and self-aligning torques, and two optical counters were applied to calculate forward and angular velocities. Response Surface Methodology was used to execute experimental design and to analyze the collected data. Afterwards, reduced form of a 2 Factor Interaction model was extracted to predict rolling resistance using seven factors. The test results show that increasing the normal load and side slip angle will cause an increment of rolling resistance. The incremental growth rate of the rolling resistance due to the normal load increment was observed. At higher cone index values, increasing the angular velocity reduces the rolling resistance, although at lower cone index values, the effect of angular velocity on rolling resistance is in reverse order. In addition, the increasing moisture content effect on rolling resistance at lower side slip angle values was observed. [ABSTRACT FROM AUTHOR]

In this research, friction coefficients of composite materials such as gear fiber reinforced plastic (gear fiber) and glass fiber reinforced plastic (glass fiber) are investigated and compared. In the experiments, gear fiber and glass fiber slide against different austenitic stainless steels such as stainless steel 201 (SS 201) and stainless steel 301 (SS 301). Experiments are carried out at low loads 2, 4 and 6 N, low sliding velocities 0.2, 0.4 and 0.6 m/s and relative humidity 70%. The obtained results reveal that in general, friction coefficient of gear fiber and glass fiber increases with the increase in normal load and sliding velocity. Results show that friction coefficient of glass fiber-SS 201 pair is the highest and gear fiber-SS 301 pair is the lowest within the observed range of normal load and sliding velocity. On the other hand, it is found that friction coefficient of glass fiber-SS 301 pair is slightly higher than that of gear fiber-SS 201 pair. During the running-in process, friction coefficient of gear fiber and glass fiber steadily increases with the increase in rubbing time and after certain duration of rubbing, it remains constant regardless of the counterface material. The obtained results reveal that for the observed range, the influence of normal load on the frictional properties of gear fibe and glass fiber is greater than that of sliding velocity. At identical operating conditions, the magnitudes of friction coefficient of gear fiber and glass fiber are different depending on normal load, sliding velocity and counterface material. [ABSTRACT FROM AUTHOR]

In this work, a fundamental study of the wear transition regimes with respect to sliding speed and normal load was carried out for a pin-on-disk sliding couple, involving hardened steel (disks) and titanium base titanium carbide composite coatings (pins). The coating was deposited using a Tungsten Inert Gas (TIG) welding torch melting process. The sliding speed was varied from 0.38 to 1.5 m s −1 , and the normal load was varied from 10 to 50 N. Dry sliding wear behavior of the disks was characterized by abrasive–oxidative wear at lower normal loads while adhesive–oxidative wear predominated at higher loads with iron oxide transfer to the TiC composite coated pins. In contrast, micro-polishing and adhesive wear predominated the TiC composite coatings pins along with very mild abrasive wear in evidence. However, the wear regime depended on the conditions with a range of transitions observed over the sliding speeds and loads tested. Wear maps have been constructed to represent the wear mode transitions and the observed wear mechanisms have been discussed. [ABSTRACT FROM AUTHOR]

In this paper, two mathematical models were developed to predict side slip angle for driven wheels of a prototype tractor. The independent variables involved moisture content, cone index, normal load, inflation pressure, angular velocity, and steer angle with three levels. Response Surface Method (RSM) was used to extract models and to analyze results. The tractor was driven in circular paths to the left side to apply all experimental treatments. Among different mathematical models, the linear models with coefficient of determination of 0.8978 and 0.8983 were chosen to predict side slip angle of the left and right wheels, respectively. The mathematical models analysis of variance showed that all factors had significant effect on tire side slip angle. In all experiments the inner wheel side slip angle was lower than that of the outer. An inverse effect for cone index and wheel normal load was observed while other factors effects were found to be direct. Also, the angular velocity had the highest impact on side slip angle for both rear wheels. [ABSTRACT FROM AUTHOR]

This paper proposes a nano-groove depth prediction model for the atomic force microscopy (AFM)-based multi-passes scratching method in which the AFM tip is considered as a cone with a spherical apex. The relationship between the normal load applied on the sample and the depth of the machined nano-groove is systematically investigated for the multi-passes scratching process. Nano-grooves are fabricated with several normal loads and two passes scratches on a 2A12 aluminum alloy surface to verify the developed models. Results show that the hardness may become larger near the machined region after one pass scratching test and a correction factor is introduced into the two passes scratching model which is as a function of the first pass machined depths of the nano-grooves. Based on the correction model, several nano-grooves with an expected depth are machined with different normal loads for each pass in the two passes scratching tests and the difference between the experiment results and the expected values is less than 10%. Actually, to machine a nano-groove with a desired depth, this method has the potential to distribute the appropriate normal load applied for each pass to reduce the tip wear and be used for nano-groove depth correction using the multi passes scratching technique. [ABSTRACT FROM AUTHOR]

Abstract: A nanochannel depth prediction model for an atomic force microscopy (AFM)-based nano-milling fabrication technique was developed and the relationship between the normal load, tip feed, and nanochannel depth was systematically investigated. Machining experiments were performed to fabricate nanochannels on a 2A12 aluminum alloy surface, the depth of which was compared with that calculated with the model. The model predicted the depth of the nanochannels accurately when a feed in the range of 40–90nm was used during machining. The cutting mechanism played an important role in determining the consistency of the experimental and theoretical results. In addition, the effects of normal load and feed on the surface quality of the machined nanochannel were studied. A nanochannel with a 3D structure at the bottom is presented by the nano-milling method. To machine a nanochannel with a desired depth, the appropriate normal force and feed to guarantee the surface quality can be selected simply and easily using this model. [Copyright &y& Elsevier]

Abstract: In biomedical research, because of the critical nature of corrosion within the environment of motion and interactions of weight bearing surfaces, scientists and engineers have shown a growing interest in the area of “tribocorrosion”. Fundamentally, tribocorrosion combines the disciplines of tribology and corrosion and their concurrent exchanges. Principles of tribocorrosion are applied in a range of industries including the offshore, space and biomedical (for example, dental and orthopedic) industries. One of the challenges in tribocorrosion research is the lack of standard testing equipment and methodologies, particularly in its bio-medical applications. Many investigators and research laboratories are engaged in modifying their existing classical tribology apparatus, whether they are commercially available or customized tribometers, to integrate electrochemical monitoring. The current work is focused on developing a tribocorrosion test apparatus to model the contact situation of artificial hip joints with particular attention on possible erroneous issues that could arise in such a system. The test system facilitates studies on the tribocorrosion behavior of implant metals or coatings as a function of diverse tribological and electrochemical parameters, and allows monitoring temperature, pH and dissolved oxygen level during testing. Initial results on the HC CoCrMo alloy in bovine calf serum (BCS), indicate that the proposed system is reliable and useful for the investigation of tribocorrosion behavior of implant materials. Electrochemical impedance spectroscopy tests (EIS) are employed before and after the sliding tests. The usefulness of such tests in understanding the changes in the surface chemistry and corrosion kinetics are explained, with the help of optimum EIS models. An attempt has also been made to highlight the relevance on wear quantification methods and possible ways to analyse the synergistic interactions of wear and corrosion for the biomedical tribosystems in orthopedics and dentistry. [Copyright &y& Elsevier]

Abstract: With a worldwide increase in the ingestion of acidic soft drinks, the wear of human teeth in acidic oral environment is becoming normal, especially in young people. Previous studies mostly focused on the friction and wear behaviours of human teeth in the artificial saliva, and wear behaviours in acidic environment are still far from being well understood. In this paper, the wear behaviour of human tooth enamel against titanium alloy in 0.001M citric acid solution (pH 3.20) has been investigated using a reciprocating wear machine. Three normal loads, 10N, 20N, and 40N, were used. Results showed that in the citric acid solution, a honeycomb-like structure appeared on the surface of enamel as a result of severe dissolution of the enamel rods, and the surface hardness decreased. Under a low normal loading level, the surface softening of enamel caused by erosion dissolution played a significant role in its wear behaviour, and the wear mechanism of enamel was dominant by adhesion delamination. Hence, enamel wear in the citric acid solution was significantly higher than in the artificial saliva. With the load increasing, brittle fracture by the loading force aggravated, and obvious flake delamination happened. Enamel wear tended to be characterized mainly by mechanical removal. As a result, both the wear morphology and wear loss of enamel in the citric acid solution were similar to those in the artificial saliva at a high normal load of 40N. These results indicated that there existed a competitive mechanism between the mechanical action and the chemical action when enamel wear occurred in the citric acid solution. The effect of erosion on the wear behaviour of enamel was more obvious under a low load than under a high load. The results would help extend understanding of the tooth wear process in acidic environment and provide a more rational explanation for the mechanism of tooth wear in the mouth. [Copyright &y& Elsevier]

Abstract: Metal-on-metal (MOM) hip prosthesis bearings have enjoyed renewed popularity, but concerns remain with wear debris and metal ion release causing a negative response in the surrounding tissues. Further understanding into the wear and corrosion mechanisms occurring in MOM hips is therefore essential. The purpose of this study was to evaluate the tribocorrosion behavior, or interplay between corrosion and wear, of a low-carbon CoCrMo alloy as a function of loading. The tribocorrosion tests were performed using two tribometer configurations. In the first configuration, “System A”, a linearly reciprocating alumina ball slid against the flat metal immersed in a phosphate buffer solution (PBS). In the second configuration, “System B”, the flat end of a cylindrical metal pin was pressed against an alumina ball that oscillated rotationally, using bovine calf serum (BCS) as the lubricant and electrolyte. System B was custom-built to emulate in vivo conditions. The tribocorrosion tests were performed under potentiostatic conditions at −0.345V, with a sliding duration of 1800s and a frequency of 1Hz. In System A the applied loads were 0.05, 0.5, and 1N (138, 296 and 373MPa, respectively) and in System B were 16, 32, and 64N (474, 597, and 752MPa, respectively). Electrochemical impedance spectroscopy (EIS) and polarization resistance were estimated. The total mass loss (K wc) in the CoCrMo was determined. The mass loss due to wear (K w) and that due to corrosion (K c) were determined. The dominant wear regime for the CoCrMo alloy subjected to sliding changes from wear–corrosion to mechanical wear as the contact stress increases. An attempt was made to compare both system, in their tribochemical responses and formulate some insights in the total degradation processes. Our results also suggest that the proteins in the serum lubricant assist in the generation of a protective layer against corrosion during sliding. The study highlights the need of adequate methodology/guidelines to compare the results from different test systems and translating in solving the practical problems. [Copyright &y& Elsevier]

Abstract: Nanocrystalline Al based composite containing 39vol.% AlN nanoparticles with enhanced mechanical properties was subjected to friction and wear characterization under various conditions. It is interesting to found that an oxygen-rich tribolayer was formed at all sliding speeds and normal loads, which seems to control the tribological behavior of the composite to a large extent. Friction coefficient of the composite increases with the increase in sliding velocity, while the wear rate decreases in a hyperbolic trend. The improved wear resistance at higher velocity was attributed to increased tribolayer coverage on the worn surface, and the enhanced tribo-oxidation. Increase in the applied load leads to steady reduction of the friction coefficient, and slight variation of the wear rate in load range of 5–25N. In this range, tribolayer is the most import feature on worn surface, and wear occurs mainly by delaminating within the tribolayer. Whereas, when the load increases up to 35N, both the friction coefficient and wear rate steeply increase up to high values, indicating a severe wear. At this load, tribolayer only covers small part area of worn scar, and delamination happens within the composite underneath the tribolayer. [Copyright &y& Elsevier]

Abstract: A new coupled-oscillator model, in which the relationship of the lateral force and the normal force are considered, is proposed for the interfacial friction. The Maugis-Dugdal model is used to approximately substitute the Lennard-Jones potential of the interfacial friction in the new model. The mechanism of microscopic sliding and the energy dissipation mechanism of friction are discussed. The formulae for frictional force calculation are then deduced. Based on theoretical analysis, it is found that the frictional force increases approximately as 2/3 the power of the normal force for a tip-sample contact system, which is same as the classical conclusion of nano-tribology. A comparison between the theoretical results and the experimental data obtained with an atomic force microscope is presented at the end of the paper. The result has shown that the new coupled-oscillator model and the formulae deduced are feasible. [Copyright &y& Elsevier]

Abstract: Friction and wear behavior of a peak aged Cu–0.65wt.%Cr alloy was investigated. The friction and wear experiments were run under ambient conditions with a pin-on-disk tribometer. Experiments were performed using various applied normal loads and sliding velocities. The tribological behavior of the studied alloy was discussed in terms of friction coefficient, wear loss and wear mechanism. Friction coefficient and wear loss have shown large sensitivity to the applied normal load and the sliding velocity. At the sliding velocity of 0.3m/s weight loss increased from 6.9 to 51mg by increasing the normal load from 20 to 40N. At higher sliding velocity minimum weight loss is achieved at 60N normal load. So it can be seen that with increasing normal load wear rate decreases due to the formation of a continuous tribofilm which consists of Fe–Cu intermetallic. Varying of friction coefficients in different conditions of normal load and sliding velocity is correlated to the wear behavior. The analysis of worn surfaces by XRD and SEM showed that an increase in normal load and sliding velocity creates an intermetallic wear-induced layer, which modifies the wear behavior of the alloy. The XRD result indicates that new phase of Cu9.9Fe0.1 is generated on worn surfaces of the pin specimens during the wear tests. There is a significant correlation between the micrograph of worn surfaces and the wear rate of specimens. [Copyright &y& Elsevier]

The influence of normal loads and mating pairs on the tribological properties of diamond-like carbon (DLC) film was investigated using atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman characterizations. The friction and wear behaviors of DLC film are not only related to the friction pairs but also related to the normal loads. The friction coefficient and wear rate of DLC film are smaller with the increase of load, but the wear degree of DLC film is more serious with the increase of hardness of the friction pair. Raman analysis indicates that the structural evolution of DLC film should have a limited contribution on the different friction behaviors of DLC film against ZrO 2 , bearing steel and SiC balls. The results of TEM and Raman characterizations further show that the key factor determining the low friction of DLC film is the graphitization degree of the transfer layer rather than the formation continuity and compactness of the transfer layer. The results further deepen our understanding of the related mechanisms between DLC films and friction pairs under different normal loads as guidance for the application of DLC films in future industrial fields. • Friction behavior of DLC film depends on not only load but also friction pair. • Load and friction pair affect the formation degree and structure of transfer layer. • Friction heat plays a key role in the graphitization degree of transfer layer. • High hardness of friction pair can lead to the more serious wear of DLC film. [ABSTRACT FROM AUTHOR]

Abstract: The paper reviews some important published papers on the effects of railway track imperfections on track dynamic behavior, and investigates the effect of unsupported sleepers on the normal load of wheel/rail in detail through a numerical simulation. The numerical simulation is based on a coupling dynamic model of vehicle–track. In the model, the vehicle is modeled as a multi-body system, and the track is considered as a 3-layer model with rails, sleepers, and ballast masses. Each rail of the track is modeled with a Timoshenko beam resting on discrete sleepers. The lateral, vertical, and torsional deformations of the beam are taken into account. The sleepers are assumed to move backward at a constant speed to simulate the vehicle running along the track at the same speed, and therefore such a track model can consider the effect of the discrete support by sleepers on the coupling dynamic behavior of the vehicle and track in the simulation. In calculating the coupled vehicle and track dynamics, Hertzian contact theory and the theory by Shen et al. are, respectively, used to calculate the normal forces and the creep forces between the wheels and the rails. The motion equations of the vehicle–track are solved by means of an explicit integration method. A nonlinear spring and a nonlinear damper are used to simulate a gap between the unsupported sleeper and the ballast mass. The numerical results obtained indicate that the gaps between the unsupported sleepers and ballast masses have a great influence on the normal load of the wheel and the rail. [Copyright &y& Elsevier]

Abstract: The effect of normal load on the tribo-electrification mechanism is investigated for the five soft metals, namely Pb, Ag, Cu, Zn and Al, sliding on iron for complete contact under dry severe wear in a reciprocating friction tester. Results showed that the amplitude of the resultant tribo-electrification increased with increasing normal load. As the dynamic real contact area increased with increasing normal load, the tribo-electrification voltage was therefore not linearly proportional to the normal load. It was revealed that the tribo-electrification caused by material transfer was proportional to the (1+ n) power of normal load, and that caused by Seebeck effect was proportional to the square root of normal load. Consequently, the average interface temperature rise caused by Seebeck effect was also proportional to the square root of normal load. The error in estimating the interface temperature rise was found to increase with decreasing normal load. When the normal load exceeded 40N, the temperature rise reached saturation in the range from 15% to 45%. The main finding was thus that the effect of material transfer on the interface temperature rise was critical and cannot be neglected. An empirical formulation was proposed to predict the average interface temperature rise T S for the reciprocating friction process in terms of the friction coefficient f, the normal load L, and the average speed as , where b is 15 for the first cycle and 35 for the quasi-steady-state cycle. [Copyright &y& Elsevier]

Abstract: The fretting corrosion behavior of tin-plated brass contacts is studied at various normal loads (0.25, 0.5, 1.0 and 1.5 N) and fretting frequencies (1, 3 and 8 Hz). The typical characteristics of the change in contact resistance with fretting cycles and time are explained. Irrespective of the frequencies under study, 1 N normal load suppressed the fretting corrosion of tin contacts by maintaining the mechanical stability and good electrical contact between the contacting members which makes less accumulation of wear debris at the fretted zone. For a given normal load, the fretting corrosion of tin-plated contacts occur much faster at higher frequencies as it provides more fresh metal for oxidation and generates more accumulation of oxide wear debris at the contact zone. The failure time, i.e. the time for contact resistance at the fretted surface to reach 0.1 Ω is delayed with increasing normal loads at the studied frequencies. For a given normal load, the failure time reaches early at 8 Hz, i.e. at higher fretting frequencies. The fretted surface is examined using laser scanning microscope, scanning electron microscope and energy dispersive X-ray analysis to assess the surface profile, extent of fretting damage, extent of oxidation and elemental distribution across the contact zone. From the surface profile data, the fretted area and the wear rate is calculated and correlated with the observed extent of oxidation and earlier failure of electrical contacts. The surface morphology and EDX analysis results of the fretted surface clearly revealed the severe fretting damage at 0.25 N and 8 Hz. [Copyright &y& Elsevier]

Tribological tests were conducted on different pairs of the bearing steel 100Cr6 and a commercial alumina under unlubricated and oil lubricated conditions where the transition behaviour from static to kinetic friction as a function of surface finish and contact pressure was of particular interest. Static and kinetic friction were measured using a laboratory tribometer with ball-on-plate geometry at an extremely low tangential speed of 8.3 μm/s applied to the plate specimen. Results showed a significant effect of surface finish on both the static and kinetic coefficients of friction as well as on the transition behaviour. Transition behaviour was strongly dependent on the materials mated. The self-mated steel pairs showed greater values of kinetic than static coefficient of friction while no significant difference of both friction values was measured on self-mated alumina pairs with ground plate specimens. Influence of oil lubrication on static friction was small compared with that on kinetic friction where the last one was substantially reduced by oil lubrication with 100Cr6/100Cr6 and 100Cr6/Al2O3 pairs, particularly. Stick-slip effects occurred with both unlubricated and lubricated pairs under high normal loads and depending on surface finish. [Copyright &y& Elsevier]

The steering forces on an undriven, angled wheel mounting a 6-16 8PR tire were measured on a wheel test carriage at zero camber angle and at 1.5 km/h forward speed in a soil bin with sandy clay loam soil. The lateral force developed was found to be a function of slip angle, normal load, and inflation pressure for a particular soil condition. An exponential relationship could estimate the coefficient of lateral force of the 6-16 tire. The coefficients of this equation were found to be linearly related to inflation pressure. Rolling resistance of the wheel tested was found to be a function of slip angle, normal load, and inflation pressure for the soil condition tested. A linear relationship existed between the rolling resistance and slip angle, where the coefficients were found to be a function of inflation pressure and normal load. The generalized equations developed in the present study for estimating coefficients of lateral force and rolling resistance by taking both the tire and operating parameters into account, were found to be reasonably good by looking at the high coefficient of determination between experimental and estimated values. [Copyright &y& Elsevier]

The nanotribological properties of magnetron sputtered sulfur deficient MoSx (x=1.3) coatings were investigated by oscillating lateral force microscopy (LFM) in ambient air of 50% relative humidity (RH) and 23 °C. The formation of a transfer film on the Si3N4 cantilever tip originated from the MoSx coating is found. The effect of that transfer film on the adhesive and frictional force between the MoSx coating and the cantilever tip, is reported for this nanoscale friction tests. Wear of the MoSx coatings was found on sliding against a Si3N4 cantilever tip. The sliding behavior of MoSx coatings in oscillating nano- and macro-sliding tests performed at different oscillation frequency is discussed. [Copyright &y& Elsevier]

The interparticle friction behavior of saturated clay controls its mechanical properties, but remains unclear at nanoscale. As one of major clay minerals, the hydrated montmorillonite (MMT) is selected to investigate the nanoscale friction characteristics using Molecular Dynamics simulation method. Two portions of MMT representing two particles with a water film in the middle are created to simulate an undrained system. A virtual spring is applied on the upper MMT portion to provide the sliding with a constant velocity relative to the bottom portion. The effects of normal load and sliding velocity on the frictional behavior are then investigated. The friction coefficients of hydrated MMT at different cases were measured and compared with other experimental and simulation results for the validation. The evolution of normal load with the number of hydrogen bonds for hydrated MMT was finally analyzed. All simulation results indicated that the friction load fluctuated periodically with a cycle of about 9.10 Å at sliding velocity inferior 0.001 Å•fs−1, which was nearly equal to montmorillonite's lattice constant along the sliding direction; the fluctuation amplitude of the friction load increased with the decreasing sliding velocity; the relationship between the average friction load and the logarithm of sliding velocity followed a power function; the friction coefficient and the cohesion were found to increase approximately linearly with sliding velocity. [Display omitted] • The friction load fluctuates periodically at low sliding velocity. • The cycle equals to montmorillonite's lattice constant along the sliding direction. • Friction coefficient and cohesion increase almost linearly with sliding velocity. • Bound water increases with normal load till a certain level. [ABSTRACT FROM AUTHOR]

Diamond-like carbon films (DLC) as a prominent surface protection coating are used to enhance the tribological performance of engineering components due to their low coefficient of friction (COF), high wear resistance and excellent chemical stability. In recent years, researchers have separately studied the environment-dependent and load-dependent tribological behaviors of DLC films. However, there is few studies about the influence of corrosive media on the load-dependent tribological behaviors of DLC films. In this paper, we explained the tribo-mechanism of chromium doped DLC film (Cr-DLC) in hydrochloric acid (HCl) by combing the effect of load-dependent. For comparison, tribological behaviors of Cr-DLC films in ambient air and distilled water environment were also investigated. It is found that the coefficient of friction of Cr-DLC film complies with the Hertzian elastic contact model in HCl solution. In other words, the COF decreases with the increase of normal contact load except 2 N. The Cr-DLC film shows the highest wear resistance with wear rate of 1.79 × 10−7 mm3/Nm at 5 N. The main wear mechanism at 2 N and 5 N is governed by the formation of a graphite-like transfer film and tribochemical products on friction interfaces; Whereas the wear mechanism for 7 N and 10 N contact load is attributed to the superimposed effect of the high Hertzian contact stress and the corrosion function of chloride ion. The insights acquired from this study can be used to the design of high-performance of DLC coated tribo-systems in corrosive environment. [Display omitted] • Tribo-mechanism of Cr-DLC in HCl solution by combing environment and load effects is investigated. • The transfer layer with tribochemical products lead to the lower Cr-DLC wear at low applied loads. • The synergistic effect of Hertzian contact pressure and chloride ion corrosion influences the wear behaviors at high loads. [ABSTRACT FROM AUTHOR]

Electrical railways produce airborne wear particles (AWPs) in significant amounts from wheel–rail contact. Herein, the effects of wet conditions on the AWPs released from the wheel–rail contact under different normal loads were investigated using a twin-disk rig. A fast mobility particle sizer and an optical particle sizer were used to measure number concentrations (NCs) of AWPs. Analyses of NCs and particle size distribution were conducted. The findings suggest that significantly fewer AWPs are generated under wet conditions than under dry conditions. Marked differences were also seen in the size distribution peaks between wet and dry conditions: under dry conditions, the largest peak corresponded to fine particles, whereas under wet conditions, the largest peak corresponded to ultrafine particles. The generation trends of AWPs were different for dry and wet conditions: AWPs increased and then decreased after maximal NCs under dry condition; however, they increased and then remained nearly constant under wet conditions. The total amount of ultrafine, fine, and coarse particles increased with normal load under wet conditions, indicating that AWP generation is significantly influenced by the normal load. Furthermore, water can significantly reduce the amount of ultrafine, fine, and coarse particles generated from the wheel–rail contact. • AWP generation was examined for various normal loads under wet conditions. • The applied normal load influenced the generation of all particle size categories. • Water significantly reduced the generated amount of all particle size categories. • Under wet condition, ultrafine particles were dominantly generated. [ABSTRACT FROM AUTHOR]

Contact between a deformable, random rough surface and a rigid flat counterpart was investigated experimentally and theoretically. In the experimental setup, two modes of rough surface flattening were performed, namely normal compression and sliding (tangential load) in presence of normal compression. The load-approach relationship and friction coefficient were measured. After unloading the surface, the topography was measured using scanning profilometry. The deformation of roughness zone, i.e. evolution of the real contact area (RCA), and roughness parameters were analysed. A model based on statistical analysis and a finite element solution was applied to predict the load–approach relationship. The numerical and experimental results were compared. The effect of friction coefficient was discussed. • The contact of rough surface under tangential and normal load is investigated in elastic-plastic range. • The variation of roughness parameters, real contact area, approach and friction coefficient are specified experimentally. • The local deformation of single asperity is analysed numerically for different modes of load. • The model of contact is proposed in which two kinds of load are taken into account. [ABSTRACT FROM AUTHOR]