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The effect of normal loads on the tribological properties of Fe-based bulk metallic glass composites (BMGCs) are studied in this paper. Fe-based BMGCs were firstly fabricated by water-cooled copper mold suction casting method. Then the phase composition and microhardness of the samples were characterized by X-ray diffraction and microhardness tester, respectively. The friction and wear properties of the samples were tested by the reciprocating friction and wear tester. The results show that there are both crystalline and amorphous phases in alloys, and the proportion of amorphous phase is 55.65%. In addition, compared with traditional crystalline materials of the same composition, Fe-based BMGCs possess higher hardness. With the increasing of the normal load, the wear mechanism of the sample surface gradually changes from abrasive wear and oxidation wear to the coexistence of abrasive wear, oxidation and fatigue wear mechanism, as well as the wear quantity increases further. [ABSTRACT FROM AUTHOR]

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]

The experiment was carried out in a reciprocating lubricated conformal sliding contact between steel discs of the same hardness. The effects of disc surface texturing on the friction coefficient at various operating conditions (temperature, normal load, and frequency of oscillations) were studied. Under various conditions, surface texturing caused friction reductions of sliding pairs. The largest reduction was 4.6 times at a lower temperature and 2.5 times at a higher temperature. The effect of the pit area ratio on the friction reduction was visible at a higher temperature. The highest dimple density of 25% corresponded to a lower coefficient of friction than the smallest density of 9%. The sliding pair with a dimple density of 17% led to large variation of the friction force. At lower temperatures, the coefficients of friction were lower compared to tests at higher temperatures. [ABSTRACT FROM AUTHOR]

Analysis of non-local, phase-lag, and temperature-dependent properties of modified couple stress thermoelastic diffusive medium is examined in conditions of exciting by thermomechanical sources. The governing equations are framed involving non-local, phase-lag, and temperature-dependent properties. These equations are simplified by using the potential functions and employing the Laplace and Fourier transforms for further study. The problem is solved by deploying suitable thermomechanical loads. A specific type of normal and thermal loading of the ramp-type is considered. The transformed components of the physical field like the displacements, stresses, temperature change, and chemical potential are derived. A numerical analysis is performed for these quantities using the numerical technique. The graphs of the resulting quantities are shown to analyze the impact of non-local, phase-lag, and temperature-dependent properties. The specific cases are also mentioned. [ABSTRACT FROM AUTHOR]

Multiple passage of power machinery system particularly heavy machines with high wheel loads creates sub-soil compaction which results into increasing in soil bulk density & penetration resistance and reduction in crop germination, growth as well as yield. This study was conducted to determine the wheat crop growth and yield models could be developed to predict growth as well as yield of crop considering normal load and number of passes of tractor. A 36-plot experiment consisting of 12 treatments with three replications were set up using a randomized block design in a uniform field of Division of Agricultural Engineering, IARI, New Delhi. Prediction models were developed between compaction parameters (normal loads and number of passes) and crop parameters like (a) plant height, (b) number of plants per meter, and (c) yield. Further, another relationship between crop yield and sub-soil bulk density and penetration resistance were established and their sensitivity analysis was done. The best fit model for plant height and number of plants per meter row was quadratic. However, the best fit models between yield vs soil bulk density and yield vs penetration resistance for critical layer and whole soil were exponential and quadratic, respectively. The developed model is not more sensitive for number of plants per meter row and yield vs soil bulk density. However, model was more sensitive to plant height model as well as yield vs soil penetration resistance. [ABSTRACT FROM AUTHOR]

The friction behavior of high-purity titanium in microscale was experimentally investigated by using atomic force microscopy to scan two neighboring grains with different crystalline orientations across the grain boundary. The results indicated that there were obvious friction contrast (FG1/FG2) between the two neighboring grains. The friction contrast became more obvious when higher normal load was applied, and this trend could be well reproduced by using the DMT model. Based on this, the interfacial shear strength was identified as the main factor causing the friction contrast. Higher scan velocity reduced the friction, which was regarded as the result of the decreased ability to maintain a stable capillary bridge in ambient atmosphere. However, the scan velocity had no significant effect on the friction contrast. [ABSTRACT FROM AUTHOR]

Surface texturing plays an increasingly important role as it can directly influence the tribological behaviors of contact surfaces in many ways. In texturing, tribological performance depends upon the types of design texture used and methods of fabricating texture on the surface. In this research work, friction and wear behaviors of surface textured EN-31 alloy steel have been investigated under wet condition by using DUCOM linear reciprocating tribometer. The textured dimples are fabricated by micro-EDM and micro-drilling CNC machining processes. Initially un-textured surfaces of EN-31 alloy steel with EN-8 steel pin are experimented for friction and wear under ISO-68 and ISO-220 lubricant oil. Under the different normal load conditions, it has been observed that the ISO 68 oil gives better friction-wear characteristics with respect to ISO-220 oil. Then after textured surfaces were investigated by using optimized ISO-68 lubricant. Experimental results on micro-EDM provide better friction and wear properties with respect to micro-drilling CNC. Surface texturing may act as wear debris trapper, lubricant reservoirs, hydrodynamic lift, and retarding the lubricant molecules flow in a particular path where potential exists. In this work, it has observed that texturing by micro-EDM enhanced mixed lubrication and minimum friction in mixed lubrication leads to decrement of wear loss. [ABSTRACT FROM AUTHOR]

Purpose: Random point-contact between the space bearing retainer and the rolling elements may cause wear of the space bearing retainer. The paper aims to clarify the friction and wear behaviors of polyimide bearing retainer under point-contact condition. Design/methodology/approach: Space bearing retainers were cut into flat specimens and the tribological behaviors of the specimens were studied under point-contact condition using a friction and wear testing machine. Different sliding velocities and normal loads were used to simulate the running state of space bearing retainer. The wear behaviors of the space bearing retainer were analyzed by SEM and white light interferometer. Findings: The friction coefficient of the polyimide composites decreased with increase in sliding velocity from 1 to 5 mm/s. Moreover, with increase in sliding velocity and normal load, the wear rate of the polyimide composites decreased and increased, respectively. Moreover, the wear behaviors of the polyimide composites were mainly determined by the combined actions of ploughing friction and adhesive friction. The lubricating properties of transfer film and wear debris were limited under point-contact condition. Practical implications: The paper includes implications for the understanding of the wear mechanism of the polyimide composites space bearing retainer under point-contact condition and then to optimize space bearing retainer materials further. Originality/value: Under point-contact condition, wear debris can hardly participate in the friction process because of limited contact area. Consequently, the wear debris has limited impact on the wear process to decrease the friction and wear. Peer review: The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-01-2020-0017/ [ABSTRACT FROM AUTHOR]

Applying accurate normal load to a specimen in direct shear tests under constant normal stiffness (CNS) is of importance for the quality of the resulting data, which in turn influences the conclusions. However, deficiencies in the test system give rise to a normal stiffness, here designated as system normal stiffness, which results in deviations between the intended and actual applied normal loads. Aiming to reduce these deviations, this paper presents the effective normal stiffness approach applicable to closed-loop control systems. Validation through direct shear tests indicates a clear influence of the system normal stiffness on the applied normal load (13% for the test system used in this work). The ability of the approach to compensate for this influence is confirmed herein. Moreover, it is demonstrated that the differences between the measured and the nominal normal displacements are established by the normal load increment divided by the system normal stiffness. This further demonstrates the existence of the system normal stiffness. To employ the effective normal stiffness approach, the intended normal stiffness (user defined) and the system normal stiffness must be known. The latter is determined from a calibration curve based on normal loading tests using a stiff test dummy. Finally, a procedure is presented to estimate errors originating from the application of an approximate representation of the system normal stiffness. The approach is shown to effectively reduce the deviations between intended normal loads and the actual applied normal loads. [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 present study consists in analyzing the friction coefficient as a variable of the normal load and slip speed for 5 % Cr steel, by applying the Block-on-Disk method according to ASTM D2714. The friction coefficient increases linearly 23,25 % from 0,214 to 0,266 when the load is increased from 196 N to 392 N; the same phenomenon is observed when the friction coefficient increases 47,82 % from 0,23 to 0,34 when the slip speed increases from 0,18 m/sec to 0,54 m/sec. The friction coefficient increased by 23,25 % for an increase in the load from 196 N to 392 N, which corresponds to 100 %, while an increase of the friction coefficient of 47,82 % occurred by increasing the speed from 0,18 m/sec to 0,54 m/sec which corresponds to 200 %. [ABSTRACT FROM AUTHOR]

In this study, an in-situ Single Wheel Tester (iSWT) was used to investigate the effect of the most important independent variables on the lateral force of a high lug agricultural tire. The variables include some soil parameters and tire characteristics. Response Surface Methodology (RSM) also was used to construct a design of experiment and propose the lateral force model as a function of seven variables (various combination treatments of soil characteristics and tire parameters). A total of 152 experimental tests were conducted to find the appropriate model. Reduced form of a quadratic model was selected to predict the tire lateral force (R2>0.9). The results showed that the side slip angle and the moisture content had major and minimal effect on tire lateral force respectively. When all other variables were in their middle points, the 95% increase of the normal load would cause the 112% increment in the lateral force. Also, the difference between the lateral force when the moisture content is at minimum and maximum levels, is about 4%. Noteworthily, increasing the tire normal load, not only increases the maximum value of tire lateral force attainment, but also pushes the peak value of the lateral force to higher slip angles. [ABSTRACT FROM AUTHOR]

Purpose The purposes of this paper include studying the friction coefficient of polyoxymethylene (POM) under a broad range of normal load and sliding velocity; developing a mathematical model of friction coefficient of POM under a broad range of normal loads and sliding velocities; and applying the model to dynamic finite element (FE) analysis of mechanical devices containing POM components.Design/methodology/approach Through pin-on-disc experiment, the friction coefficient of POM in different normal loads and sliding velocities is investigated; the average contact pressure is between 5 and 15 Mpa and the sliding velocity is from 0.05 to 0.9 m/s. A friction algorithm is developed and embedded in the FE model to simulate the friction of POM in different normal loads and sliding velocities.Findings The friction coefficient of POM against steel declines with the increase of normal loads when the contact pressure is between 5 and 15 Mpa. The friction coefficient of POM against steel increases markedly when the sliding velocity is between 0.05 and 0.15 m/s, it decreases sharply between 0.15-0.45 m/s and then it stabilizes at high sliding velocity between 0.45 and 0.9 m/s. The friction coefficient of POM in different working operations has a significant effect on contact stress and shear stress. The simulation data and experiment data of POM friction force fit very well; therefore, it can be concluded that the friction algorithm and FE model are accurate.Originality/value The friction coefficient of POM under a broad range of normal loads and sliding velocities is investigated. The friction coefficient model of POM is established as a function of normal loads and sliding velocities in the dry sliding condition. A friction algorithm is developed and embedded in the FE model of the friction of POM. The mathematical model of the friction coefficient accurately agrees with the experiment data, and simulation data and experiment data of the POM friction force fit very well. [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 interface bonding condition between asphalt concrete layers is unquestioned as one of the significant factors influencing pavement performance. In order to research the interface characteristics of asphalt pavement under traffic loading, this paper presents a procedure to estimate shear strength between the asphalt layers via a direct shear test where both normal and shear stresses are applied to the interface. To do this, first, laboratory testing was conducted under varying conditions of temperature, vertical pressure, and tack coat application rate to analyze the influence of each parameter on interface shear strength. Second, the results from the tests were statistically analyzed for the significance level of each factor on interface shear strength. Finally, prediction models describing interface shear strength under the combined effect of shear and normal stresses at three different temperature levels are proposed and their feasibility is validated. All things considered, findings of this study hope to lead to a greater understanding of the monotonic behavior of interface performance in asphalt pavements. [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]

In this paper, the effect of rock bridge surface on the shear behavior of planar non-persistent joints under low and high normal loads has been investigated using particle flow code in 2 Dimensions. PFC2d was calibrated with respect to the data obtained from experimental laboratory tests to ensure the conformity of the simulated numerical models response. The models consisting non-persistent joint were simulated and tested by shear loading under low and high normal loads. The discrete element simulations demonstrated that the failure pattern was mostly influenced by normal load, while the shear strength was linked to the failure pattern and failure mechanism. The failure patterns were found reasonably similar to the experimentally observed trends. [ABSTRACT FROM AUTHOR]

Shear failure of rockmass along a weak plane occurs frequently in rock slopes and in underground tunnels. To study the shear behaviour and acoustic emission (AE) characteristics of joints under different experimental conditions (asperity height, shear rate, and normal load), irregular artificial saw-tooth joints with different asperity heights were sheared in the laboratory, and the AE signals were detected and analysed. The results demonstrated that the strength of the joints increased with increases in normal load and asperity height, while the strength of the joints first increased and then decreased when the shear rate was elevated. The ideal curve of the cumulative hits could be divided into a quiet period, a slow rise period, and a sharp growth period, which could be used to monitor and predict the potential shear failure of the joints. The higher the asperity was, the higher the peak energy rate and the lower the peak hit rate, and cumulative hits at failure were because of differences in asperity size and number. The peak hit rate and cumulative energy tended to increase gradually with the decrease in shear rate, and the minimum peak hit rate and energy rate at failure were both attained at the maximum shear rate. In addition, curves of the energy rate and hit rate showed large fluctuations at higher shear rates. The peak energy rate and cumulative energy under low normal stress could be greater than the peak energy rate and cumulative energy under high normal stress, and the peak hit rate and cumulative hit number under high normal stress could be larger than that of under low normal stress. [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 paper investigates the dry sliding wear behaviour of AA6061/ZrB2/SiC hybrid composite prepared by the stir casting. A pin-on-disc wear apparatus was used for this study. The effect of ZrB2 and SiC particulate content and normal load on wear rate was analyzed. The insitu fabricated ZrB2 and the reinforced SiC particles enhance the wear resistance of the AA6061 composite. The worn surface analysis of the composite as a function of ZrB2 and SiC particulate content and normal load are also presented. [ABSTRACT FROM AUTHOR]

Cu and Cu-MoS coatings were fabricated by cold spray, and the fretting wear performance of the two coatings was compared. A mixture (95 wt.% Cu + 5 wt.% MoS) was used as feedstock for the composite coating. Coatings were sprayed with identical gas flow conditions on the substrates pre-heated to approximately 170 °C. The morphology of coating top surface and polished cross sections was analyzed by scanning electron microscopy (SEM) and light optical microscopy (LOM). The influence of MoS on Cu deposition was examined. The local MoS concentration within the coating was found to affect the hardness. Fretting tests were carried out at two different normal loads, and the influence of MoS on friction and wear was studied. The morphology and elemental compositions of the wear scars and wear debris were observed by SEM and energy dispersive x-ray spectroscopy (EDS), respectively. [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 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]

Understanding the scratch behaviour of the composite materials is of primary importance. The scratch behaviour of glass fibre reinforced polyester composite was investigated. A new scratch device is designed and developed. Scratch tests at room temperature were carried out, using conical indenter and constant scratching velocity. The main aim of this study is to investigate the effects of tribological parameters on wear mechanisms and friction coefficient. Particularly, the effects of the normal load and the attack angle were presented. The different wear scenario of the composite material during the scratch test was deduced, then supported by an analysis of the glass fibre reinforced polyester composite wear modes using scanning electron microscopy (SEM) observations was performed. The correlation between tribological parameters and wear mechanisms was highlighted through the composite scratch map. Results were compared with those previously published associated with the metallic and the polymeric materials. [ABSTRACT FROM AUTHOR]

In the present study, friction coefficient and wear rate of stainless steel 304 (SS 304) sliding against mild steel are investigated experimentally. In order to do so, a pin on disc apparatus is designed and fabricated. Experiments are carried out when smooth or rough mild steel pin slides on SS 304 disc. Experiments are conducted at normal load 10, 15 and 20 N, sliding velocity 1, 1.5 and 2 m/s and relative humidity 70%. Variations of friction coefficient with the duration of rubbing at different normal loads and sliding velocities are investigated. Results show that friction coefficient is influenced by duration of rubbing, normal load and sliding velocity. In general, friction coefficient increases for a certain duration of rubbing and after that it remains constant for the rest of the experimental time. The obtained results reveal that friction coefficient decreases with the increase in normal load for SS 304 mating with smooth or rough mild steel counterface. On the other hand, it is also found that friction coefficient increases with the increase in sliding velocity. Moreover, wear rate increases with the increase in normal load and sliding velocity. The magnitudes of friction coefficient and wear rate are different depending on sliding velocity and normal load for both smooth and rough counterface pin materials. [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]

Studying shear behavior and failure mechanism of concrete-rock interfaces between concrete structures and rock mass is highly important. To this end, laboratory direct shear tests were conducted on several bonded interfaces. The effect of normal stress, displacement rate, and bonding percentage on shear behavior of bonded joints were also evaluated. The results showed that the adhesive bond between concrete and rock has the most important effect on shear mechanism of concrete-rock interfaces. When the normal load is low, the asperities do not contribute in shear process, so the shear strength of the joints is governed by adhesive bond. The only contribution of the asperities is that they show a small peak after bond breaking peak. When the normal load is high, the asperities break simultaneously with adhesive bond and they show their contribution in the whole shear process. In a constant normal load, by decreasing bonding percentage, the brittle failure of the bonded joints is changed into softening failure because of less contribution of the adhesive bond and more contribution of the rough asperities in joint failure. Acoustic emission (AE) monitoring of bonded joints showed that for all different tested samples under various loading conditions, there has been no or very few AE activities before adhesive bond breaking point, indicating that other parameters (roughness, normal load, displacement rate) are impressed by adhesive bond and maximum shear strength is determined by this parameter. [ABSTRACT FROM AUTHOR]

It has been shown theoretically and experimentally that the friction properties of rubber are controlled by multiple parameters, such that coefficient of friction (COF) calculations for rubber differ from the traditional, metallic-based laws of friction. A previously formulated hypothesis details a unified theory of rubber friction that incorporates adhesion forces, surface deformation hysteresis, bulk deformation hysteresis and cohesion losses from rubber wear. This paper discusses experimental data that support the theory of surface deformation hysteresis (F Hs) and the implications for footwear testing. The experimental results indicate that surface deformation hysteresis on smooth test surfaces can contribute 6.5–42% of the total frictional forces depending on the applied load and outsole compound. This contribution is specific to the experimental dataset reported, and can vary with the rubber compound used. The data also show that F Hs is a constant, resulting in a decreasing contribution to the total COF (µ T) with increasing normal load. The combination of these two facts has implications in the establishment of standard test methods and the application of minimum COF requirements for footwear. [ABSTRACT FROM AUTHOR]

Purpose – The present paper seeks to report the effect of duration of rubbing on friction coefficient for different polymer and composite materials. Variations of friction coefficient and wear rate with the normal load are also investigated experimentally when stainless steel (SS 304) pin slides on different types of materials such as cloth-reinforced ebonite (commercially known as gear fiber), glass fiber-reinforced plastic (glass fiber), nylon and polytetrafluoroethylene (PTFE). Design/methodology/approach – A pin on disc apparatus is designed and fabricated. During experiment, the rpm of test samples was kept constant and relative humidity was 70 percent. Findings – Studies have shown that the values of friction coefficient depend on applied load and duration of rubbing. It is observed that the values of friction coefficient decrease with the increase of normal load for glass fiber, nylon and PTFE. Different trend is observed for gear fiber, i.e. coefficient of friction increases with the increase of normal load. It is also found that wear rate increases with the increase of normal load for all the materials. The magnitudes of friction coefficient and wear rate are different for different materials. Practical implications – It is expected that the applications of these results will contribute to the design of different mechanical components of these materials. Originality/value – Within the observed range of applied normal load, the relative friction coefficient and wear rate of gear fiber, glass fiber, nylon and PTFE are experimentally investigated. [ABSTRACT FROM AUTHOR]

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]

In the present paper the influence of the addition of chromium carbide (CrC) particles on the microstructure, microhardness and abrasive wear behaviour of flame sprayed Ni-Cr-Si-B coatings deposited on low carbon steel substrate has been reported. Wear behaviour of the coatings was evaluated with a pin-on-block wear system against SiC abrasive medium (120 & 600 grades) over a range of normal load (5–20 N). It was observed that the wear behaviour is governed by the material related parameters (microstructure, microhardness of coating) and test parameters (abrasive grit size and normal load). The addition of CrC reduces the wear rate three to eightfold. Wear resistance was greater against coarse abrasives at high loads than against fine abrasives. Heat treatment of both unmodified (1004) and modified powder (1004-10%CrC, 1004-20%CrC) coatings deteriorated the abrasive wear resistance. SEM study of wear surfaces showed that wear of the coatings largely takes place by groove formation, plowing and scoring. Electron probe micro analysis (EPMA) of the coating was carried out for composition and phase analysis. [ABSTRACT FROM AUTHOR]