Evolution Of Friction Coefficient Research Articles

Friction Coefficient Evolution of Si3N4 Binary Coating with a Stoichiometric Ratio of 57/43

Friction coefficient depends on various factors or surface characteristics during tribological testing, and this friction coefficient can be modified by altering the properties of one of the two contacting surfaces. It is crucial to monitor the friction coefficient continuously, not only at the conclusion of the test. This research examined the evolution of friction coefficient of silicon nitride (Si3N4) coating and H13 steel over different sliding distances (250, 500, 750, 1000 m). The study assessed surface wear and oxidation through three-dimensional profilometry and SEM/EDX. The findings indicated a reduction in friction coefficient by 22%, a decrease in wear rate by 88%, and a reduction in wear volume by 87% when comparing the silicon nitride coated steel to the uncoated steel. Furthermore, the changes in friction coefficient provided insights into the timing of the complete fracture of the hard coating.Graphical abstract

Frictional mechanisms of hydrated montmorillonite under normal loading

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.

Mechanical characteristics of die-wall friction on the compaction process of metal nano-powders

The die-wall friction is one of the influential mechanical factors in the metal nano-powder compaction process that can noticeably impress the densification behavior of nanoparticles. In this study, the uniaxial cold compaction process of aluminum nano-powders, which are initially loaded into the nickel die-walls, is analyzed using the molecular dynamics (MD) method. The influence of die-wall friction is studied on the nano-powder compaction process. The results illustrate the effect of frictional die-walls on the characteristics of relative density–pressure and stress–strain curves, the pressure distribution of nano-powders, and the final green product. The evolution of the die-wall friction coefficient on the compaction velocity and temperature is determined during the compaction process. Applying the nonlinear regression method, an empirical relation is derived to estimate the frictional behavior of die-walls at different relative densities. It is demonstrated that the proposed computational model has acceptable accuracy to evaluate the die-wall friction during the nano-powder compaction process.

In situ SEM side observation of asperity behavior during sliding contact

The performance and evolution characteristics of the friction interface are crucial for the design optimization of material friction and wear, as well as the revelation of the mechanism of seismic sliding motion. To understand the friction behavior of rough surfaces, it is essential to understand the physical process of interaction among asperities. However, visualizing the contact of asperities is challenging because materials are typically opaque, and the dimensions of asperities are usually in the micron range. This study developed an in-situ scanning electron microscope friction device and a micro speckle fabrication method to measure the strain field of regular asperities from the side, and synchronously measure the macroscopic friction coefficient. In-situ friction experiments were conducted on brass and silicon materials. The results indicate that directly correlating the friction coefficient with the deformation and failure images of asperities can effectively explain the evolution process and differences in friction coefficient for the two materials, validating the performance of the device. Different failure modes of asperities were observed, including severe plastic deformation in brass asperities and fracture-producing large particles in silicon asperities. The critical transition of asperity failure modes in the experiments was analyzed based on a theoretical model. The phase diagram of asperity failure modes and friction coefficient evolution was plotted, providing potential explanation for the evolution of friction coefficient in friction experiments on randomly rough surfaces. The developed device in this study can be used for non-transparent materials and helps reveal the microscopic mechanisms behind experimental phenomena.

Cyclic shear behavior of en-echelon joints under constant normal stiffness conditions

To reveal the mechanism of shear failure of en-echelon joints under cyclic loading, such as during earthquakes, we conducted a series of cyclic shear tests of en-echelon joints under constant normal stiffness (CNS) conditions. We analyzed the evolution of shear stress, normal stress, stress path, dilatancy characteristics, and friction coefficient and revealed the failure mechanisms of en-echelon joints at different angles. The results show that the cyclic shear behavior of the en-echelon joints is closely related to the joint angle, with the shear strength at a positive angle exceeding that at a negative angle during shear cycles. As the number of cycles increases, the shear strength decreases rapidly, and the difference between the varying angles gradually decreases. Dilation occurs in the early shear cycles (1 and 2), while contraction is the main feature in later cycles (3−10). The friction coefficient decreases with the number of cycles and exhibits a more significant sensitivity to joint angles than shear cycles. The joint angle determines the asperities on the rupture surfaces and the block size, and thus determines the subsequent shear failure mode (block crushing and asperity degradation). At positive angles, block size is more greater and asperities on the rupture surface are smaller than at nonpositive angles. Therefore, the cyclic shear behavior is controlled by block crushing at positive angles and asperity degradation at negative angles.

Atomistic Insights into Interfacial Optimization Mechanism for Achieving Ultralow-Friction Amorphous Carbon Films under Solid-Liquid Composite Conditions.

The combination of fluid lubricants and textured amorphous carbon (a-C) can provide an ultralow friction state, which can improve the reliability and service life of dynamic machinery. However, the coupling effects of the contact pressure and oil content on the friction-reducing efficiency is still lack of study, and the corresponding friction mechanism is also not fully understood, which cannot be achieved by experiment due to the limitation of in situ characterization. In this study, using the reactive molecular dynamics simulation, the insight into the evolution of interfacial structures induced by both contact pressures and oil contents on a-C surface was systematically investigated to explore the fundamental mechanism. In particular, the friction difference between textured and untextured a-C films was evaluated comparatively. Results indicate that the tribological performance strongly depends on the interfacial lubrication state, which is jointly determined by the oil content and contact pressure; the best operating condition to achieve ultralow friction coefficient (0.002) is obtained, and the evolution of friction coefficient with oil content and contact pressure is highly dominated by the lubricant mobility, cross-linking between mating a-C surfaces, or competition/synergy of the H stress state from the lubricant with interfacial passivation. Furthermore, the difference in friction reduction between textured and untextured systems is unveiled; with the increase of contact pressure, the role of texturing a-C surface in antifriction changes from positive to negative effect, which is related to the transformation of interfacial hybridized structure and anomalous flow of lubricant. These results can significantly enhance the understanding of composite lubrication systems through computation and also provide a roadmap for the R&D of the advanced lubrication system according to the working conditions.

Grain size and scratching depth dependent tribological characteristics of CrCoNi medium-entropy alloy coatings: A molecular dynamics simulation study

In this work, we have investigated the impact of grain size and scratching depth on the tribological behavior of nanocrystalline CrCoNi medium-entropy alloy (MEA) coatings during scratching by molecular dynamics (MD) simulations. Regardless of the grain size, a monotonically increase in both friction coefficient and wear rate with increasing the scratching depth was observed in the nanocrystalline CrCoNi MEA coating. When its microstructure becomes coarser, the friction coefficient becomes lower. As a function of grain size, the evolutions of friction coefficient and wear rate can be divided into three regions, namely high friction coefficient (or wear rate) region in a grain size range of below 6 nm, relatively stable friction coefficient (or wear rate) region in a grain size range of 6–12 nm, and a relatively low friction coefficient (or wear rate) region in a grain size range of larger than 12 nm. According to the MD simulation results, the higher wear rate for a finer CrCoNi MEA coating can be explained by its lower hardness. In contrast, the reduction of friction force was mainly attributed to the scratching induced grain boundary (GB) sliding and dislocation emissions from GBs. Moreover, the scratching induced phase transformation from crystalline to amorphization and plastic deformation mode transitions also affect its tribological properties.

Multiscale analysis of nano-powder compaction process using the FEM–MD technique

In this paper, a continuum–atomistic (FEM–MD) multiscale technique is developed for modeling the compaction process of metallic nano-powders. Nano-powders are assumed as the continuum body that are modeled using the nonlinear finite element method (FEM). Representative volume element (RVE) consists of a number of nanoparticles to reflect the discrete nature of nano-powders. The atomistic RVEs are modeled using the molecular dynamics (MD) method to capture the plastic deformation of nanoparticles under the compaction process. The MD analysis of atomistic RVEs provides the feasibility of modeling the friction and interaction of nanoparticles. A parametric study is performed over the atomistic RVEs to evaluate the influence of various parameters, such as the type of boundary conditions, width of boundary layer, size and number of nanoparticles, and compaction velocity on the densification behavior of nano-powders. The effect of die-wall friction is investigated on the compaction behavior of nano-powders by evolution of die-wall friction coefficient for different values of compaction velocity and temperature. The accuracy of multiscale analysis is verified by comparing the numerical results with those of experimental data for the uniaxial compaction test. The performance of proposed computational method is finally presented in modeling the micro-cylindrical bush and micro-tablet components during the compaction process of micro-scale metallic nano-powders.

Validation of a new methodological approach for the selection of wire-coil inserts in thermal equipment

• New methodology to select wire coils for thermal equipment enhancement. • The estimation of TSP for a given insert categorizes its hydraulic performance. • Method experimentally validated for heat transfer enhancement in a solar collector. • 4 wire-coils selected with differentiated behaviours in their friction factor curves. • Pressure drop and heat transfer tests were performed in collectors with wire coils. The use of wire-coils is especially relevant at low Reynolds numbers (below the critical number to turbulent flow in smooth tubes) according to its inherent positive features such as the advance of transition onset and, if they present suitable geometric characteristics, the establishment of an extended transitional flow in a critical Reynolds number interval [ Re CL - Re CT ], with a predictable friction coefficient and Nusselt number. This paper presents the experimental validation of a new methodology based on the evaluation of a non-dimensional geometry-based parameter: the TSP (Transition Shape Parameter) that allows to predict the friction coefficient evolution with wire-coil inserts and enables to compute the extension of the transitional flow region. The close relationship between hydraulic and thermal performance of wire-coil inserts makes this methodology a valuable tool for selecting the most appropriate wire-coil geometry for a given tubular heat exchanger. It is observed that to promote an increase in heat transfer, the value of the Re CL of the wire coil must be less than the operating Reynolds number range of the equipment. Thus, the Re CL - Re CT interval of the insert should fall into this range. In order to validate the methodological approach, an application to harp-type solar thermal collectors with typical Reynolds number range [40-6000] is presented. Four representative wire-coils, with a wide geometrical range characterized by TSP values of 759, 196, 35.3 and 3.1 (exhibiting significant differentiated behaviours in their friction factor curves and critical Reynolds numbers) were inserted inside the risers of a modified solar collector and experimentally tested at laboratory conditions. Static temperature at different locations at the absorber plate, and pressure drop were measured to obtain friction factor and Nusselt number inside riser covering the laminar, transitional and low-turbulent regions. For a general application with friction factor constraints the most suitable wire-coil geometry is the TSP W 02 = 196 with a range of critical Reynolds number of Re CL = 663 and Re CT = 2286 and Nu ¯ w 02 / Nu ¯ s = 2.21 for Re = 300 - 3000 with f ¯ w 02 / f ¯ s = 3 , 82 . However, for the case study presented (a harp-type solar collector) it is feasible to insert the third wire-coil geometry TSP W 03 = 35.3 due to its early transition, with a range of critical Reynolds number of Re CL = 364 and Re CT = 2324 , and Nu w / Nu s = 1.35 for Re = 300 with a high friction factor augmentation f ¯ w 03 / f ¯ s = 18.84 . This geometry also promotes the highest absorber temperature reduction. The greatest temperature reduction is observed in the range of Reynolds numbers [700-2000], reaching approximately 6 °C, which represents approximately 15%.

Effect of Tooling Temperature on the Transient Lubricant Behavior in Hot Metal Forming Processes

In hot metal forming processes, the temperature of the forming tool progressively increases under serial production conditions. Water-based two-phase lubricants may be applied to cool the forming tool and moderate temperature, in which the liquid agent would evaporate or decompose rapidly with dry matter deposited on the tooling surface during the dwelling time before the forming process commences. Herein, an interactive friction model for a two-phase lubricant is developed to predict the transient lubricant behaviors, i.e., predicting the effects of tool temperature and dwelling time on the friction coefficient evolution and lubricant breakdown. Friction tests between a warm pin and hot aluminum workpiece are conducted using the advanced friction testing system, TriboMate, to validate the modeling results.

The influence of the pavement friction coefficient evolution caused by traffic flow on the risk of motorway horizontal curves.

The friction coefficient between the tire and the road is one of the key parameters affecting road traffic safety. The purpose of this paper is to quantify the risk of skidding for the vehicles due to the friction evolution caused by the traffic polishing in the horizontal curve. Based on the reliability theory, an innovative dynamic risk assessment model is developed in the present study for passenger cars and trucks. The influence of two traffic characteristics for pavement friction is quantified: cumulative traffic volume (CTV) and annual average daily traffic of trucks (AADTT). The speed distribution on the horizontal curve of the motorway is obtained through field experiments as the basic parameter of the model. The Hasofer-Lind Method is adopted to solve the reliability and the risk probability of vehicle skidding. The results show that in the traffic characteristics, the AADTT has a significant impact on the pavement friction; When the AADTT on the road exceeds 2000 veh/d, the increasing CTV leads to friction decrease rapidly and therefore has a significant impact on the risk of horizontal curve. Especially for roads with more than 50 million vehicles of the CTV, the risk of the horizontal curve shows a sharp increase with CTV rising. The research results can provide reference for the road maintenance department to determine the timing of road maintenance.

Study on the microstructure and wear behavior of Mg-containing Al–12Sn–4Si alloys

Al–Sn–Si alloys have been widely used as bearing alloys in automotive engines. In those alloys, β(Sn) phase serves as the solid lubricant and Si particles are used to improve the hardness, fatigue resistance as well as seizure resistance. With the development of machinery, bearing alloys with higher load-carrying capacity and better tribological properties are demanded to meet the application under severe conditions. Adding appropriate Mg to the Al–Sn–Si alloy can effectively optimize the tribological properties. In this paper, the microstructure, mechanical properties and tribological properties of the Al–12Sn–4Si-xMg (x = 0, 0.5, 1, 1.5, all in wt.%) alloys were studied. The Al–12Sn–4Si-1.5 Mg alloy had the smallest grain size and more uniform microstructure than other specimens. The Al–12Sn–4Si-1.5 Mg alloy showed significantly better wear resistance than the Al–12Sn–4Si alloy at 2 N–70 N. The volume wear rates and friction coefficients at 15 N–70 N were lower than those at 2 N–8 N. The evolution of volume wear rate and friction coefficient with load was related to the evolution of tribolayer with load. The wear mechanism of the alloys was mainly oxidation wear, adhesive wear and fatigue wear, accompanied with abrasive wear.

ASSESSMENT OF CONJUGATE AND CONVECTIVE HEAT TRANSFER PERFORMANCE OVER A PARTITIONED CHANNEL WITHIN BACKWARD-FACING STEP USING THE LATTICE BOLTZMANN METHOD

This research concerns the quality of heat transfer across a two-dimensional backwards-facing step in the presence of vertical or inclined partitions in a rectangular channel. Convective and conjugate heat transfer input using constant temperature and heat flux were successfully achieved for low Reynolds number values. The actual code based on the double multiple relaxation time of the lattice Boltzmann method was employed to model numerically the physical problems related to the fluid flow and heat transfer, where the obtained results had a good agreement with previous experimental research. During this study, parametric variations were carried out according to the inclination and position of the partitions, as well as the thermal conductivity ratio in the case of the conjugate heat transfer problem. The numerical results show that the involvement of the partitions and the thermal conductivity ratio has a significant effect on the fluid flow and heat transfer. This impact controls the evolution of local Nusselt number, bulk temperature, and skin friction coefficient. The improvement of heat transfer is remarkable by the introduction of vertical or inclined partitions such that the quality of heat exchange reaches about 7%. While the presence of two inclined partitions increases the heat transfer by about 6% in case of constant temperature of studied hot wall. The implementation of constant heat flux for the hot wall increases the quality of heat transfer by about 9%-10% by the presence of inclined or vertical walls, 11%-13% with double inclined partitions.

Effects of Contact Load and Torsion Angle on Crack Propagation Behaviors of Inclined Crossed Steel Wires during Tension–Torsion Fretting Fatigue in Acid Solution

The hoisting rope in the kilometer-deep coal mine exhibits the tension–torsion fretting fatigue behaviors of inclined crossed steel wires in acid solution. Distinct contact load and torsion angles of steel wires in the rope cause different crack propagation behaviors, which greatly affect the fatigue lives of steel wires. Therefore, the effects of contact load and torsion angle on the crack propagation behaviors of inclined crossed steel wires during tension–torsion fretting fatigue in acid solution were investigated in the present study. The three-dimensional X-ray tomographic micro-imaging system was used to reveal evolutions of crack profiles and crack propagation depths during the test. The evolution of friction coefficient between steel wires during the test is presented. The three-dimensional white light interference microscope, electrochemical analyzer, and scanning electron microscope were employed to investigate the wear depth profiles, Tafel polarization curves and impedance spectra, and wear scar morphologies, respectively, of steel wires. Effects of contact load and torsion angle on crack propagation behaviors of inclined crossed steel wires during the tests were explored through analyses of friction and wear mechanisms and electrochemical corrosion damage. The results show that as the contact load and torsion angle increase, the crack propagation depth and rate of steel wire both increase and the fatigue life of steel wire decreases. Those are mainly attributed to the increases in the average tangential force between steel wires, wear depth, electrochemical corrosion tendency, and surface damage of steel wire as well as the decrease in corrosion resistance.

A novel synthesis method of carbide-derived carbon (CDC) in open air for hip implants

The inferior tribocorrosion behavior of commonly used biomedical alloys has led to the early failure of total hip replacements (THRs) and serious complications. Our previous results have shown that carbide-derived carbon (CDC) produced by direct chlorination can provide excellent protection to Ti6Al4V and has a high durability.In this study, a novel method has been developed to synthesize CDC on Ti6Al4V substrates by electrolysis from LiCl-KCl with a low melting point. Compared to previous preparation processes, this newly developed approach eliminates the exposure to chlorine gas and the requirement of the inert gas environment, which makes the synthesis process more controllable and the CDC layer more uniform. X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM) techniques were utilized to characterize the products. Based on the characterization results, the produced CDC has a porous structure which may contain nanocrystalline graphite (NCG) and amorphous carbon (a-C). The theoretical thickness of the sample is estimated by a calculation which is approximately 1.44 μm. In addition, the performance of the new layer was tested in a tribocorrosion hip simulator. A system has a pin on ball contact and immersed in bovine calf serum of 30 g/L protein concentration, with a pH of 7.6 and a temperature of 37 °C. We applied a normal force of 16 N and ran the test for 3600 cycles with 1 Hz. According to the recordings of normal and tangential forces, the evolution of friction coefficient is deducted and reported in the results.The findings have shown that the CDC samples prepared by electrolysis method exhibit smaller friction coefficient (approximately 0.12) and potential drop (less than 0.1 V compared to 1.1 V for substrate). Therefore, it is promising that CDC prepared by the novel electrolysis approach can protect Ti6Al4V substrates from the tribocorrosive damages.

Anisotropic friction behavior of aligned and oriented graphite flakes/copper composite

Graphite flakes as lubricant are widely used for friction materials, but little is known about the friction behavior at sliding surface oriented at some angle to the basal plane of graphite. In this paper, oriented graphite flakes/copper (volume ratio of 7/3) composite was prepared by vacuum hot-pressing sintering. The friction coefficient evolution of the composite with the dihedral angle (φ) between sliding surface and graphite basal plane was estimated based on the numerical solutions of the anisotropic stiffness at the contact plane, and measured experimentally. Theoretical estimates suggested an increase of the friction coefficient induced by adhesion with raising the dihedral angle, and a decreased one caused by ploughing. A pronounced trend of the former happened with the dihedral angles of more than 75°, while for the latter it was conspicuous with the dihedral angles of less than 15°. Through ball-on-disk friction experiments, the friction coefficients were fairly low with the dihedral angles from 30° to 60°, which was consistent with the theoretical prediction, and a monotonous decrease of the wear rate was exhibited with increasing the dihedral angle due to increased shear modulus and hardness. This anisotropic friction behavior may provide new ideas for the designs of friction materials.

Transitions of Wear Characteristics for Rubber/Steel Seal Pairs During the Abrasive Wear Process

Abrasive wear resulting from the microclastic rock is a common failure phenomenon in the drilling environment that often limits the sealing ability and the service life of seals. In this study, the friction and wear process of fluoro rubber (FKM) seals against 304 stainless steel (SS304) after one single entry of SiO2 abrasives were investigated. The influence of the changes in particle state on friction coefficient evolution, wear loss evolution, wear morphologies, and wear mechanisms were discussed in detail. The results indicate that the presence of abrasive particles dispersed between the sealing interfaces clearly improves the friction performance of the seal pairs and deteriorates the wear performance of the metal counterpart. The movement and breakage of particles after one single entering into the sealing interface were obtained. And on this basis, the stable wear process can be divided into three stages. In addition, the main causes contributed to this change of wear mechanisms are the random movement and process of continuous breakdown of abrasive particles. Furthermore, the transition of the wear mechanism that clearly describes the wearing behavior of the seal pairs under these abrasive wear conditions was identified. The results of this study enhanced our understanding of the abrasive wear degradation of rubber seal in practical drilling applications.

Rheology of granular liquids in extensional flows: Beyond the μ(I)-law.

The Granular Integration Through Transients (GITT) formalism gives a theoretical description of the rheology of moderately dense granular flows and suspensions. In this work, we extend the GITT equations beyond the case of simple shear flows studied before. Applying this to the particular example of extensional flows, we show that the predicted behavior is somewhat different from that of the more frequently studied simple shear case, as illustrated by the possibility of nonmonotonous evolution of the effective friction coefficient μ with the inertial number I. By the reduction of the GITT equations to simple toy models, we provide a generalization of the μ(I)-law true for any type of flow deformation. Our analysis also includes a study of the Trouton ratio, which is shown to behave quite similarly to that of dense colloidal suspensions.

Design of Sand‐Based, 3‐D‐Printed Analog Faults With Controlled Frictional Properties

AbstractLaboratory experiments with surrogate materials play an important role in fault mechanics. They can improve the current state of knowledge by testing various scientific hypotheses in a repeatable and controlled way. Central in these experiments is the selection of appropriate analog, rock‐like materials. Here, we investigated the frictional properties of sand‐based, 3‐D‐printed materials. We performed uniaxial compression tests, direct shear, and inclined plane tests in order to determine a) the main bulk mechanical parameters of this new analog material, b) its viscous behavior, c) its frictional properties, and d) the influence of some printing parameters. Complete stress‐strain/apparent friction‐displacement curves were presented including the post‐peak, softening behavior, which is a key factor in earthquake instability. Going a step further, we printed rock‐like interfaces of custom frictional properties. Based on a simple analytical model, we designed the a) maximum, minimum, and residual apparent frictional properties, b) characteristic slip distance, c) evolution of the friction coefficient with slip, and d) dilatancy of the printed interfaces. This model was experimentally validated using interfaces following a sinusoidal pattern, which led to an oscillating evolution of the apparent friction coefficient with slip. Our approach could be used for simulating earthquake‐like instabilities in the laboratory and mimic the periodical rupture and healing of fault sections. Additionally, our tests showed the creation of a gouge‐like layer due to granular debonding during sliding, whose properties were quantified. The experimental results and the presented methodology make it possible to design new surrogate laboratory experiments for fault mechanics and geomechanics.

High-temperature friction characteristics of N-BK7 glass and their correlation with viscoelastic loss modulus

It is fundamentally important to investigate the interfacial friction between softened glass and rigid molds for controlling the product quality in glass hot forming techniques. However, the underlying mechanism has not been fully understood in terms of the contribution of viscous dissipation to the viscoelastic friction of softened glass. In this paper, a dry rotary sliding scheme using the ball-on-disc pair is employed to investigate the high-temperature friction characteristics of a typical borosilicate crown glass N-BK7 from 500 °C to 660 °C. Experimental results show that the friction coefficients in the steady stage remarkably increase with the applied temperatures. For softened glass, friction could originate from adhesion, ploughing and dissipation. Furthermore, with careful estimations on both the scratch deformation frequency and the induced flash temperature during sliding, the increase in friction coefficient with temperature is well correlated with the loss modulus ratio, which presents the viscous energy dissipation by macroscale ploughing. Additionally, the friction coefficients at 500 mm/s are slightly larger than those at 250 mm/s, because the effect of more frictional heat prevails. Lastly, the influences of temperature and frequency shifting on the loss modulus ratio are summarized to explain the experimental evolution of friction coefficient. It is confirmed for N-BK7 glass that the dependences of friction coefficients on temperature and sliding velocity, also follow the classical superposition master curve of viscoelastic friction.