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The wear behavior and mild–severe (M–S) wear transition of Mg–10Gd–1.5Y–0.4Zr alloy were investigated within a temperature range of 20–200 °C. The morphologies and compositions of worn surfaces were examined to identify the wear mechanisms using scanning electron microscope and energy dispersive X-ray spectrometer. The microstructure and hardness in the subsurfaces were analyzed to reveal the M–S wear transition mechanism. Under a constant loads of 20, 35 and 40 N, each wear rate–test temperature curve presented a turning point which corresponded to the M–S wear transition. In mild wear, the surface material was plastically deformed and hence was strain- hardened, whereas in severe wear, the surface material was dynamically recrystallized and consequently was softened. It has been found that the critical temperature for M–S wear transition decreases with increasing the normal load, and the normal load exhibits an almost linear relationship with critical temperature for M–S wear transition. This work reveals that the M–S wear transition of the studied alloy conforms to the surface DRX temperature criterion.

Solid-liquid separation aims to increase the solids concentration. The dewatering extent can be parameterized through the compressive yield stress, py(ϕ) whereby to further consolidate a suspension, higher stress has to be applied to the material. In a variety of dewatering devices, shear (in addition to unilateral compression) is applied to the material and is demonstrated to be of benefit to the process, albeit without a basis for optimisation. To address this fundamental issue, two novel measurement techniques, namely the Vane-in-a-Filter (ViaF) and the Vane-under-Compressional-Loading (VuCL), are presented, allowing measurement of the shear stress response with different normal loads under fixed solids concentrations. Furthermore, high solid concentration shear yield stress measurements are possible. The results demonstrate that the shear stress response increases with normal load (at a fixed state of compression) and the peak shear stress has a minimum at shear rates of 0.01–0.1 s−1.

The viscosity of the ice premelting layer in contact with silica in the temperature range of -18 to -1 °C was studied by resonance shear measurement (RSM). The viscosity of the ice premelting layer was determined to be ∼5 orders of magnitude greater than that of the bulk liquid water and continuously decreased with the increasing sliding speed between the two surfaces over the temperature range employed in this study, which was the same behavior as for the typical confined liquids including water. On the other hand, the normal load and the contact pressure did not influence the viscosity, indicating that the premelting layer behaved differently from the typical confined liquids.

A constitutive model is developed that incorporates the wavy elastic response of an arc-shaped metallic glass (MG) ribbon subjected to applied progressive normal load. This model is based on the ev...

The paper assesses and compares the friction and wear behavior of SAE-AISI 1055 steel and brass (CuZn39Pb2) under dry sliding condition. The tribological behavior was investigated and compared by conducting two different experiments, the first experiment conducted on a CSM tribometer, and the second experiment was carried out on a test bench in horizontal lathe machine where device holder pin is fabricated and mounted on a test bench and a rotating disc, varying the normal load exert on the disc by the pin and the rotation speed of the disc. These tests consisted of measuring friction coefficient and wear loss of samples. Experiments are carried out in normal load 5-10 N, sliding speed 0.24-0.35 and 0.48 m/s. Variations of coefficient of friction during sliding at different initial surface roughness, normal load and sliding speed are investigated. Results show that the two alloys had different friction and wear behavior, for steel friction coefficient increase slowly with the increase of normal load and sliding speed. For brass friction coefficient decreases with the increase of normal load and sliding speed. On the other hand, it is also found that wear loss increase with sliding distance. Microscopic of worn surfaces for each alloy were carried out and compared.

An investigation was conducted to determine the extent to which the microstructure of NiCrSiBC alloy coatings influences micro-abrasive wear resistance. The wear performance of the complex microstructure in coatings is strongly dependent on nature of the strengthening particles. Processing conditions can be altered to manipulate these particles and therefore to design coatings to address specific service demands. The purpose of this investigation was to evaluate the relationship between microstructure and micro-abrasive wear performance of NiCrSiBC coatings. Differences in the microstructure of coatings were imposed by processing with Plasma Transferred Arc (PTA) and High-Power Diode Laser (HPDL). The microstructures of NiCrSiBC coatings were evaluated using EDX, XRD, SEM techniques and hardness measurements. Wear performance was assessed by micro-abrasive wear tests in which normal load, sliding distance and ball sliding speed were varied. A suspension of alumina in water was used as the abradant. The features of the strengthening particles were observed to depend on the imposed solidification/cooling rate. A faster cooling rate, produced by HPDL processing, displayed block shape chromium borides (CrB) in finer structure coatings. This contrasted with PTA-processed coatings which exhibited floret-shape chromium borides (Cr5B3) in a coarser microstructure. Higher hardness coatings with finer microstructure exhibited lower abrasive wear resistance than the coatings with coarser microstructure.

This contribution presents an asymptotic formulation for the stick-slip behaviour of incomplete contacts under oscillatory variation of normal load, moment, shear load and differential bulk tension. The asymptotic description allows us not only to approximate the size of the slip zones during the steady-state of a cyclic problem without knowledge of the geometry or contact law, but provides a solution when all known analytical solutions for incomplete contacts reach their limitations, that is, in the presence of a varying moment and a differential bulk tension large enough to reverse the direction of slip at one end of the contact. An insightful comparison between the mathematically explicit analytical solution and the asymptotic approach is drawn using the example geometry of a shallow wedge., 21 pages, 11 figures

Nanoscale wear behaviour of copper-based nanocomposite with Al2O3 nanoparticles have been investigated with the help of the circular mode atomic force microscopy (CM-AFM). The occurrence of running-in and steady-state wear regimes is similar to macroscopic behaviour described by Barwell. Archard's macroscopic wear equation, which states that the wear value is proportional to the applied load and independent on the sliding speed, is also valid at the nanoscale, with the limitation that the normal load should reach a threshold value to generate wear. Eventually, it is shown that the wear value at the nanoscale is highly dependent on the nature of the counter-body (AFM tip) material.

With the increasing of quality of life, consumer’s expectations on high value products are not only on functionality and usability, but also on emotional characteristics. In this work, we developed an in vivo methodology, based on a self-assembled finger friction test apparatus and systematical influence factor investigation, for studying the interfacial interactions between human skin and hair, with respect to tactile friction and sensory properties of human hair bundles before and after using hair care products. It was found that, the tactile friction reduced with the increasing of normal load and product usage. Backward sliding showed higher tactile friction than forward sliding mode. Damaged hair exhibited lower tactile friction than virgin hair under wet situation, while showed higher tactile friction than virgin hair under dry situation. The developed methodology has been used preliminarily to investigate the tactile friction of hair after using several well-designed hair care products.

The fretting wear behavior of ultra-high molecular weight polyethylene (UHMWPE) against a GCr15 steel ball was investigated using an Optimal SRV-IV oscillating reciprocating friction and wear tester (Optimal Corp., Germany). The influence of temperature, stroke size and frequency was studied in detail under a normal load of 10 N. The results showed that the friction coefficients initially decreased and then increased afterward with the increasing of temperature, having the lowest value at 0°C. The wear loss of UHMWPE at a stroke of 100 µm showed a similar tendency as the friction coefficients, but monotonously increased with increasing of temperature for a stroke of 200 µm. According to an analysis of the worn surface, it was concluded that abrasive wear accompanied with local plastic deformation dominated the wear mechanism in the process of the fretting test at −30°C. The plastic deformation was slightly less at 0°C than that at −30°C. With the increasing of temperature, the hardness of UHMWPE d...

This work characterizes the wear behavior of pearlitic SAG Mill liner material for in-service and laboratory worn samples. The abrasion tests (Dry Sand/Rubber Wheel Abrasion Test – DSRW) were carried out on samples removed from the undeformed region of the same liner – applying different normal loads (from 22 N to 380 N). The electron microscopy techniques (FEG-SEM, FIB-SEM) were used to characterize the microstructural and wear micromechanisms. A cross sections analysis of both samples highlighted the presence of a deformed layer. The hardness of the original microstructure (undeformed pearlite) was 360 HV 10 , however, this work shows that the typical operational conditions in the mining process increased the hardness in a sub-superficial layer up to 580 HV 10 . The thickness of the deformed layer was determined to be approximately 300 µm and 3 µm in-service and laboratory worn samples, respectively. The in-service worn samples showed scratches and micro-indentations along the surface. For the laboratory tests, the predominant wear mechanisms were micro-cutting with and without micro-ploughing and micro-indentation. It was shown that the normal load in the laboratory abrasion tests did not significantly affect the deformed layer formation. For the various normal loads applied, the thickness of deformed layer remained practically constant, around 3 µm. On the other hand, regarding wear mechanisms, a change in the normal load affected the indentations/cutting ratio: for lower loads micro-indentations prevailed whereas increased loads (above 130 N) indicated the presence of micro-cutting. Therefore, on the basis of these observations, it was possible to conclude that the DSRW represented a suitable alternative to simulate the abrasion component occurring in liners for SAG Mills once a higher load was applied (200 N to 280 N).

Stick-slip, manifest as intermittent tangential motion between two solids, is a well-known friction instability that occurs in a number of natural and engineering systems. In the context of adhesive polymer interfaces, this phenomenon has often been solely associated with Schallamach waves, which are termed slow waves due to their low propagation speeds. We study the dynamics of a model polymer interface using coupled force measurements and high speed \emph{in situ} imaging, to explore the occurrence of stick-slip linked to other slow wave phenomena. Two new waves---slip pulse and separation pulse---both distinct from Schallamach waves, are described. The slip pulse is a sharp stress front that propagates in the same direction as the Schallamach wave, while the separation pulse involves local interface detachment and travels in the opposite direction. Transitions between these stick-slip modes are easily effected by changing the sliding velocity or normal load. The properties of these three waves, and their relation to stick-slip is elucidated. We also demonstrate the important role of adhesion in effecting wave propagation., Comment: 22 pages, 9 figures

When subjected to shear loading condition, a steel rock bolt will become bent in the field close to the loading point in situ. The bolt is deformed as the joint displacement increases, which can mobilize a normal load and a shear load on the bolt accordingly. In this work, the relationship analysis between the displacing angle and loading angle is carried out. By considering elastic and plastic states of rock bolt during shearing, the rotation of bolt extremity can be calculated analytically. Thus, the loading angle is obtained from displacing angle. The verification of analytical results and laboratory results from reference research implies that the analytical method is correct and working. In terms of in-situ condition, the direction of the load acting on steel bolt can be predicted well according to the direction of the deformed rock bolt with respect to original bolt axis.

Phosphorene, a new two-dimensional material, has attracted tremendous attention in recent years due to its superior physical and electrical properties. Despite the fact that it may have a strong impact in future flexible electronics, there has been so far little or no atomic-level understanding of the failure process of phosphorene under nanoindentation. Here, we report a systematic study of the deformation and failure mechanism of phosphorene under nanoindentation by using molecular dynamics simulations. Three different regimes of deformation behaviors can be identified. The bump behavior in the first regime is different from that in the graphene and other 2D materials. Our simulations reveal the strong correlation between the normal load and the number of C–P pairs with repulsive force across the contact interface. Furthermore, the failure mechanism does not change with increasing tip size or defect concentration. However, it is found that the failure load decreases by 30% even though the defect concent...

The vacancy-enhanced contact friction of graphene is mainly attributed to the vacancy-enhanced out-of-plane deformation flexibility of the graphene and the climbing of the tip out of the vacancy trap (which actually acts as a step edge). However, this mechanism does not apply for explaining the enhanced friction caused by small-sized vacancies that are unable to accommodate the tip, such as single vacancy and double vacancies, which also commonly exist in the graphene. In the present study, by performing a set of classic molecular dynamics simulations, we demonstrated that the double-vacancy defect in graphene substantially enhanced the contact friction when the tip slides over it and the pinning effect of the reconstructed lattice of the double-vacancy defect with atoms at the bottom of the tip dominated such an influence. The underlying mechanism of such an atomic pinning effect and the influence of the normal load, sliding direction, and the sliding velocity were unveiled by analyzing the obtained friction evolution and the atomic configuration and interaction between the tip and the graphene. We believe that the findings presented in this study complete the state-of-art understanding of the nanoscale friction behaviors of vacancy-defected graphene, which is essential for the implementation of their potential control.

Frictional properties of interfaces with dynamic chemical bonds have been the subject of intensive experimental investigation and modeling, as it provides important insights into the molecular origin of the empirical rate and state laws, which have been highly successful in describing friction from nano to geophysical scales. Using previously developed theoretical approaches requires time-consuming simulations that are impractical for many realistic tribological systems. To solve this problem and set a framework for understanding microscopic mechanisms of friction at interfaces including multiple microscopic contacts, we developed an analytical approach for description of friction mediated by dynamical formation and rupture of microscopic interfacial contacts, which allows to calculate frictional properties on the time and length scales that are relevant to tribological experimental conditions. The model accounts for the presence of various types of contacts at the frictional interface and predicts novel dependencies of friction on sliding velocity, temperature, and normal load, which are amenable to experimental observations. Our model predicts the velocity-temperature scaling, which relies on the interplay between the effects of shear and temperature on the rupture of interfacial contacts. The proposed scaling can be used to extrapolate the simulation results to a range of very low sliding velocities used in nanoscale friction experiments, which is still unreachable by simulations. For interfaces including two types of interfacial contacts with distinct properties, our model predicts novel double-peaked dependencies of friction on temperature and velocity. Our work provides a promising avenue for the interpretation of the experimental data on friction at interfaces including microscopic contacts and opens new pathways for the rational control of the frictional response., Comment: 42 pages,12 figures

Negative friction coefficients, where friction is reduced upon increasing normal load, are predicted for superlubric graphite-hexagonal boron nitride heterojunctions. The origin of this counterintuitive behavior lies in the load-induced suppression of the moir\'e superstructure out-of-plane distortions leading to a less dissipative interfacial dynamics. Thermally induced enhancement of the out-of-plane fluctuations leads to an unusual increase of friction with temperature. The highlighted frictional mechanism is of a general nature and is expected to appear in many layered material heterojunctions., Comment: 34 pages, 16 figures, (file includes Supplemental Material)

The single-asperity friction of polished single crystal silicon using a cone with a spherical apex tip was quantitatively studied concerning the influence of normal load and scratching cycles. Specific friction equations were presented and the interfacial friction equations were determined by fitting the experiment data obtained in the scratching tests. The influence of different scratch conditions to both the interfacial fiction and the plowing friction was analyzed. This paper aims to understand the mechanisms of friction and material removal of single crystal silicon in micro and nanocutting or polishing.

The reduced wear rate of hip-joint bearing components is the main issues in biotribology researches for the biomedical implants. Ultra-high molecular weight polyethylene (UHMWPE) is extensively used as a bearing material because of having its great properties of friction and wear. Surface texturing can be selected as a method of enhancing tribology properties of engineering surfaces. In this present study, surface texturing examination was performed by 3D finite element method under influence of normal load. Stress distribution of UHMWPE surface under the influence of texturing with the variation in diameter cavities and separation was investigated. The result showed that the surface texturing provided the higher stress distribution of the lower separation in the cylindrical cavities.

The tribological behavior of aged Al-Sn-Cu alloy rubbed in the presence of lubricant over a range of sliding velocities and normal loads was investigated. The results showed that peak-aged (PA) alloy had a better tribological behavior than under-aged (UA) and over-aged (OA) alloys, which could be attributed to the optimized strength-ductility matching and a better hardness under PA condition. Wear rate and friction coefficient showed great sensitivity to applied sliding velocity and normal load. The wear rate and friction coefficient of the alloy exhibited a reduction trend with the increase in sliding velocity. The low wear rate and friction coefficient of alloy at high velocities were due to the effectively protected film and homogeneous Sn on surface. However, an increase in normal load led to an obvious increment in wear rate. The friction coefficient exhibited a fluctuant trend with the increase of normal loads. The seriously destroyed film and abraded Sn resulted in poor tribological behavior at high normal loads. The Sn particles and lubricant film which includes low shear interfacial lubricating layer and oxide tribolayer are the key to the tribological behavior of Al-Sn-Cu alloy.

In order to investigate the deformation characteristics and material removing mechanism of the single crystal silicon carbide at the nanoscale, the nanoscratching tests were conducted on the surface of 6H-SiC (0 0 0 1) by using Berkovich indenter. In this paper, a theoretical model for nanoscratching with Berkovich indenter is proposed to reveal the relationship between the applied normal load and the machined depth. The influences of the elastic recovery and the stress distribution of the material are considered in the developed theoretical model. Experimental and theoretical machined depths are compared when scratching in different directions. Results show that the effects of the elastic recovery of the material, the geometry of the tip and the stress distribution of the interface between the tip and sample have large influences on the machined depth which should be considered for this kind of hard brittle material of 6H-SiC.

A two-dimensional contact problem of a trapezium shaped punch pressed into a frictionless, elastically similar half-plane and subject sequentially to the normal load and bending moment is considered. The model of a tilted flat punch is used to evaluate the pressure distribution and the contact deformation within the contact zone. Comparisons of the results generated by the analytical technique to those computed by the finite element method demonstrate the high level of accuracy attained by both methods. The presented numerical results illustrate the effects of the normal load, bending moment, and internal angles of the punch geometry on the contact stresses.

Little attention has been paid to the role of stacking fault energy (SFE) in the two-body abrasive wear of austenitic steels. Using a pin-abrasion test with 220 grit garnet paper as the counterbody, three austenitic steels of different SFEs were compared. The steels were: (i) FeMnAlC, (ii) Hadfield steel, and (iii) AISI 316 L steel. Following a pre-conditioning procedure, the normal loads on the 3 mm diameter test pins were 5 N, 10 N and 15 N, and the sliding speed along a spiral track of total length 430 m was 0.158 m/s. Data showed that the FeMnAlC steel had a higher wear resistance than AISI 316 L steel but lower wear resistance than the Hadfield steel. However, at the highest test load, all three steels had similar wear resistance. The steel with the lowest SFE had the highest abrasive wear resistance and the steel with the highest SFE had the lowest abrasive wear resistance. The main wear mechanisms were microcutting and microploughing. There was a transition from microploughing to microcutting as the normal load was increased.

Pure magnesium and its composites reinforced with 5, 10 and 15 vol% SiC particulates with a mean diameter of 7.8 µm were fabricated by a powder metallurgy process. The tribological behavior of the samples was investigated under normal loads of 5–60 N at sliding speed of 0.4 m/s and at wear temperatures of 25–200 °C. At the wear temperature of 25 °C, results showed almost close wear rates under normal loads of 5 and 20 N. However, under higher normal loads the composites showed lower wear rate than that of the unreinforced magnesium. At the higher wear temperatures of 100, 150 and 200 °C, a significant lower wear rate was observed for the composites compared to the pure magnesium. Increasing the normal load resulted in a transition from mild to severe wear at all wear temperatures. Below the transition load oxidation was a dominant wear mechanism, while above that severe plastic deformation and adhesion were the main wear mechanisms. The results also showed wear rate improvement of the composites with increasing SiC content. Finally, a wear map showing mild and severe wear regimes as a function of load and wear temperature was adopted for the composites.

The initial transient, or running-in, period during abrasive wear has been largely overlooked in the literature, with the focus instead being on steady-state conditions. However, the running-in period of abrasion might determine the evolution to steady-state wear behaviour, as is known to occur in sliding wear conditions. To this end, the running-in period during the abrasive wear of the austenitic stainless steel AISI 316L and Hadfield (15%Mn-1.5%C) steel was analysed through the testing pin (flat-ended)-abrasive paper wear configuration. The effects of the normal load, the size of the abrasive, and the type of material were evaluated, and the wear rate and the strain hardening of the matrix were recorded. The steady-state wear might be influenced by the phenomena occurring during the running-in period. Thus, the hardening, roughness, friction coefficient and wear micromechanisms were correlated with the wear rate to elucidate these possible relationships.

Ti6Al4V coatings were made from spherical and irregular powders manufactured using plasma gas atomization and Armstrong processes, respectively. The coatings, made from the two different powders, were distinctly different in terms of porosity content and hardness. Sliding wear tests were performed at different normal loads on both Ti6Al4V coatings. Despite low porosity and high hardness, irregular powder (IP) deposited coatings showed higher wear rates compared to spherical powder (SP) deposited coatings. In case of IP coatings, abrasive ploughing by the wear debris led to high wear rates, whereas presence of porosity in SP coatings, entrapped the wear debris generated during sliding and decreased the wear rate. Increase in normal load led to a decrease in wear rate and CoF in both coatings. The decrease in wear rate was due to tribo-oxidation in IP coatings whereas the combined effect of entrapment of debris along with tribo-oxidation resulted in much lower wear rates in SP coatings. Transmission electron microscopy (TEM) analysis showed that the third bodies filled in the pores consisted of highly deformed material with ultrafine grain microstructure and micron sized particles from the counterface while the wear track had comparatively coarse grain microstructure. Results indicated that despite low hardness of SP Ti6Al4V coatings, presence of porosity facilitated for the entrapment of wear debris generated from first bodies which inhibited abrasive ploughing and contributed to low wear rates.

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

There exists a family of methods for finding the extent of partial slip in contact problems between elastically similar bodies, capable of idealisation by half-planes. Closed form solutions are given to problems subject to a constant normal load and subsequent application of either an increasing shear force or differential bulk tension parallel with the surface. The starting point may be either a sliding contact (as is customary), or a fully adhered contact. The corrections required to impose a point-wise interpretation of Coulomb’s law of friction may be either in the form of a shear traction distribution or as dislocation arrays. The latter, when applied to the fully adhered contact, has the merit of automatically preserving the locked-in relative surface strains.

International audience; The response of glass to a scratch experiment is first reviewed. Then the influence of the applied load on the microcracking pattern, the apparent friction coefficient, and the strength (post scratch test) are studied and discussed for a soda-lime-silica glass (standard window glass composition). As the normal load increases, the depth of the radial crack seems to stabilize at about 100 μm length. Correlatively it is observed that the residual strength of the scratched glass remains at about 40 MPa whatever the scratch load above 0.5 N. Generally, this work contributes theoretically in the understanding of the existing factor controlling scratch in glasses.

To investigate the effects of a developed friction film on clutch system performance, clutch samples tested in three different field conditions were characterized. The wear mechanisms identified were simulated in laboratory. For this, a commercial truck friction material and cast iron were tested in a pin-on-disc tribometer. Tribological tests were carried out at standard procedures and by cleaning the wear track of the counter-face. For both lab test conditions, the normal load ranged from 200 N to 450 N at constant sliding speed of 2.05 ms −1 and 3.27 ms −1 . These parameters correspond to PV values between 3.08 and 11.08 (MPa ms −1 ), which cover part of the power release in real clutch systems (between 1.3 and 20 MPa ms −1 ). The friction film simulated in laboratory showed similar characteristics with those observed in field and its development and morphology depend on the test severity. Standard tribometer tests presented a transition in the wear rate at temperatures of 250 °C. Removing the wear debris, the wear rate decreased almost 31% at the lowest level and 45% at the highest level of energy. The friction material tested traditionally revealed a stable friction film for tests performed to 7.88 (MPa ms −1 ) and a damaged film was found in severe wear regimes. When removing the wear debris under the same conditions, a stable friction film was observed and a less developed friction film was noted on the disc surfaces by eliminating the wear debris. The friction coefficient decreased with increasing testing temperature and cleaning the sliding surface increased the friction levels.

The WC particle-reinforced copper matrix composites with WC content in the range of 5 vol%–30 vol% were prepared and investigated by micro-scratch tests under the loads of 1–4 N. The influences of normal load and WC content on friction coefficient and wear volume of the composites were studied. The hardness and scratch resistance of the composites are greatly improved by the addition of WC. The penetrate depth and residual depth increase near-linearly with normal load increasing and decrease with WC content increasing, and the friction coefficient of the composites also shows the same variation trend. The valid material removal mechanism is found to transform from micro-plowing to micro-cutting as the normal load increases.

Non-stationary effects in the friction-induced dynamics of a two-degree-of-freedom brake model are examined in this paper. The belt–spring–block model is designed to take into account variations of the normal load during the braking process. It is shown that due to the adiabatically slowing down velocity of the belt, the system response experiences specific qualitative transitions that can be viewed as simple mechanical indicators for the onset of squeal phenomenon. In particular, the creep-slip leading to a significant widening of the spectrum of the dynamics is observed at the final phase of the process.

The dry sliding wear behavior of Cu-Cr-Zr alloy prepared by electromagnetic horizontal continuous casting was investigated. The wear behavior of the studied alloy was discussed in terms of friction coefficient, mass loss/sliding, specific wear rate and wear mechanism. The results indicate that with the increasing normal load and sliding velocity, the friction coefficient of Cu-Cr-Zr alloy decreased monotonically, the mass loss/sliding and specific wear rate increased. By wear surface morphology and composition analysis, the wear mechanisms were discussed preliminary. Oxidation and abrasion mechanisms dominated at the lower sliding velocities and loads. Increasing loads and velocities led to a combination of oxidation and adhesion. Plastic deformation was dominant for the higher applied load and sliding velocities.

We performed the resonance shear measurement (RSM) for evaluating the nanorheological and tribological properties of model lubricants, hexadecane and poly(α-olefin) (PAO), confined between iron surfaces. The twin-path surface forces apparatus (SFA) was used for determining the distance between the surfaces. The obtained resonance curves for the confined lubricants showed that the viscosity of the confined hexadecane and PAO increased due to liquid structuring when the surface separation (D) decreased to a value less than 24 and 20 nm, respectively. It was also determined that the iron surfaces were lubricated by the hexadecane when normal load (L) was less than 1.1 mN, while the confined hexadecane behaved almost solid-like and showed poor lubricity when L was greater than 1.1 mN. In contrast, PAO between the iron surfaces showed high lubricity even under the high load (L2 mN). The surface separation of hexadecane and PAO at a hard wall contact between the iron surfaces was determined to be 4.6 ± 0.5 and 5.0 ± 0.4 nm by applying the fringes of equal chromatic order (FECO) for half-transparent iron films deposited on mica surfaces as substrates. We also characterized hexadecane and PAO confined between mica surfaces for studying the effect of substrates on the confined lubricants.

In the case of a general symmetric incomplete contact, if we can deduce the partial slip solution when it is subjected first to a normal load (held constant) and then to a monotonically increasing bulk tension, we show, here, how to obtain the solution when the normal load and bulk tension vary with time in an arbitrary manner. The procedure is demonstrated for a Hertzian contact where the constant normal load solution is known in closed form. It reveals the extent of slip and the shear traction distribution at all points within the contact at any instant. It was discovered that the size of the permanent stick zone, for a given cyclic loading trajectory, is unique in the steady state. The steady state is established after one cycle of loading: it is independent of the transient loading prior to reaching the steady state cycle, which is also observed in the case of varying normal and shear loading ( P − Q ). An example is given to illustrate the type of behaviour that is to be expected.

Contact self-cleaning of gecko-inspired synthetic adhesives with mushroom-shaped tips has been demonstrated recently using load-drag-unload cleaning procedures similar to that of the natural animal. However, the underlying mechanics of contact cleaning has yet to be fully understood. In this work, we present a detailed experiment of contact self-cleaning that shows that rolling is the dominant mechanism of cleaning for spherical microparticle contaminants, during the load-drag-unload procedure. We also study the effect of dragging rate and normal load on the particle rolling friction. A model of spherical particle rolling on an elastomer fibrillar adhesive interface is developed and agrees well with the experimental results. This study takes us closer to determining design parameters for achieving self-cleaning fibrillar adhesives.

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.

Reciprocating ball-on-disk tests were performed to investigate the effects of sliding velocity and ambient temperature on the tribological characteristics of a diamond-like carbon (DLC) coating under a boundary lubrication regime. The coating consisted of sequentially deposited gradient Cr/CrN, a-C:H:W, and a-C:H layers; the counter surface was an AISI 52100 steel ball. Average sliding velocities (range: 0.01–0.04 m/s), temperatures (range: 27–160 °C), and a normal load of 293 N were selected as the test conditions. One fully formulated motor oil containing a glycerol mono-oleate was used as a lubricant. Wear rates of the coatings increased with increasing temperature. As temperature increased from 27 to 55 °C, friction coefficients increased, but wear rates of the ball decreased. With an increase in temperature from 55 to 160 °C, friction coefficients decreased until 110 °C then slightly increased, and wear rates of the ball increased. Increasing sliding velocity led to a decrease in friction coefficients and wear rates of both friction surfaces. Through surface analyses and observations, it was confirmed that graphitization of the worn DLC coatings, reactive tribofilms on the worn ball specimens, portion of the surface asperities in dry sliding contact, and corrosive wear of the ball specimens affect tribological characteristics.

A model for interactions of a single large impurity particle with the optic and pad during pad-polishing has been developed to detailedly illustrate the scratch formation process. Equations for calculating the normal load applied on the optic by the impurity particle are provided, and the friction between the optic and impurity particle has also been considered in scratch formation. Initiation and evolution of the scratches under the combined effects of the normal and friction forces are illustrated in the model. The most important factors affecting the normal load and thus scratch formation have been analyzed and verified using published experimental results. Based on the model, the implications in reduction of scratch formed on optics during pad polishing have been analyzed.

The wear test of copper (Cu)–niobium diselenide (NbSe2)–silicon carbide (SiC) composite were conducted on a pin on disc under different normal loads and sliding speed. The morphologies of the worn surface of the composite were observed by using scanning electron microscopy. Experiment results show that the friction coefficient and wear rate of (NbSe2+SiC)/Cu composite increased with increasing of the normal load and the sliding speed. The addition of NbSe2 can effectively reduce the plastic deformation of the substrate, making the friction and wear properties of composite has greatly improved. Mild adhesive wear and abrasive wear were the main wear mechanisms during the sliding process.

Although both wear and fatigue are inevitably associated with the wheel/rail contact, they are normally studied as different phenomena and both are commonly considered excluding. Wear of railway wheels and rails is usually studied by relating wear rate to the T γ or T γ / A parameters or by plotting the so called “wear maps”, where different rolling parameters are related to the wear rate. On the other hand, fatigue in wheel/rail contact is mainly studied from the surface and subsurface stress or deformation fields and from the crack growth rate. However, one of the main causes of wear in rolling contact is the loss of material due to delamination processes that have their origin in the presence of surface fatigue cracks. So, if wear and fatigue are related, it should be possible to study both of them using the same method. In this paper, the fatigue index, based on the shakedown theory and used to predict the surface initiated fatigue of railway wheels, is proposed as a means to study the wear rate in rolling contact. The fatigue index is directly related to the apparition and growth of fatigue cracks at the surface and, thus, directly related to the peeling and spalling processes that constitute the severe and catastrophic wear mechanisms. The data collected from a series of twin-disc tests show that the wear rate correlates with the fatigue index following an exponential evolution independently of the normal load. Furthermore, a fatigue index around 0 marks the onset of severe wear.

TaN–(Cu,Ag) nanocomposite films were deposited by reactive co-sputtering on Si(001) and M2 tool steels. The films were then annealed using RTA (Rapid Thermal Annealing) at 200 °C–400 °C to induce the nucleation and growth of metal particles in TaN matrix and on film surface. After deposition, structures, surface morphologies, and mechanical properties were analyzed, the samples were tested for their anti-wear and anti-bacterial behaviors against Gram-negative Escherichia coli. It is found that the antibacterial efficiency against E. coli can be much improved for TaN–(Cu,Ag), comparing with that of TaN–Ag or TaN–Cu films. This could be attributed to large number of surfaced Ag and Cu particles. The tribological properties could also be improved, and are similar to TaN–Ag under a normal load of 5 N. In general, the annealing temperature for TaN–(Cu,Ag) can be as low as 200 °C. Being annealed at this temperature, the film still shows good antibacterial and anti-wear behaviors.

We studied the effect of mating materials on the tribological properties of DLC film using mating materials of martensite stainless steels (hardened and annealed AISI440C), austenite stainless steel (AISI304), and bearing steels (hardened and annealed AISI52100). The reciprocating sliding tests were performed with ball-on-plate friction tester in dry air. Analyses of friction surface of specimens were carried out using optical microscopy and micro-laser Raman spectroscopy. With the exception of the annealed AISI52100 ball, the friction coefficient of DLC film decreased with increasing normal load. The friction coefficient for the annealed AISI52100 ball showed a high friction coefficient under wide normal load conditions. Analysis using micro-laser Raman spectroscopy showed that the difference between annealed AISI52100 and others materials appears mainly due to structural changes in the film. Tribological properties of DLC film may thus be affected by a property change in mating materials rather than by mating ball hardness.

The surfaces of fibrous materials like textiles are characterised by emergent superficial fibres forming hairiness. Hairiness is an important factor for the frictional and tactile properties of fabrics. In this study, we investigated the friction mechanisms of hairy fabrics by sliding probes with varying geometry over the textile surfaces. The total friction between a hairy fabric and a cuboid-shaped slider is given by two main contributions—one due to the contact of hairs with the front of the slider and the other one resulting from the hairiness underneath the slider. By changing the slider geometry, the relative importance of these two contributions can be systematically varied, so that conclusions about adhesion and deformation mechanisms involved in the friction of hairy fabrics can be drawn. For a given slider area and a given normal load, the force underneath the slider was found to be independent of the slider width while the force in front of the slider increased linearly with the slider width. The major contribution of the force underneath the slider is highlighted. This force is influenced by the material and surface roughness of the slider, indicating that adhesion mechanisms play a significant role in the friction.

Copper single crystals grown according to the Bridgman method and having their axes 1 ¯ 1 1 or [1 1 0] aligned with the normal load axis were used for dry sliding experiments. As shown, sliding-induced severe plastic deformation occurred in the subsurface of single crystals and caused formation of a lip by mechanism of texture formation. The SEM structure of this lip was found to be composed of fragments with their shapes dependent on the single crystal orientation with respect to normal load and friction force. These fragments may be referred to as wear particles generated by shearing in planes parallel to the worn surface.

A molecular dynamics simulation of confined n -alkanes has been conducted to investigate the effects of the chain length of lubricant molecule and normal load on friction and asperity contact. The density distribution and surface coverage of lubricant atoms were calculated. The simulation results indicate that longer chain n -alkanes can provide more atoms for the mono-layer in the asperity contact interface than shorter ones, which, as a result, significantly reduce the friction force when the asperity contact occurs. C64-alkanes molecules can form a mono-layer of lubricant to avoid direct asperity contact at pressures up to 1000 MPa.

The surfaces of materials differ in topography and compatibility with human skin. Those differences were distinguished by measuring the friction coefficients between a human finger and different types of surfaces. Sample groups used in these tests were glass, plastic, and fabric. In the case of fabric, the following materials were used: linen, canvas, felt, silk, velvet, and cotton. Experiments were conducted using a specially-designed apparatus under constant temperature and relative humidity. Two factors were identified using the finger friction measurements. First was the friction level and second was the variance in friction. In Particular, it was found that different surfaces show significantly different variance in their friction coefficients. Fitting parameters from a linear regression of the relationship between friction coefficient and normal load were used to map materials' responses.

This paper investigates the effect that contact geometry on fretting wear rates and mechanisms for non-conforming contacts. Tests have been conducted in the gross sliding regime under a constant normal load in a cylinder-on-flat configuration with a high strength steel for a range of cylinder radii. Two distinct damage mechanisms have been identified, with the operative mechanism depending upon contact geometry. Fretting of less conforming contacts results in higher wear rates compared to that associated with more conforming contacts where the wear rate is significantly lower (between 20% and 50% of that of the less conforming contacts) in addition, much higher levels of adhesive transfer are observed in more conforming contacts, with beds of metallic debris particles being evident. It is proposed that the dependence of wear rate and mechanism on the contact geometry is not associated with differences in contact pressure (although these do exist), but is associated with the influence of contact geometry on debris flow out of the fretting contact and on the effectiveness of oxygen being excluded from the centre of the fretting contactpatch.

International audience; Cu and Cu-MoS2 coatings were fabricated by cold spray, and the fretting wear performance of the two coatings was compared. A mixture (95 wt.% Cu + 5 wt.% MoS2) was used as feedstock for the composite coating. Coatings were sprayed with identical gas flow conditions on the substrates pre-heated to approximately 170 degrees 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 MoS2 on Cu deposition was examined. The local MoS2 concentration within the coating was found to affect the hardness. Fretting tests were carried out at two different normal loads, and the influence of MoS2 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.

We present a general formulation for the stick slip behaviour of incomplete contact under oscillating loading, but with a constant normal load. An asymptotic description of the contact traction very close to the contact edges is used. The slip zones present in the steady state with cyclically varying bulk tension and shear force (with an arbitrary phase shift) are found. The range of the variation of the state of stress near both of the contact edges and the respective slip zone sizes are defined in terms of the loading parameters, including the phase angle. The quality of the approximations used by the asymptotic approach and the range of applicability of the method is also analysed in detail in this paper.