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The present work investigates the effect of process parameters (normal load, sliding velocity, and sliding distance) on wear behaviour of Ti-3Al-2.5 V alloy under dry sliding condition. The wear te...

The purpose of this paper is to compare the influence of the normal load on the friction and wear behaviour of nickel-based alloys, which is use communally in the industry. We chose two samples of nickel-based alloys (NI-Cr-B-Si-C-W) elaborated by isostatic compression with a difference in their microstructure composition and quantity of micro constituent. The friction coefficient, the wear loss and the wear rate are evaluated using a tribometer (pin on disc) and a test bench created to reproduce the same industrial environment. Experiments showed that the applied normal load affects differently on the behaviour of the samples. The results conclude that the friction and wear resistance of the alloy do not just depend on the increase of the normal load but also on the distribution, the location and the morphology of the hard compounds formed in the alloys.

The temperature effect on the frictional characteristics of a silver-silver friction pair was experimentally studied. These characteristics are as follows: Amontons’ and Coulomb’s friction coefficients and specific friction force parameters. The applicability of the Coulomb’s friction law to the description of the friction force dependence on the normal load in the studied temperature range is substantiated. In addition, the experiments showed that the contact frictional characteristics are significantly affected by its temperature prehistory and the temperature variation direction during the experiment—from room to maximum or from maximum to room.

In this paper, the nonlinear partial slip contact problem between a monoclinic half plane and a rigid punch of an arbitrary profile subjected to a normal load is considered. Applying Fourier integral transform and the appropriate boundary conditions, the mixed-boundary value problem is reduced to a set of two coupled singular integral equations, with the unknowns being the contact stresses under the punch in addition to the stick zone size. The Gauss–Chebyshev discretization method is used to convert the singular integral equations into a set of nonlinear algebraic equations, which are solved with a suitable iterative algorithm to yield the lengths of the stick zone in addition to the contact pressures. Following a validation section, an extensive parametric study is performed to illustrate the effects of material anisotropy on the contact stresses and length of the stick zone for typical monoclinic fibrous composite materials.

The pin-on-disc test model system was used to simulate contact wear between wheel flange and rail gauge corner. Pins were extracted from a class C cast steel wheel and discs were extracted from premium and intermediate steel grades rail, both hypereutectoid and with hardness between 321 and 392 HB. Rails are pearlitic and the bainitic wheel is in its first life. Dry and lubricated tests were performed with normal load variation (40, 80 and 120 N). Under dry condition, there was no influence of the pair hardness to mass loss of the tribosystem studied, contrary to what is stated by literature. For the lubricated tests, mass loss was significantly lower and regardless of both, hardness ratio and normal load, than in the other tests. This shows the technological importance of: (i) implementing wheel flange lubrication (locomotive embedded process); (ii) improving rail lubrication techniques (mobile or way side process) and/or friction coefficient measurement techniques and (iii) need for revision of wheel standards and/or specifications.

In the present study, experiments have been carried out by sliding hard elastic material on ductile flat surface. The different state of stress has been achieved by changing the geometry of hard material and different orientation of changed geometry with respect to sliding direction. Experiments have been carried out using pin on disc wear test rig. The tests were conducted with Al6061 alloy disc and EN8 alloy steel pin with different tip geometries and orientations. The obtained results revealed that coefficient of friction was higher at lower loads compared to higher loads in hemisphere pin geometry. An exception has been found at 90° orientation at low loads. It has been attributed to adhesive effect. The state of stress and its configuration with respect to direction of sliding was found to influence the sliding phenomenon. Micrograph studies revealed damages due to adhesion, abrasion and extrusion.

Surface texturing plays a significant role on the tribological performance of contact surfaces. The tribological characteristics are mostly dependent upon the different pattern and methods of surface texturing. This research works investigates lubricated wear behaviour on circular dimples textured EN-31 alloy steel materials using DUCOM linear reciprocating tribometer. The surface dimples are fabricated by micro-EDM and micro-drilling CNC methods. Initially plane surfaces are tested under different lubricants ISO-68 and ISO-220 oil on a couple of EN31 plates with EN8 steel pin. At loading conditions the results clearly shows that the ISO 68 oil gives better performance than ISO-220 oil in terms of frictional force and COF. The operation during test under ISO-220 oil observed is very noisy and wear rate is also more. Then after experiments under different load conditions on textured surfaces were investigated using optimized ISO-68 oil. An experimental results on m-EDM provides superior frictional characteristics with respect to m-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. Here micro-dimples EDM surface textures can enhance mixed lubrication and in mixed lubrication the friction is least and this may leads to decrement of wear loss.

Two-body abrasive wear tests of Fe-B alloys with various matrix microstructures were performed using a pin-on-disc tribometer at a normal load of 3 N. The wear behavior was analyzed using s...

The paper investigates sliding friction of the α-Al2O3/α-Al2O3 and α-Fe2O3/α-Fe2O3 contacts by using the spring interfacial potential. It is found that at micronscale the friction properties of the oxides are almost independent of the coarse-graining and are the same in the different sliding directions. Even the hardness contacts friction coefficient shows a decrease with increasing intensity of the normal component of the interfacial interaction force. This result is as an implementation for the previous observations of sliding friction of various materials that showed that a drop of friction coefficient with increasing externally applied normal load has originated from deformation of interfaces or occurrence of debris at contact, indicating an unsteady contact.

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.

We developed a method to simulate the polytetrafluoroethylene (PTFE) tribology by estimating the molecular interaction parameters on the basis of experimental verification. We fitted the molecular interaction parameters between coarse-grained PTFE beads using the iterative Boltzmann inversion method and multi-centered Gaussian-based potentials. The fitting parameters were subsequently validated by comparing an all-atom PTFE model and its corresponding coarse-grained model. A two-layer PTFE friction model was then built on the basis of the estimated parameters to study the effect of normal load on the friction coefficient and wear depth of PTFE. The simulation results showed that the friction coefficient decreases and the wear depth increases as the normal load increases. Moreover, the reasonability of the simulation results was verified through experiments.

There have been insistent demands for effective lubricants as well as additives for reducing friction and wear in various applications. Recently, graphene oxide (GO) and nanodiamond (ND) have been observed to be promising materials to reduce friction and wear in tribological studies. Herein, a novel lubricant has been developed by using GO and ND together as lubricant additives in water in order to enhance tribological performance. The tribopairs employed were Si3N4 balls and Si wafers, and the applied normal load was 5–80 mN. The lowest friction coefficient of approximately 0.03 was obtained for water with 0.1 wt.% GO and 0.5 wt.% ND, while the corresponding depth of wear track was as low as about 5 nm. Analysis of the tribological mechanisms elucidated that the sliding-induced nanostructured tribofilm, the low shearing resistance between graphene sheets, and the possible ball bearing effect of ND all contributed to the remarkable tribological behaviors of the lubricant.

This study investigated the tooth friction force and transmission error (TE) of spur gears due to sliding friction under quasi-static condition. The sliding velocity and friction force of spur gears and mesh compliance during meshing were calculated. The load–deformation relations between the tooth normal load, tooth errors, and mesh compliance, and moment equilibrium equation, including friction force, were derived. The friction force, tooth load, and TE of unmodified and linear tip-relief modified spur gears were analyzed by using the derived equations. Results indicated that the friction force, tooth load, and TE increased during approach and decreased in recess regardless of tooth modification, particularly in the single-mesh region. Friction caused larger peak-to-peak change of TE than that without friction. Xu’s friction coefficient generated smooth TE and tooth load transitions near the pitch point, and BK’s friction coefficient remained approximately constant, except for the sharp increase in tooth load and TE near the pitch point.

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.

Surface topography is a critical parameter that can influence friction and wear in engineering applications. In this study, the influence of surface topography directionality on seizure and scuffing initiation during tribological interactions is explored. Hence, unidirectional sliding wear experiments were carried out in immersed lubrication conditions for various normal loads. The tribological interactions were studied using EN31-60 HRC flats and SAE52100-60 HRC pins in a sphere on flat configuration. The results show that, in some cases, the sliding interactions in the initial cycles lead to a high friction coefficient of up to ∼0.68 in lubricated conditions, which was termed as “peak friction”, and this was accompanied by scuffing. The existence of peak friction was found to be dependent on surface topography directionality, especially when the directionality in topography was parallel to the sliding direction. Continuous ratchetting was found to be the cause of peak friction, which was accompanied by seizure and scuffing. When the topography directionality was perpendicular or independent of the sliding direction, elastic shakedown occurred at earlier cycles and prevented peak friction initiation and scuffing and also facilitated for higher steady-state friction values.

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

The similarity in implied process zones, based on violations of the plastic yield condition, between the state of stress at the edge of an incomplete contact subject to reciprocating shear and tension and a crack suffering pure mode II loading are compared. This is done for both small and large amounts of edge slip, i.e. when the frictional slip region is either enveloped by the plastic region or envelops it. The results show that for realistic coefficients of friction strong agreement is unlikely to occur, so that attempting to analyse the nucleation of fretting cracks as a form of crack branching/turning is inappropriate and the effect of normal load on crack nucleation is an important consideration.

A multifunctional hypothesis of filler selection for PTFE suggests ultralow wear (10−7 mm3/Nm) depends on the filler’s ability to both support the normal load by being large in size and to induce favorable tribochemistry by being weak in aggregate strength. This study is the first to test the independent effects of preferential load support and tribochemistry on PTFE wear by studying the wear reducing properties of a porous GaN fiber, a fully dense SiO2 fiber and microsized GaN particles. The results found the only filler that induce ultralow wear is the GaN fiber which was microns in size and friable under shear stresses. This study reinforces the leading hypothesis that a mechanistic synergy is required for ultralow wear PTFE composites.

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.

This paper presents the results of the tribological experiments conducted using an oscillating wear tester under lubricated conditions. Specimen from piston ring slid against counter-specimen from cylinder liner. Various surface textures of cylinder liners resulted from honing and/or plateau honing. Tests were carried out for two normal loads of 100 and 300 N at a temperature of 80 °C, the stroke was 3 mm, the frequency was 10 Hz. Each experiment was divided into four time intervals. During breaks between the particular parts of the test, the surface topography of cylinder liners was measured by a white light interferometer. It was found that an increase in the normal load led to an increase in changes of the liner surface textures.

The problem of the forced non-axisymmetric vibration of stiffened ellipsoidal shells under nonstationary load is formulated. A numerical algorithm for solving the problem is developed, and the results obtained are analyzed.

The tribological behavior of different polyaryletherketones (PAEKs) and their composites was studied under reciprocating sliding conditions at room temperature as well as at elevated temperature (120 °C). The effect of glass, carbon fiber and solid lubricants (PTFE and graphite) filled in PAEK matrix was investigated at different normal loads. It was found that friction and wear characteristics are strongly influenced by the type of PAEK matrix and the material composition. The temperature has a more pronounced influence on the coefficient of friction and specific wear rates of PAEKs and their composites than the normal load. Friction and wear mechanisms are discussed, using scanning electron micrographs of the worn surfaces. A relationship between the average coefficient of friction and specific wear rates of PAEKs and their composites is also reported.

The contact problem concerning sliding of a solid surface with a regular relief along a fixed ring-shaped elastomer under normal load and rotational moment is studied. The concept of an effective (integral) coefficient of friction is introduced, and the aim of this study is to analyze the dependence of this value on the density of contact spots. The problem is solved by direct computer modeling in finite-element formulation. It is shown that dependences of an effective friction coefficient on time have a nonmonotonic cyclic character with transition to the steady-state regime. The amplitude values of an effective friction coefficient are higher for a lower density of contact spots at a fixed value of the normal load on the sample, whereas its averaged value is higher for a higher density of contact spots.

To investigate the nano-scale scratch and friction behaviors of polycarbonate (PC), atomic force microscopy (AFM) was utilized to conduct the nano-scratch tests under different normal loads with a probe of 20 nm tip radius. Then a probe of 1 nm tip radius was employed for the scanning of scratched surface morphology. The stick-slip phenomena are found for all studied normal load levels. The nano-scratch deformation at different normal loads can be categorized into three modes: sliding on asperities, elastic deformed groove and permanent scratch groove. After the analysis of amplitude-frequency characteristics of friction force, a good correlation is found among scratch deformation, friction force and stick-slip. Those findings can be a good guidance to understand and improve the nano-scratch performance of polymers.

Abrasion is one of the most common failure forms of metals and alloys. The study of the friction and wear behavior of metals and alloys is greatly beneficial to improve their long life and safe service. The friction coefficient, weight loss, specific wear rate, and wear mechanism of Ti-20Zr-6.5Al-4V (TZ20) alloy after various treatments were investigated using a pin–disk-type wear apparatus at various normal loads. The wear results revealed that at a normal load, the friction coefficient steadily decreased, whereas the weight loss increased. By contrast, different specimens displayed distinct variation trends of specific wear rate with respect to the normal load. Further investigations indicated that the effects of hardness and toughness on the wear properties were markedly influenced by the normal load. The main characteristic of the abrasion surface gradually changed from grooves to plastic deformation as the normal load increased. The findings promote the practical application of TiZrAlV series alloys and expand wear theory.

The fracturing of well-logging steel wire occurred during downhole operation in a gas injection well in China. The failed well-logging steel wire material was austenitic stainless steel manufactured by cold drawing. In this study, the failure cause was investigated based on visual inspection, chemical composition analysis, metallographic observation, mechanical performance testing, SEM and EDS analysis. The results showed that the failure reason was primarily Cl−-induced stress corrosion cracking which made the steel wire unable to bear the normal load of well-logging operation. The steel wire had experienced SCC during previous operation in a production well where downhole mediums contained high Cl− concentration, and then cracks-containing steel wire was used for this operation. Most of the cracks initiated within surface scratches caused by cold drawing where corrosion pits were present. In addition, cold drawing process of steel wire had a significant influence on SCC propagation mode, and the secondary cracks mainly extend along deformed austenitic grain boundaries in cold drawing direction.

Wet skid resistance is one of the basic properties for tread rubber, which plays an important role in driving performance of vehicles. Firstly, a new test device has been developed for testing wet skid resistance of tread rubber. Then, effect of filled nano silicon, water film depth, normal load and sliding velocity on friction properties of tread rubber have been studied, and contact pressure under different pavement parameters of Rsm, Ra also has been investigated. Finally, friction surface morphology of tread rubber has been investigated by OLYMPUS-DSX510 Optical Digital Microscope. Results indicate that nano silicon instead of silica (white carbon black) can significantly improve wet skid resistance with the same mechanical properties, replacement ratio of 40% is the best; friction coefficient of tread rubber block decreases when the normal load increases, however, it increases and then decreases when sliding velocity increases under dry and wet conditions; contact pressure increases with the increase of Rsm, however, it decreases when Ra increases.

Many wear failures are caused by a combination of fretting wear and sliding wear. In this study, the torsional fretting and torsional sliding wear properties of CuNiAl against 42CrMo4 were comparatively investigated under dry condition using a flat on flat contact tester. Experimental results showed that the sliding friction coefficients declined more dramatically than the fretting friction coefficients when the normal load increased. The fretting wear rate was lower than the sliding wear rate, which was partly due to the solid lubrication effect of the wear debris and strain hardening of the worn surfaces. The dominant wear mechanisms for the fretting tests were oxidation, cracks and delamination, while for the sliding tests were abrasion combined with plastic deformation.

Fretting crevice corrosion has become a major concern in orthopedic medical devices when passivating alloys are used alone or in combination. In this work the fretting crevice corrosion response of 316 L stainless steel was investigated when the counterface consisted of different alloys and the effects of potential and normal load were investigated. Fretting initiated crevice corrosion was observed in all stainless steel containing couples, but not in the Ti6Al4V_CoCrMo combination. The coefficient of friction (COF) varied with the applied potential, reaching a maximum of about 0.6–0.7 and decreasing on either side. In summary, normal load and electrode potential have significant effects on fretting crevice corrosion and fretting initiated crevice corrosion of SS316L and its mix-alloy couples.

The effects of normal load and velocity on the friction and wear behavior of single-phase Fe2B bulk have been investigated by optical microscopy, X-ray diffraction, scanning electron microscopy, an...

Most previous studies on fretting fatigue have been accomplished under constant normal loading and less attention has been paid to cyclic normal loading. An innovative test apparatus was specially designed and manufactured for fretting fatigue tests under cyclic loading in this work and the fretting fatigue behavior of Al7075-T6 was investigated at different normal load frequencies. A finite element model was developed to study the effect of normal load frequency on the contact stress distribution. It was found that the cyclic normal load has a more damaging effect on fretting fatigue life compared to constant normal load, particularly at lower frequencies. The results showed that at the normal load frequency of f = 1 Hz, fatigue life decreased by 52% in the high cycle fatigue regime and 28% in the low cycle fatigue regime. The experimental results also indicated that at the normal load frequency of 80 Hz, the fretting fatigue life converged to its corresponding life under constant normal load con...

An effective and simple method is reported to enhance the property and economics of graphene lubricants, that is, 3D porous graphene and T161 are proposed as hybrid lubricant additives. Compared with PAO 6, 3D PG-T161 can reduce the friction coefficient of 316 ASS (Austenitic stainless steel) by about 4.0%, 26.1% and 73.1% respectively under the normal load of 5 N, 50 N and 100 N, and the wear rate is reduced by about 28.9%, 65.6% and 97.8% respectively. The excellent tribological performance is attributed to the formation of uniform protective lubricating film. These findings are of great significance for enhancing the tribological properties of ASS and expanding its application range.

Experiments were carried out using a pin-on-disc tester in conformal contact starved lubrication and unidirectional sliding with constant normal load of 20 N and sliding speed of 0.8 m/s. Different combinations of co-acted samples were studied: textured specimen and untextured counter-specimen, both samples textured and untextured. Samples with the following densities of dimples were tested: 2.25%, 5%, 9%, 14% and 20.25%. The best tribological performance was achieved for both surfaces textured with the smallest pit-area ratio. For large area densities tribological behaviors of sliding pairs with one sample textured were better than those with both samples textured.

To improve the wear resistance of Al-Si alloys, different types of reinforcing particles such as SiC, TiC, ZrO2, and B4C were used to produce matrix composites by friction stir processing (FSP). First, microstructural properties of different locations of stir zone (SZ) in the FSPed specimens such as advancing side, retreating side, shoulder-affected area, and pin-affected area were investigated. The results demonstrate that Si particles size is not the same in different SZ subdomains. SEM investigation was performed in order to investigate the particles distribution in different areas of the SZ as well as bonding quality between particles and metal matrix. Hardness and wear tests were carried out to determine mechanical and wear properties of the composites. The pin-on-disk wear tests were performed at room temperature, with the normal applied loads of 5, 10, and 20 N and sliding speed of 1 and 2 m/s. All fabricated composites show higher resistance in wear than A356 alloy. Wear test results show, by increasing the normal load and sliding velocity, the wear loss weight of all composites increased gradually.

Fretting experiments were run below fully developed friction load levels, and the stability of friction was investigated. It was observed that stable friction behavior can be achieved if friction load per normal load is limited to maximum of about 0.5. Exceeding this value leads to increasing instability in friction until gross sliding is achieved. Minute surface sliding, in range of few micrometers, occurred even at these load levels, which may contribute to friction instabilities. publishedVersion

In this study, we investigated the effect of temperature on the friction and wear of rice bran (RB) ceramics, a hard porous carbon material made from rice bran, sliding against alumina, stainless steel, and bearing steel balls under dry conditions. Friction tests were performed using a ball-on-disk-type friction tester wherein a ceramic heater was installed in the rotational stage. The surface temperature of the RB ceramic disk specimens was controlled at 20, 100, 150, or 200°C. The normal load was 1.96 N, sliding velocity was 0.1 m/s, and number of cycles was 20,000. The effect of surface temperature on the friction and wear of RB ceramics substantially differed among the ball material types. The friction coefficient for the RB ceramics sliding against an alumina ball decreased with increasing temperature and exhibited an extremely low value (0.045) at 200°C. The friction coefficient in the case of the RB ceramics sliding against a stainless steel ball exhibited a stable value as the temperature ...

This paper presents an investigation into the effect of unexpected overloading on scuffing initiation with a twin-disc test device. An existing twin-disc test device was modified to be suitable for applying overloading dynamically to the disc contact. Three test series including three pairs of discs in each series were subjected to combined normal force and rolling/sliding loading. First a base level for scuffing initiation was defined by increasing the normal load stepwise until failure, while keeping the rolling and sliding between the discs constant. In the subsequent test series, the overload cycles were applied at a significantly lower level in two different patterns leading to scuffing at both an earlier and later stage than was observed for the base level. acceptedVersion

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

This study is concern with tribocorrosion behaviour of Ti6Al4V-HA biocomposites. The Ti6Al4V composites reinforced with different contents of hydroxyapatite (HA) particles 5–15%, vol%) were produced by hot pressing technique. The tribocorrosion tests were performed by using a ball-on-plate configuration in artificial saliva at 37 °C. The tests were carried out under open circuit potential (OCP), with a sliding duration of 1800 s, 1 N normal load and 1 Hz frequency. The open circuit potential and wear mechanisms for all tested biocomposites are presented and discussed. The results suggest that HA plays a relevant role on tribocorrosion behaviour of Ti6Al4V-HA composites. All composites samples presented better wear resistance and also a relatively lower tendency to corrosion with increasing HA content.

In this paper, the effect of joint separation on the shear behavior of planar non-persistent joints under high normal load has been investigated using PFC2D. Initially, calibration of PFC was undertaken with respect to the data obtained from experimental laboratory tests to ensure the conformity of the simulated numerical models response. Furthermore, the simulated models were cross checked with the results of direct shear tests performed on non-persistent jointed physical models. Through numerical direct shear tests, the failure process was visually observed, and the failure patterns were found reasonably similar to the experimentally observed trends. The discrete element simulations demonstrated that the failure pattern was mostly influenced by joint separation, while the shear strength was linked to the failure pattern and mechanism. Исследовано влияние разъединения соединения при сдвиге плоского нестабильного соединения под действием интенсивной нормальной нагрузки с использованием программного обеспечения PFC2D. Первоначально калибровка PFC2D проведена на основе экспериментальных данных для согласования с моделируемым числовым результатом. Достоверность моделируемых моделей проверена при сравнении с результатами прямых испытаний на сдвиг, выполненных на нестабильных сочлененных физических моделях. Благодаря числовому прямому испытанию на сдвиг процесс разрушения был отмечен визуально, и разрушенные образцы отбирались подобными экспериментально наблюдаемым тенденциям. Дискретное моделирование элемента показывает, что на разрушение образца главным образом влияет разъединение соединения, в то время как прочность при сдвиге связана с моделью разрушения и механизмом разрушения. Досліджено вплив роз’єднання з’єднання при зсуві плоского нестабільного з’єднання під дією інтенсивного нормального навантаження з використанням програмного забезпечення PFC2D. Спочатку калібрування PFC2D проведено на основі експериментальних даних для узгодження з модельованим числовим результатом. Достовірність модельованих моделей перевірено при порівнянні з результатами прямих випробувань на зсув, що виконані на нестабільних зчленованих фізичних моделях. Завдяки числовому прямому випробуванню на зсув процес руйнування було відмічено візуально, і зруйновані зразки відбиралися подібними експериментально спостережуваним тенденціям. Дискретне моделювання елемента показує, що на руйнування зразка головним чином впливає роз’єднання з’єднання, тоді як міцність при зсуві пов’язана з моделлю руйнування і механізмом руйнування.

The fretting wear behavior of the nuclear power material Incoloy 800 was investigated in this study. A PLINT high-temperature fretting tester was used on an Incoloy 800 cylinder against a 304SS cylinder at vertical cross contact under different temperatures (25, 300, and 400°C). During testing, a normal load of 80 N was applied, and the displacement amplitudes ranged from 2 to 40 µm. The fretting wear mechanism at high temperatures and the kinetic character of the materials of the Incoloy 800 steam generator tube were analyzed. Results showed that the fretting running regimes varied little with ncreasing temperature, and some microcracks were observed in both the mixed fretting regime (MFR) and the partial slip regime (PSR) at high temperatures. Slight abrasive wear and microcracks were the main wear mechanisms of the Incoloy 800 alloy in PSR, whereas those in the MFR and the gross slip regime were oxidative wear, abrasive wear, and delamination.

A new solution for general half-plane contact problems subject to a constant normal load together with alternating shear loads and tension in the steady state is presented. The method uses a formulation where a displacement correction is made to the fully stuck contact solution. There will be two outer regions of slip and a central permanent stick zone, which is explicitly established. Thereby, the maximum extent of the slip zones is effectively specified. Cases of small and large tension are studied, that is when the direction of slip is the same or opposing at the ends of the contact, respectively., Comment: 11 pages, 5 figures

[EN] In fatigue problems, an accurate estimation of the propagation direction is important for life prediction. We identify the most relevant factors that affect the crack orientation during the propagation stage of fretting fatigue cracks, arising from complete contacts. Contrary to what initially expected, parameters such as normal load, cyclic bulk load, etc. do not have a noticeable influence on the orientation. However the relative Young's moduli of indenter/specimen materials, the indenter width and the surface coefficient of friction are the most influencing factors. Analyses are performed through the extended finite element method (X-FEM) and an orientation criterion for non-proportional loading proposed by the authors. Experimental fretting fatigue tests confirm the predicted trends. An explanation of this behaviour is also given., The authors gratefully acknowledge the financial support given by the Spanish Ministry of Economy and Competitiveness and the FEDER program through the projects DPI2017-89197-C2-1-R and DPI2017-89197-C2-2-R. The support of the Generalitat Valenciana, Programme PROMETEO 2016/007, is also acknowledged. The authors thank the collaboration of Mr. Francisco Gelardo Rodriguez

Wear behaviour of ZE41A magnesium alloy coated with WC/Cu using electrical discharge coating (EDC) has been investigated in this study. The wear test was conducted with pin-on-disc machine using different conditions such as normal load of 1.5 kg, 2.5 kg, 3.5 kg and sliding speed of 100 rpm, 200 rpm, 300 rpm and sliding time of 3 min, 5 min, 7 min through L9 Taguchi orthogonal array to study the effect of sliding parameters on the wear characteristics. A wear transition map was constructed to describe the different wear conditions such as mild wear, severe wear and ultra severe wear. The main wear mechanisms for each wear regime were identified and classified through wear mechanism map. The worn-out specimens were characterized using scanning electron microscope (SEM) to identify the various wear mechanisms.

In this article, the authors have experimentally determined the friction coefficient in the polyoxymethylene gear pairs lubricated by a dry film lubricant. The obtained expression increases the precision of the frictional power loss calculation, which is essential when the frictional power loss is used as the optimization criterion. The friction coefficient was characterized for three influencing parameters: radius of relative curvature, sliding velocity, and normal load. The full factorial organization was used for the experiment design; five curvature radii levels, four sliding velocity levels, and three load levels were used. Each of the runs was recorded three times, which resulted in a total of 180 experimental runs. The resulting expression for the friction coefficient is valid for the gear modules between 1 and 4.5 mm and sliding speeds of up to 2.7 m/s. The normal load was found to have the greatest influence on the friction coefficient, while the sliding velocity influenced only the specimens running under lower load levels. A further increase in the values of radii of relative curvature above 5 mm had no effect on the friction coefficient. The experimental data is provided in full.

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

Solution procedures for establishing the regimes of stick and slip for frictional contacts capable of idealisation within the framework of half-plane elasticity and subject to complex loading regimes are reviewed, starting with the well known fundamental problems and going on to more complicated ones. These include problems where the normal load, shear force, applied moment and differential remote tensions all vary with time. Transient and steady state solutions are discussed., Comment: 16 pages, 9 figures, Tribology International

In this work, we have investigated anisotropic frictional behaviour of patterned silver surfaces. Anisotropy in coefficient of friction (COF) is observed depending on the sliding direction on one-arm and two-arm columnar structures as a function of applied normal load. Anisotropy in COF values for one-arm Ag nanocolumnar surface was observed to be as large as 42% when measured between along and transverse to the columnar tilt direction. However, the percentage change in COF values were as high as 67% for two-arm (zig-zag) nanocolumnar surface along the tilt direction of the columns between trace-retrace directions. There was a significant variation in COF of about 149% by changing geometry from one-arm to two-arm (zig-zag) nanorods, which is explained through a force balance model.

A marker layer (Au80Pd20) combined with four different techniques was used in order to estimate the real contact area between static steel–steel contact pairing (substrate: AISI304 and ball: AISI 52100). The experiments were done with polished references (substrate and ball) and with substrates having a laser-patterned line-like surface topography using two loads (1 and 5 N) and two pattern periodicities (9 and 15 µm). Irrespective of the normal load, all techniques led to the same results within the standard deviation for the polished surfaces. For the laser-patterns, only the combination of foot-printing, scanning electron microscopy and image segmentation is capable to provide meaningful results, since the other techniques significantly overestimate the real contact area.