Search

The random distribution of graphene in epoxy matrix hinders the further applications of graphene-epoxy composites in the field of tribology. Hence, in order to fully utilize the anisotropic properties of graphene, highly aligned graphene-epoxy composites (AGEC) with horizontally oriented structure have been fabricated via an improved vacuum filtration freeze-drying method. The frictional tests results indicated that the wear rate of AGEC slowly increased from 5.19×10−6 mm3/(N·m) to 2.87×10−5 mm3/(N·m) with the increasing of the normal load from 2 to 10 N, whereas the friction coefficient (COF) remained a constant of 0.109. Compared to the neat epoxy and random graphene-epoxy composites (RGEC), the COF of AGEC was reduced by 87.5% and 71.2%, and the reduction of wear rate was 86.6% and 85.4% at most, respectively. Scanning electron microscope (SEM) observations illustrated that a compact graphene self-lubricant film was formed on the worn surface of AGEC, which enables AGEC to possess excellent tribological performance. Finally, in light of the excellent tribological properties of AGEC, this study highlights a pathway to expand the tribological applications of graphene-epoxy composites.

Ceramic composites are extremely sensitive to the surrounding environment. Their tribological performance may degrade drastically if they are polluted by some external agent. The present article was aimed to investigate the unidirectional sliding characteristics of brake oil treated woven carbon-carbon (C/C), and carbon-carbon-silicon carbide (C/C-SiC) composites. The laminate orientation, surface conformity, normal load, and sliding velocity were varied. It was depicted that the quantity of oil absorbed by the composites depends on the laminate orientation. The decay rate in friction coefficient revealed that C/C composites are more prone to brake oil in comparison to C/C-SiC composites. The results showed that it is difficult to make an assumption regarding the wear and friction behavior of laminated composites in low conformity contacts concerning normal load and sliding velocity. However, friction coefficient increased with increment in sliding velocity and normal load, for non-conformal Hertzian contacts. The generation of friction film was arduous in the presence of absorbed oil. SEM analysis of worn surfaces revealed different surface morphology for low, and non-conformal contacts which depicted distinct tribological mechanisms.

The shearing behaviour of reproduced flat LBS grains artificially bonded with ordinary Portland cement (OPC) and plaster of Paris (PP) was examined using micromechanical experiments. Monotonic shearing tests showed a distinct variation in the load–displacement relationship at low, medium and high normal loads, and a nonlinear shear strength envelope was proposed. For OPC-bonded sand grains, a brittle–ductile transition at 20–30 N normal load was observed and three breakage mechanisms in shearing (chipping, shear cracks and crushing) were distinguished in accordance with the changes in the load–displacement curves. OPC-bonded sands showed a predominant dilation at lower normal loads, whereas PP-bonded sands were highly compressive. Based on previously published works using element-scale tests, a new mechanism for dilation under micromechanical testing was proposed in the study. Cyclic shearing tests were conducted on OPC-bonded sands, and the effects of increased displacement amplitude and normal load were highlighted.

Alloys used as bearings in aircraft landing gear are required to reduce friction and wear as well as improve the load-carrying capability due to the increased aircraft weights. Cu-15Ni-8Sn-0.8Nb alloy is well known for possessing good mechanical and wear properties that satisfy such requirements. In this study, the microstructure, mechanical properties, and grease-lubricated sliding wear behavior of Cu-15Ni-8Sn-0.8Nb alloy with 0.8 wt% Nb are investigated. The nanoscale NbNi3 and NbNi2Sn compounds can strengthen the alloy through the Orowan strengthening mechanism. A Stribeck-like curve is plotted to illustrate the relationship among friction coefficient, normal load, and sliding velocity and to analyze the grease-lubricated mechanism. The wear rate increases with normal load and decreases with sliding velocity, except at 2.58 m/s. A wear mechanism map has been developed to exhibit the dominant wear mechanisms under various friction conditions. When the normal load is 700 N and the sliding velocity is 2.58 m/s, a chemical reaction between the lubricating grease and friction pairs occurs, resulting in the failure of lubricating grease and an increase in wear.

This paper refers to studying the effect of microgravity on the friction behavior of friction units and to further analyzing the influencing mechanism. The potential factors affecting the friction behavior of the moving joints of space mechanisms in microgravity environment were analyzed. A friction apparatus was designed and manufactured by reducing the complexity and costs. A microgravity test program was designed and performed using the Beijing Drop Tower platform with designing three positioning styles for the friction apparatus by changing the relationship between the normal load and gravity. For each positioning style of the friction apparatus, the friction behavior of friction unit in gravity and microgravity environment was compared. It was found that microgravity can not only change the value of friction, but also affect its dynamic characteristics. For different positioning styles of the friction apparatus, the influencing mechanism of microgravity on friction behavior was different. The normal load or the relative position between the connected bodies of the friction unit could be changed by microgravity, resulting in that the friction behavior was different from that in gravity environment.

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.

Wear behaviors of a peak-aged Cu-15Ni-8Sn alloy fabricated by powder metallurgy were investigated. The results indicated that the friction coefficients and the wear rates of Cu-15Ni-8Sn alloy within a normal load range of 50-700 N and a sliding speed range of 0.05-2.58 m/s were less than 0.14 and 2.8×10−6 mm3/mm, respectively. Stribeck-like curve and wear map were developed to describe the oil-lubrication mechanism and wear behavior. The equation of the dividing line between zones of safe and unsafe wear life was determined. Lubricating oil was squeezed into micro-cracks under severe wear conditions. In addition, the lubricating oil reacted with Cu-15Ni-8Sn alloy to generate the corresponding sulfides, which hindered the repair of micro-cracks, promoted cracks growth, and led to delamination. This work has established guidelines for the application of the Cu-15Ni-8Sn alloy under oil-lubricated conditions through developing wear map.

Rock bridges are commonly encountered in rock engineering practices and play a significant role in the stabilization of rock masses. Under shear loading, they are supposed to provide resistance not only at before-failure stages, but also at after-failure stages, and this is not fully understood yet by now. To promote the solving of this problem, the present study tried to discover the geometric and mechanical properties of a shear-formed fracture occurring in a rock bridge between discontinuous open joints through laboratory experiments. After careful preparation of sandstone specimens containing discontinuous open joints, a series of direct shear tests were carried out under constant normal load (CNL) conditions. Once the rock bridge failed, another two sets of cyclic loading were subsequently applied to the specimen to examine the after-failure properties. The geometric properties of the newly formed fracture in rock bridge area, including undulating angle, fracture roughness, and fracture aperture, were systematically examined and analyzed with respect to the applied normal stress and joint persistence. Furthermore, the failure mode was dependent on the normal stress and joint persistence and also influenced the properties of the shear-formed fracture. Accordingly, different fracture types occurring to the rock bridges were summarized and described in terms of geometric morphology. The findings in present study would enhance our understanding of the influence of rock bridges, especially at after-failure stages, on engineering practices such as rock slopes.

Pumice has a large potential to be used in structural and geotechnical engineering projects as a lightweight aggregate, and it is a material of unique characteristics. Gravel-sized pumice from Yunnan, China, was examined in the present study with a custom-built micromechanical apparatus investigating the properties of pairs of grains at their interfaces subjected to monotonic and cyclic tests under different saturation states. Based on interferometer measurements, the vesicular surface of the pumice was characterized to be very rough. From the interface micromechanical experiments, it was shown that the virgin surfaces appeared to be very soft under normal loading tests, during which micro-asperity breakage was caused. After preloading and preshearing, the normal load–displacement curves became much smoother and the Young’s moduli, which remained relatively low in magnitude compared with other geo-materials previously studied, could be quantified applying the Hertzian model. Large hysteretic loops were observed from cyclic normal load tests. Plastic displacements dominated the first normal loading cycle, while elastic displacements prevailed in the subsequent cycles. High values of the apparent coefficient of friction were observed from the monotonic shearing tests. A modification of the power value of the Mindlin and Deresiewicz model was necessary to be applied during the fitting of the experimental data so that theoretical and laboratory curves could match. It was shown that this power had a direct correlation with the coefficient of friction.

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.

Finite element analysis is used to investigate an elastic-plastic coated spherical contact in full stick contact condition under combined normal and tangential loading. Sliding inception is associated with a loss of tangential stiffness. The effect of coating thickness on the static friction coefficient is intensively investigated for the case of hard coatings. For this case, with the increase in coating thickness, the static friction coefficient first increases to its maximum value at a certain coating thickness, thereafter decreases, and eventually levels off. The effect of the normal load and material properties on this behavior is discussed. Finally, a model for the static friction coefficient as a function of the coating thickness is provided for a wide range of material properties and normal loading.

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.

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.

Direct shear tests under constant normal loads were conducted to investigate the effects of the asperity angle and the infill thickness on shear characteristics. The results showed that the increase of the asperity angle frequently led to the degraded area, whereas the opposite occurred for the increase of the ratio of asperity height to infill thickness (t/a). In addition, tensile fracture of the asperity occurred for the high asperity angle and the low t/a ratio. On the contrary, shear abrasion prevailed with the small asperity angle and the high t/a ratio. The degradation transition may result from the variation of the effective contact area between two blocks and deserves further studies. It can be further concluded that the asperity angle and infill thickness significantly determine shear characteristics of the infilled joint. For joints with thick infill materials, the improvement of the mechanical properties of the infill will contribute to promoting the shear strength.

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.

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.

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.

CoCrMo alloy and UHMWPE have been widely used in knee joint prosthesis implantation materials. In this paper, swing friction behavior of the contact interface between CoCrMo alloy and UHMWPE is studied under dynamic loading. Swing friction characteristic and damage mechanism are discussed. The results show that swing friction coefficients increase with the rising of maximum normal load and swing angular amplitude. Unloading-standing could play alleviative roles in friction and wear to a large degree. As the cycle number gradually increases, the surface roughness of UHMWPE decreases, while the roughness of CoCrMo increases. During the swing friction, the main damage mechanism of CoCrMo is abrasive wear and the main damage mechanisms of UHMWPE are abrasive wear, fatigue wear and plastic deformation. Besides, it is easier to generate surface damages with small angle and heavy load.

In this study, “within the environment” and “within the contact” in situ tribology techniques are combined in order to study the interfacial processes in lubricated metallic (i.e., aluminum-based) sliding conditions. The evolution of the roughness follows the trend of the coefficient of friction closely, with initially low values followed by higher roughness during steady state. Similarly, the transfer film behavior correlates well with the roughness of the worn surfaces and the subsurface microstructure of the worn surfaces. The effect of normal load on the running-in behavior is also studied in terms of differences in the interfacial processes.

In this study, the effect of porosity on the dry sliding fiction of ethylene-vinyl acetate (EVA) foams was investigated under different normal load conditions. EVA foam blocks with varying porosities were slid against a smooth stainless steel plate under dry conditions. The friction coefficient increased with increasing porosity under all of the normal load conditions. In addition, the contact area was estimated using a contact model considering elastic buckling of the cell walls (elastic collapse). The elastic collapse area in the anterior portion of the EVA foam block increased with increasing normal load and porosity, which resulted in an increased contact area. Furthermore, the friction coefficient was positively correlated with the estimated contact area divided by the normal load, indicating that adhesion friction increases with increasing porosity of polymer foams. These results may contribute to the design of high-friction, lightweight shoe sole tread blocks prepared using polymer foam blocks.

Tool–chip friction of high-speed cutting (HSC) is complex and involves nonlinear problems. This paper proposes a new friction model having multiple factors for HSC and investigates the multiple effects affecting the tool–chip friction coefficient. An experimental method of high-speed friction and an inverse method of obtaining the temperature of the friction interface are proposed. The proposed friction model reveals the inherent relationship between the friction coefficient and the multiple factors, and describes tool–chip friction in the HSC of aluminum alloy 6061-T6 accurately and improves the cutting simulation accuracy. It is also found that an increase in friction speed increases the friction coefficient and that an increase in friction temperature reduces the friction coefficient. However, an increase in ambient temperature clearly increases the friction coefficient at high temperature, and an increase in the normal load reduces the friction coefficient at low temperature and increases the friction coefficient at high temperature. The results provide a clear understanding of the mechanisms of friction speed and temperature, the ambient temperature, and the actual contact area in tool–chip friction of HSC.

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.

This paper presents the results of an intensive series of experiments on various types of sands using cyclic direct shear testing apparatus. Two different grain shapes (rounded and angular) of sands having distinct sizes (0.25–0.5 and 1.0–2.0 mm) were tested in an automated constant normal load cyclic direct shear testing apparatus at an 100 kPa vertical effective stress, and two rates of shearing (2.0 and 0.025 mm/min). Experiments on the specimens resulted in sharp decreases followed by gradual increases in shear stress values. The results suggest that these successive drops and formations give a stick–slip nature to the fluctuations, which might be attributed mainly to the rate of loading, shape of grains, size of grains, and fines contents.

Shear behaviour of rock joints can be studied under both constant normal load (CNL) and constant normal stiffness (CNS) boundary condition. CNS condition is suitable for non planar and reinforced rock joints whereas CNL condition is suitable for planar and non reinforced rock joints. In the present study shear behaviour of modelled rock joints with different asperity have been experimentally investigated under both CNL and CNS boundary conditions. Test results indicate that CNS boundary conditions gives higher shear strength as compare to CNL boundary condition when other parameter of testing is kept same. But, this effect tends to diminishes with increase in normal stress on the shearing plane and at high normal stress both CNL and CNS boundary conditions gives same shear strength. A new shear strength model is proposed for both the boundary condition and the proposed model is validated by comparing the predicted shear strength with present experimental results and results available in the literature for natural and artificial rock joints with different asperity and roughness. The new study and model will be useful for safe and economical design of underground openings in jointed rocks, stability analysis of rock slopes, design of foundation on rock and design of rock socketed piles.

A novel laboratory technique has been used to measure the rolling resistance of tyres using a force transducer. The effect of inflation pressure, normal load and speed has been studied for two different bias-ply tyres—with and without treads. As anticipated, the results indicate that a treaded tyre has higher value of the rolling resistance as compared with a tyre with treads removed.

Shear failure of rockmass along a weak plane occurs frequently in rock slopes and in underground tunnels. To study the shear behaviour and acoustic emission (AE) characteristics of joints under different experimental conditions (asperity height, shear rate, and normal load), irregular artificial saw-tooth joints with different asperity heights were sheared in the laboratory, and the AE signals were detected and analysed. The results demonstrated that the strength of the joints increased with increases in normal load and asperity height, while the strength of the joints first increased and then decreased when the shear rate was elevated. The ideal curve of the cumulative hits could be divided into a quiet period, a slow rise period, and a sharp growth period, which could be used to monitor and predict the potential shear failure of the joints. The higher the asperity was, the higher the peak energy rate and the lower the peak hit rate, and cumulative hits at failure were because of differences in asperity size and number. The peak hit rate and cumulative energy tended to increase gradually with the decrease in shear rate, and the minimum peak hit rate and energy rate at failure were both attained at the maximum shear rate. In addition, curves of the energy rate and hit rate showed large fluctuations at higher shear rates. The peak energy rate and cumulative energy under low normal stress could be greater than the peak energy rate and cumulative energy under high normal stress, and the peak hit rate and cumulative hit number under high normal stress could be larger than that of under low normal stress.

The wear behavior of two high-speed steels and a high-chromium cast iron with cryogenic treatments was evaluated using a pin-on-disc configuration. An ASTM A36 steel disc, oxidized previously in a furnace at 950 °C, was used as a counterbody with the goal of developing an accurate representation of the industrial calamine formed on the surface of steels under industrial conditions. This is a new perspective to evaluate the real contribution of the normal load, sliding velocity and heat treatments on the wear phenomena of hot rolls. Optical microscopy, scanning electron microscopy and X-ray diffraction were used to correlate the wear rate, friction coefficient and wear mechanisms to the microstructure, hardness and types of oxides that formed on the counterbody. Experimental results showed a complex relation between the normal load, the sliding velocity and the interaction with counterbody, which establishes the wear rates and friction coefficients.

Frictional behavior of wax–oil gels against steel has been investigated using a homemade tribometer to estimate the frictional resistant force produced by wax plugs in pipelines pigging. Experimental results show that there are several impact factors on frictional behavior of wax–oil gels against steel, including wax concentration, normal force and velocity. Coefficient of sliding friction increases with wax concentration. Normal load and velocity have opposite impacts on the coefficient of sliding friction. The outflow mineral oil has an important impact in the frictional process. A mechanism model of wax–oil gels against steel is built based on the results.

Tribochemically active Ti–C–S coatings are nanocomposite coatings containing a S-doped titanium carbide, from which S can be released in a tribological contact. This work studies tribochemical reactions between a Ti–C–S coating and various counter surface materials, and their effect on the tribological performance. Tribological tests were performed in a ball-on-disc set-up, using balls of five different materials as sliding partners for the coating: 100Cr6 steel, pure W, WC, 316-L steel and Al2O3. For W balls, a WS2 tribofilm was formed, leading to low friction (down to µ = 0.06). Furthermore, increasing normal load on the W balls was found to lead to a strong decrease in µ and earlier formation of the low-friction WS2 tribofilm. Similar WS2 and MoS2 tribofilms were, however, not formed from WC- and Mo-containing 316-L balls. The performance when using WC and Al2O3 balls was significantly worse than for the two steel balls. It is suggested that this is due to sulphide formation from Fe, analogous to formation of anti-seizure tribofilms from S-containing extreme pressure additives and steel surfaces. The tribochemical activity of Ti–C–S coatings, with the possibility of S release, is thus beneficial not only for pure W counter surfaces, but also for Fe-based sliding partners.

Infill materials in rock joints usually cause a reduction in the joint shear strength. The shear behavior of rock discontinuities depends upon whether they are clean and unfilled or filled, so this concern invites accurate understanding of the shear behavior and strength of infilled joints. A series of constant normal load direct shear tests was performed to investigate the shear strength of artificial samples with infilled rough joint surfaces having different asperity and infill characteristics. The current study focuses on the effects of factors that influence the shear strength of infilled rock joints samples, with emphasis on forward and reverse shearing. In the forward cycle, the front joint wall is compressed and possibly sheared, and the back side fill is unbonded from the joint surface and slightly disturbed. In the reverse cycle, the disturbed and weakened back side fill is under shearing. The effect of the normal stress on the joint is studied, as this factor plays an important role on the shear behavior of infilled rock joint samples. The results show that joints with low asperity angle exhibit higher shear strength during the forward shearing cycle than the reverse cycle, but in joints with steeper asperity angle, the reverse cycle exhibits greater shear strength. In the reverse cycle, the joint infill has less influence compared to the effect of the rougher surface and higher asperity inclination, even in higher normal stress.

A study highlighting the aspect of the generation of aerosol wear particles during abrasion is presented. The substrate chosen is a masonry brick which is reinforced with TiO2 nanoparticles. This is done using a pin on plate arrangement. The material removal mechanism via fracturing is first understood. The parameter chosen for the study is the normal load. The formed aerosols are then characterized by their number concentration, particle size distribution, individual particle shape, size and chemical composition. Having irregular shapes, the aerosol wear particles have unimodal size distributions with 5–7 % (in mass) of Ti content. The size mode increases with the increase in normal load. However, at higher normal loads, while there is an unexpected increase in the wear mass, the maximum concentration of the aerosol particles saturates. During the whole study, no free nanoparticles of TiO2 were found.

During sliding low elastic modulus, materials can form ridges and distinct zones of contact vice the commonly observed Schallamach wave phenomena. It was shown that at low velocities (0.001 and 0.01 m/s), the contact geometry retained a single region of contact and at higher velocities (0.1 and 1 m/s) the contact separated into two distinct zones. It was also shown that this phenomenon was independent of the applied normal load.

Sliding friction between cellulose Iβ nanocrystals is studied using molecular dynamics simulation. The effects of sliding velocity, normal load, and relative angle between sliding surface are predicted, and the results analyzed in terms of the number of hydrogen bonds within and between the cellulose chains. We find that although the observed friction trends can be correlated with hydrogen bonding, it may not be the most significant factor in determining frictional behavior on cellulose nanocrystal surfaces.

This study investigates how Cu content affects thermal stability and wear behavior of Al-12.5Si-1.0Mg alloy, by adding 2.55 and 4.53 wt.% Cu. The low-Cu and high-Cu alloys were isothermally heat-treated at 300 °C for 100 h. The results indicated that the amount of eutectic Al2Cu and Al5Cu2Mg8Si6 particles in the high-Cu alloy was more than that in the low-Cu alloy. These hard particles retained in the Al matrices during isothermal heat treatment, maintaining a relatively stable hardness. Therefore, the hardness of the high-Cu alloy was superior to that of the low-Cu alloy in as-cast condition and after isothermal heat treatment. For wear behavior, both isothermal heat-treated alloys showed the same wear rate with 10 N normal load. The wear rate of Al-12.5Si-1.0Mg alloy was independent on the copper content under 10 N load, but the wear rate at a load of 40 N decreased with increasing Cu content in Al-12.5Si-1.0Mg alloy.

Al-doped zinc oxide films (AZO) of different thicknesses were deposited by reactive magnetron sputtering on glass substrates. Fracture toughness and adhesion of transparent AZO films were measured by indentation in air and water. Fracture toughness of glass is about 0.63 MPa center dot m(1/2). Under the same normal load, radial crack length shortened with the increase of AZO film thickness. When indented under deionized water, cracks in both glass and AZO films got longer, signifying corresponding decrease in the fracture toughness.

The characterization of the rolling resistance is an important issue when designing mechanical systems in order to predict the force needed to run a given mechanism. Frictional forces in bearings and rolling resistance in rollers and wheels are the most important factors contributing to the overall resistance to the movement. As the rolling coefficient varies as a function of the material nature, normal load, and rolling velocity, it is desirable to test the designed system under real conditions. Mechanical testing machines, present in most mechanical laboratory tests, usually work in uniaxial mode. In this work a simple testing device to be used with uniaxial mechanical testing machines is presented.

Several real life reinforced soil structures such as traffic supporting embankments and pavements are subjected to cyclic loads. The behaviour of soil-reinforcement interface under cyclic load is different from that under monotonic load. In the investigation reported herein, cyclic pullout tests were conducted on geogrid embedded in sand. A load controlled pullout test apparatus complying with the dimensions suggested by ASTM D-6706-01 was designed and fabricated in-house. Modular units for applying both cyclic and monotonic load were also designed and fabricated. Cyclic load was applied through a pneumatic double acting air cylinder. In addition, the setup consisted of a signal generator and a filter lubricator regulator volume booster. The effect of normal stress and cyclic load on the pullout behaviour was studied. It was found that cyclic loads of a lower magnitude than the monotonic capacity could cause failure. In the case of cyclic loads of small magnitude, the displacements showed tendency to stabilize. In the case of higher magnitudes of cyclic loads, the displacements progressively increased to failure. An initial stiffness was noticed in the system due to the initial densification achieved near the interface due to the dynamic nature of the cyclic pullout forces. Normal load as well as cyclic load played important roles in the number of cycles to cause failure. Post-cyclic monotonic tests showed the effect of degradation of the soil-geosynthetic interface after subjecting the same to certain number of load cycles.

From the early tribological studies of Leonardo da Vinci to Amontons' law, friction has been shown to increase with increasing normal load. This trend continues to hold at the nanoscale, where friction can vary nonlinearly with normal load. Here we present nanoscale friction force microscopy (FFM) experiments for a nanoscale probe tip sliding on a chemically modified graphite surface in an atomic force microscope (AFM). Our results demonstrate that, when adhesion between the AFM tip and surface is enhanced relative to the exfoliation energy of graphite, friction can increase as the load decreases under tip retraction. This leads to the emergence of an effectively negative coefficient of friction in the low-load regime. We show that the magnitude of this coefficient depends on the ratio of tip-sample adhesion to the exfoliation energy of graphite. Through both atomistic- and continuum-based simulations, we attribute this unusual phenomenon to a reversible partial delamination of the topmost atomic layers, which then mimic few- to single-layer graphene. Lifting of these layers with the AFM tip leads to greater deformability of the surface with decreasing applied load. This discovery suggests that the lamellar nature of graphite yields nanoscale tribological properties outside the predictive capacity of existing continuum mechanical models.

The present study is focused on the failure mechanisms of an automobile clutch disc/pressure plate assembly due to abnormal operation of a passenger vehicle. Terminal malfunction of the part took place about one year after an accidental overloading incident. This component operates under sliding friction conditions, during which the pressure plate is pressed against the clutch disc with normal load that varies in a non-predictable manner, depending on the driving conditions. Commonly, the clutch disc is considered as “consumable” part that can be replaced when malfunction occurs, whilst the metallic pressure plate is considered to remain intact. However, in this particular case, visual inspection, macro-examination and microscopic observation using principally light and scanning electron microscopy of such a cast iron pressure plate revealed its severe long-term progressive failure, through extensive deep grooving and oxidation of the surface and the development of subsurface shear cracks.

Elevated-temperature wear tests under atmospheric conditions at 400 °C were performed for a hot working die steel H21 on a pin-on-disk wear tester. The phase and morphology of worn surfaces were examined using XRD and SEM, and the relation of wear resistance to tempered microstructures was studied for H21 steel. XRD patterns exhibit that oxidative wear is a predominated wear mechanism with Fe 3 O 4 and Fe 2 O 3 on worn surfaces. It is found that with increasing normal load, obvious plastic deformation of substrate appears on worn surfaces. Microstructures start to affect apparently wear resistance of the steel with an increase of load. Under loads of 50–100 N, wear losses of steel retain low values and relatively approach for steels with various microstructures. As loads are increased to 150 – 200 N, wear losses of steel start to increase obviously and present apparent difference for steel with various microstructures. Wear resistance is found to increase in the sequence as follows: tempered sorbite, tempered martensite, tempered troostite without secondary hardening and tempered troostite with secondary hardening or upcoming one. Higher strength and microstructural stability are required for steels with excellent wear resistance.

An important issue concerning the use of soft contact lenses is comfort, which, among other factors, has been related to the level of friction between the anterior side of the lens and the inner eyelid. Although several studies have been carried out to investigate the frictional properties of contact lenses, these have not taken the physiological environment of the eye into account. In use, lenses are in contact with proteins present in tears, with corneal cells and with the palpebral conjunctiva (clear membrane on inner eyelid). The focus of this study was to establish a biologically relevant measurement protocol for the investigation of friction of contact lenses that would mimic the eye’s physiological environment. By optimizing parameters such as the composition of the friction counter surface, the lubricant solution, the normal load and the velocity, an ideal protocol and setup for microtribological testing could be established and used to perform a comparative study of various commercially available soft contact lenses.

In this article, morphology of damage inflicted on the polymethylmethacrylate (PMMA) plate in static point lubricated contact with a ball made of five different materials under cyclic normal load has been presented. It is argued that the counterface material and lubricating film play an important role in the damage process and the extent of damage inflicted on the PMMA plate.

The article presents asymptotic modeling of the running-in wear process with fixed contact zone under a prescribed constant normal load or an imposed contact displacement. The wear contact problem is formulated within the framework of the two-dimensional theory of elasticity in conjunction with Archard’s law of wear. The running-in process is considered at the macro-scale level, while the micro-processes associated with roughness changes, tribomaterial evolution, and microstructural alteration in the subsurface layers as a first approximation are neglected. The setting of the steady-state regime for the macro-contact pressure evolution is chosen as the criterion to characterize the completion of running-in. Simple closed-form approximations are derived for the running-in period and running-in sliding distance. The obtained results can be used for estimating the running-in period in wear processes where the evolution of the macro-shape deviations at the contact interface plays a dominant role.

In this study, an ultra-thin (~20 nm) functionalized polyethylene (fPE) film is successfully attached to Si substrate via a reactive benzophenone (Ph2CO) layer. The presence of fPE promotes wear durability of Si/Ph2CO/fPE to 1,000 cycles compared with 100 cycles for Si/Ph2CO and nearly zero wear life for bare Si in a ball-on-disk (4-mm-diameter Si3N4 ball) wear test under 40 mN applied normal load and 500 rpm sliding speed. As an enhancement to the wear life, perfluoropolyether (PFPE) is applied as a top mobile lubricant layer coated onto Si/Ph2CO and Si/Ph2CO/fPE. A significant improvement in the wear durability is observed as Si/Ph2CO/PFPE fails at 250,000 cycles and Si/Ph2CO/fPE/PFPE does not fail until one million cycles. Si/Ph2CO/fPE/PFPE can withstand a minimum applied load of 150 mN at a sliding speed of 0.052 ms−1 without failure, providing a PV (pressure x velocity) limit of greater than 106.6 MPa ms−1.

This work examines the variation of stick–slip amplitude with the variation of frequency of vibration and relative humidity on a mild steel disc. A pin-on-disc machine, developed by the authors, capable of vibrating the disc at different frequency is used for the experiments. During the experiments, normal load, speed, relative humidity and frequency of vibration were varied. The results reveal that, depending on the conditions, a different stick–slip behavior can arise in the same system. It is found that the rate of reduction of stick–slip amplitude has a particular relationship with the frequency of vibration and the relative humidity. It is also observed that the transition from irregular to regular stick–slip behavior is load dependent, while the transition from irregular to smooth sliding is load independent.