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The hot forging process has better formability than cold forging, however, the hot forging die sustains higher temperature and coupled pressure and temperature effect. The die wear is faster than those of cold forging. The objective of this research is to combine the previous experimental techniques in wear coefficients, numerical method, and wear model to predict the wear behavior of hot forging die in 3D, obtain the most critical wear points and geometry types on the die surface and investigate the effect of these geometrical parameters on the most critical wear points of the die surface. These investigations are done based on the combined effects of the pressure, temperature, and velocity as functions of step time and contact geometry. DEFORM FEM code is used to analyze the hot forging die in 3D and Archard wear model is applied to obtain the wear surface of the die. To deeply investigate the geometrical effects of the surface slope angle of the contact surface and fillet radius, a 2D FE model is implemented in ABAQUS commercial code and a velocity field distribution on the die surface is defined and contact temperature effect in wear coefficient and contact pressure for each node at each step time is considered to obtain the final wear depth. Although increasing the surface slope angle from 0° to 45° usually moderates the highest amount of wear and the corresponding position on the die, the surface slope angle of 30° shows a reverse trend. A statistical analysis based on the Taguchi method is carried out and an empirical model to predict the wear on hot forging dies is presented.

The contact geometry of rough surfaces markedly affects the functional properties such as sealing and lubrication. The effect of surface roughness on the percolation characteristic of elastic contact was studied. The elastic contact of randomly rough surfaces with a glass plate was performed using four different surface roughnesses of silicone rubber blocks as specimens. The results illustrate that the percolation threshold was significantly affected by the valley morphology of a surface. The increase in depth and void volume of valleys improved the connectivity between valleys, but impeded the coalescence of contact clusters, resulting in the extinction of the spanning void cluster allowing fluid flow when the relative contact area was large. Furthermore, the critical pressure and connectivity at the percolation threshold were related to the maximum peak height and autocorrelation length of a surface, respectively.

Gears are important motion and power transmission components and enhancing their machining accuracy and performance. In this work, a novel processing of ultrasonic vibration-assisted forming grinding gear (UVAFGG) is proposed, which involves the superimposition of ultrasonic vibration on the gear, and the thermo-mechanical coupling effect on surface residual stress is explored as well. Firstly, based on the local contact geometry relationship for forming grinding, the three-dimensional (3D) distribution model of the heat flux density along the tooth profile is established. In addition, the mathematical relationship models between the grinding force, grinding heat, residual stress, and grinding parameters are also given. Then, the effect of grinding parameters such as spindle speed and ultrasonic amplitude for the characteristics of residual compressive stress accumulation is obtained by comparing the single factor experimental and finite element simulation results. It is found that under the action of ultrasonic vibration, the grinding heat and grinding force reduce by 38.7% and 40.5 % and 40.5 %, respectively. The surface residual compressive stress gradually decreases with the increase in spindle speed, while with the increase in ultrasonic amplitude, it increases first and then decrease. Comparing with the conventional grinding gear (CGG), the average rate of increase of the surface residual compressive stress under the both parameters is 17.8 %–31.3 % and 20 % respectively. Meanwhile, the residual compressive stress along involute shows a negative correlation with the increase in the rolling angle. At the same time, the experimental and simulation results agree well with the theoretical model. In conclusion, the results of this study provide a novel process technology for gear machining, and a reference for the investigation on residual stress during UVAFGG.

Wet adhesion is widely adopted in biological adhesion systems in nature, and it is beneficial to design new materials with desired properties based on the underlying physics of wet adhesion. The aim of this work is to develop a design criterion to regulate the wet adhesion. The effects of different contact shapes (flat and sphere) and morphologies of the substrate (smooth, microstructure and nanostructure) on the adhesion force are investigated. Combining with the theoretical models, the dominated factors in the separation process and isolating the viscous contributions from the capillary interactions are evaluated. The results demonstrate that the adhesion mechanisms depend significantly on the capillary numbers of the interstitial liquid and the contact geometry, and the ratio of capillary force to viscous force is a key to regulate the wet adhesion mechanism. These findings can not only explain some phenomena of wet adhesion to organisms, but also provide some inspirations to design new adhesion technology for robotic fingers that can grasp objects in wet environments.

Polyacrylamide (PAAm) hydrogels with brush-covered or crosslinked surfaces were produced and their tribological behavior was studied over a wide range of sliding speeds for two different contact geometries: sphere-on-flat and flat-pin-on-flat. Irrespective of the contact geometry, the brushy hydrogel surfaces displayed up to an order of magnitude lower coefficients of friction mu (COF) compared to the crosslinked surfaces, even achieving superlubricity (mu < 0.01). In general, a hydrogel sphere showed a lower coefficient of friction than a flat hydrogel pin at a similar contact pressure over the entire range of sliding speeds. However, after normalizing the friction force by the contact area, the shear stress of hydrogels with either crosslinked or brushy surfaces was found to be similar for both contact geometries at low speeds, indicating that hydrogel friction is unaffected by the contact geometry at these speeds. At high sliding speeds, the shear stress was found to be lower for a sphere-on-flat configuration compared to a flat-pin-on-flat configuration. This can be attributed to the larger equivalent hydrodynamic thickness due to the convergent inlet zone ahead of the sphere-on-flat contact, which presumably enhances the water supply in the contact, promotes rehydration, and thus reduces the friction at high sliding speeds compared to that measured for the flat-pin-on-flat contact., Friction, 10 (3), ISSN:2223-7704, ISSN:2223-7690

Lubrication is often employed in fretting contacts to reduce wear and stresses associated with high friction. Owing to the very small displacements associated with fretting, penetration of lubricating oils into the contact may not be effective. The efficacy of the penetration of the lubricant into the contact is very difficult to observe experimentally, and accordingly, this paper presents a numerical simulation of a lubricated fretting contact using a Coupled–Eulerian–Lagrangian (CEL) finite element method. Meso-scale CEL finite element models are developed to simulate the cylinder-on-flat arrangement used experimentally at the University of Nottingham in which the roughness of contact surfaces is characterized as fractal geometry by the Weierstrass-Mandelbrot (W-M) function. The fluid–solid and solid–solid contact in the lubricated fretting contact are simulated, and from these, wear and friction coefficients are determined. The effects of contact geometry on lubricated fretting contacts and lubricant on fretting wear are modelled and compared with experimental observations. Results indicate that oil lubrication reduces fretting wear and friction effectively in the less-conforming contacts but has little effect in the more-conforming contacts.

Fretting wear is a common phenomenon that happens between contact parts when there is an oscillatory relative movement. To investigate wear characteristics history in the fretting process, the finite element method (FEM) is commonly applied to simulate the fretting by considering the wear in the model. In most literature publications, the wear coefficient is considered as a constant, which is not a real case based on the experimental results. To consider the variation of wear coefficient, a double-linear model is applied in this paper, and the tribologically transformed structure (TTS) phase is considered in the study of the wear coefficient variation model. By using these models for variable wear coefficient for both flat and cylinder, the difference of wear characteristics, plastic strain, and stress between variable wear coefficient model (VWCM) and constant wear coefficient model (CWCM) are analyzed. The results show that the variable wear coefficient has no significant effect on the wear characteristic at the end of the process in the gross sliding regime. However, in the partial slip regime, the effect of variable wear coefficient on wear characteristics is significant. Due to the difference in contact geometry in the fretting process between VWCM and CWCM, the tangential and shear stress and equivalent plastic strain also show differences during the fretting process.

In the presented work, tribological interactions of tetrahedral amorphous carbon (ta-C) coatings with variations of sp3 content have been systematically studied in the boundary lubrication regime with emphasis on fatty acid-based lubricants. Manifold phenomena regarding wear and friction have been observed that exceed the conventional understanding of lubricant-surface interaction. Distinct superlubricity behavior of ta-C was found when lubricated with glycerol or oleic acid. It was shown that both the acid function and the double bond in the lubricant molecules are essential features to achieve superlubricity. Superlubricity is not confined to self-mated ta-C surfaces, but also happens in contact to steel. The contact situation has important influence on tribological behavior. Wear rates change by 1–2 orders of magnitude by altering the contact geometry and loading conditions of an oleic-acid-lubricated ta-C/steel pairing. This ta-C wear phenomenon is attributed to tribochemical wear.

Superalloy coatings of the CoMoCrSi family (e.g. Tribaloy® T800) are applied to mitigate wear effects at high temperature. These coatings are extensively used on the contact surfaces of the shroud of turbine blades. If severe wear occurs on these contact surfaces the blade interlocking decreases, reducing the stiffness of the assembly, altering its dynamic behaviour, and increasing the risk of fatigue failure. Fretting is the expected damage mechanics on these mating surfaces. The study presented in this paper investigates the fretting damage mechanism of interfaces coated with CoMoCrSi alloys. The experimental plan includes fifteen combinations of the test parameters: two contact geometries, three deposition processes, four temperatures, three normal loads and three strokes. Wear at different number of cycles was also explored. Moreover, two types of contact geometries were investigated, namely point contact (sphere-on-flat) and flat-on-flat. The friction coefficient was computed using the hysteresis loops measured during the fretting tests. The topography of the contact surfaces was measured at different fretting wear cycles to estimate the volume loss. Wear grooves were observed by scanning electron microscopy. Results of point contact experiments at room temperature exhibited a steady friction coefficient independent of the normal load. Wear volumes showed a sharp increasing in wear rate at high dissipated energy while the trend was linear at lower dissipated energy. Oxidation was found more dependent on substrate than on temperature, stroke and wear cycles. Wear volumes and wear rate on flat-on-flat specimens coated with welded T800 were higher at 400 C than at room temperature and at high temperature (800 C). At room temperature, wear volumes of welded T800 applied by single layer were much higher than in dual layer. At room temperature and at low dissipated energy the wear rate of the point contact geometry was lower than flat-on-flat. At high energy, the wear rate of point contact tends to the flat-on-flat wear rate.

To investigate the effects of the contact geometry, interfacial friction, and substrate recovery on the behavior of polymer scratching using a conical tip, an analytical model is proposed. The normal stress acting on the contact surface between the tip and the substrate is described as a function of the included angle θ, representing the angle between two planes across the axis of the conical tip, and the attack angle β, representing the angle between the conical surface and the substrate material surface. The effects of the rear contact geometry on the scratch friction between the tip and substrate, represented by recovery angle φ, owing to the instantaneous elastic recovery of the polymer substrate, are also introduced. Validated by the experimental and numerical results from the literature, the proposed analytical model can describe well the scratch coefficient of friction (SCOF), which is defined as the ratio of the tangential force to the normal force. Meaningful guidance is provided to understand the scratch friction behavior.

This study investigates the thermal response of titanium alloy fretting sliding through a numerical approach with the consideration of the friction dissipation, plastic dissipation, surface roughness and wear. The fretting sliding of a cylindrical pad on a specimen with surface roughness is modeled to predict the temperature distribution, the evolution of the wear profile, and the fields of surface and subsurface stresses. Specifically, a thermo-elasto-plastic constitutive model considering the thermal-induced softening of material properties is developed to evaluate the contact pressure and the tangential stress on the surfaces as well as the subsurface stresses in the contacting components. The friction and plastic energy dissipations function together as the heat source to cause the temperature rise in the contact zone. A modified Archard model is used to predict the wear profile of the contact surface and change the contact geometry. The models are then incorporated in the finite element analysis of the fretting sliding to evaluate the thermal response. The influences of the plastic dissipation and the surface roughness on the temperature rise in the fretting sliding are discussed.

The friction properties of a dilute solution of block copolymer additives in poly-α-olefin (PAO) confined between mica surfaces were investigated using the surface forces apparatus. Friction measurements were made for poly(11-acrylamidoundecanoic acid)-block-poly(alkyl methacrylate)-block-poly(11-acrylamidoundecanoic acid) with an alkyl chain length of C12 (A5L946A5) in the poly(alkyl methacrylate) blocks. The results were compared to those for a PAO solution of a similar triblock copolymer with an alkyl chain length of C18 (A5S992A5) examined in our previous work (Yamada et al., Langmuir 2015). The friction coefficient μ for A5L946A5/PAO was approximately 0.5, which was one or two orders of magnitude larger than that for A5S992A5/PAO (μ of 0.002–0.039). Detailed analysis of the dynamic thickness and contact geometry enabled us to investigate the difference in the friction reduction abilities of the two additives. A5L946A5 has a high affinity to PAO and the molecules have expanded conformation in PAO; extensive chain entanglement effect results in the formation of a thick and viscous lubricant layer, which gives rise to high friction. On the other hand, A5S992A5 has low affinity to PAO, and the shrunken A5S992A5 molecules adsorbed on the surfaces form a rigid slip plane at the interface, which gives rise to low friction. Our results provide insights into the oil/additive combinations used for low friction in oil-based lubrication. The friction properties of a dilute solution of block copolymer additives in a base lubricant oil confined between mica surfaces were investigated using the surface forces apparatus. Friction force, sliding film thickness, and contact geometry were evaluated in detail, and the results were compared to those of the solutions of a different block copolymer additive having similar molecular structure. The relationship between the chemical structure of additives, confined structures between surfaces, and friction mechanisms were discussed, which shed new light of the design of low-friction surfaces in oil-based lubrication.

Numerous biomimetic structures made from elastomeric materials have been developed to produce enhancement in properties such as adhesion, static friction, and sliding friction. As a property, one expects adhesion to be represented by an energy per unit area that is usually sensitive to the combination of shear and normal stresses at the crack front but is otherwise dependent only on the two elastic materials that meet at the interface. More specifically, one would expect that adhesion measured by indentation (a popular and convenient technique) could be used to predict adhesion hysteresis in the more practically important rolling geometry. Previously, a structure with a film-terminated fibrillar geometry exhibited dramatic enhancement of adhesion by a crack-trapping mechanism during indentation with a rigid sphere. Roughly isotropic structures such as the fibrillar geometry show a strong correlation between adhesion enhancement in indentation versus adhesion hysteresis in rolling. However, anisotropic structures, such as a film-terminated ridge-channel geometry, surprisingly show a dramatic divergence between adhesion measured by indentation versus rolling. We study this experimentally and theoretically, first comparing the adhesion of the anisotropic ridge-channel structure to the roughly isotropic fibrillar structure during indentation with a rigid sphere, where only the isotropic structure shows adhesion enhancement. Second, we examine in more detail the anomalous anisotropic film-terminated ridge-channel structure during indentation with a rigid sphere versus rolling to show why these structures show a dramatic adhesion enhancement for the rolling case and no adhesion enhancement for indentation.

The friction behavior of soft, aqueous, biotribological contacts depends on contact geometry, speed, and pressure. Previous efforts to measure the surface profile of soft sliding contacts have been stymied by the matching index of refraction of aqueous materials submerged in water. Here, hydrogel surface deformations were imaged using confocal microscopy to experimentally investigate the contact geometry as a function of sliding speed. In situ fluorescence confocal microscopy measurements of the contact deformation during unidirectional friction experiments revealed the existence of a front/back asymmetry that increased with increasing sliding speed. A polyacrylamide hydrogel disk (96.5% water) was polymerized with fluorescent dyes and used as a rotating countersample below a polished glass hemispherical pin (1 mm radius of curvature). All experiments were performed submerged in a dilute (0.7 wt%) suspension of 1 μm red fluorescent microspheres in ultrapure water. Imaging of the contact was performed using a confocal microscope in situ with unidirectional sliding at 0.1, 1, 10, and 100 mm/s. The friction coefficient increased monotonically with increasing speed from μ ~ 0.04 at 0.1 mm/s to μ ~ 0.20 at 100 mm/s. All imaging was performed relative to the stationary glass probe under steady-state conditions. The contact line measured by connecting the leading and exiting contact points was nearly perpendicular to the loading direction at 0.1 mm/s sliding speed, but distorted as sliding speed increased. Consistent with the theories of viscoelastic contributions to polymer friction, the tangent of the contact-line angle correlated with friction.

An appropriate clearance is required between mating gears for effective power transmission. In the present work, the effect of radial misalignment on mild wear of spur gear is modeled and simulated. The changes in the gear tooth contact geometry due to progressive wear have also been accounted in the model. The simulation results show that in the pinion dedendum region, pressure angle increases, while the contact pressure and wear depth decrease with increase in number of wear cycles. As far as the effect of radial misalignment is concerned, contact pressure decreases and wear depth increases with the increase in misalignment. In the pinion addendum region, pressure angle and wear depth increase with the increase in wear cycles, while the contact pressure and wear depth decrease with the increase in misalignment. The pressure angle and pitch point change from initial 20° (23rd pitch point) to 20.95° (26th pitch point) and 21.86° (28th pitch point) due to 0.5- and 1-mm radial misalignment, respectively. The wear depths after 50,000 wear cycles are − 1.32 × 10−2 μm, − 3.33 × 10−3 μm and − 4.39 × 10−4 μm at 23rd, 26th and 28th pitch points, respectively. The effect of radial misalignment on backlash, pressure angle, pitch point, contact ratio, double tooth contact region and speed ratio is also discussed.

This paper presents an experimental investigation into the effects of contact conformity, contact pressure and displacement amplitude on the gross-slip fretting behaviour grease-lubricated cylinder-on-flat contacts in the context of flexible marine riser pressure armour wire, and compares behaviour with that observed in unlubricated conditions. Characterisation of friction and wear is critical to fretting fatigue life prediction in flexible risers since friction directly controls trailing-edge fretting stresses and hence fatigue crack initiation, on the one hand, and on the other hand, directly affects wear via relative tangential slip (displacement). Wear can have a beneficial or detrimental effect on fatigue crack initiation and propagation, depending on relative slip and slip regime. It is shown that friction and wear are higher for dry conditions than for grease-lubricated conditions. For grease-lubricated conditions, behaviour is determined by whether grease can be retained in the contact (as opposed to being extruded out). Retention (or replenishment) of grease in the contact results in low rates of wear and low coefficients of friction; these conditions are favoured by fretting displacements above a critical value, by low contact conformity, and by low applied loads. The authors would like to thank the Irish Research Council and Wood Group Kenny for funding of this project through the Enterprise Partnership Scheme (EPSPG/2013/638), the National University of Ireland for funding through a NUI Travelling Scholarship. We also wish to acknowledge the help and support we have received from Dr. Adrian Connaire and Mr. Kieran Kavanagh (Wood Group). The authors also wish to acknowledge the support of the Faculty of Engineering at the University of Nottingham where this research was conducted. peer-reviewed

Fretting can lead to surface wear and/or crack nucleation depending on the sliding condition and contact geometry. To formalize this aspect, Ti-6Al-4V cylinder/plane and punch/plane contacts were investigated. The experimental crack nucleation domains in partial and gross slip conditions were established and simulated combining SWT cumulative damage analysis with FEM surface wear simulations. Good correlations were achieved if the experimental boundary conditions including system tangential accommodation and micro-rotations measured using DIC analysis, were considered. Fretting maps were simulated by predicting partial slip and gross slip displacement amplitudes above which cracks were respectively nucleated and removed by surface wear. Finally, it was shown that whatever the contact geometry, the gross slip cracking domain was inversely proportional to the surface wear rate.

Water-lubricated ceramic bearings are currently being used in engineering applications like water pumps, air compressors and marine systems, due to their higher efficiency, reduced environmental impact and simpler constructive designs. In search of low-cost and high-performance tribosystems, bearings made of dissimilar ceramics are not frequently studied, but have potential to show good tribological compatibility. In this study, tribological behavior of two dissimilar ceramic pairs, ZrO2-Al2O3 and ZrO2-ZTA (10 vol% ZrO2), was investigated. Water-lubricated ball-on-disc tests under loads ranging from 6.0 to 24.6 N and sliding speeds from 0.1 to 2.0 m/s were performed. Worn surfaces were characterized by optical microscopy, SEM and contact-type profilometry. Thermal (Sc,t) and mechanical (Sc,m) severity of contact models explained the mild to severe wear transition as function of tribosystem's variables such as load, speed, distance, materials’ properties, contact geometry and environment. During mild wear regime, wear rates were smaller than 10⁻⁷ mm³/Nm, friction coefficients remained low (0.1

This research is dedicated to the effect of underwater contact explosions on steel rods. The work is inspired by problems the Dutch Army Engineers experienced with the effective demolition of steel structures under water. Selected parameters which could affect incomplete demolition were investigated experimentally. These parameters included the amount of explosive, the contact geometry and the thickness of the spacer between explosive and steel. For the experiments a plastic explosive and cylindrical steel bars of 100 cm length and 5 cm diameter were used. A main cause of failing demolition can be attributed to insufficiently following the guidelines. The explosive charge should be in close contact with the target material. Furthermore, the optimal geometric arrangement of the plastic explosives was found to be a 120 degrees covering around the steel bars. The latter strategy achieved a demolition of the rods at half the amount of explosive as prescribed.

The application of polymers in power-transmitting machine elements, e.g., gears, is limited by moderate thermo-mechanical properties and the detrimental accumulation of contact heat, even with external lubrication. Hence, polymer rolling–sliding elements are often prone to thermo-mechanical overload or abrasive wear. Diamond-like carbon (DLC) coatings are well known from steel applications for enhancing wear resistance and reducing friction. Since preliminary results indicate promising results for such coatings for polymers as well, their influence on the behavior of lubricated polymer contacts is investigated by numerical simulation. For polymer–steel contacts, the mechanical and thermophysical properties of coating and polymer are varied. The contact geometry is dominated by a local conformity, in which most of the deformation is related to the polymer. The DLC coatings affect film thickness and hydrodynamic pressure only little even for untypical high coating thicknesses. In contrast, the contact temperature decreases already for very thin coatings due to enhanced heat removal. Hence, DLC coatings can act as a thermal barrier protecting the polymer from detrimental heat and protecting the polymer from abrasive wear.

The blade-disc dovetail interface in an aero-engine compressor is characterized by a non-uniform pressure distribution which can be obtained by an equivalent flat with round edge-on-plate configuration. The contact tractions for a mating pair are affected by many parameters which include, contact geometry, loading conditions, and material properties; with contact geometry being one of the prominent factors. In the present work, a 2-D elastic and elasto-plastic finite element analysis has been carried out for a rounded contact geometry to study the influence of the radius of the corners ‘R’ and length of the flat region ‘2a’. It is observed that the peak tensile stress in the fretting direction was found to decrease with increasing ‘a’ (for constant ‘R’) which is likely to delay the crack initiation. Also, as compared to elasto-plastic analysis, elastic analysis overestimates peak tensile stress and possibly give a conservative estimate for the fretting fatigue life. Further, the effect of modelling elastic-plastic behaviour is significant for low a/R ratio (for constant ‘R’). However, opposite trend was observed when ‘R’ was varied keeping ‘a’ constant. Also, it is found that the effect of contact geometry cannot be characterized using a single parameter like a/R ratio or contact area.

Plint (IMechE 182 (1):300–306,1967) reported a reduced coefficient of friction with increasing roller sizes in EHL traction measurements. In this study, a similar scale effect has been observed when comparing measured traction curves at the same operating conditions between a ball-on-disc rig and a twin-disc machine of different geometrical sizes. This scale effect has been studied numerically for point contacts of different radii of curvature Rx based on thermal EHL simulations. Results show that the reduced friction for large Rx is caused by an increase in film thickness and the enhanced thermal effects. The mechanisms are: (1) heat is hard to conduct across a thicker EHL film due to bad thermal conductivity; (2) shear is mainly localized in the middle film.

Here we compare the wear performance of silica and polyurethane in a vacuum gripper. The problem is modeled numerically by evaluating the energy dissipated at the interface, and experimentally by examining surface damage in contact subjected to cyclic normal loading. In the numerical model, the energy dissipated during unloading is found to be negligible with respect to that of loading. Changing the contact geometry has a good effect in terms of frictional work reduction, but this is not as significant as the effect obtained by changing the material. Replacing silica with polyurethane reduces the frictional work by a factor of at least 20. The latter finding is qualitatively validated in experiments.

A transient three-dimensional thermal elastohydrodynamic calculation model is introduced to simulate the contact of tooth flanks of helical gear pairs with involute gearing. At any time, contact geometry is derived from the real involute curves of gears. In addition to the consideration of mass conserving cavitation and the non-Newtonian fluid behavior, models to capture conditions of mixed friction and microhydrodynamic effects are included. To consider the non-Newtonian fluid behavior, a shear thinning fluid model with limiting shear stress is applied. A selected helical gear pair is used as example to examine the influence DLC coated tooth flanks have on tribological behavior. It shows that friction and temperature behavior of the gearing can be significantly influenced with DLC coating.

In this study the influence of tool inclination angle on deformation of thin wall cantilever shape part has been analyzed using finite element numerical simulations. Polycrystalline diamond, PCD tool has been chosen as a tool material in this study because PCD tool has given better results in terms of surface roughness, tool life and productivity in end milling of titanium alloy Ti-6Al-4V. Firstly, in this study, the effect of tool inclination angle on tool contact geometry, cutting speed and cutting forces has been discussed. Then, finite element numerical simulations (FEM) have been carried out in AdvantEdge® for the prediction of cutting forces with PCD tool at four different tilt angles viz. 70°, 75°, 80° and 85° and the results have been compared to the coated carbide tool. Then the maximum magnitude of the forces which occurred in tangential direction (FY) were input in the Abaqus® software as a load acting on the thin long cantilever part and deformation results were analyzed. Results show that PCD tool due to its high hardness, strength and better wear resistance produce lower cutting forces at all angles studied and at an angle near the perpendicular to the surface being machined both tools have lower values of the deformation. The FEM simulation results match well with the theoretical study as theoretical analysis also shows that at angles nearer to perpendicular to the surface being machined, the tool will have lower effective cutting speed & forces and hence proved as the key to achieving better accuracies for long thin wall parts.

Friction welding is a solid-state bonding process that presents itself as an interesting option as it generates less residual stress, less distortion, and crack formation when compared to the fusion welding process. The characteristics of this process also allow satisfactory welding of dissimilar materials, with good results in terms of mechanical strength. In this work, the butt welding of aluminum ASTM A6351-T6 and SAE 1020 steel was carried out aiming at evaluating the effects of the initial contact geometry on the mechanical properties of the welded joint. The methodology consisted of friction weld aluminum bars with different initial contact geometry with steel bars in a machine specially developed for the application. The results indicated the influence of this parameter on the mechanical properties of the welded joint.

The authors carried out matched experiments and molecular dynamics simulations of the compression of nanopillars prepared from Cu|Au nanolaminates with up to 25 nm layer thickness. The stress–strain behaviors obtained from both techniques are in excellent agreement. Variation in the layer thickness reveals an increase in the strength with a decreasing layer thickness. Pillars fail through the formation of shear bands whose nucleation they trace back to the existence of surface flaws. This combined approach demonstrates the crucial role of contact geometry in controlling the deformation mode and suggests that modulus-matched nanolaminates should be able to suppress strain localization while maintaining controllable strength.

The present study was undertaken to evaluate the effect of surface hardening technology on dynamic frictional rolling contact behavior and degradation of corrugated rail in Shenzhen. Characteristic parameters such as length and depth of corrugation were analyzed by means of a continuous measurement method based on the corrugation analysis trolley. The explicit finite element method for material hardening characteristics and real contact geometry was adopted to set up the 3D transient FE model of wheel and rail, after which the value and distribution of stress/strain as well as contact solutions could be obtained during frictional contact, and then the Archard wear model and simplified wear superposition method are integrated as a numerical simulation tool for rail wear after hardening. The simulation results show that laminar plasma surface hardening technology can increase residual stress and shear stress in quenched zones, leading to local stress concentration at their boundaries; the plastic strain in the matrix material is higher than that in the quenched zones, while the strain concentration is mainly focused on the matrix material. The hardening can remarkably reduce the rail wear along the corrugation wave, and the wear depth of material with hardening technology is about 36% of that of nonhardening material. Laminar plasma surface hardening technology can therefore restrain the development of rail corrugation.

Surface texturing is considered to be a promising method to improve the tribological properties. Depending upon the experimental conditions, the effect of texturing varies from favourable to unnoticeable to detrimental. In this work, surfaces with micropillars are studied under elastic dry reciprocating contact. An array of micropillars with different pillar heights are generated on stainless steel using wire-cut electrical discharge machining. The effect of stiffness of the micropillars on friction is investigated, keeping the number of micropillars in contact with a flat aluminium alloy (Al6061) slider and contact geometry constant. Reciprocating experiments are carried out against a flat surface such that about 81 micropillars are in contact. From the experimental results, it is found that the coefficient of friction is independent of the stiffness of the texture elements. However, work done per cycle significantly varied with the stiffness of texture element and applied normal load. A lumped system model with Coulomb friction shows that the work done per cycle varies quadratically with the normal load. The experimental results agree with this simplified model except in the incipient sliding regime. These results show how the work done per cycle varies, for different contact stiffness under elastic contact even though the coefficient of friction remains constant. The implication of this study for a macroscopic measured coefficient of friction as a function of microscopic asperity level friction is discussed.

The purpose of this study is to investigate friction characteristics according to micro-dimple patterns. The surface texturing of micro-dimple patterns was tested to examine the friction of pin-on-disk using flat-on-flat contact geometry. The patterns of both dimple circle and groove pattern were adopted to carry out the effect of those ones. In the low loads, such as 13.8N and 27.7N, the friction coefficients of groove pattern were lower than those of dimple circle pattern. In many other comparisons of normal loads, the groove pattern had lower friction forces, which showed the effect of surface texturing. The relationship between sliding time and friction forces showed that the increase of friction forces of groove pattern were relatively lower than those of dimple pattern. In conclusion, the dimple patterns of dimple-circle pattern and groove pattern strongly contributed to reducing the friction between contacting materials.

Adhesion forces were determined by recording force–displacement curves using a flat tip with an atomic force microscope. The outcomes show that the adhesion behavior at a single location depends largely on contact geometry, surface topography, instrumental parameters, and the environment. Moreover, these factors are mutually coupled with each other to determine the final adhesion force. A special surface structure will lead to special adhesion behaviors by repetitive measurements. The adhesion forces are grouped into several levels. In dry nitrogen (with charges removal), the adhesion force in each level first increases and then decreases slightly. In dry nitrogen and under ambient conditions, the adhesion force in different levels increases with measurement number, and jumps between different levels. The increasing trend was ascribed to the charge accumulation and the increased size of the capillary meniscus, respectively. The charges will increase only after contact and separation and have an additive e...

Sub-surface cracks can be initiated due to surface fatigue, which eventually reach the surface, leading to pitting, spalling and removal of material. In this investigation, a new approach based on shear stress reversal at the crack tips is implemented to propagate a subsurface initiated crack under fretting load. Hertzian line contact geometry is used to investigate the effects of different factors such as Hertzian pressure, coefficient of friction, displacement amplitude, and depth of the initial crack. Crack propagation paths and propagation life of the cracks under the surface are investigated in detail. Once the crack reaches the surface, it is assumed that the material enclosed is spalled and thus removed. Wear volumes and wear rates are calculated and effect of each variable on wear is analyzed. Wear rates increase with increase in maximum Hertzian pressure, displacement amplitude, coefficient of friction and depth of initial crack. The wear rates are also correlated to crack propagation rates and are in agreement with the dissipated energy wear coefficients obtained from literature.

Tribological studies on neat polyetherketone (PEK) and glass fiber reinforced PEK composite were carried out at room and elevated temperature (120 ℃). The objective of this study was to characterize for friction and wear properties under dry reciprocating sliding condition at different experimental conditions. The polymer specimens were made to oscillate against steel cylinder as a counterpart. This kind of contact condition are frequently found in bushes, sliding bearing, electronic parts, seals, etc. The friction and wear behavior of neat PEK and composite was quite different at room temperature and elevated temperature. It was observed that glass fiber reinforcement is beneficial in controlling the wear of PEK matrix at room temperature than at elevated temperature. The test results are discussed by considering the surface properties i.e. material removal and film transfer formation. Scanning electron micrographs and optical micrographs of the worn polymer and steel cylinder was used to study the wear mechanisms.

In this investigation an approach to fretting fatigue life prediction is developed with consideration of damage-coupled elastic–plastic constitutive model and wear. Nonlinear kinematic hardening is employed in the analysis of elastic–plastic damage, and the total damage is divided into two parts, elastic damage and plastic damage, which are related to the cyclic stress and accumulated plastic strain, respectively. Wear is modeled by the energy wear law to simulate the evolution of contact geometry. A two dimensional plane strain finite element implementation is presented for fretting, including the case of partial slip and gross sliding. The progressive fatigue damage and wear is simulated and the results are compared with experimental data from the literature.

The present study reports on the 'wear damages' caused at inner surfaces (and sub-surfaces) of extruded zircaloy-4 tube specimens (as half cylinders) during interactive linear reciprocating sliding against outer surfaces of D2 steel half cylinders for different durations (up to 120 s) at high contact pressures (of similar to 1 GPa) in the absence/presence of mineral oil lubricant (having chlorinated extreme pressure additives) and graphene-based lubricants/coatings [viz., CVD-grown films of well-ordered few layers graphene (similar to 7 layers) and thicker graphite (similar to 1400 layers)]. The tests were conducted using a custom-made friction-and-wear tester, which also replicates the conditions (in terms of contact geometry, pressure, stroke length and frequency) relevant to the tribological phenomena occurring at the inner walls of tubes subjected to cold pilgering process. Even though none of the lubricants/coatings had any effect on coefficient of friction, presumably because of 'boundary lubrication' at such high contact pressures, only the graphene-based lubricants/coatings slowed down the evolution of surface roughness and wear damage. Observations of worn (top) surfaces and cross-sections indicate that 'wear damages' at such contact pressures are primarily caused by severe sub-surface deformation and defect/crack initiation/propagation, followed by lateral chipping/delamination; which could be suppressed by the graphene-based lubricants/coatings (but not by the oil-based lubricant).

Graphene was grown by low-pressure chemical vapour deposition on polished steel samples using ethanol as a precursor at different pressures: 200 and 400 mTorr. The samples were characterized by Raman spectroscopy, and the Raman maps clearly show the formation of an inhomogeneous graphene coating with surface regions covered with single-layer, bilayer, and multilayer graphene, the last one covering most of the surface area of the sample when the sample was prepared at 400 mTorr. Scanning electron microscopy revealed the partial coverage of the sample surface when the graphene coating was prepared at a pressure of 200 mTorr. Friction measurements were taken using a micro-tribometer in a ball-on-flat contact geometry with a stainless steel ball as the counterbody. An important reduction in the coefficient of friction was verified even with partial coverage of the steel surface, and it was attributed to the formation of nanographitic structures in the wear track region and partially transferred to the counterbody.

We report a study of the effect of different Schottky contact orientations on maximum current achievable before failure and also temperature distributions in vertical geometry Ga2O3 rectifiers. Due to the strong anisotropy of thermal conductivity in Ga2O3, asymmetrical Schottky contacts are needed to provide higher current density with enhanced lateral thermal dissipation, symmetrical temperature profile and lower junction temperature at a specific diode current density compared to symmetrical contacts. Devices with rectangular contacts fabricated on (001) orientated wafers with their long axis perpendicular to the [010] crystallographic direction show much greater resistance to thermal degradation under forward bias conditions than either square contact rectifiers or those oriented with their long axis oriented perpendicular to the [100] direction. An optimized contact orientation can produce a 25% increase in maximum forward current. Practical operating conditions for Ga2O3 power devices will need to encompass all aspects of thermal management, including these geometric factors as well as active and passive cooling.

The effect of reducing adhesion force by coating with a metal-containing diamond-like carbon (DLC) film has been studied by recording force–displacement curves with an atomic force microscope. A flat tip, a spherical tip, and some sharp tips were applied to mimic the different contact geometries. The results show that both under ambient conditions and in dry nitrogen, the DLC film can effectively reduce the adhesion force for different contact geometries. The reduction of the adhesion force was attributed to the decrease of the surface free energy and the increase of the contact angle for water. The reduction ratio of adhesion is closely related to contact geometry, the roughness of DLC film, material characteristics paired with DLC film and the environment. These factors are mutually coupled to determine the final reduction ratio. Under both conditions, the DLC film also plays a role in reducing the wear and tear when measuring the adhesion forces.

A new silicon-based thermogenerator with a reduced parasitic contact resistivity and a large temperature working range from 250 K up to 1000 K is presented. Three-dimensional contact geometry and additional doping reduces the contact resistivity from 12.7 MΩ μm 2 to 57.6 kΩ μm 2 . In this way, for all temperatures above 250 K the contact resistivity is negligible with respect to the inner resistivity of the thermoelectric materials. At room temperature it is only 1% of the total resistivity. The thermogenerator was embedded in metal using the method of metal-spray-deposition, which ensures high yield of the successfully embedded thermogenerators (30 functional of the total of 32) due to relatively low mechanical forces applied during the embedding process and reduced thermo-mechanical stress during the following cool-down phase. Our results demonstrate that it is possible to embed silicon microsystems in metals during high-temperature metallurgic primary forming processes.

Stress distribution at the contact edge plays a dominant role in fretting fatigue strength. In the previous studies, based on the stress distribution at the contact edge, a generalized tangential stress range–compressive stress range (TSR–CSR) diagram has been proposed as a fretting fatigue fracture criterion. It has been also confirmed that the proposed diagram would be very useful to predict the fretting fatigue strength regardless of contact geometry, loading condition, material strength, environment, etc. for laboratory-type specimens. In the present study, fretting fatigue strengths of actual components, such as a dovetail joint and a bolted joint, have been predicted based on the generalized TSR–CSR diagram. To verify the effectiveness of the prediction based on the generalized TSR–CSR diagram, the fretting fatigue tests of dovetail joints and bolted joints were carried out. The fretting fatigue strengths of dovetail joints and bolted joints predicted based on the generalized TSR–CSR diagram were in good agreement with the experimental results.

The present work is an experimental study of adhesion between an elastomer and a rigid cylindrical indented The experimental characterization was carried out using a specially developed apparatus. Adhesive force was measured as function of contact geometry, pull-off velocity, normal force, temperature, waiting time, and material properties. Experimental results are compared with existing theoretical models for adhesion of elastic and viscoelastic bodies. Our study shows that the adhesive force between the studied elastomer and a steel cylinder is determined by completely different mechanisms than assumed in the Kendall’s theory. In particular, it does not depend on the surface energy and is almost entirely dominated by the viscosity of the elastomer.

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.

The mixed-lubrication model of the planetary roller screw is proposed considering contact load, contact geometry of thread, surface roughness and transient effects. The influences of rotational speed of the screw, axial load and surface roughness on lubrication are discussed. Increasing rotational speed has a significant effect on the lubrication of contact areas. As the speed increases, the film thickness increases while the friction coefficient and contact-area ratio decrease, and the lubrication mode changes from mixed to full-film. As the load increases, the film-thickness ratio decreases while the friction coefficient and maximum subsurface von Mises stress increase. High-precision machining can improve lubrication by properly reducing the roughness of the machined surface, which would increase film thickness and decrease the contact-area ratio.

The effect of heating, the normal load, and the radii of the members in contact on the static coefficient of rolling friction is considered. The design of a new apparatus for measuring the parameters of rolling friction over an inclined plane at elevated temperatures is presented. A need to specify the effect of the type of the materials in contact, as well as the roughness and contact geometry, on the static coefficient of rolling friction is emphasized.

One of the major causes of tool failure in sheet metal forming is wear in the form of galling. Galling is gradual buildup of adhered sheet material on the tool and leads to unacceptable scratches on the sheet surface and to components that fail to meet tolerances. Because it is difficult to reproduce operational and interactional conditions in laboratory test equipments it is hard to test, model, and predict galling initiation. Here the authors examine how changes from elliptical to line contact geometry influenced galling initiation under dry sliding by using a slider-on-flat surface (SOFS) wear tester. A micro clean tool steel was tested against ferritic low-strength and martensitic high-strength steel sheets. The sliding distance to galling initiation was extracted from friction data and verified by scanning electron microscopy (SEM) observations. The presence of adhesive wear on worn tools after completed tests was used as a criterion. Experimental results showed that the elliptical contact causes gal...

The study investigate three kinds of interfacial patterns (convex, corrugated, grid-like) were designed so as to improve the interfacial bonding strengths of hydroxyapatite (HA)/ZrO2 biomaterials. The thicknesses of prepared HA coatings were 50–250 μm. Both shear and tensile bonding strengths of the composites with different interfacial patterns were improved compared with the tiled samples because of the different contact geometry. The analysis on the fracture surfaces by SEM and XRD confirmed that the adhesive modes of HA and ZrO2 were the combination of mechanical and physical bonding. The research is beneficial to the clinical application of HA for a layered composite.

In this work, the back surface field (BSF) formation of locally alloyed Al-paste contacts employed in recent industrial passivated emitter and rear cell solar cell designs is discussed. A predictive model for resulting local BSF thickness and doping profile is proposed that is based on the time-dependent Si distribution in the molten Al paste during the firing step. Diffusion of Si in liquid Al away from the contact points is identified as the main differentiator to a full-area Al-BSF; therefore, a diffusion-based solution to the involved differential equation is pursued. Data on the Si distribution in the Al and the resulting BSF structures are experimentally obtained by firing samples with different metal contact geometries, peak temperature times and pastes as well as by investigating them by means of scanning electron microscopy and energy dispersive X-ray spectroscopy. The Si diffusivity in the Al paste is then calculated from these results. It is found that the diffusivity is strongly dependent on the paste composition. Furthermore, the local BSF doping profiles and thicknesses resulting from different contact geometries and paste parameters are calculated from the Si concentration at the contact sites, the diffusivity and solubility data. These profiles are then used in a finite element device simulator to evaluate their performance on solar cell level. With this approach, a beneficial paste composition for any given rear contact geometry can be determined. Two line widths are investigated, and the effects of the different paste properties are discussed in the light of the solar cell results obtained by simulation. Copyright © 2013 John Wiley & Sons, Ltd.

Modularity of femoral stems and neck components has become a more frequently used tool for an optimized restoration of the hip joint center and improvement of patient biomechanics. The additional taper interface increases the risk of mechanical failure due to fretting and crevice corrosion. Several failures of titanium alloy neck adapters have been documented in case-reports. An experimental fretting device was developed in this study to systematically investigate the effect of micromotion and contact pressure on fretting damage in contact situations similar to taper interfaces of modular hip prostheses under cyclic loading representative of in vivo load conditions. As a first application, the fretting behavior of Ti–6Al–4V titanium alloy components was investigated. Micromotions were varied between 10 μm and 50 μm, maximum contact pressures between 400 and 860 N/mm2. All modes of fretting damage were observed: Fretting wear was found for high micromotions in combination with low contact pressures. Fretting fatigue occurred with reduced movement or increased contact pressures. With small micromotions or high normal pressures, low fretting damage was observed. The developed device can be used to evaluate taper design (and especially contact geometry) as well as different materials prior to clinical use.

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