Friction Curves Research Articles

A Traction (Friction) Curve Is Not a Flow Curve

With the uncertainty regarding the global energy future, the ability to lubricate concentrated contacts with sufficiently thick liquid films while minimizing friction is of extreme importance. The assumptions of classical elastohydrodynamic lubrication have remained unchanged since the early days. It has not been possible to test many of these assumptions without the measurement of the viscosity at elastohydrodynamic lubrication (EHL) pressures, and viscometer measurements have been ignored. One of these assumptions has been the equivalence of a traction curve to a rheological flow curve for the lubricant. This notion should have been discarded forty years ago, simply because it required the pressure–viscosity behavior to be unlike the behavior observed in viscometers. At the heart of the problem is the fact that the pressure within the EHL contact is not homogeneous and the liquid properties are highly dependent on pressure, making the contact a very poor rheology laboratory. These past failures must be avoided in the future.

Coefficient of friction behavior of gear oils and significance for the meshing process of spur gears

The mechanical properties of oils are determined using test methods. There are standardized test methods for determining viscosity and density. The characterization of transmission oil based on its dynamic viscosity alone is not sufficient for the physical explanation of different levels of noise emissions in vehicle transmissions. For this reason, the test procedure for determining the coefficient of friction is used in the following to enable a further differentiation between the oils according to mechanical properties.In gear transmissions with involute gear teeth, rolling friction occurs in the gear pair meshing along the line of action due to the variation in the equivalent curvature radii throughout the meshing cycle. This is rolling friction on which a sliding friction component, so-called slip, is superimposed. Pure rolling friction only occurs in the pitch point. From the pitch point to the start and end of the meshing, there is a superimposed sliding friction component that increases with increasing distance from the pitch point. Slip values occur in the range of 5–50% depending on tooth geometry.These friction conditions during tooth flank lubrication can be assessed using the Stribeck curve. The Stribeck curve represents the coefficient of friction as a function of the speed. A mini traction machine from PCS Instruments with a ball/plate measurement setup was used to determine the coefficient of friction behavior of gear oils. This allows the coefficient of friction of an oil to be assessed at low speeds in the range from boundary and mixed friction to elastohydrodynamic fluid friction at high speeds.The investigations show that the coefficient of friction behavior of a gear oil depends on the oil viscosity and above all on the chemical composition. The lower the coefficient of friction, the less energy is required to shear the lubricating film and the lower the power transmission through the fluid. The coefficient of friction is a property that is dominated by the type of base oil and the type of VI improver in the area of mixed and fluid friction, especially with additional sliding in contact.It will be shown in the paper that the use of a gear oil that has been optimized with regard to the coefficient of friction curve can reduce the entry impacts of meshing gear pairs under vibration excitation and the gear transmission thus generates lower noise emissions.

Advantages of Using Triboscopic Imaging: Case Studies on Carbon Coatings in Non-Lubricated Friction Conditions.

Triboscopy focuses on the analysis of the temporal evolution of a tribological system, combining local and time-resolved information, most commonly the evolution of friction. In this work, this technique is applied on measurements, which were carried out with a custom-built ultra-high vacuum tribometer in ball-on-disc configuration. Based on these experiments, an extended classification to distinguish different triboscopic features is suggested, depending on the persistence in both track position and time: Uniform, Global, Local, and Sporadic. Further, a filter technique for quantifying triboscopic data regarding this classification is introduced. The new and improved triboscopic techniques are applied to various dry friction measurements of hydrogen-free carbon coatings under varying humidity and pressure. The resulting specific triboscopic features are correlated to wear phenomena, such as counter body coating abrasion, inhomogeneities in the wear track, non-uniform track wear, stick-slip and debris in the contact area, demonstrating the increased analysis and monitoring capabilities when compared to conventional friction curves and wear track images.

COMPUTATIONAL FLUID DYNAMICS ANALYSIS BASED ON THE FLUID FLOW SEPARATION POINT ON THE UPPER SIDE OF THE NACA 0015 AIRFOIL WITH THE COEFFICIENT OF FRICTION

A new method that is more practical, efficient and applicable is proposed to track the position of fluid flow separation on the upper side of NACA 0015. The proposed method is the coefficient of friction curve (Cf) method on the airfoil's upper side. The approach used is a computational fluid dynamics (CFD) approach. The governing equation used is the Reynolds Averaged Navier-Stokes (RANS) equation. is the turbulence model implemented in this study. The research is conducted on the low Reynolds number category. The low Reynolds number is in the range of values from 104 to 3Í105. Cf can predict the location of fluid flow separation more practically, efficiently, and applicable than the fluid flow velocity profile method. Flow separation begins to form at =8° at position x/c=0.8. The location of the fluid flow separation continues to move closer to the leading edge as the airfoil increases. Through the Cf curve, the location of the fluid flow separation is when the Cf curve experiences a sudden decrease and approaches the x-axis. If the separation points are described in the form of velocity profiles and fluid flow velocity contours, it will form an extreme decrease.

PSO Optimized Active Disturbance Rejection Control for Aircraft Anti-Skid Braking System

A high-quality and secure touchdown run for an aircraft is essential for economic, operational, and strategic reasons. The shortest viable touchdown run without any skidding requires variable braking pressure to manage the friction between the road surface and braking tire at all times. Therefore, the manipulation and regulation of the anti-skid braking system (ABS) should be able to handle steady nonlinearity and undetectable disturbances and to regulate the wheel slip ratio to make sure that the braking system operates securely. This work proposes an active disturbance rejection control technique for the anti-skid braking system. The control law ensures action that is bounded and manageable, and the manipulating algorithm can ensure that the closed-loop machine works around the height factor of the secure area of the friction curve, thereby improving overall braking performance and safety. The stability of the proposed algorithm is proven primarily by means of Lyapunov-based strategies, and its effectiveness is assessed by means of simulations on a semi-physical aircraft brake simulation platform.

Preload degradation mechanism of double-nut ball screws revealed by an innovative wear distribution model

The preload of ball screws is generally used to confirm the ball screws with a high axial contact stiffness and a high positioning accuracy. Aiming at the preload degradation due to wear on the raceway of double-nut ball screws, a wear distribution model was proposed by calculating the contact stress distribution and the inhomogeneous relative sliding velocity distribution on the contact region to determine the wear profile of the raceway. Firstly, the contact stress distribution and the relative sliding velocity distribution on the contact surface were determined using the kinematics analysis and the force balance analysis. Subsequently, a wear distribution model was established to determine the wear depth distribution of the contact surface based on elastic-plastic contact mechanics of asperity with consideration of the microscopic characteristics of the raceway. Finally, the three-level step-stress accelerated degradation test based on the simulated model was designed and implemented on a self-designed bench to validate the proposed model. Experimentally measured friction torques of the ball screws were used to reflect the preload degradation, and they were compared with the results of the simulated model. The results showed that the degradation curves of the experimental friction torques and the wear profiles of screw raceways were in good agreement with the numerical results, confirming the correctness and effectiveness of the proposed model. The results indicated that the wear profile of the raceway could be accurately calculated based on the proposed model, clearly describing the wear process of the raceway surface and providing a more precise approach to predict preload degradation for ball screws.

Dry Sliding Behavior and Particulate Emissions of a SiC-graphite Composite Friction Material Paired with HVOF-Coated Counterface

Dry sliding wear tests and corresponding particulate matter (PM) analysis were conducted on a newly developed SiC-graphite-based composite friction material, paired with two types of HVOF counterface/discs: WC-CoCr and WC-FeCrAlY coatings, with a conventional martensitic stainless steel counterface as a reference. The trials were conducted on a pin-on-disc testing equipment at room temperature and a constant sliding velocity and contact pressure of 7 m/s and 0.5 MPa, respectively. The coefficient of friction (CoF) curves with the uncoated disc exhibited considerable fluctuations. On the other hand, the coated discs featured an increase in the CoF at the beginning of the tests, followed by either a continuous reduction until the end of the testing duration or the attainment of a steady state regime. The pin wear and emissions with both coatings were appreciably lower when compared to the trials with the uncoated disc. The evaluation of the friction layer observed a significant contribution of the counterface for all the pairings. The PM analysis was conducted on the particles that were lying in the range of 10 μm and 2.5 μm on a scanning electron microscope (SEM), and particles from 2.5 μm and 1 μm on transmission electron microscope (TEM), with an emphasis on the particles that were detached from the pin surface and friction layer to explain the wear mechanisms for each pairing. Through this, the need for the proper selection of both friction material and counterface to avoid the emission of harmful compounds in the environment was highlighted.

DEM Analyses of Cemented Granular Fault Gouges at the Onset of Seismic Sliding: Peak Strength, Development of Shear Zones and Kinematics

Fault zones usually present a granular gouge, coming from the wear material of previous slips. This layer contributes to friction stability and plays a key role in the way elastic energy is released during sliding. Considering a mature fault gouge with a varying amount of mineral cementation between particles, we aim to understand the influence of the strength of interparticle bonds on slip mechanisms by employing the discrete element method. We consider a direct shear model without fluid in 2D, based on a granular sample with angular and faceted grain shapes. Focusing on the physics of shear accommodation inside the granular gouge, we explore the effect of an increase of cementation on effective friction (i.e. stress ratio) within the fault. We find that brittleness and the overall shear strength are enhanced with cementation, especially for dense materials. For the investigated data range, three types of cemented material are highlighted: a poorly cemented material (Couette flow profile, no cohesion), a cemented material with aggregates of cemented particles changing the granular flow and acting on slip weakening mechanisms (Riedel shear bands R), and a highly-cemented material behaving as a brittle material (with several Riedel bands followed by fault-parallel shear-localization Y). Effective friction curves present double weakening shapes for dense samples with enough cementation. We find that the effective friction of a cemented fault cannot be directly predicted from Mohr–Coulomb criteria because of the heterogeneity of the stress state and kinematic constraints of the fault zone.

Simultaneous Physico-Mechanical and In Vivo Assessment towards Factual Skin Performance Profile of Topical Polymeric Film-Forming Systems.

Topical film-forming systems (FFS) change drastically after solvent displacement, therefore indicating their skin metamorphosis/transformation as a property of special regulatory and research interest. This paper deals with the lack of suitable characterization techniques, suggesting a set of methods able to provide a comprehensive notion of FFS skin performance. After screening the physico-chemical, mechanical and sensory properties of FFS and resulting films, an elaborate three-phase in vivo study was performed, covering skin irritation, friction and substantivity. Upon removal of 24-hour occlusion, no significant change in erythema index was observed, while the film-former type (cellulose ether, acrylate and/or vinyl polymer) affected transepidermal water loss (TEWL); hydrophobic methacrylate copolymer-based samples decreased TEWL by 40–50%, suggesting a semi-occlusive effect. Although both the tribological parameters related to the friction coefficient and the friction curve’s plateau provided valuable data, their analysis indicated the importance of the moment the plateau is reached as the onset of the secondary formulation, while the tertiary state is still best described by the completion of the film’s drying time. The final part of the in vivo study proved the high in-use substantivity of all samples but confirmed the optimal 4:1 ratio of hydrophobic cationic and hydrophilic polymers, as indicated during early physico-mechanical screening.

Reentrant strain glass transition in Ti-Ni-Cu shape memory alloy

Reentrant glass transition, referring to a reversible transition from a high-temperature ordered phase to a low-temperature glassy phase, is unusual and difficult to be understood, because at low temperature the glassy phase is usually less thermodynamically stable than the ordered phase. In this work, we report a systematic study of a reentrant strain glass (RSTG) transition between a strain-ordered martensitic phase and a strain glass in a Ti50Ni34Cu16 shape memory alloy. This transition is characterized by a temperature invariance of average B19 martensitic structure, a deviation in heat flow and resistivity curves, a frequency-dependent behavior of storage modulus and internal friction curves, and a peak in a zero-field cooling curve. In-situ microstructural observations show that the B19 martensitic domain pattern keeps unchanged upon cooling while nanodomains with local 4H symmetry gradually emerge and grow in large martensitic domains. Moreover, the RSTG transition exhibits low modulus (∼24 GPa) and high damping (tan δ > 0.075) over a large temperature range. Based on experimental results, we established a new Ti50Ni50-xCux phase diagram with the RSTG state included. This phase diagram helps understand the abnormal formation of RSTG: since Cu dopants stabilize the B19 phase instead of B19’ phase, with sufficient Cu dopants (x = 16) the B19-B19’ transition is thermodynamically suppressed and the RSTG transition appears, which resembles the conventional strain glass formation in a Ti50-xNi50+x phase diagram. The microstructural evolution of RSTG with nanodomains embedded in large domains explains novel properties and puzzles in Ti-Ni-Cu alloys. We further predict more unusual properties could be found in RSTG materials.

Study on the Liquid-Liquid and Liquid-Solid Interfacial Behavior of Functionalized Graphene Oxide.

With the rise of carbon neutrality, the applications of carbon-based materials are gaining considerable attention. Graphene oxide (GO) is a two-dimensional sheet with epoxy and hydroxyl groups on the basal plane and carboxyl groups at the edge. In order to change the oil/water (o/w) interfacial activity, GO was controlled and modified by dodecylamine to get two kinds of functionalized GOs (fGOs), named as basal plane-functionalized GO (bGO) and edge-functionalized GO (eGO), respectively. The interfacial tension measurement showed that fGOs could reduce more interfacial tension at the poly-α-olefin/water interface than those at synthetic esters or aromatic compounds/water interfaces. Besides, eGO can reduce more poly-α-olefin-4/water interfacial tension compared to bGO. The interfacial dilatational rheology of eGO and fatty alcohol polyoxyethylene ether-4 (MOA4) showed that MOA4 gradually replaced eGO at the interface with the increase of MOA4, until the interface was completely occupied. eGO and MOA4 complex emulsion exhibited the best friction-reducing performance at 250 rpm. The coefficient of friction (COF) curves of the emulsions with eGO showed two platforms, with the COF reduced by 37.42% at the most. The rheological results of emulsions showed that the addition of eGO increased the elasticity of the emulsion. Emulsions showed shear-thinning and friction-thickening properties, which make it easier for the emulsion to form a lubricating film on the metal surface. Our research results suggested that the functionalization on the edge of GO will change the interfacial properties significantly, which have widespread applications in the encapsulation of active materials, surface protection, adsorption, and separation of pollutants.

Inhibition of cold‐welding and adhesive wear occurring on surface of the 6061 aluminum alloy by graphene oxide/polyethylene glycol composite water‐based lubricant

In this paper, graphene oxide/polyethylene glycol (GO/PEG) composite water‐based lubricant was prepared by an ultrasonic dispersion method, and characterized and analyzed by Fourier transform infrared (FT‐IR), Raman spectroscopy, X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The suspension performance of GO/PEG composite water‐based lubricant in water was verified by static sedimentation and centrifugation, and then, the prepared GO/PEG composite water‐based lubricant was added into 304 stainless steel and 6061 aluminum alloy, and the coefficient of friction (COF) curve, average COF value, average wear rate, corresponding photomicrographs of balls and disks after wear, and energy‐dispersive spectrometer (EDS) elemental analysis were used to illustrate the lubrication effect and lubrication mechanism. The results show that the GO/PEG composite water‐based lubricant possesses excellent suspension ability in water, and the average COF value and wear rate of GO/PEG composite water‐based lubricant are reduced by 78.8% and 88.8%, respectively, compared with water lubrication. The excellent lubrication effect of GO/PEG composite water‐based lubricant can effectively reduce the cold‐welding and adhesive wear phenomenon, mainly because GO/PEG composite water‐based lubricant first fills the uneven surface of friction mating to form a high‐quality lubricating film and then because of the special space structure of GO and the low shear between GO layers and the synergistic lubrication effect of GO/PEG.

Lubrication and Sensory Properties of Emulsion Systems and Effects of Droplet Size Distribution.

The functional and sensory properties of food emulsion are thought to be complicated and influenced by many factors, such as the emulsifier, oil/fat mass fraction, and size of oil/fat droplets. In addition, the perceived texture of food emulsion during oral processing is mainly dominated by its rheological and tribological responses. This study investigated the effect of droplet size distribution as well as the content of oil droplets on the lubrication and sensory properties of o/w emulsion systems. Friction curves for reconstituted milk samples (composition: skimmed milk and milk cream) and Casein sodium salt (hereinafter referred to as CSS) stabilized model emulsions (olive oil as oil phase) were obtained using a soft texture analyzer tribometer with a three ball-on-disc setup combined with a soft surfaces (PDMS) tribology system. Sensory discrimination was conducted by 22 participants using an intensity scoring method. Stribeck curve analyses showed that, for reconstituted milk samples with similar rheological properties, increasing the volume fraction of oil/fat droplets in the size range of 1–10 µm will significantly enhance lubrication, while for CSS-stabilized emulsions, the size effect of oil/fat droplets reduced to around 1 µm. Surprisingly, once the size of oil/fat droplets of both systems reached nano size (d90 = 0.3 µm), increasing the oil/fat content gave no further enhancement, and the friction coefficient showed no significant difference (p > 0.05). Results from sensory analysis show that consumers are capable of discriminating emulsions, which vary in oil/fat droplet size and in oil/fat content (p < 0.01). However, it appeared that the discrimination capability of the panelist was significantly reduced for emulsions containing nano-sized droplets.

The Friction and Wear Behaviours of Inconel 718 Superalloys at Elevated Temperature

Machine parts made of nickel-based alloys usually work in high-temperature service environments such as aircraft turbines. The mechanical properties and antioxidant properties of materials tend to be reduced at high temperatures. Therefore, it is of great practical significance to reveal the wear mechanisms of materials at different temperatures. In the present investigation, the tribological behaviour of an Inconel 718 superalloy at different temperatures was investigated. First, the coefficient of friction curves obtained under different test conditions were analysed in detail to illustrate the dynamic change process of friction at high temperature. Next, the morphology of the wear surface, surface morphology of friction pairs and material transfer during friction were analysed via scanning electron microscopy 3D morphology and energy dispersive spectroscopy measurements to reveal the wear mechanisms of materials in a high-temperature environment. Finally, the microstructure of the cross section of the wear tracks was analysed by using optical microscopy electron back-scattered diffraction etc., to clarify the mechanisms of crack initiation and material removal. The results show that the friction properties of the Inconel 718 superalloy have differences at different test temperatures. Although increasing the test temperature does not necessarily aggravate the wear of the material, the oxidation of the wear surface during the friction process significantly increases. In addition, when the contact load increases, the thickness of the oxide layer and wear of the material simultaneously increase.

Wear assessment of Cr2O3‐/TiAlN‐coated DAC‐10 tool steel against steel and Al2O3 counterbodies

AbstractTiAlN film was deposited on Cr2O3‐coated plasma‐nitrided DAC‐10 tool steel to obtained multilayer Cr2O3/TiAlN coating layer using cathodic arc deposition technique. The structural make‐up of the coating was characterized using Atomic Force Microscopy (AFM) and X‐ray diffraction methods, and the mechanical properties were evaluated using nanoindentation and nanoscratch test. The structural phases of the coating indicated the presence of crystalline CrO structure and cubic TiAlN phases. The coating showcased improved hardness (38 GPa), elastic modulus (387 GPa), and adhesion along with appreciable H/E (0.09) and H3/E2 (0.366 GPa) attributes. Further, friction‐induced wear behavior of the coating was investigated against steel and Al2O3 counterbodies under dry sliding conditions. The wear behavior of the coating was greatly influenced by its hardness and deformation properties and frictional behavior of the counterbodies. More spikes and fluctuation were observed in the frictional curve against Al2O3 counterbody attributed to the emanation of TiO2, Cr2O3, and Al2O3 compounds due to dry sliding leading to the formation of flakes and delamination induced debris. Against the steel counterbody, the coating mainly formed a typical smooth glossy surface ascribed to the formation of Fe2O3 compound on the worn surface.

An improved contact model considered the effect of boundary lubrication regime on piston ring-liner contact for the two-stroke marine engines from the perspective of the Stribeck curve

The prediction of lubrication performance is required to be the basement of friction optimization for marine engines. This paper simulates the lubrication performance of marine engines based on statistical models which have the advantages of fast, efficient, and macroscopic fault location. Boundary lubrication exists in the piston ring-cylinder liner (PRCL) of two-stroke marine engines because of the harsher load, lower speed, and larger structure. It has been proposed that there would be tribofilm under boundary lubrication which has a significant influence on the contact. To understand the boundary lubrication, it is necessary to study the lubrication regime transition. In this paper, firstly, the coefficient of friction curve combined with the thickness ratio embodies the lubrication regime transition process of two-stroke engines under work conditions. However, the phenomenon that the coefficients under boundary lubrication are smaller than that of other regimes shows the non-objectivity of this curve. Therefore, the Stribeck curve is introduced for objectively evaluating the transition. Then, the calculation of asperities contact pressure under boundary lubrication, which Wen proposed, is introduced into the classic Greenwood-Williamson model, the problem that the original model cannot reflect the boundary lubrication regime in the form of the Stribeck curve is improved. Finally, the results are compared before and after modifying the model to verify this study’s practicability. It provides more precise asperities contact pressure for the tribofilm growth calculation from the perspective of the Stribeck curve under the PRCL statistical model in future work.

Transfer layer evolution during friction in HIPIMS W–C coatings

The investigation of the evolution of transfer layer in the contact area in W–C coating/steel ball system during interrupted friction tests in humid air revealed that fast oxidation occurs simultaneously with water vapor dissociation and carbon hydrogenation. These mechanochemical reactions produced a mixture of hydrogenated carbon, ferritungstate with minor amounts of single tungsten (sub-)oxides and iron oxides forming transfer layer preferentially adhered to steel ball. The steady coefficient of friction (COF) was attributed to the development of a thin continuous tribo-film with higher hardness and stiffness in the central zone of the wear track facing the thickest zone of transfer layer on the ball. Despite direct experimental confirmation is necessary, tribo-film was proposed to consist of ordered rehydrogenated carbon film. Friction would be controlled by shear interactions between transfer layer and thin tribo-film on top surface of W–C coating. The effect of humidity on COF can be explained by a control of oxidation, dissociation and hydrogenation reactions and subsequent influence on the composition of transfer layer and tribo-film in the wear track. The existence of phases identified in transfer layers during friction were confirmed via modelling of parallel and mutually interacting mechanochemical reactions based on the minimization of free Gibbs energy. The ability to apply equilibrium thermodynamical calculations to dynamical processes in friction contact was related to very fast reactions among microscopic asperities eliminating kinetical factors necessary for thermodynamical equilibrium at macroscale. Based on the experimental observations and modelling, three stage model of transfer layer evolution was proposed to explain run-in and steady periods on friction curves.

Study of the structure and properties of cermets based on the NiAl–Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; system

The powder metallurgy method including mechanical activation of powders in a planetary mill and spark plasma sintering at 1470 °C in an inert atmosphere was used to obtain NiAl–45vol.%Al2O3 cermet samples with the addition of nanoparticles of magnesium aluminum spinel in an amount of 0.05 vol.%. The features of their microstructure were investigated. Spinel nanoparticles are located at the boundaries between the grains of composite components. The results of X-ray phase analysis at t = 25 and 800 °C were obtained. The main components of the material at t = 20 °C are α-Al2O3 and NiAl. The dependence of internal friction on temperature in the range of 20–900 °C was studied, and the influence of magnesium aluminum spinel nanoparticles on the nature of its change was established. The internal friction curve shows that vibration damping occurs up to 600 °C. Dependences of the ultimate bending strength of cermets at t = 20÷750 °C were determined. The positive effect of introducing a small amount of magnesium aluminum spinel on the elastic properties of composites was established. The best mechanical properties were demonstrated for NiAl–42vol.%Al2O3–0.05vol.%MgAl2O4 samples. On average, the ultimate bending strength of this material was 8–15 % higher compared to samples without nanoparticles. The materials obtained in this research had an ultimate bending strength under normal conditions of 460–490 MPa. A summarizing analysis of NiAl–Al2O3 cermet researches was carried out to determine the nature of the ultimate bending strength dependence on the ratio of components. It was found that it has an extreme nature: the maximum is observed when using the ratio of aluminum oxide to aluminum nickel equal to 0.5.

Effect of groove surface texture on the fretting wear of Ti–6Al–4V alloy

Groove surface textures with different orientations and spacings were prepared on the Ti–6Al–4V alloy by using a laser. Fretting tests (rubbed with a Ti–6Al–4V ball) were conducted under different textures, and the coefficient of friction (COF) curves, system deformation and wear morphologies were analysed in detail. The groove orientations to the fretting direction affected the fretting wear behaviour through the following ways: (1) Fretting tests on different groove orientations resulted in different fretting loop shapes at the initial stage of fretting wear. (2) The COF curves of fretting test on perpendicular grooves and 45° grooves were similar. COF presented a sharp increase after approximately 1000 cycles under relatively low loads. The COF curves of fretting test on parallel grooves and untextured surface were similar. Only a small oscillation of COF was observed during the whole fretting tests. (3) When fretting was run in gross slip regime, the magnitude of system deformation was perpendicular grooves, no-texture samples, 45° grooves, and parallel grooves in order. The above results may have resulted from the influence of surface textures on the formation and development of wear debris. For the perpendicular grooves, high pressure was recorded in partial slip regime cracks, and peeling appeared on the convex platform of samples with the groove spacing of 80 μm, and this phenomenon promoted the formation of debris. Moreover, the perpendicular grooves prevented debris from spilling out of contact areas. For the parallel grooves, the filling mechanism of grooves was found, in which the material in the upper layer gradually squeezed the material in the lower layer toward the centre and bottom of the groove, and the parallel grooves accelerated debris from spilling out of contact areas. Therefore, groove surface textures may adjust the fretting regime of titanium alloys by changing the orientation and spacing, which could provide a new tribology design for fretting friction pairs.

Generalised scaling law for soft contact tribology: Influence of load and asymmetric surface deformation

Finite Element Analysis is used to investigate surface deformation associated with traction and normal forces within soft contact tribology (SCT). Significant deviation from the Hertzian contact model occurs, including asymmetric deformation in soft substrates. A cavity is predicted in front of the contact that has a characteristic length scale significantly larger than the film thickness expected in lubricated SCT. This cavity may facilitate fluid entrainment and squeeze out dynamics in boundary-mixed lubrication regimes that would explain many anomalous results in SCT studies. Analysis of contact mechanics reveals scaling parameters for the boundary-mixed and EHL regimes in SCT, which are used to generalise Stribeck friction curves onto a “single” curve as a function of a modified Hersey number at varying loads.