Friction Evolution Research Articles

Transient evolution of permeability and friction in a slowly slipping fault activated by fluid pressurization

The mechanisms of permeability and friction evolution in a natural fault are investigated in situ. During three fluid injection experiments at different places in a fault zone, we measured simultaneously the fluid pressure, fault displacements and seismic activity. Changes in fault permeability and friction are then estimated concurrently. Results show that fault permeability increases up to 1.58 order of magnitude as a result of reducing effective normal stress and cumulative dilatant slip, and 19-to-60.8% of the enhancement occurs without seismic emissions. When modeling the fault displacement, we found that a rate-and-state friction and a permeability dependent on both slip and slip velocity together reasonably fit the fault-parallel and fault-normal displacements. This leads to the conclusion that the transient evolution of fault permeability and friction caused by a pressure perturbation exerts a potentially dominant control on fault stability during fluid flow.

Intermittent lab earthquakes in dynamically weakening fault gouge.

Large and destructive earthquakes on mature faults in Earth's crust occur as slip in a layer of a fine granular material-fault gouge-produced by comminution during sliding1,2. A range of insights into the frictional resistance of faults-one of the main factors controlling earthquake nucleation, dynamic propagation and arrest, and hence the destructive ground shaking of earthquakes2,3-has been obtained in experiments with spatially uniform slip imposed in small samples2,4-21. However, how various features of gouge friction combine to determine spontaneous progression of earthquakes is difficult to study in the lab owing to substantial challenges with sample sizes and adequate imaging22. Here, using lab experiments, we show that spontaneously propagating dynamic ruptures navigate a fault region with fine rock gouge through complex, intermittent slip processes with dramatic friction evolution. These include repeated arrest of rupture propagation caused by friction strengthening at lower slip rates and dynamic earthquake re-nucleation enabled by pronounced rapid friction weakening at higher slip rates consistent with flash heating8,12,23. The spontaneous repeated weakening and strengthening of friction in fine rock gouge highlights the fundamental dependence of friction on slip rate and associated processes, such as shear heating, localization and delocalization of shear, and dilation and compaction of the shear layer6,7,9-21. Our findings expand experimental support9,11 of the concept that co-seismic weakening may enable earthquake rupture to break through stable fault regions24,25, with substantial implications for seismic hazard.

Microscopic mechanisms of frictional aging

Frictional aging is observed at a wide range of length- and time-scales, and plays a crucial role in functioning of micro- and nanomachines, as well as in the nucleation and recurrence of earthquakes. Here, we developed an analytical model for description of frictional aging mediated by dynamical formation and rupture of microscopic interfacial contacts. The model accounts for the presence of various types of contacts at the frictional interface and exhibits three different aging regimes: (i) linear aging at short hold times, (ii) logarithmic (or logarithmic-like) aging for intermediate time scales and (iii) levelling off in the static friction for long hold times. It is demonstrated that the linear aging regime is a universal feature of frictional aging for the interfaces including various types of contacts, and the slope of variation of the static friction with the hold time depends on a distribution of energy barriers for contact formation. The conditions for the existence of a pronounced logarithmic aging regime, covering a long-time interval, have been established. Frictional aging has been found to manifest itself not only in slide-hold-slide measurements, but also in sliding experiments exhibiting stick-slip mode of motion, and a relationship has been established between these two regimes of aging. The predicted dependencies of frictional aging on the normal load and temperature are in good agreement with the experimental observations. Our work shows that experimental studies of load and temperature dependencies of aging, carried out over a wide range of time scales, offer promising opportunities for understanding the microscopic mechanisms of frictional aging and revealing the physical meaning of state variables that determine temporal evolution of friction described by phenomenological rate and state laws.

Contact size effects on the friction and wear of amorphous carbon films

Since different properties of coating systems influence their friction and wear at different length scales contact size can play a critical role in microtribological experiments. In this study the behaviour of 3 different types of coating system which vary in terms of their thickness, substrate and mechanical properties has been investigated. The coatings were chosen for either their industrial relevance in automotive or MEMS applications, or as model coating systems. A wide range of nano/microtribological tests have been performed with different indenter geometries (tip sharpness), including single and repetitive scratch tests with unidirectional contact, and reciprocating wear tests, with depth and friction evolution monitored so that the relationships between failure mechanism and friction in coating systems with differing mechanical properties could be explored. The influence of surface topography on friction has been shown in ramped and constant load scratch tests. When fracture occurred resulting in a sudden increase in probe depth there was an abrupt decrease in friction which is ascribed to a contact area effect. In contrast, where deformation progressed through micro-wear a more gradual increase in depth can be associated with higher contact area and higher friction.

Effect of Friction Modelling on Damage Prediction in Deep Drawing Simulations of Rotationally Symmetric Cups

The damage state in a formed component has a significant influence on the performance of the component in service. Controlling damage evolution during forming through specific modifications of the process parameters will therefore allow an improvement of this performance. The evolution of the stress-strain state during the forming process is the primary influencing factor of the resulting damage state. The stress-strain state is influenced by the friction between tools and workpiece. To investigate the cause-effect relationship between friction and damage evolution in the deep drawing process, Finite Element simulations of the deep drawing of rotationally symmetric cups were performed. Punch velocity and blank holder force were varied. Damage was predicted using a Lemaitre damage model. The damage states predicted using a Coulomb friction law and a model incorporating a dependence on contact normal stress and relative velocity were compared. The parameter-dependent friction model predicted a change in the damage distribution after forming when varying the process parameters, which was not found using the Coulomb friction law.

Relationship between corrosion and nanoscale friction on a metallic glass.

Metallic glasses are promising materials for microdevices, although corrosion and friction limit their effectiveness and durability. We investigated nanoscale friction on a metallic glass in corrosive solutions after different periods of immersion time using atomic force microscopy to elucidate the influence of corrosion on nanoscale friction. The evolution of friction upon repeated scanning cycles on the corroded surfaces reveals a bilayer surface oxide film, of which the outer layer is removed by the scanning tip. The measurement of friction and adhesion allows one to compare the physicochemical processes of surface dissolution at the interface of the two layers. The findings contribute to the understanding of mechanical contacts with metallic glasses under corrosive conditions by exploring the interrelation of microscopic corrosion mechanisms and nanoscale friction.

Microstructure evolution and hardness distribution of linear friction welded AA5052-H34 joint and AA5083-O joint

The microstructure evolution of the AA5052-H34 joint and AA5083-O joint fabricated by linear friction welding (LFW) was evaluated in a wide range of interfacial temperatures, i.e., applied pressures. The grain refinement of Al alloys with high stacking fault energy was found to be mainly dominated by discontinuous dynamic recrystallization (DDRX) at low temperatures. While, it was clear that grain refinement at high temperatures was dominated by continuous dynamic recrystallization (CDRX). In the joint of work hardened AA5052-H34 fabricated at low applied pressure of 50 MPa (above the 0.5 Tm), the annealing effect was more significant than the grain-boundary strengthening effect at the WC, leading to the softening. However, the increased applied pressure to 100 MPa resulted in the flat hardness distribution due to the increased strengthening by grain refinement. On the other hand, in the joint of annealed AA5083-O, the flat hardness was obtained at the lower pressure of 50 MPa, because both the low angle grain boundary length per unit area and dislocation density at the WC was almost equivalent to that of base material.

Interfacial microstructure evolution and mechanical properties of inertia friction welded aluminium alloy/stainless steel joint with preheat treatment

This paper reveals the evolution of interfacial microstructure and its effect on the mechanical properties of Al alloy/stainless steel joint made by inertia friction welding with different surface roughness (as-machined and polished) followed by preheat treatment. At the central interface region, an unbonded area is witnessed for the Al alloy/as-machined steel (AA-MS) joint, while an intimate contact is realised for the Al alloy/polished steel (AA-PS) joint. The 1/2 radius (1/2R) region is characterised by the formation of the Fe–Al reaction layer, and no obvious layer exists at the peripheral region. Further microstructural characterisation indicates the reaction layer consists of a partially nanoscale crystallised layer of Fe2Al5 and a nanoscale amorphous layer in the as-welded joint induced by the severe shear effect. Preheat treatment promotes the growth of the crystallised layer and suppresses the amorphous layer due to the enhanced atom mobility. Preheat treatment improves the local interfacial bonding strength by reducing the unbounded area while maintaining a high bonding quality at the 1/2R region in the AA-MS joint. However, the enlarged brittle peripheral region is responsible for a slightly lowered joint strength in the preheated AA-PS joint.

Evolution of static and kinetic friction in a horizontal well using an adaptive model-based observer: Field validation

Evolution of static and kinetic friction coefficients across the depth of a horizontal wellbore and field validation against the downhole parameters is presented in this paper. It has been observed in the literature that stick–slip oscillations exist in drill strings even with bit off-bottom and persist with the bit on bottom. In the past, various lumped mass models have failed to explain this phenomenon, as they have only considered bit–rock interaction to be the main reason for the stick–slip oscillations. The model considered in this study was used previously to estimate the friction factors at particular connections with bit off-bottom. For effective estimation of the downhole parameters, the model employs an adaptive soft sensor, robust to capture the disturbances occurring at the downhole. Using only the topside measurements (i.e., surface measurements), downhole RPM is estimated along with the evolution of friction coefficients (static (μs) and kinetic (μk)) using the soft sensor. For obtaining the friction factors profile, those values of μs and μk that converge and remain stable for a specified time are considered. The model is first validated against the downhole RPM data obtained for a well in North America. It was observed that the estimates of the downhole RPM obtained using the model are in close agreement with that of the actual data. Next, the model was employed to obtain reasonable downhole RPM estimates for another well in North America where the downhole data is unavailable.

Experimental and modelling studies of the transient tribological behaviour of a two-phase lubricant under complex loading conditions

The transient tribological phenomenon and premature lubricant breakdown have been widely observed in metal forming, leading to excessive friction at the contact interfaces. In this research, the transient tribological behaviour of a two-phase lubricant were studied under complex loading conditions, featuring abrupt interfacial temperature, contact load, and sliding speed changes, thus representing the severe interfacial conditions observed in warm/hot metal forming applications. The strong experimental evidence indicates that the evolution of friction was attributed to the physical diminution and chemical decomposition effects. As such, a visco-mechanochemical interactive friction model was developed to accurately predict the transient tribological behaviour of the two-phase lubricant under complex loading conditions. The new friction model exhibited close agreements between the modelling and experimental results.

High Performance Accelerated Tests to Evaluate Hard Cr Replacements for Hydraulic Cylinders

To prolong the lifetime of hydraulic cylinders, a wear-resistant low-friction surface is required. Until now, hard Cr coatings were the best materials for this. However, in recent years, there has been an increasing pressure on the manufacturing of hard Cr plating and plated products, because of environmental and health hazards. The replacement of these coatings by alternatives has not been highly successful yet, because it requires extensive component testing, which is costly and time-consuming and thus not appropriate for material development. For this reason, there is a high need to develop tribological methods that simulate hydraulic cylinders’ component-testing closely. In addition, these new methods should also provide additional information (e.g., friction evolution) that can assist in the further development and optimization of alternative coatings. Having the above in mind and building on an existing method from the American Society for Testing and Materials (ASTM G133), a new test method that allows users to test directly on hydraulic cylinders was developed. This method can provide a relative ranking of both the wear resistance and frictional performance of alternative coatings in direct comparison to state-of-the-art hard Cr. Importantly, the method is repeatable and has a much shorter test duration than full-scale component tests, thereby accelerating material development significantly.

Defect formation, microstructure evolution, and mechanical properties of bobbin tool friction–stir welded 2219-T8 alloy

Bobbin tool friction stir welding (BT–FSW) is a novel and effective welding method for aluminum alloys, but microvoid defects are usually formed in the nugget zones (NZ). Therefore, it is important to understand the formation mechanism of microvoid defects and their effect on the fracture behavior of joints. In the present study, 6 mm thick 2219–T8 aluminum alloy plates were subjected to BT–FSW under a constant rotation rate of 300 rpm and welding speeds of 150–500 mm min–1 with the objective of analyzing the evolution of microvoid defects and their effect. The results showed that, lower welding speeds resulted in microvoid defects in the band pattern structure on the retreating side (RS) of the NZ, and the size of the defects decreased with increasing the welding speed, while sound joints were obtained under higher welding speeds. In contrast to conventional FSW precipitation–hardened aluminum alloys with the lowest hardness zone (LHZ) at the heat affected zone (HAZ), the LHZs of the BT–FSW 2219–T8 joints were located at the thermal–mechanically affected zone (TMAZ). Digital image correlation (DIC) of large tensile samples showed that the microvoid defects exhibited an obvious effect on the tensile deformation and fracture behavior of the joints due to the decrease in the maximum strain and lack of necking. Mini-samples in different layers further confirmed that cracks initiated from the microvoid defects in the NZ. However, the sound BT–FSW 2219–T8 joints fractured along the LHZs on the RS, with a maximum joint strength coefficient of 78.1% being achieved.

On the constitutive relationship between solidification cells and the fatigue behaviour of IN718 fabricated by laser powder bed fusion

IN718 combines excellent mechanical properties with a good weldability and is therefore an ideal alloy for laser powder bed fusion (LPBF). Knowledge of the relationship between its as-built microstructure, particularly solidification cells, and its fatigue properties is needed to better utilise additively manufactured microstructures and guide their further optimisation. This study presents a comprehensive investigation of the as-built microstructure and the associated monotonic and fatigue properties of LPBF IN718 aimed at highlighting the influential effect of solidification cells on monotonic and cyclic plasticity. In monotonic tension, cells induced pronounced strain hardening and good ductility by acting as strong yet not impenetrable obstacles to dislocation slip. In fatigue loading, cyclic hardening followed by cyclic softening was linked to the stability of the as-built solidification cells, the high initial dislocation densities and the subsequent rearrangements of such dislocations during cyclic loading using the similitude relation and the evolution of friction and back stresses. By thoroughly investigating the evolution of the cyclic response of samples printed using two different scanning patterns, the relationship between process (scanning line length and thus local substrate temperature), microstructure (dislocation cell size and their spatial arrangement) and mechanical properties (cyclic hardening and softening responses) was comprehensively discussed.

The evolution of rock friction is more sensitive to slip than elapsed time, even at near-zero slip rates

The evolution of rock friction is more sensitive to slip than elapsed time, even at near-zero slip rates

Investigating load-dependent wear behavior and degradation mechanisms in Cr3C2–NiCr coatings deposited by HVAF and HVOF

Wear resistant coatings that comply with non-toxic environment goals are highly desirable. Cr3C2–NiCr is a promising alternative to the toxic, ‘Co’- containing WC–Co coatings to mitigate wear. The purpose of this study was to examine the suitability of Cr3C2–NiCr coatings for automotive brake disc application by systematically investigating their dry sliding wear behavior at different test conditions. Therefore, High Velocity Air Fuel (HVAF) and High Velocity Oxy Fuel (HVOF) were employed to deposit Cr3C2–NiCr coatings. The powder feedstock and as-deposited Cr3C2–NiCr coatings were characterized for their microstructure and phase composition using SEM and XRD. Mechanical properties (hardness, fracture toughness), porosity and surface topography of the as-deposited coatings were evaluated. The coatings were subjected to sliding wear tests at different normal loads (5 N, 10 N and 15 N) using alumina ball as the counter surface. Coefficient of friction (CoF) evolution of HVAF and HVOF deposited coatings, along with their wear performance, was obtained for different normal load conditions. The wear performance ranking of HVAF and HVOF processed coatings was influenced by the test conditions, with HVAF coatings demonstrating better wear resistance than HVOF coatings at harsh test conditions and the HVOF coatings performing better under mild wear test conditions. Detailed post-wear analysis of worn coatings, the alumina ball counter-body and the resulting debris was performed to reveal the degradation mechanisms at different test conditions. Findings from this work provide new insights into the desirable microstructural features to mitigate wear, which can be further exploited to deposit wear-resistant coatings.

Microstructure Evolution, Mechanical, and Corrosion Behavior of Cryogenic Friction Stir‐Processed AA2014 Alloy

Herein, a large‐area stir zone (SZ) is fabricated in AA2014 alloy via overlapping friction stir processing (FSP). The microstructure and texture evolution of AA2014 alloy during FSP is examined using electron backscattered diffraction (EBSD). EBSD maps reveal recrystallized fine grains with high‐angle grain boundaries in SZ due to dynamic recovery (DRV) and continuous dynamic recrystallization (C‐DRX). Severe plastic straining induced by the tool leads to the formation of the A‐(110) <111> component. The texture component is further investigated with X‐ray analysis, and the results are reported. Mechanical testing reveals that ductility significantly increases after FSP, whereas the hardness and strength remain lower than that of base metal (BM) due to thermal softening. Corrosion studies show that the corrosion attack is significantly controlled after FSP due to the development of a homogeneous fine‐grained structure and uniformly distributed fine precipitates.

Fractal variation of three-dimensional surface topography during sliding wear under mixed elastohydrodynamic lubrication

The surface morphology variation of the friction pair during the wear process directly affects the local lubrication effect and contact strength, and this variation is difficult to quantify in mixed lubrication. In this study, a numerical simulation and characterization model method of sliding wear surface morphology evolution based on deterministic elastohydrodynamic lubrication (EHL) model and surface three-dimensional morphology fractal method is established. The effectiveness of the lubrication model is verified by the film thickness measurement, and the actual surface wear coefficient is obtained through the wear experiment. On this basis, the variation of surface morphology and fractal parameters in the sliding wear process is investigated, and the effect of contact load and sliding speed on surface lubrication characteristics and fractal parameters is analyzed. The results show: wear will cause the higher wave peaks on the ground surface to be gradually worn away and the fine structure of the surface is reduced, and the fractal dimension of the morphology is therefore reduced. In addition, the contact area ratio and contact load ratio, as well as the coefficient of friction and surface evolution under different loads and sliding speeds are consistent.

Effect of humidity on friction, wear, and plastic deformation during nanoscratch of soda lime silica glass

AbstractPhysical flaws and defects on glass surfaces are known to reduce the mechanical strength and chemical durability of glass. The formation of surface defects depends not only on the mechanical conditions of the physical contact but also on the environment in which the contact is made. In this study, the nanoscratch behavior of soda lime silica (SLS) glass was investigated in 10% and 60% relative humidity (RH) conditions. Based on the evolution of friction and scratch depth, the deformation of SLS glass surface could be divided into four regimes: elastic deformation and recovery (E), RH‐independent mild plastic deformation (P‐1), RH‐dependent intermediate plastic deformation (P‐2), and RH‐independent severe plastic formation (P‐3). It is quite surprising to observe that plastic deformation of the glass surface has dependence on RH of the environment (outside the glass) because plastic deformation is the process occurring below the surface (inside the glass) by the externally applied load. From this result, it can be inferred that frictional energy dissipation mode at the sliding interface, which is a function of adsorbed water molecules, influences the subsurface deformation mode. Although friction, wear, and subsurface deformation/damage are all coupled, there is no direct one‐on‐one correlation among them.

Frictional Behavior of the Stressed Basalt Fracture

The frictional properties and sliding stability of rocks are critically important in several fields, e.g., earthquake dynamics, extraction of underground resources, and sequestration of carbon dioxide. Here a self-developed triaxial apparatus was employed to conduct sliding tests under the coupled hydro-mechanical (HM) conditions. We determined the evolution of frictional strength in the stressed basalt fracture. The relations were established through velocity-stepping (VS) experiments on split basalt core subject to various stress, infill, and hydraulic conditions. The basalt exhibited velocity-strengthening behavior under cyclic velocities of 1–30 μm/s. Moreover, the frictional property was observed to be stress- and infill-dependent, showing lower frictions at both larger effective normal stresses and larger infill thickness. Besides, the sliding stability of the basalt fracture is found to be velocity- and stress-dependent. In particular, an unstable slip of a stick-slip was observed at a small velocity of 1 μm/s, but stabilizes with increasing shearing velocities at about 6 μm/s. Meanwhile, there is a transition from stable slip to unstable slip with the increasing effective normal stress, which is 5 MPa. Whereas, the thickness of the gouge as large as 2 mm does not seem to influence the sliding stability, even though the gouge thickness will significantly weaken the frictional strength of basalt fracture. The results suggested that the evolution of basalt fracture friction is controlled considerably by hydro-mechanical conditions, the sliding velocity, and gouge thickness. The frictional strength tends to vary with specified hydraulic, mechanical, gouge-bearing conditions. Furthermore, the frictional behavior transits between stable- and unstable-slip based on the velocities, stress pore pressure, and gouge thickness. These conclusions are drawn from constrained conditions.

Evolution of sub-surface microstructure under linear reciprocating wear of nanostructured bainitic steel

Nanostructured bainitic steels have high strength as well as a combination of significant ductility, fracture toughness and sub-critical fatigue crack growth life. Hence, they can be potentially useful in structures suffering rolling and/or sliding fatigue e.g. rails, mining components, bearings and shafts. However, such applications require high wear resistance which is not solely dependent on the initial strength since repetitive sliding often leads to sub-surface microstructural evolution that alters the tribological response with time. In the current study, three nanostructured bainitic blocks with lath thickness of 45, 54 and 77 nm have been made from steel of composition: Fe-0.86C-1.45Si-1.8Mn-0.47Al-1.88Co-0.23Mo (wt%) by austempering at 250, 300 and 350 °C respectively. We have investigated the friction and microstructural evolution under reciprocating sliding motion of a 10 mm spherical tungsten carbide indenter at 10 kN load and correlated the findings with the initial morphology of bainite. All specimens exhibited hardening after multiple cycles of deformation. However, the steel austempered at the lowest temperature showed the highest sub-surface hardness and lowest friction coefficient and was the most resistant to wear when tested under sliding fatigue for up to 500 cycles. Repeated sliding resulted in formation of two kinds of layers: white etching layer (WEL) and severely deformed layer (SDL). WEL exhibited very fine microstructure whereas the SDL showed alignment of retained austenite and bainitic laths along the direction of highest shear stress. There was a predominant presence of WEL in the specimens austempered at the lowest temperature and SDL in the specimens transformed at the highest temperature. Atomic probe tomography (APT) was used to investigate the distribution of carbon in the deformed layers just below the wear track after 500 sliding cycles for 250 and 350 °C austempered condition. No significant change in carbon has been detected in any of the austempered specimens after wear.