Initial Friction Coefficient Research Articles

Synergistic reinforcement mechanisms of Cu/graphite composites with MAX phase Ti2AlN and ZrO2-B4C ceramics: Enhancements in mechanical, thermal, and tribological properties

In braking materials field, MAX phase has been introduced into Cu-based composite materials to address issues such as inadequate wetting between ceramic particles and the metal matrix. This study focuses on preparing Ti2AlN-ZrO2-B4C reinforced Cu-based materials via the powder metallurgy method, and investigates the effects and mechanisms of different Ti2AlN/ZrO2 ratios on the mechanical, thermal, and tribological properties of these materials. The addition of an appropriate amount of Ti2AlN (3 wt%) can enhance the densification and mechanical properties of Cu-based materials, as the diffusion of Al atoms promotes the sintering process. Similarly, the diffusion of Al also improves the thermal conductivity at grain boundaries, resulting in samples with higher Ti2AlN content exhibiting superior thermal diffusivity. When the Ti2AlN content reached 7 wt%, the material exhibited optimal hardness (43.35 ± 1.05 HBW), moderate shear strength (19.84 ± 1.86 MPa), as well as excellent high initial braking speed friction coefficient and stability. The reinforcement primarily arises from the diffusion bonding between Ti2AlN and Cu, leading to the formation of Cu-Ti compounds at the interface, which facilitates Ti2AlN in providing frictional force during the friction process and promotes the formation of a friction film due to the “pinning effect”. Additionally, the layered structure of Ti2AlN and the subsequent formation of amorphous Al2O3 after oxidation contributed to the enhancement of friction stability.

Analysing crack evolution in wind turbine bearings using an oscillation-unified model

In wind turbine bearings subjected to oscillating motion, cracks undergoing reciprocating rolling contact loading experience more complex stress changes compared to those under constant rotational motion. However, oscillation is less explored. This study introduces an Oscillation Unified Surface-Initiated Crack Finite Element Model (OU-CFEM) to address this gap. The model is validated through mesh sensitivity analysis and Oscillation Contact Fatigue (OCF) experimental data. The OU-CFEM enables a detailed investigation into crack evolution characteristics under varying initial angles and friction coefficients. The findings reveal that crack behaviors significantly differ across different oscillation phases, except in cases involving vertically oriented cracks or fully lubricated conditions. The study highlights that larger initial angles and higher friction coefficients promote crack extension, with the initial angle playing a more critical role in crack evolution than the friction coefficient. This research provides new insights into the behavior of cracks in oscillating conditions, offering valuable contributions to the understanding of wind turbine bearing durability under such loading scenarios.

Effect of lateral stress on frictional properties of a fracture in sandstone

The injection of large volumes of natural gas into geological formations, as is required for underground gas storage, leads to alterations in the effective stress exerted on adjacent faults. This increases the potential for their reactivation and subsequent earthquake triggering. Most measurements of the frictional properties of rock fractures have been conducted under normal and shear stresses. However, faults in gas storage facilities exist within a true three-dimensional (3D) stress state. A double-direct shear experiment on rock fractures under both lateral and normal stresses was conducted using a true triaxial loading system. It was observed that the friction coefficient increases with increasing lateral stress, but decreases with increasing normal stress. The impact of lateral and normal stresses on the response is primarily mediated through their influence on the initial friction coefficient. This allows for an empirical modification of the rate-state friction model that considers the influence of lateral and normal stresses. The impact of lateral and normal stresses on observed friction coefficients is related to the propensity for the production of wear products on the fracture surfaces. Lateral stresses enhance the shear strength of rock (e.g. Mogi criterion). This reduces asperity breakage and the generation of wear products, and consequently augments the friction coefficient of the surface. Conversely, increased normal stresses inhibit dilatancy on the fracture surface, increasing the breakage of asperities and the concomitant production of wear products that promote rolling deformation. This ultimately reduces the friction coefficient.

Electrodeposited MoxSyOz/Ni Tribological Coatings.

Deposition of molybdenum disulfide (MoS2) coatings using physical vapor deposition (PVD) and mechanical burnishing has been widely assessed for solid lubricants in space applications but still suffers from line-of-sight constraints on complex geometries. Here, we highlight one of the first demonstrations of electrodeposited MoxSyOz and MoxSyOz/Ni thin-film coatings from aqueous solutions of ammonium tetrathiomolybdate for solid lubricant applications and their remarkable ability to provide low coefficients of friction and high wear resistance. Characterization of the coating morphology shows amorphous microstructures with a high oxygen content and cracking upon drying. Even so, electrodeposited MoxSyOz can achieve low steady-state coefficients of friction (μ ∼ 0.05-0.06) and wear rates (2.6 × 10-7 mm3/(N m)) approaching those of physical vapor deposited coatings (2.3 × 10-7 mm3/(N m)). Additionally, we show that adding dopants such as nickel increased the wear rate (7.5 × 10-7 mm3/(N m)) and initial coefficient of friction (μi = 0.23) due to compositional modifications such as dramatic sub-stoichiometry (S/Mo ∼ 1) and expression of a NiOx surface layer, although doping did reduce the degree of cracking upon drying.

High-temperature dry sliding wear behavior of additively manufactured austenitic stainless steel (316L)

Austenitic steels are commonly used in aero-engine components owing to their higher thermal resistance to wear and corrosion. This study investigates the high-temperature dry sliding wear behavior of direct metal laser-sintered (DMLS) austenitic stainless steel (SS 316L) up to 400°C. A dry sliding reciprocating wear test conducted on a high-frequency tribometer with a ball-on-flat setup. High-hardness chrome steel (900 HV0.5) and alumina (1165 HV0.5) balls of 6 mm diameter were used as the steel and ceramic counterbodies against 316L specimen (320 ± 5 HV0.5). When tested against the chrome steel, the initial average coefficient of friction (CoF) decreased to a lower value (0.45) during the initial running-in period. However, with rising temperatures, the CoF also increased, stabilizing at 0.6 under steady-state conditions. A similar CoF pattern was observed in experiments with the alumina counterbody. Wear resistance declined at 100°C, followed by an increase at 200°C due to the development of an oxide glaze on the surface. Subsequently, wear resistance decreased as the temperature continued to rise. Up to 200°C , the wear mechanism was characterized by a combination of abrasive and oxide debris wear, transitioning to adhesive wear with plastic deformation at higher temperatures (400°C). Diffraction (XRD) analysis revealed the formation of oxide layers and martensite phase at higher temperatures, contributing to the observed improved wear resistance around 200°C . However, these oxide layers were progressively removed from the surface at 400°C, leading to an increased wear. The measured wear rates are two times lower, up to 200°C, compared to the reported values for conventional 316L due to the fine microstructure and higher hardness of DMLS SS 316L. This results in potentially rendering it suitable for applications where high-temperature (up to 200°C) wear-resistant SS is needed. This study provides further insights into the importance of post-treatment of DMLS parts for temperature beyond 400°C.

Preventing insufficient braking force in wading conditions by strategic use of surface texture on brake discs

Surface texture technology has been widely used in many industrial fields due to its outstanding antifriction and wear-resistance properties. In this study, groove textures with different quantities and inclined angles are prepared on the surface of a brake disc for high-speed railways. The effects of these textures on the braking capacity and wear resistance of the brake disc under dry and wading conditions are investigated. Results reveal that groove textures significantly reduce the damage caused by abrasive wear on the brake disc surface. The reason is that the grooves can capture and collect debris, which effectively mitigates wear. Under dry conditions, the effect of groove texture on the average friction coefficient is found to be negligible. However, under a small number of dripping conditions, the friction coefficient of the nontextured group drops to 0.04. By contrast, the groove textured groups maintain the lowest friction coefficient between 0.15 and 0.35. This excellent drainage performance of the groove textures prevents the formation of a continuous and whole water film, which ensures effective braking. Under a large number of flushing conditions, the groove textured group requires a shorter number of cycles to recover the initial friction coefficient than the nontextured group. Therefore, groove textures effectively overcome the problem of insufficient braking force during wading. In addition, no linear relationship exists between the quantity and inclined angle of grooves and their ability to suppress braking force deficiency. The groove texture is more conducive to improving braking capacity when the number of grooves is 6 and the direction is aligned with the friction direction.

Triaxial experimental study of mechanical behavior of non-persistent jointed granite subjected to slight cyclic dynamic disturbances

A slight disturbance can induce fault-slip burst failure in deep rock masses. In this paper, a series of triaxial tests were conducted to investigate the strength, deformation characteristics, and failure behavior of non-persistent jointed granite specimens subjected to slight cyclic loading. A representative stress–strain curve for non-persistent jointed rock was generated and used to study the crack evolution process (initiation → propagation → coalescence). The effects of varying the joint angle and frequency and amplitude of the disturbance were then analyzed. A theoretical (fracture mechanics) model for a non-persistent joint subject to triaxial cyclic loading was also established and used to investigate the combined effects of static confining stress and slight disturbances. Finally, the mechanical mechanism by which a slight dynamic disturbance induces fault-slip burst was discussed. Our results show that the failure mode of a non-persistent jointed rock mass depends more on the joint angle than the amplitude and frequency of the slight disturbance. The disturbance induces the initiation and extension of cracks. It can also induce fracture-slip bursting once the disturbance exceeds a certain threshold. The value of this threshold depends on the initial stress state, joint angle, length, dynamic and static friction coefficients, and fracture toughness of the rock. The study thus enriches our mechanistic understanding of the crucial ingredients involved in the triggering of fault-slip rock bursts in deep rock masses by slight disturbances.

Scaling behavior of granular columns collapse on erodible-inclined surfaces

This paper explores the effects of slope angle, initial column aspect ratio, and interparticle friction coefficient on the mobility and erodibility of granular columns collapsing on an erodible-inclined layer before spreading and depositing on a nonerodable-horizontal surface using Discrete Element Method. The numerical results showed that the column collapse with higher aspect ratio or smaller friction coefficient induces larger normalized kinetic energy, leading to larger granular masses out the inclined region and longer runout distance. The slope angle, on the other hand, shows trivial impacts on the kinetic energy and apparent friction coefficient. Remarkably, these physical quantities are nontrivially behaved by a new dimensionless number that incorporates the initial column aspect ratio, inclination angle, and interparticle friction coefficient. These findings may significantly complement the better understanding of the behavior of natural disaster events such as landslides, rock and snow avalanches, or pyroclastic flows on complex surfaces.

Research on wear rate of train brake pads driven by small sample data

Increased wear leads to the failure of brake pads. Both the International Union of Railways (UIC) and the China Railway Production Certification (CRCC) take wear rate as an extremely important parameter that characterizes the degree of wear, and its value can provide reference for the braking performance of the brake pads. Since the braking performance test of brake pads is time-consuming and costly, the sample data of wear rate is small. In this study, we explore the coupling relationship between the wear rate of train brake pads and its features such as initial braking speed (IBS), braking pressure (FB), braking temperature and average coefficient of friction (ACOF), and propose a Butterfly Optimization Algorithm-Back Propagation (BOA-BP) method suitable for small sample data for the wear rate prediction of brake pads. The grey relational analysis (GRA) and Pearson correlation analysis (PCA) are used to arrive at preferred features for the prediction of wear rate of brake pads, such as IBS, FB, braking distance and ACOF. The dynamic GM(1, N) and the least square method are used for sample expansion. Furthermore, the wear rate is designated as the output feature parameter and the preferred features are designated as the input feature parameters, a BOA-BP model is established for predicting the wear rate of brake pads. The results show that the wear rate obtained by dynamic GM(1, N) is consistent with the actual braking test, the maximum temperature and the braking pressure are important factors affecting wear rate. Compared to the comparison methods such as PSO-BP, GA-BP, and BP, the BOA-BP exhibits better advantages in prediction with small samples, and the average prediction accuracy with 33 sets of data and 99 sets of data are 95.70 % and 97.21 % respectively.

The Effect of Temperature and Load on the Current Carrying and Erosion Behavior of Metal-Impregnated Pantograph Strip

As an important current-collecting component of the electric locomotive, the pantograph strip may increase its surface temperature due to catenary vibration, friction heat generation, arc burning, catenary hard spots, and other reasons during actual operation. Given the action of electric current, an electrochemical reaction is unavoidable on the contact surface. Under low voltage DC conditions, the influence of temperature and load on relevant electrical and tribological parameters of the pantograph strip was studied using self-developed multi-functional friction and wear tester and changing forward load, surface temperature, etc. The electrochemical reaction was performed at 3.5 wt% NaCl. The results showed that with the increase in temperature, the initial contact resistance, friction coefficient, wear, and corrosion resistance of the surface material of the sample decreased, and the wear amount increased significantly.

Low-volume-loss surface polishing with a krypton fluoride excimer laser for polycrystalline diamond films

Surface polishing of hard diamond coatings is a key technique for applying such coatings in industry. In this study, we demonstrated surface modification of polycrystalline diamond films with minimal volume loss of the films by vertical irradiation of KrF excimer laser beams. Optimized laser scanning selectively removed surface asperities and reduced the surface roughness from ∼0.1 to ∼0.05 μm. Raman spectroscopic measurements revealed that laser polishing involves the phase transformation of diamond to amorphous carbon phases and thermal oxidation or evaporation of the amorphous phases. The residual amorphous carbon was almost completely removed by subsequent laser irradiation at appropriate fluences. We consider that the surface texture inherent to polycrystalline diamond films plays an important role in the concentration of laser beams on the nearby film surface. From reciprocating slide-type friction tests, we found that laser polishing decreased the initial friction coefficient from 0.5 to 0.1–0.3 against alumina balls and reduced the abrasion of the mating materials.

A new TiAlSiN coated tool and its machining performance evaluation for milling difficult-to-machining materials under MQL conditions

In this study, the Si and Al contents in a coating composition were improved, and a new TiAlSiN-coated tool was fabricated using high-power pulsed magnetron sputtering (HiPIMS). For comparison purposes, four types of coated tools, typically used in the market, were selected to mill 304 stainless steel in a minimum quantity lubrication environment. To evaluate the machining performance of the tools, the criteria of tool wear, workpiece surface roughness, and milling force were considered. The machining performance of the TiAlSiN-coated tools remained stable for all four working conditions, and it had the smallest compound milling force and tool wear in the experiment. The primary wear forms of the tools were coating peeling, oxidation wear, and adhesive wear. Under working conditions 1 and 2, due to the lower initial surface friction coefficient and thermal barrier effect, the workpiece surface roughness of the TiN-coated tools was the smallest, followed by that of the TiAlSiN. However, under inadequate working conditions, because of the good wear resistance of the TiAlSiN coating, it was not rapidly worn. This resulted in a lower average friction coefficient during processing and the workpiece surface exhibited the minimum surface roughness. A comparative analysis of the processing state of the five coatings under four groups of parameters indicated that the TiAlSiN coating had good processing stability, wear resistance, and a low surface friction value. The TiAlSiN-coated tools had better machining performance than the other four commercial-coated tools for 304 stainless steel machining, especially under working conditions 3 and 4.

Impediment effect of two-dimensional graphene on gaseous fluorine permeation during direct fluorination of polyurethane

Direct fluorination with gaseous fluorine (F2) is a simple way to prepare fluorine-containing polymers, however, partial hydrolysis of CONH- group during fluorination process often reduces the bulk properties of fluorinated polyurethane (PU). Here, two-dimensional graphene (GE) as barrier layer was introduced into the thermoplastic polyurethane (TPU) matrix followed by fluorination treatment by F2 gas. The conjugated π structure of graphene limited excessive fluoridation of the GE/TPU sample, while the large aspect ratio restrained the diffusion and penetration of F2 gas in the bulk. As a result, rich F elements were accumulated on the surface with the slightly-fluorinated bulk. The fluorinated GE/TPU sample integrated the excellent mechanical properties of TPU with friction behaviors of fluorine-containing polymers. The initial friction coefficient was 0.5, 50% lower than the original GE/TPU while maintaining the unchanged mechanical modulus and thermal stability. This research may effectively solve the issue on performance degradation caused by partial hydrolysis of polymer materials containing CONH- group during fluorination process, providing a simple and effective surface modification strategy for preparing high-performance fluorine-containing TPU.

Osteochondral junction leakage and cartilage joint lubrication

Background and ObjectivesPrevious studies have shown that there is potentially interstitial fluid exchange between cartilage tissue and the subarticular spongiosa region in the case of injury or disease (e.g., osteoarthritis and osteoporosis). Interstitial flow is also required for cartilage lubrication under joint load. A key question then is how cartilage lubrication is modified by increased interstitial fluid leakage across the osteochondral junction. Thus, the purpose of this study is to develop a numerical model to investigate changes in cartilage lubrication with changes in osteochondral junction leakage. MethodsThe multi-phase coupled model includes domains corresponding to the contact gap, cartilage tissue and subchondral bone plate region (ScBP). Each of these domains are treated as poroelastic systems, with their coupling implemented through mass and pressure continuity. The effects of osteochondral junction leakage on lubrication were investigated with a parametric study on the relative permeability between the ScBP and cartilage tissue. ResultsSignificant effects of ScBP permeability were predicted, especially during the early stage of the junction leakage development (early stage of the disease). There is a significant reduction in mixed-mode lubrication duration under the effect of increased junction leakage (the cartilage tissue mixed-mode lubrication duration is about 33% decrease for a relative permeability ratio of 0.1 between ScBP and cartilage tissue, and about 52% decrease under the osteoarthritis condition). In addition, the time for cartilage to reach steady-state consolidation is significantly reduced when ScBP permeability increases (the consolidation time reduces from roughly 2 h to 1.2 h when the relative permeability ratio increases from 0.001 to 0.1, and it reduces to 0.8 h for an advanced osteoarthritis condition). It is predicted that the initial friction coefficient could increase by over 60% when the ScBP permeability is consistent with an advanced osteoarthritis (OA) condition. ConclusionIncreased osteochondral junction leakage induced by joint injury and disease could result in increased cartilage surface wear rates due to more rapid interstitial fluid depressurization within articular cartilage.

Analytical approach to estimating the influence of friction slip contact between surrounding rock and concrete lining on mechanical response of deep rheological soft rock tunnels

For a deep-buried tunnel in soft ground, the contact condition between the surrounding rock and lining significantly affects the mechanical response. This is because the slip bond relationship on the contact surface has a direct influence on the tangential slip, resulting in the redistribution of displacement and stress on the contact surface between the surrounding rock and lining. However, previous studies have not paid sufficient attention to this problem, and a well-established method for quantitatively evaluating this influence remains unavailable. Therefore, the influence of the contact behavior between the surrounding rock and lining was examined in this study using an analytical method. First, the concept of friction slip contact and corresponding theoretical equations are proposed based on the real contact behavior between the surrounding rock and lining. Second, considering the friction slip contact between the surrounding rock and lining, a mechanical model of the tunnel excavation-support process was established, and analytical solutions for the stress and displacement of the tunnel within a viscoelastic rock material were realized. Third, the effectiveness and reliability of the proposed solutions were validated. The results could be degenerated to those that do not consider the influence of the mechanical behavior of the contact surface. In addition, there was good agreement between the analytical results and further numerical calculation results. Finally, a comprehensive parametric investigation was conducted, including the influences of the initial release coefficient, friction coefficient, and installation time of the lining. Some useful recommendations for tunnel lining design are provided herein.

Two strategies to improve the lubricating performance of WS2 film for space application

WS2 films with duplex-layer structure were deposited by magnetron sputtering. The tribological differences of as-deposited, AO-irradiated WS2 films and WS2/P201 hybrid system were mainly investigated for the same film. In comparison with as-deposited WS2 film, AO-irradiated WS2 film exhibits the lower initial friction coefficient and lower wear rate, and longer service longevity. Meanwhile, WS2/P201 hybrid system significantly extended service life further. It is found that the generated WS2 debris act as lubricating agent with different participation states and air isolation in the different lubricating systems affect the tribological property of WS2 film. Eventually, different lubrication mechanisms for AO-irradiated WS2 film and WS2/P201 hybrid system in vacuum were revealed.

Role of Environment on the Shear-Induced Structural Evolution of MoS2 and Impact on Oxidation and Tribological Properties for Space Applications.

This work investigates the role of water and oxygen on the shear-induced structural modifications of molybdenum disulfide (MoS2) coatings for space applications and the impact on friction due to oxidation from aging. We observed from transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) that sliding in both an inert environment (i.e., dry N2) or humid lab air forms basally oriented (002) running films of varying thickness and structure. Tribological testing of the basally oriented surfaces created in dry N2 and air showed lower initial friction than a coating with an amorphous or nanocrystalline microstructure. Aging of coatings with basally oriented surfaces was performed by heating samples at 250 °C for 24 h. Post aging tribological testing of the as-deposited coating showed increased initial friction and a longer transition from higher friction to lower friction (i.e., run-in) due to oxidation of the surface. Tribological testing of raster patches formed in dry N2 and air both showed an improved resistance to oxidation and reduced initial friction after aging. The results from this study have implications for the use of MoS2-coated mechanisms in aerospace and space applications and highlight the importance of preflight testing. Preflight cycling of components in inert or air environments provides an oriented surface microstructure with fewer interaction sites for oxidation and a lower shear strength, reducing the initial friction coefficient and oxidation due to aging or exposure to reactive species (i.e., atomic oxygen).

Exact analytical solution to inclined plane motion with constant resistance

This study describes the entire block-sliding movement process from various aspects to reveal the law governing its movement on a slope. An analytical solution of the block motion on an inclined plane is provided based on the variable substitution for various friction coefficient values in this study. The block movement’s initial angle and equivalent friction coefficient are determined by describing its trajectory, kinetic energy change, movement duration, maximum horizontal displacement, and other indicators. The block movement’s initial angle affects its movement duration, whereas the equivalent friction coefficient indirectly represents the degree of influence of friction on the block movement. The influence of singular points on the sliding trajectory is considered under certain initial conditions. Finally, this study provides a calculation method for assessing the block motion state with given initial conditions, which offers a theoretical reference for the application of the block slope-sliding model in the engineering field.

“Evaluation of a method to measure the friction coefficient between vital mandibular bone and biomedical materials”

PurposeThe aim of this study is to evaluate a developed experimental set-up to measure the coefficient of friction between fresh mandibular bone and biomaterials used in oral and maxillofacial surgery including a standardized routine for specimen preparation. Material and methodsFor this purpose, we developed a specialized routine for harvesting and preparation of cortical bone specimen from cadaveric porcine mandibles and modified a ball-plate tribometer. These harvested bone specimen were kept moist all the time. A total of 24 bone cylinders with 8 repetitions per material were examined for their coefficient of friction against stainless steel (1.4404), a titanium alloy (Ti6Al4V) and a cobalt‑chromium‑molybdenum alloy (Biosil F®) and subsequently analyzed by laser-scanning and scanning electron microscopy. ResultsThe lowest coefficients of friction between vital cortical bone and the investigated implant materials were found for Biosil F®, while the highest coefficients of friction were found for the titanium alloy Ti6Al4V. Significant differences for the initial coefficient of friction with p < 0.05 have been proven. Stick-slip effects occurred during the measurement and a layer of debris formed on the metallic samples. EDX analysis of the abrasion marks revealed that this layer consisted of elements contained in hydroxyapatites and carbon. ConclusionThe experimental set-up is suitable to perform reproducible comparative measurements of the coefficient of friction of different material combinations. A significant advantage of the methodology is the flexibility and scalability of harvesting and preparation of the bone specimen, which allows the simulation of realistic situations while measuring the coefficient of friction.

The Effects of Steel Core Imperfection, Gap Size and Friction Coefficient on the Behavior of All-Steel Buckling Restrained Braces

Buckling restrained braces (BRBs) which are generally composed of a steel core and a encasing(buckling restrainers) are utilized to resist lateral forces in high seismic regions since BRBs exhibit high energy dissipation capacity, ductility and stiffness. The steel core carries both compressive and tensile forces. During the compression, the core starts buckling and the encasing tries to prevent this buckling. However, due to the unbonding layer/gap between the encasing and steel core, the steel core eventually buckles and contacts with the encasing. Buckling phenomenon is also associated with the initial imperfection and gap size. In this study, the effects of the initial imperfectionof steel core, gap size (1-5 mm)and friction coefficient (0.01-0.5)between the encasing and steel core on the behavior of BRBs are investigated. Pursuant to this goal, numerical analyses using a finite element tool ABAQUS were conducted. A total of 19 numerical models were developed and monotonically loaded. Initial imperfection was implemented to the models using buckling mode shapes.The results revealed that increasing gap size leads to a reduction in load-carrying capacity. It is recommended to keep gap sizes between 1 and 2 mm. On the other hand, initial imperfection does not significantly affect load-carrying capacity and global behavior. However, it was also observed that the fluctuations in load increase as the amplitude of the mode shape and gap size increase. Moreover, the friction coefficient should be kept between 0.01 and 0.05; otherwise, undesired behaviors can be observed.