Increase In Normal Load Research Articles

Research on shear characteristics of pile–soil interface under cyclic loading

Offshore wind turbine constructions are developing rapidly in China. Generally, large-diameter rigid steel piles are used in offshore wind turbine foundations. However, existing design methods for pile foundations with bearing capacity are often based on small-diameter flexible piles and are inapplicable to real situations. Simultaneously, thick layers of clay cover the fields. Therefore, the shear characteristics of pile–clay interface require examination to optimize design methods. A large-scale interface shear test model corresponding to laboratory tests is established using PFC software, and the micro-parameter values are validated. The simulation results well follow the laboratory test results, and the model accurately reflects pile–clay interactions during shearing. Based on the simulation model, the shear characteristics of the steel–clay interface under different normal loads, cyclic load frequencies, and cyclic load amplitudes are analyzed. With an increase in normal load, the shear strength of the steel-clay interface increases gradually, whereas it slightly decreases with an increase in the cyclic load frequency and amplitude; particularly, the effect of amplitude is significant. The mechanical response of clay at a mesoscopic scale is analyzed. Results show that the greater the normal load, the greater the horizontal displacement of particles. However, the influence of cyclic load frequency and amplitude is the opposite: normal load does not change the contact force distribution of soil particles, and clay volume change is determined by the normal load. Thus, this study provides support for bearing capacity prediction of offshore wind farm pile foundations.

Study on fretting friction properties based on GCr15 alloy and liquid epoxy resin protective coatings on GCr15 alloy

GCr15 alloy steel is a representative alloy steel of high carbon chromium bearing steel, with good comprehensive performance. However, the fretting wear and fatigue at the interface of the GCr15 friction pair greatly reduce its service life. In this study, the friction pair of GCr15 alloy was prepared by the surface treatment of epoxy resin coating. The tangential fretting wear experiments were conducted on the samples with the point contact mode in an SRV-IV multifunctional tester to investigate the evolution law of coefficient of fretting wear, dissipation energy and damage varying with the normal load and displacement amplitude. The results showed that the coefficients of the friction pairs treated with epoxy resin composite materials are lower than those of GCr15-GCr15 friction pairs under the same normal load and displacement amplitude, and the friction coefficients of both types of friction pairs gradually decreased with the increase of normal load and the decrease of displacement amplitude. At the same time, epoxy resin coating treatment reduced the dissipation energy of fretting wear. From the perspective of damage protection, GCr15-GCr15 exhibits severe scratch damage, micropores, and microcracks after friction experiments, while the epoxy resin coating mainly showed furrow. In addition, the decrease in hardness of polymer composite coating increased the wear rate of the samples to a certain extent.

Tribological behavior of silver-doped eggshell-derived hydroxyapatite reinforcement in PMMA-based composite

Polymethylmethacrylate (PMMA) and hydroxyapatite (HAp) are the two most promising biocompatible materials used in biomedical applications. The current research performs the wettability and tribological characterization of the novel hybrid biocomposite of PMMA reinforced with eggshell-derived, silver-doped hydroxyapatite (HAPAg). Varying wt% of HAPAg in PMMA were analyzed using a ball-on-disk tribometer. The coefficient of friction shows an increasing trend with an increase in normal load for all the compositions while, with reinforcement of HAPAg, it increases till 5 wt% and then shows a sudden decrement at PHA7.5 due to the formation of flattened asperities at the contact surface. However, the progressive increase in hardness with the inclusion of HAPAg in PMMA correlates with the reduction in the wear rate of the composite samples. The highest wear rate was observed for PHA0 (i.e. 862.42 × 10−5 mm3/m) at 60 N. As observed, the hydrophilicity increases (contact angle changed from 96.30° ± 2.11° [PHA0] to 81.70° ± 1.01° [PHA7.5]), and the porosity decreases (≈2.86%) with the reinforcement of HAPAg in PMMA which further improves the cohesion strength and microhardness of the composite material. The X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analysis confirms the uniform reinforcement of HAPAg, and the worn surface behavior was inspected using scanning electron microscopy, Stereo zoom microscope, and three-dimensional surface profilometer. The low-specific wear characteristic at higher loads ensures the application of developed biocomposite material in dental and orthopedic applications.

Improved tribological performance of Al2O3 fiber at microscale via a WS2-PEG/WPU self-lubricating sizing agent

In this work, a new green modified tungsten disulfide (WS2-PEG)-waterborne polyurethane (WPU) self-lubricating sizing agent was proposed to improve the tribological properties of microscale alumina fibers (single Al2O3 fiber). The chemical characterization analysis confirmed the successful synthesis of WS2-PEG nanosheets with an average particle size of 211 nm, which significantly improved their dispersion in sizing agents. Subsequently, the single Al2O3 fibers were sized (denoted as WS2-PEG/WPU@single Al2O3 fibers) and their friction and wear properties were discussed. The experimental results showed that the addition of WS2-PEG to the single Al2O3 fiber decreased its friction coefficient (COF) (43.48 %), reduced wear conditions. And the COF and wear of WS2-PEG /WPU@single Al2O3 fiber decreased with the increase of normal load. The optimized concentration of WS2-PEG nanosheets was 0.05 g/L, which minimized the friction and wear of the fibers and formed a continuous, dense, and high load-carrying lubricating friction film on the fibers, improving the tribological properties of the single Al2O3 fiber.

Friction Behavior of Fingers on Micro-Textured Flexible Surfaces

Abstract Compared to rigid materials, people have a distinct tactile perception when touching flexible materials. Moreover, adding micro-patterns to the surface enhances the tactile experience even further. This sensation arises from the physical stimulation of frictional behavior between the skin and flexible materials. Therefore, this study focuses on human fingers as the research subjects and employs flexible materials with micro-textured surfaces as frictional objects. A friction test setup is designed to conduct a series of finger friction experiments, and theoretical explanations are provided to elucidate the reasons for performance variations. Research findings show that as the normal load increases, the frictional force gradually increases while the friction coefficient decreases. The former is attributed to the expanding contact area, while the latter is due to the inconsistent rate of frictional force increment with the normal load. The impact of friction velocity is mainly caused by changes in the viscous forces generated at the liquid film in the contact interface and the energy loss in elastic hysteresis. On the other hand, the effect of surface micro-topography is primarily a result of the transition between partial contact and full contact modes under the influence of normal load, leading to alterations in the contact area. Overall, during the finger friction process on a flexible micro-textured surface, changes in contact area play a vital role in modifying frictional performance, with adhesive friction exerting a more significant influence than deformation friction. This study summarizes the variations in frictional performance parameters based on experiments and analyzes the effects of contact area changes and deformation friction mechanisms from a theoretical perspective, providing a theoretical foundation for exploring the genesis of delicate tactile sensations during friction.

High thermal conductivity and ultralow friction of two-dimensional ice by molecular dynamics simulations

Two-dimensional ice (2D ice) is a relatively new family member of ice polymorphs. While the phase transitions and growth kinetics of 2D ice have aroused intensive investigation, its thermal and mechanical properties are seldom studied. The unique 2D structure of this polymorph suggests that it may possess distinct physical characteristics compared to its bulk counterparts. In this work, the thermal conductivities and friction of 2D ice were systematically studied by molecular dynamics (MD) simulations. It was found that the thermal conductivities of 2D ice show a strong dependence on their length and the extrapolated value of the infinite system can achieve ∼4.7 W/(m∙K), which is notably higher than that of bulk phase of ice Ih ∼1.5 W/(m∙K). Decreasing temperature or applying mechanical strains can both increase the thermal conductivity of 2D ice, which can be explained by structure or phonon analyses. 2D ice also exhibits an ultralow friction of ∼0.24 nN when sliding upon a graphene substrate, lower than that of ice Ih ∼0.70 nN at the same sliding conditions. With the decrease in temperature or the increase in normal load, the friction coefficient of 2D ice exhibits a downward trend, and it could be as low as ∼0.005, indicating superlubricity. The analysis on structural lubricity suggests that ordered but mismatched crystal-crystal contacts have lower friction than disordered case. The results on high thermal conductivity and ultralow friction would guide the potential application of 2D ice in efficient thermal management or design of nanofluidic devices.

Effect of the scanning strategy and tribological conditions on the wear resistance of IN718 obtained by Laser Metal Deposition

Since the advent of additive manufacturing processes, new ways of repairing damaged parts emerged. Among those, Laser Metal Deposition offers the possibility of restoration or functionalization of surface properties. In this work, we study the wear properties of an IN718 coating deposited by LMD. IN718 is deposited according to several scanning strategies. Samples are subjected to dry flat-on-flat reciprocating wear tests using the flat surface of a cylindrical pin as a counter body. Then, we investigate the impact of the scanning strategy, the normal load, and the number of cycles on wear. Finally, we compare the wear resistance of the LMD samples with the one of wrought IN718 to evaluate the competitiveness of the wear properties of the material obtained by LMD. Results show that the scanning strategy turns out to not change the wear resistance of the deposited sample despite the highly heterogeneous surface microstructure. As the chosen contact is a flat-on-flat contact, the large area covered by the pin does not perceive the local microstructural heterogeneities. It has been observed as well that the wear volume of the coating increases linearly as the normal load increases, with an evolution of the wear mechanisms. Although oxidized transfer layer is observed at lower load, the main wear mechanisms observed are abrasion and adhesion. Then, by studying the wear evolution with increasing testing time, it has been observed that after a certain amount of cycles, a protective oxide layer appears and prevents IN718 from wear. In similar tribological conditions, wrought IN718 has slightly better wear resistance than its LMD's homolog. This difference in wear between the materials obtained by two processes could certainly be explained by the high initial amount of dislocations in the LMD samples and the presence of detrimental phases in its microstructure, such as Laves phases. Nonetheless, this study highlights the good wear resistance of the IN718 samples obtained by LMD despite their non-optimized as-built properties.

Sliding friction acoustics

Contact interactions generate acoustic energy in complicated ways, dictated by dozens of system parameters. The ability to accurately predict these stochastic friction forces and resultant acoustic energy from first principles is currently lacking. A summary of experimental tests with sliding and stick-slip of metal-on-metal contact are presented to provide insight into the influence of roughness, sliding velocity, and normal load on the generated acoustics. Normal load and normal displacement increase in dynamic amplitude under stick-slip conditions relative to smooth sliding. In the current configuration, stick-slip also produces an excess of up to 30 dB SPL, relative to smooth sliding. Relationships between normal load, surface roughness, tangential velocity, and acoustic output are explored, and parameters are tuned to fit these variables using an existing empirical relationship from the literature.

Characteristic and tribological behavior of amorphous carbon–containing CrAlCN coating by cathodic arc ion plating

Amorphous carbon–containing CrAlCN coating was deposited on Cr12MoV steel by cathodic arc ion plating. The effect of amorphous carbon on the tribological performance of obtained coating under the different normal loads was investigated using a ball–on–disc tester, and the lubrication and wear mechanisms of amorphous carbon were discussed in detail. The results show that the CrAlCN coating is composed of AlN, CrN and CrC phases with the amorphous carbon. The average coefficient of friction of CrAlCN coatings under the normal loads of 2, 4 and 6 N are 0.458, 0.586, and 0.666, respectively, where the amorphous carbon plays a self–lubrication role in the friction process. The wear degree is deteriorated with the increase of normal load, and the wear mechanism is changed from abrasive wear to adhesive wear.

The dry-sliding and micro-scratching behaviors of a Mg65Zn30Ca5 bulk glassy alloy

The macro-sliding and micro-scratching behaviors of a Mg65Zn30Ca5 bulk glassy alloy (BGA) were studied in the present work. The dry sliding results show that the wear mechanism of the Mg-based BGA includes the abrasion and oxidation wear. Increasing the normal load does not change the wear mechanism of macro-sliding. The cross-sectional morphology of the subsurface under the worn scar shows obvious brittle cracks across shear bands. The micro-scratching results illustrates that as the normal load increases, plastic deformation occurs and accumulates, then micro-cracks form and propagate, and cracks generate and expand in the scratch when the load excesses 5 N. The cross-sectional morphology below the scratch scar after 1 min repeating scratching shows river pattern fracture surface with nanoscale swirling periodic corrugations, which indicates the brittle fracture characteristic of the Mg-based BGA.

The effects of normal load on the dry-sliding, micro-scratch and nanoindentation behaviors of the Zr-based bulk glassy alloys

In this work, the macro-sliding, micro-scratch and nanoindentation behaviors of the Zr55Cu30Al10Ni5 bulk glassy alloy (BGA) were investigated. It was found that the frictional coefficient and wear rate of the BGA in the dry-sliding test increase with the increasing normal load, and the wear mechanism includes abrasive and oxidative wear. The micro-scratch test results show that the plastic deformation of the alloy and the density of shear bands at the edge of scratch scar increase with the increase in normal load. The friction mechanism between the indenter and the BGA shifts from stick-slip friction to ploughing friction and to micro-cutting as the applied normal load elevates. The nanoindentation result reveals that increasing the normal load leads to the plastic deformation accumulation, and the decreasing elastic modulus and hardness, which are related to the increase in the free volume and shear bands during the deformation process.

Tribological Models for Advanced Ball Bearing Simulation

Advanced ball bearing simulations require the use of accurate tribological models, starting with the calculation of the ball-race contact load and pressure in potentially truncated contacts. Ball—cage and cage—guiding ring contact stiffness models are suggested using the Hertzian contact stiffness and/or a structural stiffness. The central ball-race film thickness, a hydrodynamic rolling force, and normal load increase are calculated using relationships respecting miscellaneous lubrication regimes, including iso-viscous-rigid or piezo-viscous-elastic. A drag force acting at the ball center is included, and its magnitude discussed for claiming that the contribution from drag to the final rolling resistance is smaller than the one from hydrodynamic rolling acting at the ball-race contact. The central film thickness and sliding speed are used for defining shear rate, shear stress, and, hence, final sliding force, using a nonlinear visco-elastic-plastic lubricant rheological model including viscosity, elastic lubricant shear modulus, and limiting shear stress, which are all functions of the pressure and temperature. Elastic effects are approximated using a simple nonlinear visco-plastic model with apparent viscosity. The temperature is defined as the sum of the operating temperature and several other sources of temperature increase. It is observed that when adopting Roelands viscosity model and a simple nonlinear visco-plastic model, the final lubricant traction coefficient and its gradient with respect to sliding speed decrease as the rolling speed increases, mostly reducing the need for a lubricant elastic shear modulus. Appropriate experimental results are highlighted to support these analytical models.

Friction Behavior and Self-Lubricating Mechanism of SLD-MAGIC Cold Worked Die Steel during Different Wear Conditions

Wear tends to shorten tool life, reduce component quality. To prevent or postpone the wear of tool steel forming tools, methods to increase wear resistance, such as increasing the material hardness, optimizing the carbide distribution and application of surface coatings, are often used. However, the formation of lubricating phases in steels leading to anti-attrition is less investigated. The friction behavior of three steels were investigated thoroughly by a tribo test with different normal loads. A Field-emission scanning electron microscope (FE-SEM) along with energy dispersive X-ray spectroscopy (EDS) were used to characterize the microstructure as well as the influence of the precipitated phases on the wear mechanisms. Results showed the friction coefficient decreased with increasing normal load, whereas the wear rate increased with increasing normal load. Compared with SKD11 and DC53 steels, the friction coefficient and wear volume of SLD-MAGIC steel were reduced by 0.1 to 0.3 and 10% to 30%, respectively. With the increase of normal load, the wear mechanism changed in order from abrasive wear, adhesive wear to oxidation wear. The more carbide contents, the rounder the carbide, the better the wear resistance of the tool steel. It can be shown that, under different normal loads, SLD-MAGIC exhibits better wear performance than SKD11 and DC53 tool steels, which is mainly due to the self-lubricating phenomenon of SLD-MAGIC steel. The self-lubricating mechanism was due to the fact that the exfoliated sulfide during wear formed a lubricating film to reduce wear.

Mechanical and Tribological Behavior of Austempered Ductile Iron (ADI) under Dry Sliding Conditions

In the current investigation, a vertically continuous casting technique was used to produce a ductile iron pipe. The ductile iron was austempered, and the tribological behavior of austempered ductile iron (ADI) was examined under various service conditions. The finding demonstrated that ADI’s tribological behaviors were significantly affected by normal loads and sliding speeds. Spheroidal graphite was preferential to be transferred from the matrix to the tribosurface in ADI under high normal loads, and high sliding speed accelerated the formation of the graphite lubricating layer on the tribosurface. Consequently, ADI’s friction coefficient dropped with the increase in normal load and sliding speed. When compared with the friction coefficient, the wear rate of ADI displayed a similar tendency in that it increased with an increase in normal load and reduced with an increase in sliding speed. The worn surface indicated that adhesive wear at low sliding speeds and abrasive wear at high sliding speeds were the primary wear mechanisms for ADI.

Effect of Grain Size on the Tribological Behavior of CoCrFeMnNi High Entropy Alloy

The effect and mechanism of grain sizes on the tribological behavior of CoCrFeMnNi high entropy alloy (HEA) were studied by friction experiments and wear morphology analysis. Under normal low load and low sliding speed, the primary wear mechanism of the HEA samples is adhesive wear. With the increase in sliding speed, the wear mechanisms of the samples are adhesive wear and oxidation wear. The oxide layer formed under the action of friction heat of the coarse grain (CG) sample is easy to break due to the softening of the CG. With the increase of normal load and sliding speed, the wear mechanisms of the HEA samples are mainly adhesive wear, oxidation wear, and plastic deformation. The oxide layer of CG sample has many cracks, and the worn surface also has plastic deformation, which leads to the increase of friction coefficient and specific wear rate and the decrease of wear resistance. Therefore, the fine grain size HEA sample has better wear resistance than the CG sample due to its high surface strength.

Influence of normal load, electric current and sliding speed on tribological performance of electrical contact interface

The purpose of this study is to investigate the effect of normal load, electric current and sliding speed on tribological performance of electrical contact interface. Sliding electrical contact tests of H65 material are carried out by using a customized ball-on-flat tribometer. Results show that with the increase of normal load, both the friction coefficient and contact voltage drop show a downward trend. Too low or too high load will not cause the generation of friction-induce vibration (FIV). Although increasing the normal load appropriately can improve the wear behavior of the interface, it will intensify wear if the normal load is overly high. Larger electric current will increase the friction coefficient, contact voltage drop and surface temperature, but reduce the contact resistance. A certain value of electric current will cause the generation of FIV. Larger electric current will destroy the surface lubricating film of interface and intensify wear. With the increase of sliding speed, the friction coefficient firstly increases and then decreases, while the contact voltage drop shows the opposite trend. Strong FIV will be generated at a certain value of sliding speed. Larger sliding speed will cause the formation of a dense film layer, which plays a good lubrication effect.

Fretting wear behavior of Inconel 718 alloy manufactured by DED and treated by UNSM

Alloy 718 is commonly used in the maritime and aerospace industries due to its strength and durability, particularly in engine rotating components such as disks, fan blades, and high-pressure compressors. As a new type of 3D printing technology, directed energy deposition (DED) can employ lasers to melt metal powders or wires to fabricate arbitrary-shaped workpieces directly from customized data, thereby making machining more synergistic and intuitive. However, the surface properties of the DED-printed alloy 718 samples, such as surface roughness and wear resistance, are typically subpar. By introducing severe plastic deformation to the near-surface, ultrasonic nanocrystal surface modification (UNSM) can be used as a post-processing method and results in altered properties. The uniaxial tensile test reveals that the UNSM-treated alloy 718 exhibits a higher mechanical property. Moreover, using a fretting test rig in accordance with the cylinder-on-plane agreement, a higher wear resistance for UNSM-treated alloy 718 is observed. This study employs the finite element method to fully comprehend the effect of UNSM on wear performance. The fretting wear process of Inconel 718 alloy is established using an energy-based finite element model. Considering the severe practical scenarios, the Johnson–Cook constitutive model is implemented, with the linear isotropic hardening model capturing the plastic behavior. In comparison to experimental measurements, the finite element results demonstrate unprecedented wear loss consistency with an error of less than 2%. Therefore, we conclude that the finite element model built in this study exhibits a high accuracy and can be used to analyze the effect of UNSM on fretting wear behavior. According to finite element analysis, as the normal load increases, the improvement in wear resistance induced by UNSM decreases. Given that the finite element model is based on the energy method, the effects of coefficient of friction (COF) and wear coefficient modified by UNSM are investigated separately. According to the findings, the UNSM-modified COF and wear coefficient play a significant role in determining the wear characteristics. Due to the removal of a substantial amount of material from the central area of the alloy 718 surface by wear, it is also possible to observe that severe plastic strains are primarily concentrated at the edges of the wear scars.

Three-body abrasive wear behavior of rice straw fibers reinforced PLA composites

The current study focused on improving the abrasion resistance of rice straw fiber reinforced polylactic acid (PLA) composites. The effect of applied load and rice straw fiber loading on three-body abrasive wear behavior of PLA composites have been investigated. Rice straw fibers were treated with 2 wt% NaOH solution. Both the untreated and treated were produced by injection molding technique. Wear testing was performed on a dry sand/rubber wheel abrasion tester. According to the experimental results, fiber loading and applied load have a significant influence on composite wear loss. The results reveal that the abrasive wear loss of the composites decreases as fiber loading rises, whereas wear loss increases as applied normal load increases. SEM images were used to examine at the worn surfaces of the abraded samples to learn more about how the material was taken away. The treated fiber-reinforced PLA composites showed improvement in wear resistance than the untreated fiber reinforced PLA composites because the NaOH treatment of the fibers increased the fiber–matrix adhesion,

Fretting behaviors of self-mated diamond-like carbon films: The evolution of fretting regime and transfer film

Diamond-like carbon (DLC) film is undoubtedly one excellent self-lubricating solid film to improve the fretting behavior due to its ultralow friction as well as high-hardness. Investigating the evolution laws of fretting running state with operating condition and the friction reduction mechanism of DLC film under fretting is vital for reducing friction and wear. Here, the fretting performance of self-mated hydrogen-containing DLC (H-DLC) and Cr-doped DLC (Cr-DLC) was studied under different normal loads and amplitudes. In addition, the mechanism of friction reduction for transfer film and the cause of discrepancy in fretting behavior were clarified through a control experiment. The results demonstrate that as the normal load increases, the relative slip decreases, the elastic deformation increases, and the fretting regime develops towards the partial slip regime, while the increasing amplitude moves the fretting regime towards slip regime. During fretting, the third body layer will transfer to the counterface, and form a uniform and graphitized transfer film. The difference in the length of run-in period and the sp2/sp3 hybrid carbon content of DLC films influences the formation rate and the graphitized degree of the transfer film, which induces the discrepancy of fretting behavior of H-DLC and Cr-DLC films. Thus, this study not only clarifies the evolution on fretting regime of DLC film with changes in working conditions, but also demonstrates the key role of highly graphitized transfer film in fretting performances of self-mated DLC films. This work demonstrates the controllability and potential of self-mated DLC films in the field of fretting.

Influence of areal surface parameters on the tribological behavior of Ti6Al4V under lubricated condition

The present work investigates the influence of areal surface parameters according to ISO 25178 on the tribological behavior of Ti6Al4V alloy. Experiments on lubricated sliding wear at varied weights and sliding speeds were conducted using the pin-on-disk tribo test with an EN31 steel disk as the counter body and SAE40W15 as the lubricant in accordance with ASTM G-99. The lowest friction coefficient was obtained at 3 m/s sliding velocity and was independent of normal load, however the wear rate increased with increase in normal load. Vvv, Svk, Spk, Sk, Sku, Sk and, Sq had shown strong correlation with CoF and wear depth. Spk had a correlation coefficient of −0.8247 with CoF, and Vvv had a correlation coefficient of - 0.8415 with wear depth. Vvv and Svk retained the lubricant and increased the life of the component.