Fluctuation Of Friction Coefficient Research Articles

Molecular dynamics study of tribological properties of Ni–Cr alloy coatings with different pore shapes

The presence of nanopores with different shapes cannot be avoided during the coating preparation process, and the pore shape is an important factor affecting the coating friction mechanism. This paper applies molecular dynamics to study the tribological properties of Ni–Cr alloy coatings containing spherical, rectangular and ellipsoid-shaped pores. It is found that the coating with spherical pores has the smallest fluctuation of friction coefficient and the shallowest depth of wear scar. The atoms near the rectangular pores are more likely to move towards the corners of the pores due to shear strain, and the stress concentration in the spherical pores is less than that in the rectangular and ellipsoidal pores. The spherical pore has the least effect on the potential energy inside the coating, and the interatomic bond strength is optimal. The ellipsoidal pore is also more effective in insulating the heat transfer to the bottom of the coating, protecting the substrate underneath the coating and prolonging the coating life. Coatings with rectangular pores generate more interstitial atoms during friction, producing Frank dislocation loop and prismatic dislocation loops.

Role of temperature in tribolayers in fretting wear of γ-TiAl alloy

The formation of tribolayers may play significant influences on fretting wear. At elevated temperature, the adhesion among wear debris and the increased diffusion rate facilitate the formation of tribolayers. However, the intensification of oxidation at elevated temperature and the low diffusion rate in oxides may play an adverse role. The present study aims to investigate the role of temperature in tribolayers in fretting wear using a γ-TiAl alloy. Scanning electron microscope, energy dispersive spectrometer, Raman spectrometer, transmission electron microscope and nanoindentation were utilized to investigate the wear debris, tribolayers, and wear scars. The fretting tests showed that, compared with that at room temperature (RT) and 350 °C, significant reduction in wear rate and decrease in the fluctuation of friction coefficient occurred at 550 and 750 °C. It was further revealed that when temperature raised from room temperature (RT) to 750 °C, the oxidation of the wear debris increased slightly and the diffusion coefficients increased prominently, which facilities the formation of well tribo-sintered tribolayers. The well tribo-sintered tribolayers presented homogenous structure, nanocrystalline grains with excellent mechanical properties, and resulted in the improvement in the fretting wear resistance of the γ-TiAl alloy at 550 and 750 °C.

Preparation and Tribological Properties of Bismaleimide Matrix Composites Reinforced with Covalent Organic Framework Coated Graphene Nanosheets.

The poor compatibility between the polymer matrix and complex modification processes greatly affects the excellent tribological properties of graphene in the polymer matrix. In this study, a covalent organic framework (COF)-coated graphene hybrid lubricating filler (G/COFs) was synthesized in situ using a sample one-step mechanochemical synthesis process. This was used to improve the tribological properties of bismaleimide (BMI) resin. The morphology and microstructure of the G/COFs hybrid were characterized, and the effect of the added amount on the tribological properties of the G/COFs/BMI composites was studied. The results showed that the G/COFs hybrid could improve the stability of the friction coefficient and decrease the volume wear rate of BMI composites. Compared to the neat BMI, the 0.6 wt% G/COFs/BMI composites showed optimal tribological performance, with the friction coefficient and volume wear rate decreasing from 0.35 to 0.14 and from 48 × 10−6 to 10.6 × 10−6 mm3/(N‧m), respectively. In addition, the G/COFs/BMI composites showed lower friction coefficient fluctuations and volume wear rates than G/BMI composites. This is mainly attributed to the fact that the deposition of COFs can not only effectively prevent the aggregation of graphene nanosheets, but can also significantly improve the compatibility and interfacial bond between the graphene and BMI matrix. Moreover, the good synergistic effect between the lamellar COFs and graphene nanosheets can generate high-quality self-lubricating transfer films during the friction process. The excellent dispersibility, efficient chemical functionalization, better friction reduction and wear-resistance properties, and facile preparation method make graphene/COFs hybrid nanoparticles promising as an excellent lubricating filler.

Effect of UV Radiation on Structural Damage and Tribological Properties of Mo/MoS2-Pb-PbS Composite Films

To investigate ultraviolet (UV) radiation effects on tribological properties of Mo/MoS2-Pb-PbS film, ultraviolet (UV) radiation exposure tests were carried out for 20 h, 40 h, 60 h and 80 h by space UV radiation simulation device developed by our team, which can reach 3 UV radiation intensity. The exposure time in test was equivalent to the radiation of 100 h, 200 h, 300 h and 400 h in the space. Then, the vacuum friction test of Mo/MoS2-Pb-PbS thin film was performed under the 6 N load and 100 r/min, and friction test time of each sample was 20 min. By SEM, TEM, XPS the composition and morphology of Mo/MoS2-Pb-PbS film surface after UV radiation were analyzed. UV radiation could change the microstructure significantly and relative content of S element and MoS2 on the surface of the films decreased, and light mass loss of the films occurred. The tribological properties will also recover with the increase of sliding time, although the friction coefficient fluctuation of the film increased at the starting stage of the friction test. The damage of Mo/MoS2-Pb-PbS under UV irradiation was mainly caused by the volatilization of the enriched S element in the surface layer due to the high temperature heating of UV irradiation.

Tribological Behavior of Two High Performance Railway Wheel Steels Paired with a Brake Block Cast Iron

ABSTRACT The wear behavior of two steels for tread-braked wheels (ER7+ and CLASS B+) against the same brake block cast iron was studied through pin-on-disc tests to understand the phenomena at the interface and compare the steels. The contact stress (0.61 MPa) and the sliding speeds (0.52 and 0.62 m/s) were set considering the UIC 510-5 (E) for technical approval of monobloc wheels. The friction coefficient, the weight loss of both steel pin and cast iron disc and the pin hardness were correlated with the damage of the pin. In addition, a preliminary thermal finite element model was proposed to predict the temperature of the pin contact surface. The experiments showed that a material transfer phenomenon from the cast iron disc to the steel pin and the subsequent detachment of the adhesion joints caused high friction coefficient fluctuations. The removal of the adhesion joints probably promoted the detachment of steel particles from the pin, which could explain the high weight loss and remarkable delaminations of the pin in some of the studied conditions. On the contrary, small friction coefficient fluctuations and red oxide formation due to the temperature increase were detected when the material transfer was very limited. ER7+ steel showed lower weight loss and less damage than CLASS B+ at the lowest sliding speed, the contrary was observed at the highest sliding speed. Finally, the simulation showed that the maximum temperature of the pin contact surface varies between 61 and 97 °C, which is consistent with the red oxide presence.

Tribological properties of graphene-modified with ionic liquids and carbon quantum dots/ bismaleimide composites

In order to improve the tribological properties of graphene reinforced polymer composites, ionic liquids (IL) and carbon quantum dots (CQDs) were deposited on the reduced graphene oxide (rGO) nanosheets surface to ameliorate the compatibility between graphene and bismaleimide (BMI) matrix, as well as the stability of their self-lubricating transfer films. Their tribological properties were evaluated, and the wear mechanism was studied by means of surface/interface analysis, respectively. It could be found that CQDs/IL/rGO could improve the stability of the friction coefficient and decrease the volume wear rate of their BMI composites. When the filler amount was only 0.6 wt%, the friction coefficient and volume wear rate of CQDs/IL/rGO/BMI composites decreased by 41.2% and 94.7% compared with the neat BMI, respectively. Also, the CQDs/IL/rGO/BMI composites exhibited the lowest friction coefficient fluctuations and volume wear rate than those of other composites. Because of the synergistic effects of the CQDs, IL and rGO, uniform, continuous and steady self-lubricating transfer film could be formed during the friction process, thus significantly improving the anti-friction and wear-resistance properties of CQDs/IL/rGO/BMI composites. More importantly, the unique combinations of advantages of each functional component can also be applied in many other fields, such as catalysis, chemical sensors, etc.

Effect of multi-pass multi-directional forging on tribological properties of Si-rich eutectoid ZA alloys

The effect of multi-pass multi-directional forge (MDF) on the tribological properties of ZA22−xSi alloy (x=0, 4, 8 wt.%) was investigated. The results indicate that MDF breaks down the cast microstructure of alloys and produces a well-modified microstructure comprising finely distributed α- and η-phases and primary Si particles. It is also found that, despite the matrix work softening, MDFed ZA22−xSi alloys show high wear resistance. The maximum wear resistance is observed in the five-pass MDFed ZA22−4Si sample, at the applied loads of 10 and 30 N, and its wear rates are lower than the wear rate of the as-cast ZA22 alloy by about 80% and 75%, respectively. MDF also significantly decreases both average friction coefficient and friction coefficient fluctuation of the sample. The high resistance of the substrate to microcracking, formation of hard Si reinforcements, fine redistribution of α- and η-phases in the microstructure, and formation of tribolayers rich in Al and Zn oxides can be considered as the main factors improving the tribological properties.

Preparation and optimization of a soft magnetic brake friction material based on permalloy additive

Purpose This paper aims to develop of magnetic field controlled friction braking technology, a novel brake friction material with magnetic was designed and prepared in this paper. Design/methodology/approach The permalloy, a soft magnetic material, was selected as an additive to design and prepare the magnetic brake material. The friction, wear performance and permeability of each brake pads were investigated by experiments. By choosing the performance of friction coefficient fluctuation, friction coefficient deviation and mean wear rate as optimization parameters, the formulation of the magnetic friction material was optimized based on Fuzzy theory by using analytic hierarchy process methods and SPSS software. Findings The results showed that the developed soft magnetic friction material has not only superior friction coefficient, permeability and inferior wear rate but also good physical and mechanical properties. Originality/value Permalloy powder was added to the formulation of friction material to achieve a new functional friction material with high magnetic permeability. It is believed that this research will be of great theoretical and practical significance to develop both new brake materials and active control technology of the braking process in the future.

Effect of micro grooves on lubrication performance of friction pairs

The influence of micro-groove parameters on the lubrication performance of plane friction pair is studied, by combining theoretical analysis with experiment. The theoretical model of single-groove is established, the variation rules of bearing capacity, maximum pressure and friction coefficient are obtained by changing the number, width and depth of groove. At the same time, micro-grooves with different parameters are processed on the surface of friction pair, the variation rules of the average friction coefficient, friction coefficient under different lubrication conditions are obtained in experiment, and the wear condition of friction pair surface is observed. The results show that the lubrication performance of friction pair increases, with the increase of the micro-groove width. The numerical value of the groove width–depth ratio will influence the friction pair, when the width–depth ratio is greater than or equal to 2.5, the dynamic pressure effect of micro-groove occupies a dominant position, which can improve effectively the lubrication performance. When the width–depth ratio is less than 2.5, the micro-groove will have a negative impact, resulting in the increase of friction coefficient. The groove density will also influence the lubrication performance of friction pair, when the density is less than 10%, the spacing between the micro grooves is too large to collect wear particles, when the density is greater than 20%, the surface roughness of friction pair will be affected, leading to the increase of friction coefficient. The optimal parameters of the micro-groove obtained are as follows: the optimal density is 10%, the width is 0.1–0.2 mm, and the ratio of width to depth is greater than or equal to 2.5. In addition, by comparing the variation of friction coefficient under different lubrication conditions, it can be concluded that adding solid lubricant in lubricating oil, can not only reduce the friction coefficient value, but also decrease the variation fluctuation of friction coefficient.

A Small-Scale Experimental Study of the Damage Due to Intermittent Shoe Braking on the Tread of High-Speed Train Wheels

Recent interest in emergency shoe braking in high-speed trains stimulated research on the damage due to sliding contact between brake blocks and wheel treads. Bi-disc tests, with rolling–sliding contact between specimens extracted from wheels and brake blocks, were previously performed in constant working conditions to reproduce the temperature of the wheel during braking. However, braking is often a discontinuous operation, obtained by intermittent contact between brakes and tread, to prevent excessive heating. In this article, braking is simulated by discontinuous tests, periodically interrupted to cool down the specimens. Three wheel steels (HYPERLOS, SANDLOS, and CLASS B) were tested against the same cast iron brake material. Measurements of the friction coefficient, contact surface temperature, and weight variation were performed. Moreover, optical microscopy observations and hardness tests were done on the cross section of the wheel specimens. Material transfer from the brake to the wheel disc with the formation of a discontinuous third-body layer was observed. This layer, when worn away, likely contributes to surface crack nucleation. In addition, the debris of both materials enhances abrasive wear. If compared with the continuous tests, the intermittent tests showed higher friction coefficient fluctuations and in some cases significantly different weight variation. Among the steels, the weight loss and subsurface hardening of HYPERLOS was higher in the discontinuous tests than in the longest continuous tests. CLASS B showed similar behavior in both tests. An effect of the temperature cycles on the friction coefficient and the wear behavior was hypothesized.

Microstructure, nano-mechanical characterization, and fretting wear behavior of plasma surface Cr-Nb alloying on γ-TiAl

In this study, surface Cr-Nb alloying was realized on γ-TiAl using double glow plasma hollow cathode discharge technique. An inter-diffusion layer was generated under the surface, composed of Cr2Nb intermetallic compounds. After Cr-Nb alloying, the surface nanohardness of γ-TiAl increased from 5.65 to 11.61 GPa. The surface H/E and H3/E2 increased from 3.37 to 5.98 and from 0.64 to 4.15, respectively. Cr-Nb alloying and its effect on fretting wear were investigated. The surface treatment resulted in improved plastic deformation and fretting wear resistance of γ-TiAl. The fretting wear test showed that an average friction coefficient of γ-TiAl against Si3N4 ball was significantly decreased after Cr-Nb alloying. The fluctuation of friction coefficient during running-in stage was significantly improved. The friction behavior of both γ-TiAl before and after Cr-Nb alloying could be divided into distinctive stages including formation of debris, flaking, formation of crack, and delamination. It was observed that the high hardness, resistance to plastic deformation, and fatigue resistance of γ-TiAl after Cr-Nb alloying could inhibit the formation of debris and delamination during friction test. The fretting wear scar area and the maximum wear scar depth were decreased, indicating that the wear resistance of γ-TiAl has been greatly improved after Cr-Nb alloying. The results indicated that plasma surface Cr-Nb alloying is an effective way for improving the fretting wear resistance of γ-TiAl in aviation area.

Investigation of the Surface Properties and Wear Properties of AISI H11 Steel Treated by Auxiliary Heating Plasma Nitriding

This paper presents an auxiliary heating method to maintain a uniform specimen temperature and precisely control nitriding temperature during plasma nitriding. The surface properties and wear properties of AISI H11 steel treated by auxiliary heating plasma nitriding are investigated. Firstly, the specimens with different diffusion layers and different hardness levels are fabricated through changing the plasma nitriding temperature. Secondly, the surface properties of the plasma-nitrided H11 steel specimens are characterized by a scanning electron microscope (SEM), X-ray diffractometer, metallographic microscope and microhardness tester. The results show that the surface hardness of the plasma-nitrided specimen is almost twice as high as that of the untreated specimen. The thickness of diffusion layer increases with the increase of nitriding temperature. However, the surface hardness firstly increases and then decreases with the increase of the nitriding temperature. Finally, the wear properties of untreated and plasma-nitrided H11 steel specimens are investigated under different friction conditions. The results show that the plasma-nitriding method can significantly improve the wear resistance of AISI H11 steel. The friction coefficient fluctuations of the plasma-nitrided specimens are all lower than those of the untreated specimens. In addition, the wear rates of the plasma-nitrided specimens rise along with load, and reduce along with the sliding speed and friction temperature.

Negative friction and mobilities induced by friction fluctuation.

We study the transport phenomena of an inertial Brownian particle in a symmetric potential with periodicity, which is driven by an external time-periodic force and an external constant bias for both cases of the deterministic dynamics and the existence of friction coefficient fluctuations. For the deterministic case, it is shown that for suitable parameters, the existence of certain appropriate friction coefficients can enhance the transport of the particle, which may be interpreted as the negative friction coefficient; additionally, there coexist absolute, differential negative, and giant positive mobilities with increasing friction coefficients in the system. We analyze physical mechanisms hinted behind these findings via basins of attraction. For the existence of friction coefficient fluctuations, it is shown that the fluctuation can enhance or weaken, even eliminate these phenomena. We present the probability distribution of the particle's velocity to interpret these mobilities and the suitable parameters' regimes of these phenomena. In order to further understand the physical mechanism, we also study diffusions corresponding to these mobilities and find that for the small fluctuation, the negative friction appears, and there coexists absolute negative mobility, superdiffusion, and ballistic diffusion, whereas all of them vanish for the large fluctuation. Our findings may extensively exist in materials, including different defects, strains, the number of interfacial hydrogen bonds, the arrangements of ions, or graphite concentrations, which hints at the existence of different friction coefficients.

A high-performance copper-based brake pad for high-speed railway trains and its surface substance evolution and wear mechanism at high temperature

High temperature is one of the most basic challenges faced by copper-based brake pads in emergency braking of high-speed railway train. Here, a novel copper-based brake pad was prepared and its tribological properties in the temperature ranging of 400–800 °C were studied by pin-on-disc tribometer equipped with heating chamber. The results show that compared with commercial copper-based brake pad, self-designed copper-based brake pad exhibits higher mean friction coefficient and smaller fluctuation of friction coefficient under all test conditions. The friction coefficient of self-designed copper-based brake pad is in the range of 0.35–0.45 at the temperatures up to 600 °C. At 800 °C, the failure mechanism of copper-based brake pad is related to the properties of graphite and copper. Graphite on the friction surface disappears after severe oxidation, resulting in the loss of lubrication of friction interface. Copper undergoes oxidation and softening. Oxidation makes copper unable to promote the formation of stable tribo-film, and the low-strength copper matrix after softening cannot effectively support the friction surface. Therefore, one of the key principles to design high-performance copper-based brake pad is to increase the oxidation resistance of graphite and the high temperature strength of copper matrix.

Improving the Tribological Properties of Al-7Fe-5Ni Alloys via Friction Stir Processing

AbstractDue to the limited solid-solubility of Fe and Ni in Al, coarse brittle intermetallics rich in Fe and/or Ni are inevitably formed in the as-cast microstructure of Al-Fe-Ni alloys. Upon formation, Fe(Ni)-rich intermetallics significantly deteriorate the properties and restrict the application of alloy in as-cast condition. The purpose of this study was to investigate the effect of friction stir processing (FSP) on microstructure and tribological properties of cast Al-7Fe-5Ni alloy. The dry sliding wear tests were done by using a pin-on-disk type machine under the applied pressures of 1, 2, and 3 MPa, sliding distance of 1000 m, at room temperature. According to the results, FSP (1250 rpm and 8 mm/min) effectively refined the microstructure, especially the coarse primary Al9FeNi intermetallics and eliminated the alloy casting-related defects. FSP also converted the large columnar grains of the cast alloys to the ultrafine equiaxed grains. These microstructural changes enhanced the substrate mechanical properties thereby increased its resistance against sliding-induced plastic flow, leading to the higher tribolayer stability on the surface, and accordingly, higher wear resistance. The results showed that applying 1-pass FSP reduced the wear by 13% and 53% under applied pressures of 1 and 3 MPa, respectively. The friction results also revealed that, compared with the as-cast sample, FSPed samples exhibited lower friction coefficient and friction coefficient fluctuations.

Mechanical and Tribological Properties of CrN Coated Inconel X750

Three CrN coatings were deposited on the Inconel X750 through the metal vapor vacuum arc ion implantation and the magnetic filtered cathodic vacuum arc deposition system (MEVVA-FCVA) with the N2 flow rates of 10, 50, and 100 sccm, respectively. The surface morphologies and cross-section morphologies of the CrN coatings were obtained through scanning electron microscopy (SEM) and an optical profilometer. The microstructures of the coatings were characterized through X-ray diffraction (XRD). The hardness and the elastic modulus of the coatings were tested by a nano-hardness tester. The adhesion strength and friction coefficients were investigated through scratch tests and ball-on-disk tests and the wear tracks were tested by the optical profilometer. The experimental results indicate that the CrN coating deposited on the Inconel X750 substrate displays a uniform thickness and a smooth surface. The mechanical properties behaves well as the N2 flow rate varies. The CrN coating significantly reduces the friction coefficient fluctuation and improves the antiadhesion and anti-wear properties of the Inconel X750.

Fretting friction and wear behaviors of spiral wound gasket (SWG) sealing surface

Abstract The vibration of the pipeline act on the gasket sealing surface in the form of fretting wear, reducing the sealing performance. In this paper, an experimental study was described on the fretting wear and frictional mechanism of interface between spiral wound gasket (SWG) and flange. The results showed that the gasket winding structure affects the variation of the friction coefficient fluctuation. Moreover, only the fretting behavior of mixed friction regime (MFR) and slip regime (SR) can be expressed stably. Additionally, fretting wear is dominated by abrasive and adhesive wear, and plastic deformation occurs under high load. The intense wear behavior mainly presented in the circumferential direction of small displacement and the radial friction process of large displacement.

Influence of Rolling Contact Surface and Contact Load on Fluctuation of Rolling Friction Force

This paper presents the experimental analysis for the fluctuation of rolling friction force, which deteriorates high precision motion in linear ball guideways. A simple drive with four tracks and four balls was used to investigate the influence of contact surface and contact load on the friction force fluctuation. To provide different surface integrity, ground, polished, and shot-peened surfaces were prepared. Experimental results showed that the friction force fluctuation was dependent on position change and influenced by the inclination angle of the rolling contact surface. The fluctuation of friction coefficient was inversely proportional to the cube root of the contact load. The wavelength analysis on the friction coefficient showed that friction coefficient was suppressed as the contact load increased. An empirical formula for estimating the friction force fluctuation was proposed based on these experimental results and a single ball friction model proposed by Ishikawa and Suda.

Understanding Wear Interface Evolution to Overcome Friction and Restrain Wear of TiAl–10 wt%Ag Composite

The main objective of this paper is to study wear interface evolution for analyzing the of friction and wear property of TiAl–10 wt%Ag composite. The results show that the friction coefficient and wear rate of TiAl–10 wt%Ag rapidly reduce at 0–25 min and rhythmically fluctuate at 25–60 min. TiAl–10 wt%Ag at 60–240 min obtains low friction and less wear. It is concluded that silver with the low shearing strength of about 125 MPa shows the eminent plastic deformation on wear interface. It effectively reduces friction resistance and material loss, cause TiAl–10 wt%Ag to obtain low friction coefficient, and less wear rate at 0–25 min. Increased silver content, reduces oxide content, and varies wear mechanisms cause the repeating variation of friction resistance and material loss, which results in the rhythmical fluctuation of friction coefficient and wear rate at 25–60 min. High silver contents exist on smooth wear interfaces, exhibit the eminent plastic deformation to lower friction and reduce wear. TiAl–10 wt%Ag obtains the low friction and less wear at 60–240 min.

Improving the Tribological Properties of Gear Synchronizations by Adjusting the Metalworking Fluid Composition of the Grinding Process

The modification of tribological properties of highly loaded components by selecting a suitable manufacturing process is state-of-the-art. Beyond the generation of microgeometrical structures, the present study investigates the potential of chemical alterations, i.e., metalworking fluid additives engaged in a grinding process to improve the chemical surface properties of machined gear synchronizations. These gearbox components ensure a uniform switching operation by harmonizing the number of revolutions between the power transmitting components by friction. A short running-in phase and a friction coefficient which is constant over the entire duration of use are required. The results show that the addition of polysulfide as well as zincdialkyldithiophosphate in the metalworking fluid could considerably reduce the friction coefficient fluctuations and the running-in phase of the generated synchronizations in later operation tests. The wear distances were lower although the machined parts revealed higher surface roughnesses as the reference workpieces, which indicates the formation of sorption or reaction layers of the additives with the metal surface.