Increase In Friction Coefficient Research Articles

Evaluation of the accuracy of adaptive responses first generation friction clutches

It is shown that when the coeffi cient of friction increases and there is no external overload in the adaptive friction clutch of the fi rst generation with differentiated friction pairs, the internal load is redistributed between the friction pairs of the leading and driven friction groups. As a result, in the absence of a clutch actuation, the predominant part of the external torque is transmitted by the friction pairs of the leading friction group, and the smaller part — by the friction pairs of the driven friction group.

Plane strain wedge indentation revisited

For a plane strain wedge indentation problem, detailed comparisons between slip-line field theory (SLFT) solutions, experimental observations and finite-element analysis results are carried out. Several past remaining questions on this problem are explored. The finite-element computations appear to account for the nature of a modified slip-line field solution (Petryk H. 1980 Journal de Mécanique Appliquée 4 , 255–282) that assumes the existence of an additional fan-shaped slip-line field just above the boundary between yielding and unyielding regions. For obtuse-angled wedges, finite-element computations show that the indentation load could be smaller for a rough wedge than for a smooth wedge. The question that was raised in the literature with respect to the reversed order of required loads for smooth and rough wedges is not necessarily a paradox. It is revealed that with increasing friction coefficient, a distinct deformation mode resembling the dead metal cap solution in the SLFT is established immediately after passing the critical friction coefficient value at which the contact mode theoretically changes from frictional sliding to perfect sticking. The profiles of the bulged-out lip material remain straight in all the finite-element computations for non-hardening materials. Nonlinearity of the lip profiles frequently observed in experiments appears in the presence of strain hardening or strain-rate sensitivity.

High-Temperature Wear Behavior of CoCr Matrix Coatings Reinforced by Niobium Element From Room Temperature to 1000 °C

Abstract The high-temperature wear resistant CoCr matrix coatings reinforced by the Nb element were fabricated on the Inconel 718 alloy by using the hot-pressed sintered method. The effects of Nb on the microstructure, hardness, and high-temperature tribological performance of coatings are also investigated in detail, and Nb content was further optimized. The tribological properties were investigated by using a ball-on-disk tribo-tester at 23 °C (room temperature), 200 °C, 400 °C, 600 °C, 800 °C, and 1000 °C. The results showed that the Nb element showed the solution strengthening effect and particle strengthening effect. The Nb greatly affected the tribological performance of the coatings. With the increase of Nb content, the wear-rate and friction coefficient initially decreased and then increased. There was a critical value of Nb content for the high-temperature wear behavior of coatings. The coating with 5.0 wt% Nb kept the best tribological properties compared with other specimens, and the wear resistance was 1.4–29 times as high as that of the substrate. It was attributed to the high hardness, high load-bearing capacity of the Nb-rich phases, and oxide lubricating film on the worn surfaces. The corresponding wear mechanism was discussed.

Tribological performance of zeolite/sodium dodecylbenzenesulfonate hybrid water-based lubricants

Water-based lubricants are a low-cost and environmentally-friendly alternative to their oil-based counterparts. Developing novel water-based formulations with improved lubricity is the key to broadening their industrial applications. We report herein two new water-based hybrid lubricants that integrate sodium dodecylbenzene sulfonate (SDBS) with 4A and 13X zeolites respectively. Their tribological performance was assessed using a block-on-ring test. The addition of SDBS mitigated stick–slip motion, leading to the reduction of coefficient of friction (COF), but caused chemical erosion of the steel surface. The addition of zeolite into water led to an increase in COF due to the occurrence of three-body abrasion. The combination of SDBS and zeolite brought about unique and superior lubrication performance attributed to the synergetic interactions between them. The application of 4A + SDBS lubricant led to a 10% reduction in COF and a four-fold increase in material removal compared to water. The angular 4A particles augmented material removal in combination with the corrosive effect of SDBS. The use of 13X + SDBS lubricant, on the other hand, resulted in a 60% reduction in COF and a 94% decrease in wear comparing to water. This can be ascribed to the “lubricated micro-bearing” effect as a result of SDBS attachment on the rounded 13X particles.

Fretting fatigue experiment and simulation of WC-12Co coating taking into account the wear effects

WC-12Co coating can be used to improve the wear resistance of mechanical connectors, but due to the fretting behavior between the contact interfaces during service, the problem of fretting fatigue cannot be ignored. Through the fretting fatigue experiment and simulation study of the WC-12Co coating, the fretting fatigue mechanism of the coating was investigated under the premise of considering the competitive or coupling relationship between fretting wear and fretting fatigue. Based on Smith-Watson-Topper (SWT) fatigue damage parameter and Miner-Palmgren (M-P) linear cumulative fatigue damage theory, a prediction model for fretting fatigue crack initiation life of WC-12Co coating was established, and the stress redistribution caused by fretting wear was analyzed by Archard wear theory. The analysis results show that the fretting fatigue cracks of the WC-12Co coating mainly initiate at the edge of the wear pits and gradually extend to the substrate; the increase of external load and friction coefficient will accelerate the fretting fatigue process of WC-12Co coating and reduce its fretting fatigue crack initiation life; the change of fretting fatigue crack initiation life and position of WC-12Co coatings caused by fretting wear depends on the change of contact state between contact interfaces.

Tribological behaviour of 3D printed materials for small joint implants: A pilot study

Additive manufacturing is a progressive method in endoprosthetics enabling customisation of implants. However, the challenge is to design articulating surfaces that are wear resistant in a long term. To tackle this challenge, it is necessary to understand the interaction between the surfaces and the lubricant synovial fluid as well as the mechanism of lubrication film formation. In this study we observed three synovial fluid constituents (albumin, γ-globulin a hyaluronic acid) in the contact area simultaneously with the coefficient of friction (CoF). Two metal alloys, CoCrMo and Ti6Al4V covered by DLC, were selected for the experiments as they are both suitable for the additive technology and commonly used in implants manufacturing. The tests were running on a custom-made pin-on-plate tribometer equipped for optical fluorescence measurements. The test apparatus allowed reciprocating motion and observation of the contact area. Our results showed differences in the pace of the CoF increase between the alloys and differences between the samples manufactured by the conventional and the additive manufacturing method. Both the conventionally and additively manufactured CrCrMo samples showed a stable CoF values from the beginning of the experiments: 0.66 (SD 0.02) for the conventional manufacturing CrCrMo samples and 0.53 (SD 0.01) for the additive manufacturing CrCrMo samples. The Ti6Al4V/DLC samples showed a stable CoF values similar to those of the CoCrMo samples not until the 240 s of experiment. These results are related to the protein formation in the contact areas as suggested by a similar increasing trend of the individual synovial fluid constituents in the contact. Increasing protein amounts in the contact led to CoF increase. There were also differences in the ratios of the individual constituents, where both the CoCrMo and the Ti6Al4V/ DLC samples manufactured additionally showed lower concentrations of γ-globulin and hyaluronic acid. These pilot results, on the one hand, support the potential of the additive manufacturing in the implantology and, on the other hand, demonstrate the application of a method suitable for the analysis of the lubricant behaviour in the contact. The method is limited in using the intensity of the emitted light to observe the behaviour of the lubricant film. Future development of the method will require a direct quantification of film thickness.

Elevated temperature contact creep and friction of nickel-based superalloys using machine learning assisted finite element analysis

Nickel-based superalloys with superior thermochemical, mechanical, and tribological properties are highly utilized for critical components in several high temperature applications such as gas turbines and nuclear reactors. Inconel 617, in particular, is considered as one of the main candidate superalloys for tribo-components in very-high-temperature gas-cooled nuclear reactors. Recent findings indicate that this alloy grows unique surface oxide especially in a high-temperature helium environment with distinctive wear, friction, and contact properties. This study investigates the high temperature contact area evolution and frictional behavior of Inconel 617 using finite element simulation and provides predictive models for the contact and friction performance at different normal loads, dwell times, and temperatures. High temperature helium-aged Inconel 617 top surface properties (up to 600 °C) are utilized along with a single asperity-based deformable elastic-plastic contact model under combined normal and tangential loading. Machine learning is used to assist the finite element results and to predict friction coefficient as well as contact area evolution. While a small difference is observed in the instantaneous friction coefficient (no dwell time) for all temperatures, friction coefficient increases considerably with dwell time. This shows that the effect of contact creep for longer dwell times significantly dominants the effect of high temperature variation in basic mechanical parameters such as modulus and yield strength. It is found that increasing temperature and dwell times lead to the friction coefficient increase, yet the dominance of dwell time effects decreases at higher temperatures and loads. While the analysis is presented for Inconel 617, the methodology is easy to be generalized and can be applied to other HT alloys.

In-situ formation, structural transformation and mechanical properties Cr[sbnd]N coatings prepared by MPCVD

This study innovatively employed the microwave plasma chemical vapor deposition (MPCVD) technique to fabricate CrN coatings. The influence of preparation temperature on chemical composition, crystalline phase, morphology, and mechanical properties of the coatings were analyzed systematically. The increase of preparation temperature resulted in the structural transformation of the CrN coatings from the duplex of Cr2N/CrN to the single-phased Cr2N. The coatings with dense and flat surface were obtained at ≤800 °C; however, the decomposition of CrN to Cr2N and N2 led to the formation of pores at ≥850 °C and consequently an increase in friction coefficient and wear. The hardness and elastic modulus increased with increasing temperature because of the structural evolution of the CrN coating and phase transformation. 800 °C was the optimal preparation temperature as the coating exhibited a dense surface and possessed the best overall mechanical properties.

Influence of nonlinear thermal radiation on mixed convective hybrid nanofluid flow about a rotating sphere

AbstractThe present study investigates the mixed convective hybrid nanofluid flow over a rotating sphere under the impact of nonlinear thermal radiation. A model is built to examine the heat transport performance of ferrimagnetic magnetite and copper nanoparticles over a rotating sphere. Nonsimilar transformations are used to nondimensionalize the coupled nonlinear governing equations and the flow model's boundary conditions. Furthermore, the nondimensional governing equations were solved using implicit finite difference approximation and the quasilinearization technique. The impacts of the flow regime on many controlling parameters are then thoroughly addressed. Temperature patterns improve when nonlinear thermal radiation and hybrid nanofluid values increase. The fluid velocity and skin friction coefficient increase in the streamwise direction while decreasing in the rotating direction. The separation of the boundary layer is delayed as the sphere's rotation weakens. The stationary sphere has a larger boundary layer separation than the revolving sphere. The velocity distribution improves with increasing rotation parameter values while decreasing with increasing combined convection parameter values in the rotating direction. An increase in the temperature ratio parameter makes the fluid get hotter, and the Nusselt number goes down simultaneously. Nusselt number and skin friction coefficient in the rotation direction increase, while skin friction coefficient in streamwise direction reduces for increasing values of hybrid nanofluid. The velocity of the fluid enhances in the stream‐wise direction while reducing in the rotational direction with the increasing values of the combined convection parameter.

Effect of Cationic/Anionic Diffusion Dominated Passive Film Growth on Tribocorrosion

Tribocorrosion behaviours of nickel (Ni) and niobium (Nb) in sodium sulfate (Na2SO4) solution under potentiodynamic and potentiostatic conditions were studied. Under the potentiodynamic condition, the passivation was early broken, accompanied by a sharp increase in frictional coefficient on Nb. The current was more fluctuant, and larger material loss appeared at the higher potential in the potentiostatic condition. However, these phenomena did not occur for Ni, and it even showed lower material loss at the higher potential in the potentiostatic tribocorrosion test. The differences in tribocorrosion behaviour had a close relationship to the passive film growth mechanism, which decided the passive film/metal interface structure. Nb with anionic diffusion dominated mechanism in passive growth would cause the accumulation of oxygen vacancies at the passive film/metal interface. This may weaken the adhesion between the metal and the passive film. However, with the cationic diffusion dominated passive film growth on Ni, cation vacancies concentrated at the passive film/tribo-film interface, and this did not affect the adhesion between metal and passive film. Ni or other passive elements with the cationic diffusion-dominated mechanism in passive film growth were recommended as the alloying element for improving the tribocorrosion resistance of alloys.

The influence of friction on the determination of rock fracture toughness

The accurate determination of fracture toughness (KIc) in rock is of great significance to the study of rock fracture characteristics. However, the effect of contact friction in the loading process on the test results of fracture toughness is rarely considered, and the tested KIc in previous studies is generally higher than the inherent KIc of the material. Based on the ABAQUS software, the influence of friction on the test results of KIc was investigated under different material elastic moduli, loading conditions and specimen sizes by the finite element analysis in this study. The results show that for the specimen with the notch semi-circular bend configuration, if the presence of friction is considered, the changes of load level, the span of roller support and the specimen size can significantly affect the test results of fracture toughness, except the elastic modulus of the material; if the influence of friction is not considered, there will be a large deviation in the research results of the size effect of fracture toughness in rock. As the friction coefficient increases, the estimated value of the fracture energy increases, while the fracture process zone length decreases for the specimen with an infinite size.

Influence of contact friction on static characteristics of multileaf bump foil bearing

In order to reveal the influence mechanism of foil contact friction on the multi-leaf bump foil bearing performance, a deformation simulation model is established under the contact constraint of multi top foils and bump foils, and the analysis of the fluid-structure interaction is realized by combining with the Reynolds equation of compressible gas. The influence of the nonlinear variation of foil stiffness caused by contact friction on the static characteristics of the bearing under different bearing numbers is numerically analyzed. The results show: the foil stiffness increases nonlinearly with the foil deformation and presents the characteristic of circumferential non-uniformity. With the increase of the deformation of foil, the influence of friction coefficient on the foil stiffness is more significant. Within the parameters studied in this paper, the friction coefficient increases from 0 to 0.5, the bearing capacity increases first and then decreases when the bearing numbers is small; the bearing capacity increases monotonically when the bearing numbers is large, the higher the bearing numbers, the more significant the growth rate. The attitude angle increases with the increase of friction coefficient, and the bearing becomes more and more unstable. When the bearing numbers is large, we should pay more attention to the influence of friction on bearing performance. The computed model emphasizing nonlinear stiffness in this paper is close to the realities, and the results can provide guidance in the design of multi-leaf bump foil bearing.

Numerical analysis of drill string whirl for vertical well based on rolling model and small-strain shell element

Whirl is a kind of serious lateral vibration. Accurate numerical simulation of whirl for drill string is important for drilling engineering. In this study, a new finite element (FE) model using shell element is established, which can accurately describe the whirl and realize the visualization of motion. Rolling model of drill string in the borehole is proposed. Kinematic contact with clearance, Coulomb friction model and initial imperfection are considered in this model. The explicit dynamic method is selected to solve the convergence difficulty caused by complex contact between shell elements. Compared with the experiment results, the accuracy and reliability of the model are verified. Meanwhile, the effects of rotation speed, friction coefficient, weight-on-bit (WOB) and clearance between the drill string and the borehole wall on the whirl of vertical wells are also studied in the numerical simulation. The rotation speed and friction coefficient have an obvious positive effect on the random motion and backward whirl of drill string: The continuous increase of rotation speed or friction coefficient can cause backward whirl, after that, velocity, acceleration of drill string, contact force and friction will increase significantly. When the rotating speed increases from 400 rpm to 800 rpm, the whirling speed increases by 160%, the average friction and contact force increase by 290%, which is much higher than the increase of 100% of the rotating speed. Instead, WOB has little influence on the drilling parameters mentioned-above and the variations of average friction and contact force for different WOB does not exceed 10%. In addition, properly increasing the distance between the drill string and the borehole wall will make backward whirl easily occur. However, when the clearance is large enough, there will be no contact and the drill string will make irregular non-contact forward whirl. What's more, forward whirl has a positive impact on eccentric wear, and backward whirl reduces WOB transmission and increases risk of sticking. Therefore, drill string whirl should be reasonably utilized or its harm should be reduced in practical drilling engineering.

Experimental and Computational Investigations on Fretting Fatigue Crack Growth in Dovetail Joints

Fretting fatigue is a significant damage mode for turbine engine blade/disk components. In the present work, fretting fatigue crack growth of a nickel-based superalloy was experimentally and computationally investigated. Fretting fatigue experiments of dovetail joints were conducted under loading ratio , and the results revealed that the fretting fatigue cracks nucleated in multiple sites on the contact edge, which was accompanied by oxide wear debris and propagated with a single through-thickness crack. The crack growth rate was determined based on the variation of applied loading ratio in the experiments. With the help of the maximum tensile stress criterion, the friction coefficient of the fretting pair was determined from simulations of the fretting fatigue crack propagation. With the increasing friction coefficient, the kinking angle of the fretting crack approaches a stable value. The stress was verified to have significant effects on the short crack kinking and implemented in the maximum tensile stress criterion. A methodology based on fracture mechanics was proposed to identify the friction coefficient on the fretting surfaces.

Elastohydrodynamic lubrication analysis of cam-roller pairs of internal combustion engines

Based on the cam-roller pair of an internal combustion engine, a mathematical model of transient thermal elastohydrodynamic lubrication under finite line-contact and the corresponding numerical solver have been established. Under an actual load spectrum and other working parameters from an industry partner, the detailed lubrication performance of the cam-roller pair has been obtained in a cam rotation cycle. Particularly, the influences of the roller convexity, the roller tilting and the skidding between cam and roller on the contact pair lubrication are presented. The lubrication performance of the cam-roller pair depends on the roller convexity markedly, and an optimal roller convexity can effectively reduce the probability of the lubrication film rupture and improve the uniformity of pressure distribution in the contact zone. The roller tilting can worsen the lubrication of the contact zone, and increase the possibility of lubrication failure. If the tilting is taken into consideration at the beginning of the roller convexity design, its negative effect can be effectively alleviated to a certain extent. The skidding results in noticeable increase in temperature and friction coefficient, which can deteriorate the contact zone lubrication. The results provide some theoretical data basis for the lubrication design of cam-roller pairs.

Experimental Study on the Influence of Slurry Concentration and Standing Time on the Friction Characteristics of a Steel Pipe-Soil Interface

In pipe jacking, the continued growth of friction resistance around the pipe may cause problems, such as an insufficient jacking force, soil collapse, and even land subsidence, which seriously endangers the structural safety of the pipe and surrounding structures. Bentonite slurry is often used as a lubrication material, but the friction resistance increases due to the inappropriate slurry concentration, and this may cause construction safety problems. In addition, the slurry standing time increases the difficulty of re-jacking construction. To further study the above problems, the friction characteristics of a steel pipe-soil interface under different slurry concentrations and slurry standing times were studied using direct shear tests. The test results show that the peak shear stress and friction coefficient of the interface first decrease and then increase with the increase in the concentration, which is less than or equal to 20%. The peak shear stress and friction coefficient increase with the increase in the concentration, which is greater than 20%, and the position of shear failure changes from between the slurry soil mixture and pipe wall to between the slurry and pipe wall, and finally to the slurry interior. The influence of the slurry standing time on the friction characteristics of the interface is as follows: For the same slurry concentration, the peak shear stress and friction coefficient of the interface increase with the standing time, approximately increasing first and then stabilizing; for different slurry concentrations, the amplification of the peak shear stress and friction coefficient increase with the increase in the concentration.

Effect of TiO2 nanoparticles on the tribological properties of lubricating oil: an experimental investigation

Nano-lubricants offer improved tribological properties in many applications, such as machines and engines. The presence of nanoparticles in the lubricating oil affects its wear, friction, thermal, chemical and physical properties in many ways. Titanium dioxide (TiO2) is a promising lubricant additive for enhanced engine efficiency. This article reports the effect of 10 W-30 pure base engine oil suspended TiO2 nanoparticles. Four different volume concentrations (0.01%, 0.025%, 0.050% and 0.075%) of TiO2 nanoparticles in the base lubricating oil are used for the analysis. The tribological tests were performed at ambient temperature as well as at 75 °C using a four ball tribometer. Scanning electron microscope (SEM) and Alicona Inginite Focus G5 microscope were used to analyze the worn surface. The results show that the surface-modified TiO2 nanoparticles can remarkably improve the load-carrying capacity, the friction reducing, and anti-wear abilities of the additive oil. The diameter of the wear trace and the coefficient of friction are the tribological properties analyzed for the nano-lubricant prepared at different volume concentration (VC). It was found that the diameter of the wear scar and the coefficient of friction increase with increasing VC of TiO2 nanoparticles in the lubricating oil. The main objective of the paper is to present the recent progress and, consequently, to develop a comprehensive understanding of the tribological behavior of engine oil mixed with TiO2 nanoparticles.

Numerical and Experimental Study on End Effect of Waste-Soil Samples under Uniaxial Compression

In order to investigate the influence of end effect on the waste-soil sample strength under uniaxial compression, the influence law of end effect on the uniaxial compression strength is numerically simulated by the FLAC3D software, and the reliability of the numerical model is verified by comparing it with the test results. Based on the above model, the influence of end effect on the lateral displacement and stress state of waste-soil samples is simulated, and the formation mechanism of end effect under uniaxial compression is revealed. The results show that, when the friction coefficient increases from 0 to 0.4, the uniaxial compression strength and percentage of triaxial compression units first increase rapidly and then slowly and then finally remain invariant. With the increase of the friction coefficient, the lateral displacement of the sample is significantly reduced. On the end surface, the nearer to the edge one is, the greater the decrease is. The triaxial compression region on the axis section is triangular in distribution; with the increase of the friction coefficient, this region increases first rapidly and then slowly and then finally remains invariant. The end effect produces lateral compression stress by reducing the lateral displacement of the end surface of samples, and lateral compression stress causes the sample to enter the triaxial compression state. With the increase of the friction coefficient, the triaxial compression region increases gradually, and thus, the uniaxial compression strength of the entire sample increases.

Performance of non-asbestos organic brake liners for light motor vehicles

Abstract In the present work, a measurement system has been designed and developed to measure friction and wear for two wheeler brake liners. Non-asbestos organic brake liners were evaluated regarding friction and wear by pressing against aluminum alloy brake drum. Each brake liner was subjected to four different applied loads (400, 500, 600 and 700 N) with six different braking times (2, 4, 6, 8, 10 and 12 minutes). The velocity of the brake drum was fixed at 1000 rpm for all test conditions. The drum temperature was measured by a thermocouple embedded in the brake drum and coupled with programmable logic controller (PLC) to record the temperatures of brake drum during braking. Test results obtained from the friction test were drum temperature (°C), friction coefficient (μ) and wear volume (cm3). From the test results, it was concluded that the increase of friction coefficient at the beginning of braking is due to the sharp edges such as zircon being ploughed onto the wear surfaces and also due to the increase in the contact area of the brake drum. The wear volume increased exponentially for the applied load more than 600 N and braking time more than 4 minutes. Below the applied load of 600 N, the wear volume increased linearly. The degradation of the organic components in the composition increased with temperature and this resulted in the reduction of composition bonding between the powders and structure integrity, thus increasing wear exponentially.

루테늄과 사마리아 도핑된 세리아의 서멧 구조 스퍼터 박막의 조성 비율 변화에 따른 스크래치 특성

Interest in the use of thin film of Ruthenium-Samaria doped ceria cermet (Ru-SDC) as anode in solid oxide fuel cells is increasing due to its high oxygen storage capacity and high chemical and thermal stability. To have enough structural integrity between sputtered Ru-SDC films and underlying substrates, good adhesion property is required. In this work, scratch resistance and failure mode for Ru-SDC films with various SDC composition were investigated using a scratch test method employing linearly increasing load from 1 to 50 N using a 200 μm radius Rockwell C indenter. Scratched surfaces were examined with a field emission scanning electron microscope. Chemical compositions in scratch tracks were analyzed by energy dispersive X-Ray spectroscopy. Critical loads for films with different SDC ratios were assessed and associated failure modes were identified. The highest scratch resistance among tested film compositions was the one that contained 50% of SDC. Failure modes of tested films regardless of the ratio of SDC were identified to be the initiation of tensile cracks with rapid increase of friction coefficient followed by chipping, and eventually the generation of a severe crack network.