Evolution Of Friction Coefficient Research Articles

Tool-life prediction under multi-cycle loading during metal forming: a feasibility study

In the present research, the friction and wear behaviour of a hard coating were studied by using ball-on-disc tests to simulate the wear process of the coated tools for sheet metal forming process. The evolution of the friction coefficient followed a typical dual-plateau pattern, i.e. at the initial stage of sliding, the friction coefficient was relatively low, followed by a sharp increase due to the breakdown of the coatings after a certain number of cyclic dynamic loadings. This phenomenon was caused by the interactive response between the friction and wear from a coating tribo-system, which is often neglected by metal forming researchers, and constant friction coefficient values are normally used in the finite element (FE) simulations to represent the complex tribological nature at the contact interfaces. Meanwhile, most of the current FE simulations consider single-cycle loading processes, whereas many metal-forming operations are conducted in a form of multi-cycle loading. Therefore, a novel friction/wear interactive friction model was developed to, simultaneously, characterise the evolutions of friction coefficient and the remaining thickness of the coating layer, to enable the wear life of coated tooling to be predicted. The friction model was then implemented into the FE simulation of a sheet metal forming process for feasibility study.

Characterisation of friction behaviour of flake, spheroidal, and compacted vermicular graphite cast irons

This paper investigates friction behaviour of flake, spheroidal, and compacted vermicular graphite cast irons used for piston rings of automobile engines. Pin–on–disc friction tests are conducted under dry and room temperature conditions. Pins are made of graphite cast irons, and discs are chromium molybdenum steels. During a test, Coulomb friction coefficient and sliding distance are recorded. Graphite size fraction on the initial surface of the cast iron is determined and its influence on friction coefficient is investigated. Results show that friction coefficient growth rate during the 'running–in' period can be expressed as a power–law function of sliding distance. Two parameters in the power–law form, the damage rate constant and the damage exponent, can describe a friction coefficient evolution during the period. It is identified that graphite size fraction has an influence on a friction coefficient evolution. The influence of graphite size in cast iron is described with the damage rate constant.

Tribo-mechanical Characterisation of Wear Behaviour for Manufacturing of Wind Turbine Gearbox Structural Parts

The main objective of this study is to simulate and compare the wear behaviours of ADI1000 and EN-GJS700-2 against 18CrMo4 for a specific key/housing geometry when surface and lubrication conditions are changed. These materials are of great interest for other structural parts, especially in the wind turbine industry. Two main aspects are going to be analysed: Wear Track Evolution and Friction Coefficient Evolution. The test consists in sliding a piece of a cylinder (on which a normal force of 685.2N is applied) in contact with the moving face of the disc. The relative displacement between the two samples is a continuous sliding with constant speed of 1.5 m/s. According to lubrication, there are two types of tests in order to identify the effect of lubrication in the wear behaviour.

Wear Behavior of Zn-15Al-1Cu-Mg Alloy after Aging

This paper studies the effect of artificial aging on the wear resistance of the recently developed Zn-15Al-1Cu-Mg alloy. Pin-on-disk tests were performed to evaluate the evolution of friction coefficient and wear rate on samples produced via spin casting. Hardness and tensile properties were also monitored as a function of aging time. After the tests, wear scares and fracture surfaces were analyzed by scanning electron microscope, in order to understand the damage and the failure mechanisms. It was found that hardness and tensile strength decrease with the aging time, while both friction coefficients and wear rates increase.

Numerical Analysis of Turbulent Forced Convection of Nanofluid Flow over a Heated Shallow Cavity

This work presents a numerical investigation of turbulent forced convection of a nanofluid over a heated cavity in a horizontal duct. Heat transfers in separated flows are frequently encountered in engineering applications, such as: heat exchangers, axial and centrifugal compressor blades, gas turbines blades, and microelectronic circuit boards. Thus, it is very essential to understand the mechanisms of heat transfer in such regions in order to enhance heat transfer. Different volume fractions of nanoparticles are presented in the base fluid and different types of nanoparticles are used. The objective of this study is to check the effect of nanofluid on heat transfer in such a configuration. Numerical simulations are performed for pure water and four nanofluids (Cu, CuO, Ag, and Al2O3). The results are analyzed through the thermal and dynamical fields with a particular interest to the skin friction coefficient and Nusselt number evolutions. The average Nusselt number increases with the volume fraction of nanoparticles for the whole tested range of Reynolds number. A correlation of average Nusselt number versus Reynolds number and volume fraction of each type of nanoparticles over the cavity wall is proposed in this paper.

A study of the frictional characteristics of metal and ceramic counterfaces against electro-deposited coatings for use on automotive seat rails

An investigation was conducted on the friction coefficient changes when ceramic balls oscillate against a series of epoxy-based cathodic electro-deposited coatings that could be used for automotive components. Reciprocating sliding tests were conducted using ball-on-plate apparatus; balls were made of silicon nitride, zirconia, and AISI 52100 (also known as GB GCr15, DIN 100Cr6, and JIS SUJ2) steel. Plates used for coating substrates were made of cold-reduced high strength steels. The Coulomb friction coefficient was measured under room temperature and ambient humidity. The endurance lives of cathodic electro-deposited coatings against various ball materials are directly compared using a plot of friction coefficient evolution. Using the basis of friction coefficient evolution, electro-deposited coatings maintained higher durability against zirconia than against Si3N4 and AISI 52100 balls. In addition, the frictional behavior is analyzed using a form of Kachanov-type damage law. It is concluded that the growth rate of the friction coefficient can be represented by the damage parameters in the law.

Preparation of the electroless Ni–P and Ni–Cu–P coatings on engine cylinder and their tribological behaviors under bio-oil lubricated conditions

Two electroless Ni–P and Ni–Cu–P coatings on engine cylinder were prepared with chemical plating nickel technology. The coatings were characterized by Energy Dispersive X-ray spectrometer (EDX), Scanning Electron Microscope (SEM) and X-ray Diffraction (XRD). The friction and wear behaviors of cylinders with these two coatings were compared with those of the cylinder without coating under bio-oil lubricated conditions. It was found that the cylinders after coating had significant improvement of tribological properties; in addition, the Ni–Cu–P coating had higher wear resistance and lower friction coefficient than those of Ni–P coating. Under the bio-oil lubricated conditions, the evolution of friction coefficient with sliding speed agreed with the classical Stribeck curves, namely, there was a transition from boundary lubrication (BL) to mixed lubrication (ML) to hydrodynamic lubrication (HL) as sliding speed increased. The lower friction coefficient in ML regimes than that in BL regimes depended on both of the adsorbed oil film and the tribo-film on the worn surfaces. The better tribological performances of Ni–Cu–P coating than those of Ni–P coating were ascribed to the formation of a tribo-film with CuO. Therefore, the Ni–Cu–P coating showed a very potential for preventing engine cylinders from the corrosiveness of bio-oil.

Identification of friction and heat partition model at the tool-chip-workpiece interfaces in dry cutting of an inconel 718 alloy with cbn and coated carbide tools

Abstract This paper aims at characterizing the frictional behaviour at the cutting tool-workmaterial interface during the dry machining of a Inconel 718 in its aged state with various coated carbide tools and c-BN tools. A specially designed open tribometer has been used to characterize friction coefficient, heat partition coefficient under extreme conditions corresponding to the ones occurring in cutting. The tribometer provides the evolution of the apparent friction coefficient and of the heat partition coefficient for a large range of sliding velocity and contact pressure. It has been shown that friction coefficient as well as heat partition coefficient decrease with sliding velocity or contact pressure. A threshold effect of the contact pressure has been highlighted. On the contrary, any sensitivity to coatings deposited on carbide has been observed, whereas c-BN leads to very low friction coefficients.

Finite element implementation of a varied friction model applied to torsional fretting wear

Coefficient of friction is a significant factor for investigating stress and strain on the contact surface for a problem on torsional fretting wear. First, in this paper, a variable coefficient of friction model related to space and time is introduced. The evolution of friction coefficient is governed by the local contact history and the slip distance. Then, this model is implemented in the finite element code ABAQUS by the user subroutine FRIC. A sphere–plane finite element model is constructed to simulate stress and strain on the contact surface under the torsional fretting loading. Comparisons between the experimental friction torque curves and the numerical data rooting from this model show a good agreement. In view of the relation between the plastic strain and friction shear stress, it is seen that damage mechanisms in the different slip regimes are different. Finally, crack initiation location in the mixed slip regime is predicted based on the Smith–Watson–Topper multiaxial fatigue criterion. The predicted results are in agreement with the corresponding experiments.

Fabric friction behavior: study using capstan equation and introduction into a fabric transport simulator

All thin materials, such as textiles, papers, polymers, or metals, are handled on rollers during manufacture and subsequent use. This requires several unwinding and winding processes. The goal of this study was to investigate the friction behavior of fabrics relative to sliding velocity and to introduce friction coefficient evolution in a fabric transport model. For the experimental part, a specific fabric/roll friction bench is presented. The friction coefficient was calculated from the capstan equation. The evolution of the friction coefficient was quantified relative to the sliding velocity for different textile fabrics and also for a polymer and a paper web. The influence of some measurement process parameters was studied: web tension, roll diameter, and wrap angle. The friction coefficient initially increased with sliding velocity and then became constant. This phenomenon can be explained by the deformation of the fabric due to friction, thereby inducing an increase in web tension with the sliding velocity. The relationship between tension and rolling friction behavior of the fabric was investigated. The variability of the friction coefficient was introduced to web/roll simulator, and improvements to the simulator are shown.

Characterization of Ni-Cr coating failures deposited on CW614 substrate using friction coefficient evolution

In this study, characterization of the total wear of coatings by the friction coefficient evolution of nickel-chromium (Ni-Cr) coatings is presented. The device used is a developed ball-on-disc tribometer. The samples tested in this study are brass discs (CW614) coated with Ni-Cr layers sliding against a steel ball. The effect of normal load, sliding velocity and substrate roughness on the wear of coatings is studied. An optical microscope (OM) and energy dispersive spectroscopy (EDX) are used to confirm the coating failures. The results show that the total wear of Ni-Cr coatings is related to the sliding distance. However, an increase in normal load and/or sliding velocity increases the total wear of the coating, whereas the reduction of the substrate roughness reduces the coating failures and increases their service behavior.

Fretting corrosion damage of total hip prosthesis: Friction coefficient and damage rate constant approach

This paper analyzes friction coefficient evolution between materials related to total hip prosthesis. Fretting corrosion tests were conducted with stainless steel and poly(methyl methacrylate) interacting surfaces. In the course of fretting corrosion tests, the Coulomb friction coefficient is determined as a function of the number of cycles. It was found that the friction coefficient growth rate can be expressed as a power-law function. The influences of ionic strength, applied potential, pH, and albumin content on fretting corrosion were then investigated on the basis of the evolution of the friction coefficient. Finally, we identify the damage rate constant as being relevant for linking the mechanical and chemical parameters in the evolution of damage.

Friction in starved hydrodynamically lubricated line contacts

The hydrodynamic friction coefficient, which determines the friction coefficient at high Sommerfeld numbers (viscosity⁎velocity/load) has a unique solution for the fully flooded case. As such the friction coefficient can be predicted as a function of the above number. As shown by Cameron (1976) [1] the friction coefficient in the fully flooded regime increases as the square root of the Sommerfeld number. For very low Sommerfeld numbers, the asperity interaction causes the friction coefficient to increase when entering the mixed lubrication regime.Even though a unique (and low) friction coefficient exists in the fully flooded regime, the situation is more complex in the starved regime. First of all the friction coefficient is higher in the starved regime, and secondly the coefficient depends on the degree of starvation.This paper analyses the load carrying capacity, the Poiseuille flow based friction and the Couette flow based friction, as a function of the degree of starvation. It is shown that the Poiseuille friction force diminishes fastest with starvation, followed by the load carrying capacity, and finally the Couette term diminishes slowest of all three terms.As a consequence the friction coefficient for sliding starved conditions is dominated by the Couette term.The current paper analyses the friction coefficient evolution as a function of starvation for line contact conditions, using analytical and numerical tools. Finally, curve fitted equations are given for the friction as a function of the starvation level.

Characterization of Friction and Heat Partition Coefficients during Machining of a TiAl6V4 Titanium Alloy and a Cemented Carbide

This article aims at characterizing the frictional behavior of a TiAl6V4 alloy and a carbide tool under extreme conditions corresponding to those occurring at the cutting tool–work material interface. A specially designed open tribometer was used to characterize the macroscopic friction coefficient, heat partition coefficient, and adhesion in the contact versus sliding velocity and contact pressure. It has been shown that titanium leads to intense adhesion, which seems to be even more intensive with high contact pressure and high sliding velocity, which limits the local sliding movement at the interface (stuck layer). However, the tribometer provides the evolution of an apparent friction coefficient and a macroscopic heat partition coefficient related to the shearing of titanium between the adhesive layer and the bulk material. An increase in sliding velocity or contact pressure induces a small decrease in the apparent friction coefficient as well as the heat partition coefficient. It has been shown that adhesion is thermally activated by a combination of contact pressure and sliding velocity, which leads to a threshold effect. Furthermore, the application of an emulsion showed a small decrease in the apparent friction coefficient associated to a decrease in adhesion. Finally, this work provides quantitative data on the apparent friction and heat partition coefficients versus sliding velocity and contact pressure that can support the development of macroscopic cutting models for titanium alloys.

Interactive mechanisms of fretting wear and corrosion of steel wires in alkaline corrosion medium

Fretting corrosion tests of steel wires inside a rope were performed on a self-made fretting wear test rig to understand the fretting corrosion behaviors of steel wires under the environment of alkaline mine water trickling using a simulated alkaline corrosion medium. The results indicate that the corrosion rate of steel wire decreases rapidly and then increases with the increase in the duration of immersion. For late immersion, the corrosion rate tends to be smooth because of the large accumulation of corrosion products. It is also found that the evolution of friction coefficient vs fretting cycles can be divided into five stages. With the increase in the fretting cycles, there is an increase in the weight loss of materials and the evolution processes of damage rate can also be divided into four stages. All the results conclude that interaction of corrosion and wear dominates the fretting corrosion and leads to material loss.

Analysis of the Biotribological Behavior for the Stainless Steel/Polyethylene Contact Using a Knee Prosthesis Simulator

In this work, a friction and wear simulator was used to reproduce the Anterior-Posterior (AP) sliding and the Flexion-Extension (FE) rotation generated in the knee joint during human gait cycle. We chose to simplify the contact geometry between the Total Knee Arthroplasty (TKA) femoral component and tibial insert. A 304L stainless steel cylinder which replaces the femoral component was loaded onto a flat High Density Polyethylene (HDPE) block which replaces the tibial insert. The tribological behavior of the considered contact was analyzed by tracking the number of cycles, the friction coefficient, the roughness of the wear track on HDPE, the HDPE weight loss and the damage mechanisms. The friction coefficient shows a gradual increase with the number of cycles for both AP and FE kinematics. The evolution of friction coefficient with the number of cycles is not affected by the value of the imposed normal load in the case of AP sliding. For the FE rotation, decreased friction coefficient is obtained when the imposed normal load increases. For both considered AP and FE kinematics, the roughness of the wear track on the HDPE is not affected by the imposed normal load. It shows a progressive decrease when the number of cycles increases. The wear of HDPE obeys the Archard law and the wear coefficient increases with the normal force. For a given value of normal load, the obtained wear coefficient for the AP sliding is larger than that obtained for FE rotation. A predominant adhesive wear mechanism was identified for both AP and FE kinematics. Under the same normal load, damage development in terms of plastic deformation, micro-cracking and debonding is more pronounced for the AP sliding if compared with the FE rotation. For a given kinematics, the damage severity increases with the normal load. This finding is in good agreement with the predicted values of the wear coefficient according to the Archard law.

Influence of the dry woven fabrics meso-structure on fabric/fabric contact behavior

The first stage of the RTM process concerns the preforming of the part. During the preforming of multilayered reinforcements, frictions between the plies occur. An experimental device designed to analyze the ply/ply, ply/tool, and yarn/yarn frictions has been built. Specific contact behavior for ply/ply friction is directly related to shocks taking place between overhanging yarns of each sample. Within the signal, two particular periods can be exhibited. This work shows for four different architecture woven fabrics that the two periods are directly related to the meso-architecture of the studied woven fabrics and more precisely to the characteristics of the fabric unit cell. The friction response appears to be very sensitive to the relative positioning and orientation of the samples. It can also lead to manufacturing defects such as unweaving or wrinkles. Up to now, finite element codes predicting the forming behavior of woven reinforcement fabrics do not take into account this friction coefficient evolution to model accurately the process. This point could therefore be addressed in future works on this topic with the view to optimize the multi-ply forming of composite parts.

Friction and wear behaviour of bacterial cellulose against articular cartilage

Bacterial cellulose (BC) is a natural and biocompatible material with unique properties, such as high water holding capacity, ultra-fine fibre network and high strength that makes it an attractive material for the repair of articular cartilage lesions. However, data on the tribological properties of BC is very scarce, particularly if natural articular cartilage is involved in the contact. In this work, unmodified BC pellicles were grown from Gluconacetobacter xylinus in order to be used as tribological samples against bovine articular cartilage (BAC) in the presence of phosphate buffered saline (PBS). The tribological assessment of the sliding pairs was accomplished using reciprocating pin-on-flat tests at 37 °C. The reciprocating sliding frequency and stroke length were kept constant at 1 Hz and 8 mm, respectively. Contact pressures ranging from 0.80 to 2.40 MPa were applied. The friction coefficient evolution was continuously monitored during the tests and the release of total carbohydrates into the lubricating solution was followed by means of the phenol–H2SO4 method as an attempt to evaluate wear losses. The morphology of worn surfaces was characterized by SEM/EDS and the main wear mechanisms were identified. Low friction coefficient values (∼0.05) combined with the preservation of the mating surfaces (BC and BAC) indicate the potential of BC to be used as artificial cartilage for articular joints.

Tribological behaviour of Pearlitic and Bainitic steel grades under various sliding conditions

Railway systems have to cope with increasing and heavier traffic conditions. New rail steel grades are designed to extend rail life and to prevent rail defects. The tribological response of such steel grades under wheel–rail contact conditions needs to be studied before its use on real track. Tests on reduced-scale simulator, i.e. roller-on-plate tribometer with wheel–rail contact like conditions, have been performed on three rail steel grades: the Pearlitic, the heat-treated Pearlitic and the Bainitic. Results show a difference in both sensitivity and mechanism of particle detachment with the steel grade microstructure, but no difference in friction coefficient evolution.

Effects of Lubrication Mode on Friction and Heat Partition Coefficients at the Tool–Work Material Interface in Machining

The quantification of friction coefficient along the tool–work material interface in machining remains an issue in tribology. This article aims at identifying the evolution of friction coefficient for a large range of sliding velocity during the machining of an AISI 4140 steel (290 HB) with a TiN-coated carbide tool. The influence of various lubricants (straight oil or emulsion) and lubrication modes (flow or mist) is investigated and compared to a dry sliding situation (dry machining). It has been shown that, in dry machining, the friction coefficient decreases with the sliding velocity until reaching a lower limit around 0.2. On the contrary, the presence of a straight oil significantly decreases friction coefficients to a value around 0.1. Emulsion enables a significant decrease of friction coefficient to around 0.2 for low sliding velocities, whereas its action is absent for higher sliding velocities. Oil mist exhibits an intermediate behavior. Finally, it has been shown that all kinds of lubrication lead to a large decrease of heat flux transmitted to cutting tools, even if heat flux is almost similar irrespective of the nature of oil and of its application mode. Straight oils decrease the heat flux transmitted to cutting tools due the decrease of friction coefficient, whereas they are not able to modify the heat partition coefficient at the interface. On the contrary, emulsions limit the transmission of heat flux to cutting tools due to their lower heat partition coefficient, even if their influence on friction is more limited.