Friction Contact Zone Research Articles

Increase in the coefficient of friction of the rolling stock disc brake via fluid cooling of its friction elements

The paper presents the results of theoretical and experimental investigations in the effect of a partial heat energy removal in the friction contact zone upon the disc brake coefficient of friction during braking. A fluid cooling system for the disc brake friction elements has been proposed based on a recuperative liquidpneumatic heat-exchange apparatus.

Structural and phase transformations in Hadfield steel upon frictional loading in liquid nitrogen

Structural transformations that occur in 110G13 steel (Hadfield) upon sliding friction in liquid nitrogen (–196°С) have been investigated by metallographic, electron-microscopic, and X-ray diffraction methods. The frictional action was performed through the reciprocating sliding of a cylindrical indenter of quenched 110G13 steel over a plate of the studied steel. A like friction pair was immersed into a bath with liquid nitrogen. It has been shown that the Hadfield steel quenched from 1100°С under the given temperature conditions of frictional loading retains the austenitic structure completely. The frictional action forms in a surface layer up to 10 μm thick the nanocrystalline structure with austenite grains 10–50 nm in size and a hardness 6 GPa. Upon subsequent low-temperature friction, the tempering of steel at 400°С (3 h) and at 600°С (5 min and 5 h) brings about the formation of a large amount (tens of vol %) of e (hcp) martensite in steel. The formation of this phase under friction is supposedly a consequence of the reduction in the stacking fault energy of Hadfield steel, which is achieved due to the combined action of the following factors: low-temperature cooling, a decrease in the carbon content in the austenite upon tempering, and the presence of high compressive stresses in the friction-contact zone.

Thermophysical, diffusion, and segregation processes in the frictional contact zone

The calculation of the temperature field for the tribosystem of wheel - composition brake shoe of the rolling stock has shown that the maximum temperature in the wheel is not on the surface, but at 200-1000 micron distance from the friction surface. Under friction, as a result of the thermomechanical destruction of the polymer shoe there evolves hydrogen that is absorbed by the contact surface of the wheel. At that, the maximum hydrogen concentration coincides with the maximum temperature. The research results of the segregation phenomena of alloying and impurity elements in the iron grain boundaries are presented. It is shown that the consistency of the rates of the transfer film formation and its destruction is the kinetic condition of self-organization on the antifriction tribo-contact. The atomic-binding energy of different elements to the grain surface is a periodic function of the atomic numbers of the elements. This may serve as a theoretical basis for predicting the strength properties of steel with a different composition of alloying and impurity elements. The obtained results indicate the promising application of XPS and AES of quantum chemistry in solving tribological tasks.

Wear mechanisms of structural steels and effect of wear on their mechanical and acoustic properties during tension

Optical and scanning electron microscopy have been used to study the wear mechanisms of structural steels with various structures and strengths, as well as to assess their mechanical and acoustic properties after friction. The prevailing wear mechanisms have been revealed; they are governed by the strength and structure of the steels and involve the refinement and rotation of grains, the formation of parallel rows of microcracks, the strain dissolution of cementite, and martensitic transformation, as well as the formation of seizure sites in the friction contact zone, shear and intergranular pores, and microcracks. The low-carbon steel with a ultrafine-grained structure has demonstrated a high wear resistance. Friction for 3000 h had a weak effect on the mechanical properties of the steels during tension.

Structural transformations and wear resistance of titanium nickelide under conditions of sliding friction at a cryogenic (−196°C) temperature

Structural transformations and tribological properties of a Ti49.4Ni50.6 alloy have been investigated at the liquid-nitrogen temperature. It has been shown that the alloy under study possesses the resistance to abrasive and adhesive wear smaller by a factor of 1.4–1.7 and the friction coefficient that is (to 1.7 times) higher, as compared to the austenitic steel 12Kh18N9. The only moderate tribological properties of the titanium nickelide are caused by an enhanced brittleness of this material under the conditions of friction-initiated severe plastic deformation. The enhanced low-temperature brittleness of the martensitic structure is seemingly explained by a low symmetry of the crystal lattice of the B19’ martensite, an atomically ordered state of this phase, and the formation of a brittle amorphous phase in the layer several microns thick near the friction surface of the alloy. The appearance of a continuous amorphous layer at the friction surface of the titanium nickelide is favored by the presence of the martensitic structure in the alloy, its stability under the friction conditions with respect to the reverse B19′ → B2 transformation, and a high intensity of the deformation processes occurring in the zone of friction contact. Below the amorphous layer, a mixed amorphous-crystal-line structure is located. The nanocrystallites are textured and range in size from a few to tens of nanometers. The formation of crystallites of the B2 phase in the amorphized layer appears to occur at the stage of warming of the alloy samples to room temperature. A similar amorphous-nanocrystalline structure arises near the abrasive-wear surface of the Ti49.4Ni50.6 alloy. It has been shown that the presence of a submicrocrystalline structure in the initial Ti49.4Ni50.6 alloy exerts no significant effect on the tribological properties and the character of structural transformations induced in the alloy by the frictional action.

О некоторых механизмах предельного трения с разрушением

A model of the process leading to formation of the third body elements at fracture of material in a dry friction contact zone was suggested. A variant of a plane element of the third body evolution was analyzed for the regime of an intensive shear action on its contact surfaces. A separated fragment can form an rolling element or remain to be a plane slipping element in dependence on a relation between normal pressure forces and shear forces in the contact zone. A criterion for twisting a plane element into a rolling element was obtained (within the framework of the plane elasticity)

Effect of pressure on chemical reactions in the zone of direct friction contact of systems with selective transfer

The paper deals with the effect of heavy pressure in the zone of real tribocontact on the rate of the reactions of complexing of copper chelates that provide the formation of a servovite film and implementation of selective transfer. It is shown that heavy pressures accelerate significantly the complexing reactions at the initial stages of servovite film formation during selective transfer. The procedure of search for potential complexing agents for implementing selective transfer can be simplified by using the developed method of mole volumes. This method allows the prediction of the efficiency of a complexing ligand at the initial stage of servovite film formation with account for heavy real pressures.

On the possibility of using acoustic spectra to study deformation processes in surface layers during friction

Elastic waves generated during friction are considered to be a source of information on the processes of deformation, fracture, and adhesive interactions in the friction contact zone. Results of computer simulations show that, in view of the dynamic nature of friction, the analysis of acoustic signals detected even in a steady-state regime requires applying frequency-temporal techniques in addition to the Fourier transform. It is concluded that the laws of frictional wear can be studied by analyzing the corresponding acoustic spectra.

Tribosynthesis of lead fluoride in friction of fluroplastic lead-containing composites and its effect on wear resistance

The processes running in the friction contact zone of lead-containing fluoroplastic composites and steel counterbody are studied. The results of X-ray phase examination of the transfer films on the steel counterbody are presented for the case of friction of lead-fluoride-containing fluroplastic composites. In friction of the given composites, the secondary structure, namely lead fluoride, is shown to arise in the contact zone. It is found that the fluorite crystalline structure β-PbF2 possesses lubricity and increases the wear resistance of PTFE-based composites.

Influence of the stressed state of the zone of friction contact on the formation of the structure of a surface layer and tribological properties of steels and alloys

An investigation has been performed of the influence of contact stresses, which appear in the zone of friction of iron-manganese alloys with 16.9–40.5 wt % Mn, wear-resistant high-manganese austenitic steels, and carbon steel U13, on the phase composition, structure, and strengthening of surface layers of these materials under conditions of dry friction at small (0.03 and 0.07 m/s) sliding velocities, when the frictional heating of the surface layer of the samples is virtually absent. A quantitative estimation of the arising compressive contact stresses (pressures) has been carried out. It is shown that the value of the compressive contact stresses approximately corresponds to the values of microhardness measured on the friction surface of the materials investigated. These compressive contact stresses initiate the occurrence a γ-ɛ martensitic transformation in the iron-manganese alloys with 16–40% Mn, which is characterized by a negative volume effect, but impedes the development of the ɛ-α and γ-α martensite transformations (in austenitic steels) which occur with an increase in the specific volume. The stresses in question favor the formation of nanocrystalline structures in a thin (≤10 μm thick) surface layer of friction steels and alloys. The contact tensile stresses following the contact compressive stresses initiate the formation and propagation of microscopic cracks in the surface layer of the friction materials, and activate the development of martensitic ɛ-α and γ-α transformations in this layer. It is shown that the structural and phase transformations initiated by contact stresses exert a substantial effect on the microhardness, friction coefficient, and resistance to adhesive wear of the steels and alloys under study.

Influence of the structure of babbitt on the stress-strain state in the zone of friction contact

The methods of mathematical simulation are used to study the process of contact friction interaction for homogeneous lead and heterogeneous intact or run-in B16 babbitt. It is shown that the hard and soft structural components of the run-in babbitt strongly decrease the size of the zones of plastic deformation in the material, improve its wear resistance, and promote the increase in the operating reliability and service life of plain bearings.

Identification of elastic waves generated in the contact zone of a friction couple

A method for evaluation of the parameters of elastic waves during simulation of the interaction between solid bodies in friction is proposed. Using an analysis of the Fourier spectra of the time series of velocity components, pressure, and stress intensity, it is possible to determine frequencies characteristic of the given numerical model, the geometry of the problem under consideration, and of the real processes taking place immediately in the friction contact zone.

Modified solid lubricating coatings based on tungsten diselenide

The dependence of the wear rate of WSe2-Ga/In coatings on the environment conditions (air, vacuum, temperature) and counterbody material is obtained. The formation of films in the friction contact zone is shown to occur owing to the transfer of individual lubricant components and their reactions with each other and with atmospheric oxygen.

Tribochemistry of n‐hexadecane in different material systems

AbstractHexadecane is widely used as the model base oil to investigate both tribological effectiveness and tribochemical reaction mechanisms of various antiwear and extreme pressure additives. On the other hand, it is well known from existing references that hexadecane, similarly to hydrocarbon oils, oxidizes during the lubrication process of tribological systems. It has frequently been pointed out that carboxylic acids are formed which — reacting with rubbing surfaces — generate soaps. Most recent studies, however, demonstrate that the oxidation process under friction conditions is very complex and the major oxidation compounds from hexadecane under boundary friction conditions relate to other oxygenates than carboxylic acids.This paper aims to examine what types of surface compound are generated from the hexadecane lubricant in the friction contact zone. Tribological tests were performed on a ball‐on‐disc tribometer. The discs used were made from several materials (steel, aluminium, brass, bronze). Fourier transform infrared (FTIR) microspectrophotometry and techniques involving a scanning electron microscope equipped with an energy‐dispersive spectrometer were used to analyse tribochemical reaction products formed on different substrates. Analytical results obtained clearly demonstrate a similar tribochemical reaction process operating on all the metals tested. The FTIR results obtained allowed new absorption bands to be discovered and accounted for. These absorption bands have been assigned to specific complex compounds. Copyright © 2006 John Wiley & Sons, Ltd.

A bridging law for creeping fibres

The problem of a creeping fibre being pulled out of a semi-infinite elastic matrix to which it is coupled by Coulomb friction is considered. Analytical approximations based on shear lag theory and the assumption of Lamé-like elastic fields lead to simple relations between the relative displacement of the end of the fibre and the surface of the matrix. The existence of a steady state configuration for the frictional contact zone between the fibre and the matrix is demonstrated, which propagates invariantly away from the surface of the matrix under constant applied loads. With trivial extensions, this relation yields a bridging traction law that would describe the relation between the crack opening displacement and the bridging tractions supplied by the fibres in the wake of a matrix crack in a composite. A useful, general result applicable to any traction history is obtained from the steady state results via a quasi-static approximation, which is valid if the tractions do not change too rapidly.

A Mechanical Model of Yarn Twist Blockage in Rotor Spinning

This paper presents a mechanical model in which dynamic equilibriums of forces and moments are considered. This model is able to quantitatively describe twist distribution in the frictional contact zone inside a rotor in terms of yam, machine, and operational parameters. Four cases of the application of this model are discussed. Three terms—the deviate angle, yarn tension, and yarn twist blockage—are given as functions of the wrap angle, contact surface factor, torsional factor, and rotary speed factor.

Structural steels surface modification by mechanical pulse treatment for corrosion protection and wear resistance

The mechanical pulse treatment is an advanced method for improving physical and mechanical properties of surface layers of machine components. This method is based on the use of high-speed pulse heating of surface layers in the friction contact zone. A special liquid is supplied to the treatment zone. During the treatment it is possible to change the chemical composition of the surface layers by saturation of alloy elements. The microhardness then increases up to 8–16 GPa. This mechanical pulse treatment provides sufficient increase of the corrosion and wear resistance of machine components.

A Mechanical Analysis of Yarn Twist Distribution in Frictional Contact Zone.

This paper presents a mechanical model in which dynamic equilibriums of forces and moments are considered.This model is capable to describe, quantitatively, distributions of four functions in the frictional contact zone, which are, deviate angle, yarn tension, frictional force and yarn twist. It can be used to characterize twist blockage between the yarn and guide in terms of yarn, machine, and operational parameters (such as twist, yarn torque, yarn torsional rigidity, yarn tension, yarn count and the wrap angle of yarn over the yarn guide). The effects of these factors are discussed for the case that the yarn twist is independent of time and the yarn speed is much lower than that of strain wave propagation.

Investigation of HDPE wear debris formed by friction against metal surfaces with different oxide film content

The work presents the results of simulation of the local oxidation process of a metal surface and its influence on the chemical composition of wear products formed by polyethylene sliding against tungsten-cobalt hard alloy in an inert gas environment. Using IR spectroscopy, pyrolytic mass spectrometry, differential thermal analysis and electron paramagnetic resonance spectrometry, it was found that the chemical composition of low-molecular weight substances dissolved in the wear debris depends on the oxide content on the metal surface. Depending on the concentration of oxide, the wear debris accumulates different low-molecular weight products including water, carbon oxides, hydrocarbons, partial products of their oxidation and metal-containing substances. The wear rate of the polymer is characterized by a linear dependence on the relative area of oxide film. Its break is presumably connected with the optimum content of associated oxygen participating in polymer spatial cross-linking by carboxylation and ester bridge formation as well as the formation of metal-containing substances of oxyhydroxide, carbonate and organic metal salt type. The change in friction coefficient correlates with the yield of low-molecular weight hydrocarbons. The formation of a local metal oxide film has a significant influence on the parameters of the tribological system owing to excitation of tribochemical processes causing the generation of polymer oxidation products in the friction contact zone.

Acoustic emission method for research and control of friction pairs : Tribology International, Vol. 24, No. 1, pp. 11–16 (Feb. 1991)

Abstract One of the new physical methods that provides much information about the processes going on in a friction contact zone in real time is acoustic emission (AE). Basic experimental results of AE in friction are presented and analysed. Using AE signals we can get an indication about the state of the friction processes, about the quality of solid and liquid layers on the contacting surfaces in real time, sometimes about intensity of wear, changing of wear and friction regimes and so on. The influence of sliding velocity, load, roughness and some physical and mechanical properties of the body on the parameters of AE are also given. Some attention to the practical implementation of these techniques for control of capability for work of friction pairs in real conditions are given. Modern methods and devices enable condition monitoring of sliding and rolling bearings.