Multi-phase materials and their surface contact behaviour with reference to friction and wear

The recent advances in the development of composite surface coatings and the increasing tribological application of multi-phase materials in engineering require a more rigorous treatment of the problem of the contact of surfaces of such materials. This paper considers the surface contact behaviour of materials having contrasting physical properties and the results presented quantitatively identify the boundaries of three contact regimes, purely elastic, elastic/plastic and predominantly plastic. The treatment also offers a method for determining the real areas of contact and corresponding supported loads associated with the modes of deformation of each phase. This is particularly important for predicting the friction and wear characteristics of multi-phase materials.     

Modeling of surface roughness effect on dry contact friction in metal forming

Interfacial conditions such as friction and roughness substantially affect the process characteristics of metal forming. This study developed a dry friction model that accounted for the adhesion and interference effects of surface roughness. A sliding friction coefficient was suggested to provide fundamental information about the interfacial conditions of the contact surface. The proposed model was easily verified by published experiments and predicted values agreed with experimental results. Accordingly, friction coefficient ?? clearly increased as relative roughness R _m (=?roughness of tool $ R_a^T $ /roughness of workpiece $ R_a^M $ , measured as interference effect) increased. Simulations confirmed that the friction coefficient ?? decreased as dimensionless stress S _m (=?contact pressure p _m /tensile strength $ \sigma_u^0 $ ) increased at small strain hardening exponent n-values. Under the conditions of large n and small R _m values, the friction coefficient ?? initially decreased and then increased. It then slightly decreased as dimensionless stress S _m increased. However, this trend became less apparent as relative roughness R _m increased since friction coefficient ?? simply decreased.     

Influence of surface preparation on roughness parameters,friction and wear

The aim of the present research was to investigate influence of surface preparation on roughness parameters and correlation between roughness parameters and friction and wear. First the correlation between different surface preparation techniques and roughness parameters was investigated. For this purpose 100Cr6 steel plate samples were prepared in terms of different average surface roughness, using different grades of grinding, polishing, turning and milling. Different surface preparation techniques resulted in different R_a values from 0.02 to 7 μm. After this, correlation between surface roughness parameters and friction and wear was investigated. For this reason dry and lubricated pin-on-disc tests, using different contact conditions, were carried out, where Al₂O₃ ball was used as counter-body. It was observed that parameters R_ku, R_sk, R_pk and R_vk tend to have influence on coefficient of friction.     

Roughness,friction and wear: The effect of contact planform

Recent theoretical and experimental findings suggest that the statistical properties which affect friction vary slowly with the dimensions of the nominal area. This variation is due to the existence of random long spatial fluctuations which are outside the bandwidth of features imposed by the surface preparation process and which longer wavelengths completely dominate contact behaviour. This leads to the suspicion that the second of Amontons' laws, i.e. that the limiting static friction is independent of the shape of the nominal area, is not absolutely true. A relation is derived which connects the limiting static friction with the statistical contact properties and with a dimension of the nominal area. In an experiment devised to test this relation, the limiting static friction was measured for three rectangular steel specimens whose nominal area and normal load were kept constant and whose length to breadth ratio varied. The limiting static friction was found to be linearly related to the specimen length in agreement with theoretical predictions. Regression analysis of 36 measurements established that the probability of this result being due to chance was less than 1 in 500. Supporting evidence for this hypothesis has also been found from wear experiments reported in the literature.     

The influence of surface roughness on friction and wear of machine element coatings

The use of low friction coatings like amorphous carbon or metal-doped carbon coatings on machine elements is constantly increasing. Most often, a surface treatment, e.g. grinding and polishing or honing, is required for optimal performance of the coated machine element. This can be time consuming and costly.In this study, the effect of surface roughness on friction and sliding wear of two different coatings, one tungsten containing and one chromium containing coating, were examined using a ball-on-disc test. Ball bearing steel plates were grinded to different surface roughnesses and coated with the two different coatings.The friction was found to depend on surface roughness where the rougher surfaces gave higher friction coefficients. The wear rate for the a-C:W coating was found to be independent of the roughness, whereas the roughness had a strong influence on the wear rate for the a-C:Cr coating. This could partly be explained by a difference in wear mechanism, where fatigue wear was observed for the a-C:Cr coating but not for the a-C:W coating.     

The effect of transfer layers on the surface contact and wear of carbon-graphite materials

Solid carbon-graphite materials are widely used in a variety of arduous tribological applications in which their ability to run either unlubricated or in the presence of fluids with unpropitious tribological properties can be exploited. The usual arrangement in such devices as seals and bearings is to run a carbon component against a much harder metallic or ceramic counterface of appropriately low surface roughness. In practice, it is often observed in these circumstances that, although the initial wear rate of the relatively soft carbon is high, a satisfactory long-term performance can be achieved if, and only if, a stable layer of carbonaceous material is generated both on the trace of the carbon and as a transfer layer on the harder counterface: in the absence of this film the high initial wear rate continues unabated. The range of carbon materials available commercially is large, but can be classified by the extent to which they have been graphitized—those with a higher graphitic content have a lower elastic modulus and generally higher wear rate than less graphitic specimens. In this paper, we consider the development of the contact conditions with component life and how these can be associated with the alleviation of the initial severity through the development of a cushioning layer of consolidated wear debris and thus offer an explanation of the experimental observations in terms of a contact mechanics model.     

Interface roughness effect on friction map under fretting contact conditions

In many industrial applications where fretting damage is observed in the contact (e.g. rotor/blade, electrical contacts, assembly joint, axe/wheel, clutch) the external loadings or geometry design cannot be changed. Therefore, the surface preparation and finishing process become essential to control and reduce the damage caused by fretting. In this paper, the authors present the experimental study of the initial surface roughness and machining process influence on fretting conditions in both partial and full sliding regimes. Surfaces prepared by milling and smooth abrasive polishing processes have been analysed. The influence of roughness on sliding behaviour and analysis of friction have been reported. Also, the contact pressure influence and qualitative analysis of fretting wear scar have been presented.     

The effect of contact stress on cartilage friction, deformation and wear

Following hip hemiarthroplasty, a metal femoral head articulates against natural acetabular cartilage. Cartilage friction and wear may be influenced by variables including loading time, contact stress, contact area, sliding distance, and sliding speed. The aim of this study was to investigate the effect of these variables on cartilage friction, deformation and wear in a simulation using idealized geometry model. Bovine cartilage pins were reciprocated against metal plates to mimic a hemiarthroplasty articulation under static loading. The effective coefficient of friction (micro elf) under contact stresses (0.5 to 16 MPa), contact areas (12 and 64 mm2), stroke lengths (4 and 8 mm), sliding velocities (4 and 8 mm/s), and loading time (1 and 24 hours) were studied. The permanent deformation of cartilage (after 24 hours of recovery) with and without motion was recorded to assess cartilage linear wear. The micro eff was found to remain < 0.35 with contact stresses < or =4 MPa. Severe damage to the cartilage occurred at contact stresses > 8 MPa and significantly increased micro eff after 12 hours of reciprocation. In long-term, contact area had no significant effect on micro eff, and sliding distance and velocity only affected micro eff under low contact stresses. The cartilage linear wear increased with contact stress, sliding distance and velocity.     

Sliding friction and wear of alumina-reinforced zirconia-toughened mullite composites

The friction and wear characteristic of self-mated alumina-reinforced zirconia-toughened mullite (ZTM/A) composites has been investigated using a block-on-ring tribometer in different lubricants at varying loads. Load-dependent wear transitions were observed for these ceramics. The wear transition was usually accompanied by an abrupt change of friction coefficient and wear rate. The addition of Al₂O₃ effectively reduces the wear rate of ZTM/A composites before the wear transition in water and at middle loads in machine oil. Scanning electron microscope (SEM) micrographs show that the main pre-transition wear mechanism of ZTM/A composites is plastic deformation, ploughing and occasionally grains pulling out, while fracture is the dominant mechanism of post-transition. Al₂O₃ can restrain the t→m phase transformation of zirconia to some extent and improve the resistance of these materials to wear by fracture, ploughing and plastic deformation.     

Experimental research of face mill wear effect to flat surface roughness

In this article, we researched the effect of wear face mills to finish the surface roughness by various conditions of cutting a steel-45 workpiece. The article shows how to affect the feed, cutting speed, and tool wear of a T5K10 carbide tool on the roughness of flat surfaces. The paper analyzes the nature the microprofile of changes in machined surfaces based on increasing the wear surface on the tooth flank.     

A thermodynamical model of contact,friction and wear: II constitutive equations for materials and linearized theories

The problem of determining the states of contacting, rubbing and wearing bodies is resolved in the present work using the constitutive equations for materials. Measures of deformation are formulated for a body and a two-dimensional micropolar interfacial layer. Definitions for the thermodynamical processes in the body and the layer are introduced. Constitutive equations and linearized theories for the thermoelastic and hyperelastic bodies are derived. Constitutive equations and linearized theories for the layer are formulated for the micropolar thermoelastic material and the micropolar fluid. The thermo viscous fluid layer is given as an example of non-polar layer theory.     

The influence of surface roughness on wear

A series of wear experiments utilizing a ball-plane configuration was performed to determine the influence of both the magnitude and anisotropy of the surface roughness on wear. It was found that: (1) wear increases with increasing roughness up to ~ V16 finish and remains relatively constant above that roughness and (2) sliding perpendicular to the lay increases wear above that for sliding parallel, with the difference becoming negligible for the finer roughness. A method of analytically relating the results obtained for ball-plane geometry to other geometries is developed.     

High temperature wear of asbestos-reinforced friction materials

The high temperature (480 °–570 °F) wear of two commercial asbestosreinforced friction materials was investigated by dynamometer testing. Wear rate constants were evaluated at different temperatures; the activation energy for the wear of each material was obtained under different loads and speeds. The results strongly support a pyrolysis mechanism proposed previously for wear at elevated temperatures.     

The effect of surface roughness on contact and stress states of spur gears

The contact characteristics of spur gears are analysed, taking into consideration real surface micro-geometry, and using numerical algorithms. The contact pressure distribution, contact area, and rigid body rotation of the gears have been calculated for different types of gear surface according to machining: shaping, grinding, or milling. Finally, the sub-surface stress state is evaluated for different pressure distributions, by finite element method.     

Sliding wear behavior of dedicated iron-based SLS materials

Systematic investigation of wear behavior of selective laser sintering (SLS) materials is lacking in SLS/selective laser melting research. The present research is an effort to fill the gap by performing sliding wear tests under plastic and elastic contact conditions upon proprietary iron-based SLS materials: LaserForm and DirectSteel. It is found that LaserForm is a better SLS wear material. It is concluded that wear performance is governed not by the hardness of the materials but by their composition.     

Tool wear and its effect on surface roughness in diamond cutting of glass soda-lime

For the technology of diamond cutting of optical glass, the high tool wear rate is a main reason for hindering the practical application of this technology. Many researches on diamond tool wear in glass cutting rest on wear phenomenon describing simply without analyzing the genesis of wear phenomenon and interpreting the formation process of tool wear in mechanics. For in depth understanding of the tool wear and its effect on surface roughness in diamond cutting of glass, experiments of diamond turning with cutting distance increasing gradually are carried out on soda-lime glass. The wear morphology of rake face and flank face, the corresponding surface features of workpiece and the surface roughness, and the material compositions of flank wear area are detected. Experimental results indicate that the flank wear is predominant in diamond cutting glass and the flank wear land is characterized by micro-grooves, some smooth crater on the rake face is also seen. The surface roughness begins to increase rapidly, when the cutting mode changes from ductile to brittle for the aggravation of tool wear with the cutting distance over 150 m. The main mechanisms of inducing tool wear in diamond cutting of glass are diffusion, mechanical friction, thermo-chemical action and abrasive wear. The proposed research makes analysis and research from wear mechanism on the tool wear and its effect on surface roughness in diamond cutting of glass, and provides theoretical basis for minimizing the tool wear in diamond cutting brittle materials, such as optical glass.     

The influence of contact geometry on friction and wear characteristics

This paper describes some results of evaluating the influence of contact geometry on tribological characteristics. Friction characteristics obtained for a distributed contact (pin‐on‐disc and block‐on‐ring), a linear concentrated contact (pin and vee‐block and block‐on‐ring) and a point concentrated contact (ball‐on‐disc and three cylinder‐cone) are presented. The wear scars were analysed using scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The results show different values of friction and wear coefficients for the different types of contact geometry. The paper also assesses the effect of non‐test characteristics, e. g., vibration.     

The influence of surface roughness and the contact pressure distribution on friction in rolling/sliding contacts

A numerical contact model is used to study the influence of surface roughness and the pressure distribution on the frictional behaviour in rolling/sliding contacts. Double-crowned roller surfaces are measured and used as input for the contact analysis. The contact pressure distribution is calculated for dry static contacts and the results are compared with friction measurements in a lubricated rolling/sliding contact made with a rough friction test rig. The mean pressure is suggested as a parameter that can be used to predict the influence of surface roughness on the friction coefficient in such contacts. The results show two important properties of the friction coefficient for the friction regime studied in this paper: (1) there is a linear decrease in friction coefficient as a function of the slide-to-roll ratio, and (2) the friction coefficient increases linearly with increasing mean contact pressure up to a maximum limit above which the friction coefficient is constant. The absolute deviation of experimental results from the derived theory is for most cases within 0.005.     

Effects of surface roughness and surface films on contact resistance

An analysis of the electrical contact resistance between two metals with a tarnish film was carried out assuming the asperities to be represented by randomly distributed cones with base angles which vary with the surface roughness, and assuming that the radius of the broken area of the film at the interface of each contact asperity is constant beyond a critical depth of penetration of an asperity.The validity of the proposed theory was confirmed by experimental data of the electrical contact resistance between a silver cone and a silver flat on which carbon films were deposited, and between a silver flat with an Ag₂S tarnish and a palladium flat without tarnish. Comparison of theoretical and experimental data shows that the critical depth of penetration U_e of an asperity varies mainly with the surface roughness, the thickness of the tarnish film and the amount of plastic deformation of the contact asperities with films, i.e.$\text{U}{}_{\mathrm{e}}\text{σ}\text{= kR}{}^{\mathrm{j}}{}_{\max }$, where R_max and σ are the maximum height and the standard deviation (r.m.s. roughness) of the profile ordinates, and j and k are constants dependent on the type of finish, the thickness of the film and the difference in the hardness of the mating surfaces.