The influence of surface roughness on abrasive wear

Analyses are given of the mechanism of friction and abrasive wear and of the effect of surface roughness on them. Theoretical expressions are derived for ploughing, adhesion and the total friction coefficient of hard conical asperities ploughing a soft metal surface, with the assumption that the asperities of the hard metal are cones with randomly distributed slopes, the mean value of which varies with surface roughness. Simple expressions for the abrasive wear rate and the mean wear particle size are also derived on the basis of a ploughing mechanism of the hard conical asperities on the soft metal surface.A comparison of calculated values based on these theories with experimental data of single-pass wear tests for various soft metals such as copper, cadmium, lead and zinc sliding on low carbon steel plates shows good agreement. The effects of surface roughness on the tangential forces under unlubricated and lubricated conditions as well as the mean wear particle size are theoretically discussed and the theoretical results are compared with experimental data.     

Influence of surface roughness and work-hardened layers on the contact between a rough and a flat surface

The validity of a theory which was derived considering the distribution of the surface slopes of conical asperities and the variation of the flow pressure of each contact due to the work-hardened layer of the softer surface was checked by comparing the theoretical and experimental results. The number, the real area and the distribution of the radii of contact points produced by impressing hard and rough surfaces into soft and flat surfaces with work-hardened layers were measured. The distribution of the surface slopes of contact asperities and the variation of the flow pressure with increasing penetration of the hard asperities were also obtained experimentally.     

Mathematical slip-line field solutions for ploughing a hard particle over a softer material

Wear mechanisms and friction in metals can be investigated by the analysis of the unit event represented by the interaction of a hard particle or asperity with a softer surface. Effective friction is the result of the interaction of many such asperities which constitute the roughness of the harder of the solid surfaces. Three types of plastic deformation at the metal surface can be identified: ploughing, edge formation and chip formation. Each mode of plastic deformation can be analysed using the slip-line field plasticity theory which requires as inputs the geometry of the hard particle and some information on the interface between the harder and the softer surfaces. The classical and the recent chord solution by Oxley assumes a sharp edge sliding against a metal surface but does not consider a curved roughness profile. However, the profiles of real asperities are more like waves with rounded summits. In the present work a new model for the asperities interaction is shown, using the slip-line field theory to calculate the friction forces, depth of sheared layer, average contact pressure and friction coefficient for a cylindrical hard particle sliding over a softer surface. The theoretical results are presented as friction graphs and maps in which the regions of elastic deformations are shown using the Hertz theory while the region of plastic strains is obtained from the present analysis. Present model results are in good agreement with experimental data obtained by Busquet et al. and are quite different from the Oxley chord model for sliding a circular particle.     

Deformation and wear behaviour of the junction in quasi-scratch friction

The initial scratching of soft metals by relatively hard metallic asperities involves considerable plastic deformation and wear of the harder metal. Thus the penetration effect on friction is reduced successively as sliding proceeds, leading to the shearing type of friction. Such a transition state of sliding can be defined as a quasi-scratch friction process because ploughing precedes the steady sliding condition.The deformation and wear behaviour at a friction junction was investigated using model experiments between a mild steel conical rider and a flat copper surface. Changes in geometry of the rider and pile-up of the flat metal were examined metallographically and with a microscope. It was found that a stable value of the friction force is determined from the geometric shape of the junction attained after the completion of transient sliding and the effect of initial asperity shape on the friction force becomes insignificant.     

Prediction of Archard's wear coefficient for metallic sliding friction assuming a low cycle fatigue wear mechanism

A recently developed model for sliding friction, in which the frictional force is assumed to result from the pushing of waves of plastically deformed material in the soft surface ahead of asperities on the hard surface, is used to calculate the magnitude of the resulting plastic strain increments which progressively deform the soft surface. On the assumption that a low cycle fatigue mechanism eventually results in detachment of wear particles from the soft surface as a result of this cyclic working of the surface, the calculated strain increments (the magnitudes of which vary with the roughness of the hard surface and the boundary lubrication conditions) are used to estimate wear rates. The results are expressed in terms of Archard's wear coefficient and, for very smooth surfaces and good lubrication, this is predicted to have extremely low values as observed in wear tests for such conditions. For rougher surfaces and less efficient lubrication it is shown that the wear coefficient can increase dramatically.     

Measurement of in-plane deformation by surface inclusion (proposed measuring method and some experimental results)

A new measuring technique is proposed for measuring the in-plane deformation of soft metal surfaces when indented by a hard indenter, using the displacement of inclusions on the soft metal surface as sensors. In in-plane deformation, the soft metal surface flows along the hard indenter surface. In this paper, the principle of this new measuring technique is explained and some example measurements obtained from the experiments with aluminum and copper beryllium alloy plates, are presented.     

Fatigue wear: A contribution to the wear phenomena of ion-plated thin metallic films

A wear model based on the fatigue failure of asperities on ion-plated surfaces is presented. It is suitable for ion-plated hard substrates sliding against hard counterfaces where asperity penetrations do not occur. The constants specific to the analysis are evaluated. A wear equation which is dependent on the mechanical properties of the system, the surface topography and the operating conditions is obtained. The wear rates are expressed as functions of the normal load, the sliding speed, the track width, the standard deviation of the surface asperities, the mean radius of the asperities, the stiffness of the asperities and the static yield strength of the multilayered material.     

Study on the Boundary-Lubricated Sliding Contact with Subsurface Plastic Deformation

In this study, low-velocity oblique plastic impact-testing experiments were conducted at different angles of incidence to investigate the consequences of the short-distance sliding interaction between a very hard (En31) ball and a relatively softer (mild steel) metal specimen. The purpose was to understand the mechanism of boundary lubrication in the metalworking processes. The specimen was mounted on a specially designed inclined-plane-type fixture so that its surface could be oriented at any desired angle against the free-falling hard ball from a predetermined height. The experimental setup included sufficient details of instrumentation to record the post-impact travel distance and time from which the average coefficient of friction was calculated using a simple methodology. The specimen surfaces were studied using the SEM for different cases of sliding experiments with and without lubricants and two different additives in the lubricants. Marked difference was observed in the nature of surfaces produced in different cases. The oblique impact process was modeled using the equations of motion of the ball and its interaction with plastically deforming specimen material. A fourth-order Runge-Kutta method was used and variations of shear and normal forces during the sliding contact were estimated. The friction behavior showed by this model is in conformance with the experimental results. In addition to that, it has been shown by this model that the coefficient of friction cannot exceed the value of one in sliding. A finite element model has been prepared to estimate the plastic deformation component of friction. Considering the soft asperities of the workpiece deforming as a wave in front of hard asperities, the steady-state Galerkin finite-element model enabled estimation of friction. The trend of the results of the FEM model seems to substantiate the experimental results.     

Explaining the mechanics of metallic sliding friction and wear in terms of slipline field models of asperity deformation

It is shown that the force which opposes the sliding of a hard relatively smooth surface over a softer surface can be explained as the force needed to push waves of plastically deformed material along the soft surface ahead of asperities on the hard surface. For rougher surfaces and/or poorer lubrication it is shown how the wave can be torn off or material removed by a chip formation process and wear particles formed. Coefficients of friction predicted from the corresponding asperity deformation models are shown to give good agreement with experimental results. For smooth well lubricated surfaces the wear of the softer surface is shown to occur as a result of the progressive damage to this surface brought about by the repeated passage of waves across it. Equations for predicting wear are derived from the asperity deformation models and a comparison made between predicted and experimental wear results. The paper ends by considering possible future trends in research into the mechanics of friction and wear.     

Friction and wear of ion-plated soft metallic films

Experimental results of the tribological behaviour of soft metallic films on hard substrates are given. It is shown that the adhesion of the film to the substrate is a critical factor determining the life of the film which illustrates the value of ion-plated films. The friction mechanism is discussed and is shown to agree with the basic premises of an existing theory. The wear mechanism appears to be an initial microcutting of the film by the mating surface asperities followed by a much less severe form of fatigue wear due to asperity-substrate contacts.     

Initial wear rates of soft metallic films

A theoretical expression was derived for the initial wear rates of hard conical asperities ploughing a soft metallic film. The theory yields a wear equation similar to the well-known existing empirical formula, although the new wear coefficient K appears to be dominated by the surface topography of the system rather than by the material properties. The initial wear rates of the film appear to be determined by the system rather than by the material. The general characteristics of the model are confirmed by experimental results for the initial wear rates of ion-plated tin and lead films.     

Deformation of single and multiple asperity models of modelling clay

Single- and multi-asperity models, made of various grades of modelling clay, were separately deformed by being pressed against a hard flat surface. The behaviour of each single conical asperity model followed a qualitatively similar pattern to that exhibited by a conical asperity on a metal surface. At low loads a clay single asperity model deformed in a similar manner to that for an identical asperity when it formed part of an abutting regular array of such asperities. However at higher loads there was a tendency for the multi-asperity model to exhibit greater flow pressures than for the single asperity model: this was due to asperity interaction.     

Finite element calculations on the influence of surface roughness on friction

Two-dimensional finite element calculations are used to describe the interaction between a hard rigid asperity and an asperity with an elastic-plastic material behaviour. In the calculations the hard asperity is moved through the soft asperity while the separation of the surfaces is kept constant. The normal and shear forces and the friction coefficient are determined for different separations. The influence of adhesive friction in the contact zone and the influence of the radii of the asperities are also investigated. The finite element model is compared with an analytical model.     

Surface coatings materials conservation and optimum tribological performance

Surface coating technology has received considerable attention in recent years. It improves corrosion, friction, wear and fatigue resistance of components with a minimum demand on scarce raw materials. It is shown that the behaviour of surface films depends on material properties, surface finish and film thickness and that friction may vary through two orders of magnitude due to such effects; the friction of soft metal films being low and that of hard films being high. Such effects are enhanced for very smooth surfaces and very thin films. It is also shown that the wear resistance is greater for thin soft metal films and less for thin hard metal films. Some experimental evidence is available to justify the theoretical proposition but much further work is required and the paper identifies the gaps in knowledge     

Studies on friction and transfer layer: role of surface texture

Friction influences the nature of transfer layer formed at the interface between tool and metal during sliding. In the present investigation, experiments were conducted using “Inclined Scratch Tester” to understand the effect of surface texture of hard surfaces on coefficient of friction and transfer layer formation. EN8 steel flats were ground to attain surfaces of different textures with different roughness. Then super purity aluminium pins were scratched against the prepared steel flats. Scanning electron micrographs of the contact surfaces of pins and flats were used to reveal the morphology of transfer layer. It was observed that the coefficient of friction and the formation of transfer layer depend primarily on the texture of hard surfaces, but independent of surface roughness of hard surfaces. It was observed that on surfaces that promote plane strain conditions near the surface, the transfer of material takes place due to the plowing action of the asperities. But, on a surface that promotes plane stress conditions the transfer layer was more due to the adhesion component of friction. It was observed that the adhesion component increases for surfaces that have random texture but was constant for the other surfaces.     

Experimental analysis of the yield criterion for a hard asperity sliding on a soft flat surface

The validity of the theoretical yield criterion proposed previously for a hard asperity on a soft flat surface in abrasive friction process was examined by the experiments in the scanning electron microscope with model asperities. The asperity specimens with six asperity angles were made of SUS304 (18Cr-8Ni austenitic stainless steel) and four types of S45C (0.45% C steel). Mating flat specimens were made of S45C and two types of 6-4 brass. The hardness ratio of those frictional pairs ranged from 0.17 to 0.91.The experimental results gave good agreement with the proposed theoretical predictions, i.e. if the asperity angle is smaller than a critical asperity angle which is determined by the hardness ratio between the asperity and the mating flat surface, the tip of the asperity deforms plastically despite its being harder than the mating flat surface.     

In-plane deformation observation of indentation along indenter surface

In this paper, the in-plane plastic deformation, which is perpendicular to the contacting load, of soft metal in contact with a hard spherical indenter is observed on a rolled aluminum surface. By measuring a displacement due to indentation of lattice points described on the aluminum surface, the in-plain deformation is obtained. Many small ferrous impurities are scattered on and in this aluminum specimen. When a hard spherical indenter is pressed onto this aluminum surface, the distance between impurities is changed by the surface plastic deformation. The change in distance between impurities is obtained using the image processing method, and it is demonstrated that the in-plane plastic deformation of surfaces caused by indentation can be observed. Results by both the lattice point method and the image processing method are compared quantitatively.     

Thermo-mechanical analysis of the magnetic head–disk interface with a fractal surface description

Rigid rough surface of a magnetic head and smooth surface of a hard disk are chosen to perform a comprehensive thermo-mechanical contact analysis at the magnetic head–disk interface, which is characterized by using the fractal geometry. The effects of mechanical and thermal surface loadings on deformation of the semi-infinite medium in normal and sliding contacts are analyzed simultaneously by developing a 2D finite element model. It is shown that frictional heating increases not only the contact area but also the contact pressure and stresses. The maximum temperature occurs at the tip of the asperities of the semi-infinite medium.     

Brake features enhancing the wear debris bimodal distribution

Investigated is wear debris separation from dry friction interface of a vehicle brake system. Owing to structural and operational properties of the interface between the disc hard surface and the pad soft surface, the study focuses on the abrasive wear process of the pad profile. A theoretical dependence is sought between debris size distribution and governing parameters such as surface roughness, unevenness, and levels of normal loading.The analysis describes interacting surface profiles in terms of discrete probabilities of their heights. Enforced by motion constraints, the disc hard profile is monitored while clearing its way through the stationary pad overlapped profile by shearing of likely encountered asperities as estimated by their conditional probability. As a result, a single tangential stroke leads to a pad redistributed profile along with a collection of separated debris. A pre-stroke approach of the pad closer to the disc simulates the effect of a normal loading increment. Accordingly, three types of shear strokes may develop, namely; the superficial, the shallow subsurface, and the deep subsurface.It is found that the range of debris size only extends up to the mean height of surface profile. The debris size distribution is generally bimodal with large-size and small-size peaks, both being distinct during superficial strokes. The position and level of either peak depend on the two interacting profiles just before each individual stroke. For a disc Gaussian profile, sharply wavy profile of the pad surface makes the debris large-size peak to lie evidently near the complementary of the disc mean height. An originally Gaussian or a disc-induced profile of the pad surface enhances the level of the debris small-size peak whose position is constant. In addition, deep subsurface strokes yield an invariable debris distribution with a single small-size peak.     

The delamination theory of wear and the wear of a composite surface

The delamination theory of wear postulates that there is a “nonworkhardening” soft surface layer which deforms continuously due to the instability of dislocations, and that the low speed sliding wear of metals is caused by the subsurface crack nucleation and propagation nearly parallel to the surface. A corollary of the theory is that when hard metal surfaces are plated with a soft metal to reduce the coefficient of friction and the wear rate, the soft metal layer must be thinner than a critical thickness so as to prevent the accumulation of dislocations in the plated layer and the formation of the delaminated layer. This corollary was investigated by plating annealed AISI 1018 steel with cadmium. The wear rate of the steel specimen plated with 0.1 μm cadmium on both of the contacting surfaces was three orders of magnitude smaller than the unplated specimen when they were tested in argon. In the case of thicker coatings, the cadmium layer wears by the delamination process which occurs within the plated layer. The very thin cadmium plate is also effective in reducing wear in inert oil, but not effective in an oxidizing atmosphere. The coefficient of friction of the 0.1 μm Cd plated steel was less than the unplated steel under all test conditions.