D.A. Rigney

Wear processes in sliding systems

Abstract A sequence of events involved in sliding wear is outlined. Local contacts cause large plastic strains in either or both solid components. The plastic deformation changes the near-surface microstructure in ways which make the material unstable to local shear. This in turn produces transfer of pieces of deformed material which are further deformed and mixed with counterface material and/or environmental components to produce ultrafine-grained material. The very fine microstructure in this transfer material is stabilized by the mixing in (mechanical alloying) of a second phase. The relative hardness of the transfer material and the adjacent substrate material affects the surface topography, the smoothness of sliding and the nature of the wear debris. Loose debris is commonly derived from the transfer material. This picture has been developed with the aid of analysis of experiments on unlubricated systems, but it is expected to be at least partly relevant to many “lubricated” systems as well.

Transfer, mixing and associated chemical and mechanical processes during the sliding of ductile materials

Abstract This paper is based on a presentation designed to stimulate discussion among participants in an international symposium held at Hyderabad, India, 14–16 December 1998. The program focused on the genesis and role of transfer and mechanical mixing in the wear of materials. It is convenient to divide the changes arising from sliding contacts into two broad categories, the first in which the average chemical composition is unchanged and the second in which the chemical composition is modified by interactions with the counterface material and the environment. These are not completely independent. In fact, plastic deformation and changes in structure during early stages of sliding may be precursors to processes such as transfer and mechanical mixing in which the chemical composition of near-surface material is modified, together with further changes in structure, and further deformation of the tribo-chemically modified material. Also, one part of a wearing surface may have a given combination of chemical composition and structure, while a nearby one may have another. Advances in understanding sliding behavior have been greatly aided by the recent availability of improved techniques for structural, chemical and mechanical characterization of materials. There is now abundant evidence for large plastic strains, allowed by the imposed compressive and shear stresses, even in materials which are considered to be brittle in simple tensile tests. Thus, sliding encourages ductility adjacent to the sliding interface. The energy dissipated during continuing plastic deformation can account for the values of friction coefficients typically measured during the unlubricated sliding of ductile materials. To improve our understanding of sliding wear processes, it is helpful to monitor changes in structure and chemical composition of sliding test specimens and wear debris, using very short to very long sliding times and using both in situ and post-test techniques. Such work has suggested that sliding wear commonly involves development of a deformation substructure which is susceptible to shear instabilities, leading to transfer, chemical reactions, mechanical mixing and fracture. The processes are similar to those which occur in the early stages of the commercial process known as mechanical alloying. Evidence for such processes has accumulated over many years and is now more than sufficient to justify major efforts to incorporate transfer and mixing in quantitative wear models. Questions designed to stimulate such efforts are included in this paper.

Plastic deformation and sliding friction of metals

Abstract Most theories of sliding friction have emphasized surface roughness or adhesion. In some cases plastic deformation has been invoked to account for energy dissipation. After a brief review of published explanations of friction, a new model is described for the source of friction during the steady state sliding of metals. It focuses on the plastic work done in the near-surface region, described in terms of work hardening, recovery and the microstructure existing during steady state sliding. The model is discussed with respect to several alternate ways in which plastic deformation has been incorporated in recent theories of friction. Reasonable results are found when the new model is used to estimate friction coefficients for metals. Also, the model appears to be consistent with a number of published observations on the relation of friction to load, sliding distance, surface temperature and microstructure, and with a model for sliding wear which has been presented earlier.

Sliding wear and transfer

Abstract It is well known that transfer of material from one component of a sliding pair to the other occurs in many tribological systems. In the present paper the authors describe their observations on transfer material and on debris particles. Detailed structural and chemical information has been obtained by using optical microscopy, scanning electron microscopy, transmission electron microscopy (TEM), scanning TEM and fluorescence analysis using energy-dispersive techniques (energy-dispersive analysis of X-rays) and wavelength analysis. The results show a clear connection between the transfer layer and the generation of loose wear debris for both unlubricated and lubricated sliding. Evidence of delamination of base material has not been observed in this work.

Orientation determination of subsurface cells generated by sliding

Abstract The near-surface material of copper samples has been examined by transmission electron microscopy (TEM) and STEM (scanning TEM) techniques after sliding tests of varying duration. The development of deformation substructure is described, and detailed orientation information is obtained with the aid of an improved technique which uses a minicomputer to analyse Kikuchi line patterns from individual cells or sub-grains. Large plastic strains and large rotation angles are achieved after very short sliding distances. The principal rotation is about the transverse axis, i.e. the axis normal to the sliding direction and parallel to the sliding interface. No tendency for alignment of close-packed planes with the sliding interface has been found.

Low energy dislocation structures caused by sliding and by particle impact

Abstract Tribological processes cause large plastic strains and plastic strain gradients adjacent to the interface between the interacting materials. For sliding wear and erosion test specimens, the dislocation structures observed in the plastically deformed material are consistent with those expected when energy is minimized. Studies of these structures can contribute to our knowledge of deformation and dislocation substructures produced at large strains under other test conditions. Tribological processes also create patches or layers of mechanically mixed material on one or both of the contacting surfaces. This material is important in the generation of typical wear debris. Studies of the mechanically mixed material may also be helpful in understanding the structure and properties of ultrafine grained two-phase materials.

The significance of near surface microstructure in the wear process

Abstract The wear process in which flake-like debris is developed and removed from the surface of metals in sliding contact is the direct result of heavy plastic deformation of a thin surface layer. The repeated ploughing of asperity contacts over a mating surface can produce high dislocation densities and eventual change in the microstructure to a cell-type structure found in heavily deformed metals. Cell sizes depend on material characteristics such as stacking fault energy, the applied stress and the temperature. It is shown that a cell structure can present many suitable pathways for subsurface crack generation and the release of thin wear flakes without the benefit of asperity cold welding and shear. Depth of crack formation and severity of wear can be associated with stacking fault energy. Changes in the microstructure caused by frictional heating or change in strain rate can cause abrupt changes in wear mode.

An energy-based model of friction and its application to coated systems

Abstract An energy-based friction model is used to develop expressions for the friction coefficient; these expressions depend on familiar mechanical parameters, stress-strain curves and microstructural features of the materials. The main assumption is that the frictional work performed is equal to the work of plastic deformation during steady state sliding. The results include as a special case the well-known expression for the friction coefficient derived from adhesion theories. However, the new model can also be applied to more complex systems such as those involving coatings. In particular, for soft coatings on hard substrates as well as for hard coatings on soft materials, the friction coefficient can be predicted as a function of layer thickness in accordance with reported observations. Furthermore, guidelines are presented for achieving low friction by using coatings. The new model can also be applied to systems involving reaction layers, transferred material and solid lubricants of varying thickness. In addition, the model offers some insight into the variation in friction coefficients with time and into the processes which occur during running-in.

Sliding behavior of metallic glass: Part I. Experimental investigations

The unlubricated sliding characteristics of zirconium-based bulk metallic glass disks have been examined in vacuum and in air using sliders made from the same material or from a hard bearing steel (52100). The pin-on-disk test system allowed collection of debris, monitoring of the friction force and, using a Kelvin probe, in situ detection of changes in the structure and chemical composition of the disk surface. Post-test characterization included microhardness testing, X-ray diffraction, SEM and EDS. Examination of worn surfaces, cross-sections and debris confirmed the importance of plastic deformation, material transfer and environmental interactions. When devitrified material was tested, sliding processes caused the near-surface material to re-amorphize. Results from sliding of bulk metallic glass specimens were compared with those from related experiments involving crystalline metals and alloys. Although bulk metallic glasses are reported to have only limited ductility in tensile tests, the friction coefficients and worn surfaces of these materials are typical of ductile materials.

The roles of hardness in the sliding behavior of materials

Abstract It is generally recognized that hardness is one of the key factors which influence the sliding behavior of different materials combinations. However, in many discussions the only hardness value considered is that of the softer of the two materials in a tribological pair. This is usually the case when a simple linear wear equation (Holm, Archard, Khruschov) is cited. Observations on many materials combinations demonstrate that the effects of hardness are much more complex. Hardness varies with position and time. It can depend on temperature, sliding speed and the chemical environment. The sign of hardness gradients adjacent to the sliding surface affects sliding behavior. Transfer and subsequent mechanical mixing strongly influence local hardness. Changes in hardness can affect transitions in friction and wear. Relative hardness values can help to explain differences in debris and in smooth and rough sliding. They can also help us to understand geometric effects such as those noted when materials are interchanged in a test system. Examples will be described.