Wear and wear particles—some fundamentals

Abstract

Each of the various processes by which material can be lost from a surface in service leaves its fingerprint both in the topography of the worn surface and in the size, shape and number of the particles which make up the wear debris. To use debris examination as a diagnostic aid in assessing the health of operating plant, which may contain many tribological contacts, requires not only careful and standardised procedures for debris extraction and observation but also an appreciation of the mechanisms by which wear occurs and the regimes in which each of the contacts of interest operates when displayed on an appropriate operational map.

Introduction

The phenomenon of wear was given a formal definition in 1968 by the OECD as ‘the progressive loss of material from the operating surface of a body occurring as a result of relative motion at its surface’ [1]. This might today be thought a slightly curious definition as there is no mention of what it is that is moving relative to the surface in question, or indeed, of the need for the transmission of some contact force between the wearing surface and its counter-face—even if this transmission is through a low-strength, intermediate film such as that formed by a lubricant. Notwithstanding this difficulty, common usage of the term wear implies some mechanical action on the wearing surface—so that, for example, corrosion per se would not be classified as a wear mechanism whereas corrosive wear involving some form of mechanical interaction in a corrosive environment certainly would be.

Having divided wear mechanisms into two principal categories, the first adequately described as ‘mechanical wear’ and a second covering those situations where there is an additional element of active chemistry (typically oxidation and/or corrosion) it is temping to subdivide the former large category into some number of smaller classes—the extent of this sub-division being to some extent a function of the enthusiasm of the author. Fig. 1 illustrates a typical set of mechanical wear processes grouped under four general headings and arranged, within each of these, in increasing order of severity. All these mechanisms have the capability of removing material from the surface in question though the rates of degradation, measured as rates of loss of mass per unit time, can vary over many orders of magnitude. All can, and often do, operate in the presence of a lubricant, very often a mineral oil, whose function may be both to limit friction and to convect away heat so reducing the severity of thermal stresses or distortions in vital machine elements. Mechanisms of wear are not mutually exclusive and in some complex pieces of machinery several may be operating simultaneously at different sites and different internal contacts. Techniques of wear debris analysis are based on the hypothesis that the morphology of debris particles examined in a representative sample of the lubricant circulating through the machine can indicate which is the most active operating wear mechanism and, furthermore, that changes in the concentration of such particles within the circulating fluid will be indicative of changes in the state of surfaces of these potentially critical components [2], [3]. A reduction in the rate of particle production and detection in the early stages of device operation is associated with the running-in process as a benign and acceptable wear regime is established; whereas a later increase in the rate of particle concentration may herald a transition to a higher wear regime as the surfaces wear out—perhaps with catastrophic results.

Surveys of industrial wear problems often highlight abrasion as being of particular concern. Within the general area of abrasive wear a distinction is often made between so-called two-body abrasion when material is removed or displaced from the softer surface by the asperities or protuberances on the harder surface and three-body abrasion when the damage is done by some form of free, discrete abrasive particles rolling and sliding between the opposing surfaces of the contact: often such particles are contaminants form the outside environment. In practice, the distinction between the two sub-divisions may be somewhat blurred, as if the free particles become lodged in one of the bearing surfaces the situation either temporarily, or even permanently, becomes one of two-body abrasion. So-called open three body abrasion occurs when the two surfaces are sufficiently far apart to be effectively independent of one another; for example, this is the type of wear to which earth moving equipment is subject when soil particles abrade the shovel faces. If the particle velocities are large, say more than a few m/s, perhaps because they are carried in a gas stream or entrained in a flow of liquid, the wear process becomes one of erosion.