Introduction

The definition and measurement of wear

Wear is the progressive damage, involving material loss, which occurs on the surface of a component as a result of its motion relative to the adjacent working parts; it is the almost inevitable companion of friction. Most tribological pairs are supplied with a lubricant as much to avoid the excessive wear and damage which would be present if the two surfaces were allowed to rub together dry as it is to reduce their frictional resistance to motion. The economic consequences of wear are widespread and pervasive; they involve not only the costs of replacement parts, but also the expenses involved in machine downtime, lost production, and the consequent loss of business opportunites. A further significant factor can be the decreased efficiency of worn plant and equipment which can lead to both inferior performance and increased energy consumption.

The wear rate w of a rolling or sliding contact is conventionally defined as the volume lost from the wearing surface per unit sliding distance; its dimensions are thus those of [length]. For a particular dry or unlubricated sliding situation the wear rate depends on the normal load, the relative sliding speed, the initial temperature, and the thermal, mechanical, and chemical properties of the materials in contact. There are many physical mechanisms that can contribute to wear and certainly no simple and universal model is applicable to all situations.