Wear of metals: Philosophies and practicalities

Innumerable papers give classifications of wear, usually including lists of possible wear mechanisms, but very few papers discuss either the rationale for the classification used or the relationship between the proposed mechanisms and other, competing mechanism lists. The aim of this paper is to synthesize the various approaches to wear classification and mechanisms into a rational system satisfying two requirements: division into a sufficient number of recognizable wear types to give adequate guidance for wear mitigation; and reasonable simplicity, to facilitate use by non-specialists. The information most relevant to wear mitigation comes from an understanding of fundamental wear mechanisms operative in a particular case, but direct investigation of these mechanisms requires specialist techniques and knowledge often not readily available in an industrial setting. A classification system is proposed based on `wear modes' - essentially an engineering description of the macroscopic situation, apparent principal agent of damage and observable manifestations of wear. To these modes can be attached a description of the microscopic wear mechanisms typically found to operate under such conditions, based upon past experience and research, without requiring microscopic investigations to be carried out for each wear problem encountered. Details of some common, industrially-important wear modes are described. Emphasis is given to the understanding of wear mechanisms, and in particular to the relationships between different possible mechanisms within a given mode. Some wear mechanisms about which there is lack of agreement in the literature are critically discussed, with an attempt to find a consensus view. Some disparities between mechanism descriptions are shown to relate to differences of opinion regarding what constitutes a `mechanism': whether the causative agent of damage should be mentioned in the mechanism description, whether elimination of debris from the system is a necessary part of wear, and the level of detail it is appropriate to describe. The different views are presented and resolutions are suggested