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Wear

Frictional contacts often involve the gradual deformation or removal of the contacting surfaces. Wear is the number one reason why machines require maintenance and has great economic relevance. A multitude of physical or chemical processes can be responsible for the removal or surface material, which is not always undesired. In manufacturing, these processes can be employed for obtain surfaces with the required surface properties, such as roughness values.

Significance

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Wear is one of the most important tribological phenomena in practice, affecting the function and the life time of many mechanical systems. It is a key factor in technical safety and determines maintenance costs of many mechanical parts in motion. Wear affects parts not only in machines, but also in medicine: artificial joints have to be replaced after approximately 10 years of service due to wear.

When controlled, wear is integral to many methods of manufacturing and material processing such as grinding, polishing or sandblasting. Many areas of technology and medicine are strongly interested in controlling wear by increasing or decreasing it depending on the particular application. Despite its importance, wear remains one of the least scientifically understood tribological phenomena. This is in part due to the complexity of the processes influencing wear. These include contact, plasticity, crack nucleation and propagation, chemical reactions, material mixing and material transfer between contact partners and lubricants as well as the formation of surface layers. For most aspects of friction, wear is not only a consequence but also a major influencing factor. For wear calculations, the most common prediction is simply that the wear volume is proportional to the normal force and sliding distance. As an empirical assumption, this law was suggested by Reye as early as 1860 but it took almost a century for wear laws to be based on models of particular conditions and to be experimentally validated. In this type of prediction there is a constant, the so called wear coefficient, whether adhesive or abrasive. In the case of abrasive wear, it is relatively well defined and ranges from approximately 10−3 to 10−2. In the case of adhesive wear, no clear physical interpretation of the wear coefficient exists. However for adhesive wear, empirically measured values of the adhesive wear coefficient are known to differ by 5 decimal orders of magnitude and also to depend on the type of materials.

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