Wear

Wear is related to interactions between surfaces and specifically the removal and deformation of material on a surface as a result of mechanical action of the opposite surface.

In materials science, wear is erosion or sideways displacement of material from its "derivative" and original position on a solid surface performed by the action of another surface.

Wear of metals occurs by the plastic displacement of surface and near-surface material and by the detachment of particles that form wear debris. The size of the generated particles may vary from millimeter range down to an ion range. This process may occur by contact with other metals, nonmetallic solids, flowing liquids, or solid particles or liquid droplets entrained in flowing gasses.

Wear can also be defined as a process where interaction between two surfaces or bounding faces of solids within the working environment results in dimensional loss of one solid, with or without any actual decoupling and loss of material. Aspects of the working environment which affect wear include loads and features such as unidirectional sliding, reciprocating, rolling, and impact loads, speed, temperature, but also different types of counter-bodies such as solid, liquid or gas and type of contact ranging between single phase or multiphase, in which the last multiphase may combine liquid with solid particles and gas bubbles.

Under normal mechanical and practical procedures, the wear-rate normally changes through three different stages(ref.4):

The secondary stage is shortened with increasing severity of environmental conditions such as higher temperatures, strain rates, stress and sliding velocities etc.
Note that, wear rate is strongly influenced by the operating conditions. Specifically, normal loads and sliding speeds play a pivotal role in determining wear rate. In addition, tribo-chemical reaction is also important in order to understand the wear behavior. Different oxide layers are developed during the sliding motion. The layers are originated from complex interaction among surface, lubricants, and environmental molecules. In general, a single plot, namely wear map. demonstrating wear rate under different loading condition is used for operation. This graph also represents dominating wear modes under different loading conditions (ref. 13).

In explicit wear tests simulating industrial conditions between metallic surfaces, there are no clear chronological distinction between different wear-stages due to big overlaps and symbiotic relations between various friction mechanisms. Surface engineering and treatments are used to minimize wear and extend the components working life.

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