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Glossary of terms
Like abrasive wear, adhesive wear can also be characterized by a single word. In the case of adhesive wear, the word is "welding," or more precisely, "microwelding." The actual micromechanism is well described by the term "adhesive wear."
When two surfaces are sliding with respect to each other, they may or may not be separated by a lubricant. When a peak, or asperity from one surface comes in contact with a peak from the other surface, there may be instantaneous microwelding due to the frictional heat. Continued relative sliding between the two surfaces fractures one side of the welded junction making the asperity on one side higher and the asperity on the other side lower than the originai height. The higher peak is now available to contact another on the opposite side.
The tip may either be fractured by the new contact or rewelded to the opposite side, and the cycle repeated. In either case, adhesive wear frequently starts out on a small scale, but rapidly escalates as the two sides alternately weld and tear metal from each other's surfaces. Also, the debris may be carried by the lubricant, if one is present, to other parts of the mechanism. Extreme wear may result; complete failure of the mechanism may result. In severe adhesive wear, the debris is composed of free metallic particles; in mild cases the much finer particles may react with the environment to form debris that is largely free oxide particles.
As should be apparent, the interface between two sliding surfaces is an extremely complex system, consisting of two metal surfaces - each with its own metallurgical, mechanical, chemical, and topographical characteristics - and usually a lubricant, which also is an extremely complex blend of physical and chemical characteristics that change with the temperature. In other words, there are both good and poor combinations of metals, and also good and poor lubricants in a given application.
The heat generated by friction is - locally - high enough to cause microwelding, as described. This means that the temperature also is high enough to cause unintended local heat treatment of the surface metal. Adhesive wear is quite similar to grinding burn in that both can cause tempering of the subsurface regions and actual rehardening of steel microstructures, resulting in white, untempered martensite, which is extremely susceptible to cracking because of its brittleness. These cracks can lead to either brittle fracture or fatigue fracture, depending on the part and the application.
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