Research
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W. Gregory Sawyer - TRIBOLOGY IS COMPACTLY defined by the Oxford English Dictionary as "the branch of science concerned with
interacting surfaces in relative motion and with associated matters (as friction, wear, lubrication, and the design of bearings." Friction and wear are not intrinsic properties of a material; they are functions of the tribological system. A tribological system is composed of three basic elements, (1) the structure - the types of materials in contact and the contact geometry, (2) the operating conditions - the gross motion, loads, stresses, and duration of operation, and (3) the environment and surface conditions - including the surface environment and chemistry, surface topography, and ambient temperature. The shear number of factors affecting performance makes fundamental studies exceedingly difficult. The studies that aim to explore these fundamentals within a tribology framework generally focus on the mechanisms of friction and the mechanisms of wear.
- Mechanisms of Friction
When two bodies are in contact and relative motion, a finite force is required to maintain this motion, the friction force . A coefficient of friction is calculated by dividing the friction force by the normal force. To date, despite considerable efforts, there is no model capable of predicting friction coefficients from first principles. Thus, careful and proven experimental techniques represent the most sophisticated and reliable technique for investigating, designing, and assessing the tribological worthiness of new materials. The tribological system must be defined well in advance of an experimental study in friction. The three basic points that are considered fundamental to studies of friction are the surface area and nature of the intimate asperity contacts, the surface adhesion and shear strength, and the nature of deformation and energy dissipation occurring at the asperity junctions.
- Mechanisms of Wear
Wear is the gradual removal of material from contacting surfaces in relative motion. Analogous to the mechanisms of friction, there are four basic wear modes that are used in the classification of wear: (1) adhesive wear, (2) abrasive wear, (3) surface fatigue wear, and (4) tribochemical wear. In adhesive wear, the junctions that give rise to the resistance to sliding can also cause removal of discrete particles at the asperity junctions. Abrasive wear is similar to the plowing contribution of friction; the plastic deformation creates wear debris that is eventually ejected from the contact. In surface fatigue wear and delamination, a variety of cyclic events initiates and propagates cracks. The cracks eventually become large enough to cause discrete regions near the surface to be ejected as debris. Tribochemical wear mechanisms involve a coupling between the mechanical and thermal processes occurring at the interface and the environment. Here the corrosiveness or reactivity of the environment is generally enhanced due to these mechanical and thermal processes.