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Wear
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Wear of Mechanical Systems
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Wear of nanoComposites
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Friction and Lubrication
- In Situ Tribology
- Gas Surface
Interactions
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Biomaterials Tribology
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Wear of Total Knee Replacements
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Hydrogels
New In Situ Experiments
RIBOLOGY 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.
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