Introduction to Biotribology

The science of Tribology (Greek tribos: rubbing) concentrates on Contact Mechanics of Moving Interfaces that generally involve energy dissipation. It encompasses the science fields of Adhesion, Friction, Lubrication and Wear.

Leonardo da Vinci (1452-1519)) can be named as the father of modern tribology. He studied an incredible manifold of tribological subtopics such as: friction, wear, bearing materials, plain bearings, lubrication systems, gears, screw-jacks, and rolling-element bearings. 150 years before Amontons' Laws of Friction were introduced, he had already recorded them in his manuscripts. Hidden or lost for centuries, Leonardo da Vinci's manuscripts were read in Spain a quarter of a millennium later.

To the pioneers in tribology one counts besides Leonardo da Vinci also Guillaume Amontons (1663-1705), John Theophilius Desanguliers (1683-1744), Leonard Euler (1707-1783), and Charles-Augustin Coulomb (1736-1806). These pioneers brought tribology to a standard, and its laws still apply to many engineering problems today. Some of their findings are summarized in the following three laws:

1. The force of friction is directly proportional to the applied load. (Amontons 1^st Law)

2. The force of friction is independent of the apparent area of contact. (Amontons 2^nd Law)

3. Kinetic friction is independent of the sliding velocity. (Coulomb's Law)

These three laws were attributed to dry friction only, as it has been well known since ancient times that lubrication modifies the tribological properties significantly. However, it took quite a long time until lubrication was studied pragmatically and lubricants were not just listed such as a "cooking formula". It was Nikolai Pavlovich Petrov and Osborne Reynolds around 1880, who recognized the hydrodynamic nature of lubrication, and introduced a theory of fluid-film lubrication. Still today, Reynolds' steady state equation of fluid film lubrication

is valid for hydrodynamic lubrication of thick films (> mm) where the frictional (drag) force, F, is proportional to both the sliding velocity, v, and the bulk fluid viscosity h, and inversely proportional to the film lubricant thickness, D. The hydrodynamic theory breaks down below a critical thickness threshold that is expressed in the Stribeck-Curve (Richard Stribeck 1902).

In the twentieth century the theories of dry friction and lubricated friction were further developed. Solid-like behavior of lubricants in the ultrathin film regime (> mm) led to theory of Boundary Lubrication, which was proposed by W.B. Hardy (1919). The adhesion concept of friction for dry friction, already proposed by Desanguliers, was applied with great success by Bowden and Tabor to metal-metal interfaces.

Adhesion is a term relating to the force required to separate two bodies in contact with each other. Desanguliers (1734) proposed adhesion as an element in the friction process, a hypothesis which appeared to contradict experiments because of the independence of friction on the contact area (Amontons 2^nd Law). Therefore the tribologists rejected Desanguliers' proposal and devoted their attention to a more geometrical hypothesis of friction, the interlocking theory of mechanical asperities. The contradiction between the adhesive issue and Amontons 2^nd Law cleared up by the introduction of the concept of the real area of contact. The real area of contact is made up of a large number of small regions of contact, in the literature called asperities or junctions of contact, where atom-to-atom contact takes place. Bowden and Tabor (1954) showed that the force of static friction between two sliding surfaces is strongly dependent on the real area of contact. A very important outcome of their work, which led to the asperity contact theory of friction, is their detailed discussion about adhesive wear. In contrast to abrasive wear which applies to the form of wear arising when a hard, rough surface slides against a softer surface, in adhesive wear, asperity junctions plastically deform above a critical shear strength, which depends on the adhesive forces of the two surfaces in contact. Assuming during a frictional sliding process a fully plastic flow situation of all asperities, friction is found to change linearly with the applied load as demanded by Amontons 1^st Law.

Bowden and Tabor investigated friction also from the perspective of a purely elastic sliding process. They used a simplified single asperity model of contact based on the Hertzian elastic theory, and found a non-linear friction-load dependence (F=L^2/3), which clearly contradicted Amontons 1^st Law and the experiments conducted at that time. It was Archard (1953), who recognized that there was no contradiction between an elastic single asperity model and Amontons 1^st law that is based on a contact that involves many asperities. Instead of assuming a constant number of asperities as Bowden and Tabor did, Archard assumed a load dependent number of asperities. With this assumption the controversy between the elastic multiple asperity hypothesis and Amontons 1^st Law could be resolved. Greenwood and Williamson further improved the method with a Gaussian and exponential distributions of asperities. With the inception of the atomic force microscope (AFM) and friction force microscope (FFM) Bowden and Tabor's single asperity elastic theory (F=L^2/3) could be experimentally verified.

Reynolds fluid film lubrication bases strongly on the assumption that no slip occurs at the fluid solid interface. The condition of no-slip, today described by physical adsorption, brought Hardy to the idea of boundary lubrication. The boundary lubrication is only of molecular thickness. In most cases the lubricant thin film, which acts like a soft solid lubricant, shows incomplete coverage. Wear occurs at these breakthroughs exhibiting complex friction-load dependences. The term boundary lubricant is used for thin organic layer lubricants which can reduce the coefficient of friction by a factor of 20, and the rate of wear by 10,000 or more. Thermodynamic activation models based on Eyring's cage model have been used to describe the frictional phenomena in boundary lubrication of Langmuir Blodgett films (Briscoe, Evans: 1981), and simple fluid lubrication such as hexadecane (He, Overney: 2000).

The shear properties of thin fluid layers under external compressions have found great interest over the last two decades. Surface forces apparatus (SFA) studies, pioneered by Tabor, Israelachvili, McGuiggan and Gee, showed liquids to behave like solids, i.e., cable of storing energy. Interestingly, liquids under these conditions exhibit very high viscosities but unexpectedly low shear resistances.