Friction arises from the interaction between moving solids in contact and hinders (sliding friction) or prevents (static friction) motion. Friction is the introduction, transformation, and dissipation of energy and may lead to loss of material. This (progressive) loss of particulate debris from the surface of a solid body due to mechanical action has been defined as wear. Although mechanical fracture of the component may also occur in extreme contact cases, wear typically results in dimensional and topographical changes as well as surface damage. This can cause secondary problems such as misalignment or—in extreme cases—seizure of the joint. The generation of wear debris, however, is in many cases even more detrimental than the actual dimensional change of components. Entrapped particles act as the interfacial medium and may change the acting wear mechanism. The effect of entrapped particles is, therefore, as important as their generation.

Although the mechanical properties of engineering materials can be described in terms of strength and toughness, friction and wear are not intrinsic material properties. Wear
and friction are characteristic to a specific tribosystem and incorporate the physical (e.g., Young modulus or thermal conduction) as well as the chemical properties (e.g., reactivity) of surfaces (4). A tribosystem consists of four principal elements: body, counterbody, interfacial medium, and environment (4,5). The operating variables load, relative speed, ambient temperature, and loading time bring about motion and work as an input to the system, resulting in motion and work as output. The loss of the system can be defined in terms of energy (heat or sound) and material (wear debris). Figure 17.1 shows a schematic of a tribosystem.

As wear and friction arise from conditions at the interface, the properties of the surfaces in contact need to be looked at. Technical surfaces are far from being smooth, and exhibit varying degrees of roughness, ranging from the nanometer to millimeter scale. Figure 17.2 highlights the differences between apparent and real contact area on the micrometer scale. The irregularities of the surface usually consist of broad-based hills with angles of inclinations of less than 15 degrees from the base (6). The ratio of real to apparent area of contact changes constantly during motion and can range from <10⁻⁴ to 1. This is dependent on the statistical distribution of topographical irregularities, the acting shear and normal contact forces, as well as the mechanical, physical, and chemical properties of the materials in contact, which might be distinctly different from the bulk properties.

Because of the complex nature of friction and wear, the problems of tribology are difficult to assess by any simple model. The system approach as depicted in Figure 17.1 is an easy-to-use approach, which can be applied for any tribologic problem from an engineering point of view. To define the limits of this approach as well as its possibilities, it is mandatory to analyze worn surfaces under real operating conditions, for example, hip joints from patients with complete medical history and clinical follow-up. Of course, this is a time-consuming and expensive process. A suitable wear-testing apparatus should then reflect the structure of the tribosystem and the type of dynamic interaction between its elements. It should be considered that the proper modeling of the input parameters, for example, loading and motion of the joint, will determine the acting wear mechanisms, while in turn a suitable wear-resistant material must be chosen with respect to them (7). In other words, the key to any solution of a tribologic problem is inextricably linked to the knowledge of the wear mechanisms as well as the sequence and interdependence of their acting. The authors stress the importance of knowledge of the acting wear mechanisms, since it is known that even for the same wear mode different design or material modifications are appropriate. For example, the parameters of the wear mode “three-body abrasion” of a MoM bearing with incorporated mineral particles can change in such a way that the mechanism “adhesion” (cold welding of surface spots by plastic deformation of surface asperities), abrasion (grooving of surfaces by plastic flow), or “surface fatigue” (predominantly cyclic elastic deformation) applies. Thus, a successful plan to improve the characteristics of the materials in contact demands an exact understanding of the structure of the tribosystem and the interaction of its elements. At first, information on prosthesis design, joint kinematics, and kinetics in the physiologic situation is requested. In the following subsections, we will introduce the relevant terminology regarding friction and wear on the basis of the historical development of today’s available systematic, empirical approaches. In doing this, the knowledge of the acting wear mechanism is essential to identify precise and purposeful measures for wear reduction and, thus, to overcome simple trial-and-error methods. Theoretical models based on mechanical finite-element or chemical molecular dynamics computer simulations will not be regarded here, because, up to date, they only investigate a very small volume of interest and do not incorporate the entire tribosystem. Nevertheless, these models are very helpful in guiding the way from the systematic engineering approaches to the atomistic, theoretical level and facilitate a better understanding of specific single effects on friction and wear (8,9).

Friction is generally understood as the introduction, transformation, and dissipation of energy. The introduction of friction is accomplished by cyclic elastic or plastic deformation of contact spots of the real contact area. Then it is transformed into elastic or plastic deformation energy within interlocking surface asperities (10,11) and/or leads to crack initiation and propagation (12,13). This describes the deformation term of friction, which might also be responsible for the generation of particles. Another contribution comes from the adhesion of the surface atoms and molecules of body and counterbody.
The probability of adhesion depends on mechanical properties (14) and the tendency of atoms and molecules to react chemically (15,16). Both the deformation and adhesion contribution to friction can be distinctly lowered by means of surface modifications or coatings as well as by lubrication. At least 90% of the introduced energy is dissipated by transformation into heat (17,18,19,20,21), leading to an increase of the temperature within the contact zone. Depending on local normal forces and the relative velocity, the average as well as the so-called flash temperatures may rise. Although the average temperature is primarily governed by the normal force, the flash temperature depends mostly on the relative velocity and lasts for only a few milliseconds. The remaining 10% is dissipated by storing mechanical energy within lattice defects generated by cyclic elastic and plastic deformation (22), phase transformation (23), or chemical reaction of body, counterbody, interfacial medium, and environment (24).

Wear can be reduced by lubrication. The principal idea behind lubrication is to interpose a material between two contacting solids to minimize interaction between them. For example, wetting of the surfaces reduces adhesion. The extent of fluid film formation plays an important role in the wear process of artificial joints in vivo. It has been pointed out that, under wet conditions, the wear rate of ultra-high molecular-weight polyethylene (UHMWPE) decreases steadily with decreasing counterface roughness, whereas under nonlubricated (dry) conditions an optimum roughness exists (due to the competing mechanisms adhesion and abrasion) (25). Therefore, an adequate surface finish of the prosthetic head (at least R_a <0.05 μm) without imperfections is recommended when paired with UHMWPE (26).

The effectiveness of a lubricant film is defined by the specific film thickness λ which depends on the viscosity of the lubricant, the relative velocity between body and counterbody, the pressure across the interface, and, last but not least, on the roughness of the mating surfaces. Thus, λ can be used to estimate the occurrence of different lubrication regimes, as shown in Figure 17.3. In the case of boundary lubrication, the lubricant adheres chemically to one of the surfaces, and there is full contact between the solids, in contrast to hydrodynamic lubrication where a total separation of the two bodies takes place. Elastohydrodynamic (EHD) lubrication occurs when the pressure in the fluid film is sufficiently high to deform the asperities of the solid surfaces. Thus, even if the thickness of the fluid film is smaller than the heights of the asperities of body and counterbody, a total separation may be still achieved. Under realistic loads and in the presence of synovial fluid, metal/plastic hip joints articulate in the mixed film (27) or boundary (28) lubrication regime. Hard/hard bearings primarily work in the EHD and mixed film lubrication regime (29); however, with increasing femoral head size (>28 mm) a shift toward full fluid film (hydrodynamic) lubrication can be observed as well (30).

Wear has been defined as removal of material from the body in contact as a consequence of mechanical action; thus, creep or plastic deformation are not forms of wear per se because they do not produce wear debris but dimensional changes of the contacting surfaces. Furthermore, corrosion is not directly related to wear because it can take place without mechanical activation at all. To be able to describe wear phenomena unequivocally, it is strongly suggested to distinguish between wear modes, wear appearances, and wear mechanisms (6).