S. Affatato and D. Brando,     Istituto Ortopedico Rizzoli, Italy

Abstract:

The term ‘wear’ could be defined as an undesirable progressive loss of material from one or both surfaces in relative motion between them. Wear is not a basic material property, but is a system response of the material; because of this, the mechanism can have a variegate nature. The mechanism of wear is very complex and there are two broad approaches to the classification of wear: the first is descriptive of the results of wear, while the second is based on the physical nature of the underlying processes. It is, however, the second form of classification that is most useful. According to this approach, five wear processes have been clearly recognized: abrasion, adhesion, fatigue, erosion and corrosion. It has been agreed that wear cannot be totally prevented, but most of these mechanisms can be attenuated and, in some cases, can be avoided. It is very important for a designer to know the amount a component can wear before it must be replaced. Since manufacturers must recommend replacement or overhaul times for the equipment they supply, they must be able to ascertain, for each component, when sufficient wear has occurred so that the component is no longer usable.

Key words

wear

wear mechanism

in vivo wear measurements

in vitro wear measurements

socio-economic wear impact

1.1 History of wear

Wear is a phenomenon that occurs between two bodies that are in contact. The term ‘wear’ could be defined as an undesirable progressive loss of material from one or both of the surfaces during relative motion between them (Bhushan, 1999; Schmalzried and Callaghan, 1999). An ancient observation that gave a good description of wear phenomena was made by Lucretius in De rerum natura, I (95–55 BC) (Dowson, 1998a):

More recently, it was Ragnar Holm, working first in the Siemens-Konzern laboratories in Berlin and later with the Stackpole Carbon Company of St Marys, Pennsylvania, who made one of the earliest substantial contributions to the study of wear (Dowson, 1998b; Holm, 1946). Initially, Holm considered wear to be a uniform atomic transfer process taking place at asperity contacts. He later introduced the concept of a uniform layer of transferred material having a thickness equal to an integral number of molecules, mainly to establish a physical appreciation of the wear process (Holm, 1946).

Wear is probably the most important aspect of tribology, yet it has remained largely unexplored until recent times. The recent overflow of publications is difficult to appreciate in historical terms, partly because of the sheer volume of material, but also because we must wait awhile before some of the basic and intrinsic concepts can be confirmed by experiment and practice as valid signposts on the road of truth.

Wear can result from a variety of different processes triggered by the sliding contact between two surfaces and can cause different effects depending upon the component in question and its functional requirements. Wear is not a basic material property, but a system response of the material so that this mechanism can have a variegate nature (Bayer, 1994).

There is no simple relationship between wear and any other materials property and the wear phenomenon can be split into two majority categories: wear dominated by the mechanical behavior of materials and wear dominated by the chemical behavior of materials (McKellop et al., 1995; Peterson, 1980). Both these effects produce changes in the appearance (the morphological features) of the bearing surfaces. During the wear process, worn material is expelled from the contact between two surfaces in the form of debris and these wear products can cause adverse reactions leading to massive bone loss around the implant and consequently loosening of the fixation (Brown and Clarke, 2006; McGee et al., 2000).

The mechanism of wear is very complex; it should also be understood that the real area of contact between two solid surfaces compared with the apparent area of contact is invariably very small, being limited to points of contact between surface asperities. The load applied to the surfaces will be transferred through these points of contact and the localized forces can be very large. The material intrinsic surface properties such as hardness, strength, ductility, work hardening, etc. are very important factors for wear resistance, but other factors such as surface finish, lubrication, load, speed, corrosion, temperature and properties of the opposing surface are equally important.

1.2 Wear mechanisms

There are two broad approaches to the classification of wear. The first is descriptive of the results of wear and the second is based on the physical nature of the underlying processes (Dowson, 1998b; Peterson, 1980). It is, however, the second form of classification that is the most useful.

A wear mechanism is the fundamental microscopic process by which material is removed from a surface (Scherge et al., 2003). Wear can also be defined as a process in which interaction of the surfaces or bounding faces of a solid with its working environment results in dimensional loss of the solid, with or without loss of material. According to this approach, five wear processes have been clearly recognized: abrasion, adhesion, fatigue, erosion and corrosion (Archard, 1980; Callaghan et al., 1995; Peterson, 1980; Schmalzried and Callaghan, 1999). These general classifications of wear were well established by the early 1970s, but much of the work supporting these views was undertaken on metals under dry rubbing conditions (Dowson, 1998b).

Both polymers and ceramics have assumed important positions in the spectrum of tribological materials in recent decades. Polymers now occupy a major position in the bearings field, while the inertness, hardness and excellent surface finish of ceramics make them particularly attractive in both monolithic and coating forms (Lawn, 2002; Martorana et al., 2009). As interest moved away from the ‘severe’ end of the wear scale, a major step forward was recorded by Suh in 1973 when he introduced the concept of delamination wear (Suh, 1973). Quinn et al. (1980) also attacked the problem of chemical or oxidative wear. The different wear modes are now explained in more detail.