Wear processes and theory for polymer implants

Polymers running under steady state conditions, display wear characteristics that are similar to those found in metals with the rate of material removal being proportional to normal load. This relationship is described by Archard s Equation (Archard, 1953)  

This equation relates the volumetric material loss per unit sliding distance (Q) to the normal load (W) and hardness of the soft surface (H). The dimensionless constant K given above is an important property that provides a measure of the severity of the interaction between the asperities of two interacting surfaces and the likelihood of this interaction generating wear. However, in engineering applications, it is often more useful to use a dimensional wear coefficient, k (mm m ), i.e. the volume of material lost to wear per unit distance slid, per unit normal load on the contact. The use of this coefficient 

Abrasion is a form of cohesive wear that can occur in two modes, viz. two-body and three-body abrasive wear. Two-body abrasion refers to a hard rough surface, of which the asperities plough through the relatively stiffer counterface. The surface penetrations cause localised plastic displacement and indentations. Three-body abrasion refers to hard particles between two sliding surfaces, ploughing through at least one of the surfaces. The two are not mutually exclusive, as two-body abrasion can often lead to three-body when hard wear particles are detached from a surface. Abrasive wear is dependent on the bulk properties of the materials and the geometry of 

In its simplest form, an abrasive wear model can be defined by considering a hard conical asperity with a slope 6 under a normal load W ploughing through a polymer surface, removing material and producing a groove. The amount of material lost by abrasive wear w is (where symbols have the same meaning as the previous equation)  

Clearly this equation has a similar form to Archard s Equation for sliding wear and so several general laws of wear can be stated  

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