An Investigation of the Fatigue Induced Failure Modes of Fiber/Elastomer ComDosites as Bearing Surfaces in Total HiD Joint Prosthesis

It is proposed to use elastomeric composites as artificial joint bearing surfaces to increase their fluid film lubrication and therefore reduce the number of failures which have been attributed to wear mechanisms. The fluid generated between the surfaces supports part of the load and prevents direct contact of the bearing surfaces. In order to obtain optimum elastohydrodynamic lubrication, elastomeric materials are selected based on their static mechanical properties such as modulus of elasticity. However, the observed permanent deformation and failure of model elastomeric composites has been associated with debonding between fibers and matrix. It is hypothesized that the operating conditions such as frequency (velocity) and temperature at the contact between the elastomeric composite and the reciprocating counter part may contribute significantly to the failure of the elastomer. The mechanical properties of elastomers as of other polymers are highly dependent on strain rate, frequency, and temperature. Polyurethane thermoplastic elastomers of three different hardnesses (85A, 93A, 100A) were characterized with dynamic mechanical thermal analysis. The coefficient of friction between these materials and metal was measured using reciprocating motion. Data showed that the lubrication mode of a metal-elastomer contact in tribological conditions observed in artificial joints is highly dependent on temperature and frequency. In this respect, dynamic mechanical analysis can be used in the selection of an optimal elastomeric material for reciprocating bearing conditions. Introduction Low modulus elastomeric coatings were proposed and investigated by several authors as a method to improve the tribological performance of bearings (Unsworth et al., 1987; Dowson et al., 1991). By deforming under pressure, elastomeric layers enhance lubrication by the formation of a fluid film through elastohydrodynamic and microelastohydrodynamic lubrication (Dowson and Jin, 1986). This occurs when the asperities of these materials flatten due to local pressure perturbations. However, when the fluid film breaks down, adhesive friction increases in presence of smoother elastomeric materials (Fuller and Tabor, 1975). Lower modulus materials provide more effective micro-elastohydrodynamic lubrication but fail due to an increase in shear strain. Dowson et al. (1991) suggested that the elastic modulus should not be lower than necessary to provide effective micro-elastohydrodynamic lubrication. However, these authors did not address the fundamental effect of temperature (heat) as experienced in tribological contacts and operating conditions on the dynamic mechanical properties of polymers and elastomers and therefore, on the lubrication mechanism and subsequent failure of the elastomers. Research on the use of LME as bearing surfaces done thus far has focused on the evaluating experimentally and theoretically the tribological behavior of the elastomer layer. However, a more in-depth understanding of the material properties of elastomers is necessary to predict the behavior of these bearings in the complex environment of the body. Two variables which can have a profound effect on the mechanical properties, such as the modulus of elastomers, and have not been considered to date in the available literature are temperature and frequency. The objective of this research was to: “ characterize the effects of temperature and frequency on the modulus of a selected set of elastomers, and to relate these parameters to both, theoretical and experimental tribological results. ” The long term goal of this research is to develop a better fundamental understanding of all of the complex modes of failure in bearing surfaces which can then be used to design new materials with sufficient longevity for use in a total National Textile Center Annual Report: November 1997 85