START UP AND STEADY-STATE FRICTION OF THIN POLYURETHANE LAYERS

The use of a thin layer of compliant material as a bearing surface in artificial joint replacements has been advocated. This study considers the frictional properties of such possible joint materials under conditions of heavy loading and low sliding velocities. The friction of hard indenters sliding on thin polyurethane layers has been studied. The effect of surface roughness, duration of static loading and layer elastic modulus on friction has been investigated. Friction was measured after a prolonged period of static loading as sliding began and the steady state friction was measured in the subsequent sliding at a constant sliding velocity. A contradiction was found in design requirements for the compliant layered joint replacement: at start up the lowest friction is obtained when the hard indenter is relatively rough whilst in the steady state it is the smoothest indenter which gives the lowest friction values in all cases. It is concluded that although compliant layers have unacceptably high friction in conditions of heavy loading with low sliding velocities, the friction in conditions of good lubrication is very low and compares favourably with existing joint materials.