Friction of hydrogel and polyurethane elastic layers when sliding against each other under a mixed lubrication regime

Abstract

The use of a compliant material to form the articulating surfaces of a replacement joint has been shown to encourage fluid film lubrication in the joint in conditions of cyclic loading and continuous joint motion. When the joint is heavily loaded with little or no movement, however, joints consisting of a metal component articulating on a compliant layer have been shown to give high friction. This study investigates the friction when two compliant layered components slide against each other in conditions of mixed lubrication, which are present in the joint during slow heavily loaded motion and compares the results with a hard metallic surface sliding on the compliant polyurethane and hydrogel materials. Start up friction on polyurethane-polyurethane contacts was extremely high (coefficient of friction up to 4) and was significantly greater than start up friction for metal on polyurethane, or polyurethane on hydrogel. Metal on hydrogel gave the lowest value for start up coefficient of friction of 0.25. The polyurethane-polyurethane combination gave stick slip motion during low velocity sliding, with peak coefficients of friction rising to 4 during the stick phase, causing damage to the surfaces. The steady state sliding coefficients of friction for the other three material combinations were similar and in the range 0.07 to 0.1. The most appropriate material combination for cushion form joints is a hard surface sliding on a compliant layer and the compliant layer should preferably be porous.