Effects of machining on tribological behavior of ultra high molecular weight polyethylene (UHMWPE) under dry reciprocating sliding

Abstract

This paper presents a study of the machining effect on tribological properties of ultra high molecular weight polyethylene (UHMWPE) in terms of coefficient of friction and wear factor under dry reciprocating sliding conditions. Machining parameters include cutting speed, feed rate and depth of cut. Polymeric structure of the semicrystalline polymer was characterized using differential scanning calorimetry (DSC). Changes in polymeric structure and surface texture caused by machining were related to the tribological behavior of machined UHMWPE. The average coefficient of friction increased quickly in the first 60 s. Then the coefficient of friction increased very slowly and reached a steady state. The initial average coefficient of friction was in the range from 0.12 to 0.15. After 1 h of dry sliding, the average coefficient of friction was in the range from 0.17 to 0.23. No significant correlations were found between depth of cut and coefficient of friction, or between cutting speed and coefficient of friction. However, coefficient of friction decreased as cutting speed increased when the same ratio of cutting speed to tool feed rate was maintained. Increase in cutting speed caused more damage on the subsurface structure of machined UHMWPE. Wear factor decreased as cutting speed increased if tool feed rate was kept unchanged. Wear factor increased as cutting speed increased when the ratio of cutting speed to tool feed rate was kept constant. There was an optimum depth of cut for the best surface roughness and wear resistance, which was about 0.127 mm under the studied condition. Optical microscopy and variable pressure scanning electron microscopy analysis showed severe plastic deformation and ploughing as the main wear mechanisms.