Influence of different environments on the sliding friction of Ultra-high-molecular-weight polyethylene (UHMWPE)

Abstract

Dynamic friction coefficient (COF) of the reciprocating sliding contact of the conventional UHMWPE, was investigated in four different environments (dry contact; distilled water; pure Ringer's solution and with PMMA particles), at five values of low normal load (0.1-1 N) and three values of sliding speed (4 - 12 mm/s). Significant differences of COF values occurred at the lowest load (0.1 N), whereas sliding speed did not influence COF values. Addition of PMMA particles in Ringer's solution produced significant increase of COF values, especially at the lowest load of 0.1 N. For the dry contact and the highest load (1 N), steady state was reached shortly after the beginning of the test and friction coefficient had uniform behaviour. In the case of wet environment and the lowest load, steady state was not reached and the friction coefficient exhibited non-reproducible random behaviour. According to the Hertz theory, 0.5 N load corresponded to the elastic stress of 48.7 MPa, thus surpassing the values of the elastic limits, hardness and true yield stress of the UHMWPE, and the behaviour of the friction coefficient was drastically different below and above this load value. It can be assumed that below the 0.5 N load, viscoelastic response, accompanied with plastic deformation is dominant, with transition to mainly plastic deformation for the higher loads.