Tribological properties and microstructure evolution of ultra-high molecular weight polyethylene

The friction and wear properties of unmodified ultra-high molecular weight polyethylene (UHMWPE) were investigated experimentally. Disks of semicrysta lline UHMWPE were slid against polished CoCrWNi pins in bovine serum at ran ges of contact pressure and sliding speed typical of those encountered in t otal joint replacements. Thr coefficient of friction was monitored continuo usly during testing, and the wear rate It as determined from surface profil ometry measurements of worn disk surfaces accounting for strain relaxation. Scanning electron microscopy (SEM) results demonstrated that surface deter ioration comprises adhesion, third-body abrasion by polyethylene wear debri s, and delamination wear. The contribution of these mechanism to the overal l wear rate and the formation of wear debris depends predominantly on the c ontact pressure and secondarily on the sliding speed. Transmission electron microscopy (TEM) yielded new insight into the evolution of the microstruct ure morphology of UHMWPE during sliding. Cross sections parallel to the wea r tracks obtained from various depths were analyzed with the TEM to develop a spatial mapping of the subsurface microstructure as a function of contac t pressure. Alignment of crystalline regions (lamellae) in the polyethylene microstructure parallel to the sliding surface was found to occur during s liding even at relatively low contact pressures. SEM observations suggested that the highly oriented microstructure is the precursor to delamination w ear, leading to the formation of wear particles larger than those produced by adhesion and third-body abrasion at the contact interface.