Research On Torsional Fretting Of Uitra High Molecular Weight Polyethylene

Ultra high molecular weight polyethylene(UHMWPE)has higher abrasion resistance,lower coefficient of surface friction,better mechanical properties and biocompatibility.The best combinations for artificial joint are UHMWPE for the cup and metal materials for the head.Wear occurs in artificial joint after being used for a long-time.Wear debris induce bone resorption,which lead to aseptic loosening and ultimate failure of artificial joint.The main modes of movement in most kinds of joints include slipping,rolling and fretting.Considering complexity of the human body,some movements are required as the rotation of the head in the acetabular cup, where torsional fretting occurs between two contact surfaces.The study of torsional fretting wear has not yet been done up to now.The torsional fretting behaviors of UHMWPE under varied angular displacement amplitudes(θ=0.2°,1°,5°,30°and 90°),and normal loads(F_n=100N,200N)were investigated.The wear behavior was investigated under a ball-on-flat configuration with two kinds of counter-pairs(Al₂O₃ and TC4).The kinetic behaviors of torsional fretting tests were analyzed in detail in combination with microscopy examinations through optical microscopy(OM),scanning electrical microscope(SEM),x-ray photoelectron spectroscopy(XPS)and x-ray diffractometer(XRD).The torsional fretting processes and mechanisms have been analyzed,and the main conclusions are as follows:1.Friction torque～angular displacement amplitudes(T～θ)curve was used to describe the running characteristics of torsional fretting.Three typical types of T～θcurves were presented for the UHMWPE,i.e.in shape of linear,elliptical and parallelogram.2.When the angular displacement amplitude was smaller(θ=0.2°),the fretting was running in partial slip regime.When the angular displacement amplitude was higher,the fretting was running in gross slip regime.Increased normal loads had almost no effect on the shape of T-θcurves. 3.The torque curves can be divided into three stages,i.e.running-in stage,ascent stage and steady stage.Under some test conditions,there was no steady stage.4.Torsional contact stiffness increased quickly with the increase of the number of cycles at the initial stage of test,and then reached the steady state.Friction dissipated energy curves first kept invariant and then rose gradually with the number of cycles.5.When the angular displacement amplitude was smaller,the wear of UHMWPE was very slight and free damage occurred in the centre of the scars.Some fined ploughs and scattered spherical debris were distributed in outer annulus of the contact area.When the angular displacement amplitude was higher,the wear scar of UHMWPE was characterized by ripples,ploughs and tears appeared in outer annulus of the contact area.The main wear mechanisms for the UHMWPE were abrasive wear,adhesive wear and oxidation wear.6.The XPS and XRD analysis found that the crystallinity of worn scar decreased compared to the original surface,and there were scission of the UHMWPE chains occurred.At the same time,the wear process accompanied with oxidation during torsional fretting.7.The variation of the T-θcurves,the torque curves,the friction dissipated energy and torsional contact stiffness were similar when the friction pairs were Al₂O₃ and TC4.The value of torque of UHMWPE/TC4 was higher at F_n=100N,but higher for the UHMWPE/Al₂O₃ contact at F_n=200N.The friction dissipated energy of the UHMWPE/Al₂O₃ contact was higher than that of the UHMWPE/TC4 contact.The morphologies of the wear scars of UHMWPE were similar for the both counter-pairs.But there was some different:the plastic flow appeared at the center of the wear scar of the UHMWPE/TC4 contact,but the phenomenon disappeared for the UHMWPE/Al₂O₃ contact.The depth and diameter of the UHMWPE/TC4 scars were lower under the same test conditions.