| Ultra-high molecular weight polyethylene(UHMWPE)is an ideal material for artificial joint,owing to its excellent chemical stability,biocompatibility,wear rsistance and low coefficient of friction,However,during the long-term service process,oxidation and wear generate wear debris which can induce osteolysis and joint loosening and ventually lead to joint failure.In order to prolong service life of artificial joint and relieve the pain of patients,improving the mechanical and tribological properties of UHMWPE become an urgent issue.In this paper,high-performance UHMWPE composites were prepared by addition of Graphene oxide(GO)nanoplateles and irradiation crosslinking.The effect of irradiation and aging on the structure,mechanical and tribological properties of the composites was investigated and the working mechanism was also studied.These studies of UHMWPE/GO composites provide important theoretical and experimental evidence for use in artificial joints in the future.(1)GO was prepared by modified Hummer's method and then irradiated by Gamma rays at the doses of 100 k Gy under vacuum.The structure,thermal stability and wettability of unirradiated and irradiated GO were characterized by XRD,FTIR,ESR,XPS,TGA and contact angle.And the mechanism of reduced GO was also studied.The results showed that after irradiation,the surface chemical bond of GO was broken,the content of the functional group of GO was reduced,the surface defect was increased,the ratio of carbon to oxygen of GO was changed,and GO was partially reduced.Vacuum irradiation increased the concentration of GO radicals and decayed exponentially in air.Irradiation reduced the wettability of GO,but improved thermal stability of GO.The mechanism of reduced GO was that high energy particles caused carbon-oxygen bonds to break and formed carbon-carbon double bonds at the fracture.(2)Under vacuum conditions,UHMWPE/GO composites were irradiated by Gamma rays at the doses of 100 kGy,and then the samples were accelerated aged for 21 days at 80°C in an air oven.The structure,thermal properties and wettability of irradiated and aged UHMWPE/GO were characterized by Raman,XRD,FTIR,ESR,TGA,DSC,gel content and contact angle,and the microscopic mechanism was also discussed.The results showed that the gel content of irradiated UHMWPE/GO was consistent with that of irradiated UHMWPE,which indicated that filling GO had no effect on the gel content of UHMWPE,but the gel content of aged UHMWPE/GO was significantly decreased.Irradiation and aging increased the wettability,crystallinity and melting point of UHMWPE/GO,but reduced the thermal stability.Initial free radicals of UHMWPE/GO were mainly composed of allyl radicals and GO radicals.When aging in air,free radicals of UHMWPE/GO were formed by peroxide radicals and GO radicals.After accelerated aging,free radicals of UHMWPE/GO were mainly formed by GO radicals.Accelerated aging resulted in oxidative degradation of UHMWPE/GO,and main ketone-based oxidation products were produced.The oxidation index of aged UHMWPE/GO was consistent with that of aged UHMWPE,indicating that the oxidation resistance of GO was weak.(3)The mechanical properties of irradiated and aged UHMWPE/GO were studied by ball indentation,compression,tensile and scratching tests and working mechanism of GO was also discussed.The results showed that irradiation and GO could synergistically enhance the mechanical properties of UHMWPE,but accelerated aging reduced the mechanical properties of the composites.Irradiation and aging resulted in UHMWPE/GO composites brittleness and reduced the toughness of the composites.Hardness,compression modulus and scratch resistance of aged UHMWPE/GO was higher than that of aged UHMWPE,indicating that GO could inhibit the reduction of the mechanical properties of UHMWPE.The working mechanism of GO was that GO could carry and transfer stress in the matrix and the interface shear stress of GO in the matrix had a close relation with its size.(4)The tribological properties of irradiated and aged UHMWPE/GO were studied by reciprocating friction and wear tester under dry friction,deionized water,physiological saline and calf serum lubrication medium,wear morphology was observed and wear mechanism was aslo studied.The results showed that irradiation enhanced wear resistance of UHMWPE/GO,but accelered aging reduced wear resistance.Wear rate of aged UHMWPE/GO was lower than that of aged UHMWPE,indicating that GO could inhibit the reduction of wear resistance of UHMWPE.In the dry friction and different lubrication media,the friction coefficient order of irradiated and aged UHMWPE/GO was: dry friction> calf serum> physiological saline> deionized water;the wear rate order was: calf serum> dry friction> physiological saline> deionized water.The wear rate order of different treated UHMWPE/GO was: aging> no irradiation> irradiation.In the dry friction,wear mode of irradiated and aged UHMWPE/GO was adhesive wear and fatigue wear.In the saline lubrication conditions,wear mode of the composites was abrasive wear.In other liquid phase lubrication conditions,wear mode of the composites was fatigue wear and abrasive wear.Irradiation increased the crosslink density of UHMWPE and the interaction between GO and UHMWPE,which effectively enhanced the effect of GO,improved the fatigue resistance of composites and reduced the peeling and transfer in wear.Aging resulted in the degradation of UHMWPE and caused the interaction of UHMWPE and GO to be reduced,which reduced the ability of the composites to resist fatigue wear and led to the increase of wear rates. |
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