Study On Mechanical And Tribological Properties Of Irradiated UHMWPE Nano-composites

Ultra high molecular weight polyethylene (simplified as UHMWPE) is a type of thermoplastic with low friction coefficient and wear loss. It has been used as bearing materials of artificial joints for fifty years. It has been clinically applied on hip, knee and wrist joints, articulated against metallic or ceramic joint. Due to the intrinsic defects on the wear surface, a large amount of wear debris will be generated, when the bearing surface of UHMWPE cup endures multidirectional wear motion under physiological impact stress condition. And it will activate local tissue macrophages which release pro-inflammatory cytokines to result in bone resorption. These actions will lead the fixed artificial joints to aseptic loosening and eventual failure, and up to 50% of artificial joint failure. It can be deduced that wear is the main reason for the failure of artificial joints. It is important to reduce wear rate of UHMWPE and enhance the bearing ablility of UHMWPE to prolong the natural life of artificial joints.Irradiation crosslinking is an effective method to enhance wear resistance of UHMWPE. But some mechanical properties and bearing ability of UHMWPE are decreased after irradiation, especially for the air irradiation condition. To avoid the effects of oxyen in the air, vacuum hot pressing, vacuum package irradiation and vacuum melting heat treatment were introduced into the preparation process. Filling nano-particles was adopted to solve the problem of softened hardness and the reduced bearing ability.In this study, three station knee joint wear rig was designed according to the motion character of natural knee. Self-molded UHMWPE were prepared, and tribological properties, mechanical properties and microstructure were investigated for self-molded and commercial extruded UHMWPE. Then irradiation crosslinking and nano-particles filling were combined to modify UHMWPE. Nano-hydroxyapatite (simplified as nano-HAP) was used as filling particles, because of its good biocompatability. Tribological properties, mechanical properties and microstructure were investigated to determine the effects of irradiation crosslinking and nano-particles filling on these properties. Finally wear resistance of UHMWPE/nano-SiO2 under multi-directional motion was investigated, and surface wear resistance layer was successfully structured.Irradiation crosslinking decreased friction coefficient of UHMWPE under linear reciprocating motion with calf serum lubrication. Wear resistance under unidirectional motion, linear reciprocating motion and knee joint motion was effectively enhanced by irradiation crosslinking. Irradiation crosslinking also enhanced the ability of resistance to adhesive and abrasive wear. The gel content of UHMWPE increased as irradiation dose increased, but crystallininty decreased significantly after irradiation. The variation of crystallinity and crosslinking degree for UHMWPE reduced plasticity and bearing ability of UHMWPE. The infrared spectrum indicated that hot pressing, package and remelting in vacuum was an effective method to avoid oxidation of UHMWPE during irradiation process.Filling nano-HAP could effectively reduce friction coefficient and wear rate of UHMWPE, enhancing adhesive wear resistance of UHMWPE. Filling 7% nano-HAP combined with irradiation at 100kGy exhibited synergistic enhancement of wear resistance under linear reciprocating condition. It should be relevant to the bearing condition of nano-particles on the contact surface. The increasing crystallinity after filling nano-particles enhanced modulus and bearing ability of UHMWPE. Tribological properties of UHMWPE/nano-HAP exhibited significant relationships with combined modulus of tribological system. Nano-particles had no effect on gel content of irradiated UHMWPE. The infrared spectrum indicated that hot pressing, package and remelting in vacuum was an effective method to avoid oxidation of UHMWPE/nano-HAP during irradiation process, and oxygen in the nano-HAP could not oxidize UHMWPE during the irradiation process.Wear rate of irradiated and non-irradiated UHMWPE/nano-SiO2 decreased as filling content increased under multi-directional motion. There were no significant differences of surface roughness for materials with different filling content, and surface roughness decreased significantly after the wear test. UHMWPE/nano-SiO2 exhibited better plasticity than UHMWPE/nano-HAP. The vacuum irradiated UHMWPE/nano-SiO2 powders did not exhibit any oxidation. The structured wear resistance layer was well combined with the matrix material, and exhibited the low wear rate. It indicated that forming a wear resistance layer was suitable to prepare a new type of artificial joint material.