Research On The Thermal Oxidation Of Zr-2.5Nb Alloy And Its Biocompatibility

Zr alloy has become a candidate for a potential new biomedical implant material because of its good mechanical properties,corrosion resistance and excellent biocompatibility.However,the application of Zr-2.5Nb alloy is restricted in the field of artificial joints as its low surface hardness and poor wear resistance.Besides,oxide ceramics have improved wear performance relative to metal alloys;nevertheless,their use has been restricted because their brittle characteristics limit implant designs and raise concerns over component fracture.Thus,the aim of the present study is to simultaneously improve wear properties of Zr-2.5Nb alloys via surface oxidation,forming a predominantly zirconia surface film by combining the fracture toughness of Zr-2.5Nb alloy with the wear performance of zirconia ceramics.The surface composition,surface morphology,mechanical properties,wettability,biotribological and biocompatibility properties of the thermally oxidized Zr-2.5Nb alloy were analyzed to evaluate the performance of the thermally oxidized Zr-2.5Nb alloy.The results show that the main composition of the thermal oxide layer is ZrO2.With the increasing of the temperature,the thickness of the oxide layer is increased greatly.However,when the thickness of the oxide layer is increased to a certain value,cracks,holes and other defects appear.In addition,the contact angle of the thermal oxide layer on the surface of Zr-2.5Nb alloy is gradually reduced,and contact angles are close to 70° under thermal oxidation treatments,which is the closest contact angle to the cell adhesion.Furthermore,the surface hardness of Zr-2.5Nb alloy is greatly improved.When the thermal oxidation temperature is 650 ?,the hardness is 4.65 times of that of Zr-2.5Nb alloy without thermal oxidation.Zr-2.5Nb alloy's nano-hardness of the test specimen was measured using an in situ nanomechanical test system,which has a significant increase by thermal oxidation treatment,but at the same time plasticity is decreased.When the thermal oxidation temperature is 650 ?,the nano-hardness of the alloy reaches at the maximum,the surface hardness is significantly improved and it is very good to retain the plasticity.Friction and wear tests show that the friction coefficients of Zr-2.5Nb alloy after oxidation are lower than that of untreated alloy under different thermal oxidation temperatures and lubricating conditions.In general,under dry friction condition,the value of friction coefficient fluctuates greatly.Under the condition of saline water lubrication,the friction coefficient of the samples under thermal oxidation treatments tends to be stable,and the fluctuation fluctuates between 0.2 and 0.4.Under the condition of 25% calf serum lubrication,the friction coefficient of the alloy tends to be stable,which is finally fluctuated around 0.2 and significantly less than the value under dry friction and saline lubrication.The wear rate of Zr-2.5Nb alloy under thermal oxidation is lower than that of untreated samples under three different conditions.The wear morphology of Zr-2.5Nb alloy without thermal oxidation has furrows and is accompanied by plastic deformation under three conditions,and the wear mechanism is abrasive wear.However,Zr-2.5Nb alloy after thermal oxidizing at 650 ? also has furrows under dry friction and there is obvious debris accumulation at both ends of the abrasive wear.Therefore,the wear mechanism is abrasive wear.The wear morphology of the furrow is shallow under the condition of saline water and bovine serum lubrication,so the wear mechanism is fatigue wear.With the increasing days of cultivation on the surface of the material,the number of cells attached to the surface of the material increases,indicating that the osteoblasts grow well on the surface of Zr-2.5Nb alloy before and after oxidation.The number of osteoblasts on the surface of Zr-2.5Nb alloy after thermal oxidation increased significantly at different temperatures,especially at the temperature of 650 ?,which is even more than that on the blank control group.The morphology of the osteoblasts is spindle-shaped,and the osteoblasts are very close contact with the surface of the Zr-2.5Nb alloy before and after thermal oxidation.Furthermore,the cell bodies are full and the pseudopods appear.From the confocal laser microscope,there are many dead cells on the surface of the untreated Zr-2.5Nb alloy.But the dead cells on the alloy surface are greatly reduced and the surface almost doesn't have dead cells after the oxidation treatment.The cells grow more densely on the surface,especially when the oxidation temperature is 650 ?.