Preparation And Properties Of TaC/TaC-Mg/Mg Gradient Coatings On Medical Magnesium Alloys

Magnesium alloys are potential biodegradable implant material due to their mechanical properties close to natural bone and excellent biocompatibility,degradability.However,their fast degradation rate of the magnesium alloys will lead to premature implant failure,which restricts their clinical applications.Tantalum carbide ceramics have excellent corrosion resistance and wear resistance.However,the physical and thermal parameters of tantalum carbide and magnesium alloy do not match,which leads to their poor bonding properties.For improving the adhesion of the coating to the substrate,TaC/TaC-Mg/Mg gradient coating was designed and prepared on the surface of ZK60 magnesium alloy by magnetron sputtering.In this work,the study on the design,preparation,microstructure,adhesive,tribological,and corrosive properties of the coatings was systematically carried out.Based on the theory of functionally graded materials,TaC/TaC-Mg/Mg gradient structure coating was designed.In the coating,Mg acts as the bonding layer;TaC is the functional layer,which plays the role of wear resistance and corrosion resistance;TaC-Mg performed the intermediate gradient layer to improve the bonding strength between the coating and substrate.The residual stress distribution of TaC monolayer,TaC/Mg bilayer,TaC/TaC+Mg/Mg multilayer,and TaC/TaC-Mg/Mg gradient coating on ZK60 surface were analyzed by finite element method.The effects of the structure parameters of the gradient coating and the substrate temperature on the residual stress of the layer were studied,and the structure of the gradient coating was optimized.The results show that:(1)the residual stress of the gradient coating is smaller than that of the single-layer and double-layer coatings;(2)the residual stress of the gradient coating was decreased with the increase in the number and thickness of the gradient layer and was increased with the rise of the substrate temperature;(3)the optimal combination of structural parameters of the gradient coating is gradient layers of 7 layers,thickness of each gradient layer is 0.7?m,and adhesive layer thickness of 0.3?m.Four kinds of TaC-based coatings,TaC single layer,TaC/Mg double layer,TaC/TaC+Mg/Mg multilayer,and TaC/TaC-Mg/Mg gradient coatings(TM1,TM2,TM3,and TM4)were deposited on ZK60 by magnetron sputtering.The microstructure,phase composition,wettability,bonding strength,tribological properties,and corrosion resistance of the coating samples were studied.The results show that:(1)the TaC-Mg gradient interlayer structure of TM4 coating is compact,and other coatings are columnar porous;(2)the surface of TM4 coating consists of Ta,Ta₂O₅,Mg,and Mg O phases,and element diffusion appears inside the coating;(3)with the increase in the number of the interlayer,both the roughness and the contact angle of TaC-based coatings increases,while the hydrophilicity decreases;(4)the adhesion of TM4 coating was 9 N,which is 140%,125%,and 50%higher than that of TM1,TM2,and TM3coatings,respectively.(5)four kinds of TaC-based coatings improved the wear resistance and corrosion resistance of ZK60,among which TM4coating exhibited the best wear resistance and corrosion resistance.Compared with ZK60 magnesium alloy,the wear rate and the corrosion current density of the TM4 specimen were decreased by one order of magnitude and 73.8%,respectively.