Preparation And Properties Of Nb₂O₅/Nb₂O₅-Ti/Ti Gradient Coatings On Medical Titanium Alloys

Recently,Nb₂O₅ ceramic has been the hot material for surface modification of metal implants because of its good anti-corrosion,anti-wear,and biocompatible performance.However,the differences in the physical parameters between niobium oxide ceramic and metal would produce considerable interface stress and coating internal stress when the ceramic is directly deposited on the metal substrate,causing the coating to crack,delaminate,and fall off easily.In this paper,Nb₂O₅/Nb₂O₅-Ti/Ti gradient coating was deposited on Ti6Al4V alloy by magnetron sputtering to improve the adhesive strength between the layer and the substrate.Residual stress,microstructure,and properties of the coating were studied with Nb₂O₅,Nb₂O₅/Ti,and Nb₂O₅/Nb₂O₅+Ti/Ti coating as the reference.According to the theory of functionally graded materials,a novel Nb₂O₅/Nb₂O₅-Ti/Ti gradient coating was designed.The Nb₂O₅ outer layer act as the functional layer with wear-resistant and anti-corrosion effects.The Nb₂O₅-Ti intermediate gradient layer and Ti adhesive layer are employed to boost the adhesive performance of the coating.Analysis of residual stress distribution characteristics of four Nb₂O₅-based coatings of Nb₂O₅ single layer,Nb₂O₅/Ti double layer,Nb₂O₅/Nb₂O₅+Ti/Ti composite layer,and Nb₂O₅/Nb₂O₅Ti/Ti gradient layer（code NO1,NO2,NO3,and NO4 respectively）using the finite element method.The effects of the number of layers and thickness of the intermediate gradient layer Nb₂O₅-Ti and the thickness of the bonding layer Ti on the residual stress of the NO4 coating was evaluated,which structure was optimized by the orthogonal method.The results of the study show a maximum residual stress of 11.4 MPa in the NO4 coating,which is 49.3%,44.8%,and 10.2%lower than that of NO1（22.5 MPa）,NO2（21.5 MPa）,and NO3（12.7 MPa）coatings,respectively.When the layer number of gradient interlayer increases from 1 to 6,the maximum residual stress of NO4 coating decreases from 12.6 MPa to 11.7 MPa.Still,it does not change significantly when the layer number rises to 8.The NO4 coating shows a decreased maximum residual stress from 12.4MPa to 11.4 MPa with the reduction of the independent interlayer’s thickness from 0.25μm increases to 0.45μm.When the adhesive layer’s thickness increases from 0.25μm to 0.45μm,the residual stress of NO4coating offers a varied range of less than 2%.For the NO4 coating,the optimal combination of structural parameters is the gradient interlayers of6,the independent layer thickness of 0.4μm,and the bonding layer thickness of 0.35μm.No1,NO2,NO3,and NO4 coatings were fabricated on the surface of Ti6Al4V by magnetron sputtering.Their microstructure and properties were studied.The results show that these coatings exhibit the columnar amorphous structure.NO4 coating has an adhesion of above 50 N,which is 21 times and 0.64 times higher than that of NO1,NO2,and NO3 layers.The order of hydrophilicity,adhesion,mechanical properties,and tribological performance from strength to weak is NO4>NO3>NO2>NO1.All coatings can provide good corrosion protection for the Ti6Al4V alloy,and among them,the NO1 film has the best corrosion resistance.