Preparation And Properties Of Hydroxyapatite Coating On The Surface Of Nitride Layer Obtained By Laser Gas Nitriding On NiTi Alloy

Due to its special shape memory effect and super-elastic properties, the shapememory alloys(SMA) can be used to make heart holder, blood vessel dilater etc forimplanting into the body. But the bio-inertia of SMA limits its application in this area,and researchers have recently reported it can be modified by preparing bioceramicscoating on its surface. Now the bonding strength between substrate and coating is oneof the focuses of biomaterials. A transitional coating TiN was prepared by LGN andetching with mixed acids to improve the bonding strength in the study.Laser gas nitriding(LGN) was conducted on the surface of NiTi alloy, thus acoating of nitride was formed. A hydroxyaptite (HA) coating on the NiTi/TiN etchedsurface was prepared through electrodeposition and sol-gel later to improve theinterface bonding strength. OM, SEM, TEM and XRD were employed to studymicrostructure, composition and morphologies of nitride and hydroxyapatite coating,and corrosion and wear properties were measured. The influence of current densityand concentration of main salts on the structure of HA coating was studied, togetherwith the influence of sintering on bonding strength and composition of the coating. Anelementary study on preparation of HA coating on NiTi/TiN was tried, and theformation mechanisms of HA coating by electrodeposition and sol-gel werediscussed.The results show that a compact TiN coating with excellent anticorrosion andwear property was formed on the surface of NiTi alloy, and the content of TiNdecreased with the increase of distance off the surface. An obvious change ofanticorrosion and wear took place after LGN. A coating of HA was obtained on thesurface of NiTi/TiN and key factors were found. The optimal process parameters,including concentration of NH4H2PO4 0.025mol/l, the current density 13mA/cm2, andthe distance of electrodes 2.5cm, and surrounding temperature 50℃, and the time 1hwere suggested. Although the sintering process improved the bonding strengthbetween coating and substrate, which increased first and then decreased with theincrease of temperature, and the optimal temperature 940 ℃ were suggested.However sintering changed the composition of coating from HA to Ca3(PO4)2 (TCP).The bonding strength of the coating on NiTi/TiN surface was higher than that onnon-LGN treated NiTi surface.HA coating was also obtained on NiTi/TiN surface using sol-gel methods whenPO(C2H5O)3 and Ca(NO3)2 used as precursors and water used as solvent. Thesuggested temperature of crystallization was 750℃. The bonding strength improvedafter the addition of bioglass (BG) in the solution.