Effect Of Microwave Plasma Oxidation/Ti-alloying On The Surface Microstructure And Properties Of NiTi Alloy

Near-equiatomic NiTi alloy has super elasticity,elastic modulus close to osteogenesis,good corrosion resistance and biocompatibility,thus is a good implant material for replacing human bone.However,Ni²⁺ions will release from Ni Ti alloy after implantation,which will impact human body negatively in a direct or a potential way.At present,researchers mainly reduce the content of Ni on the surface by preparing TiO₂ film on Ni Ti alloy,so as to reduce the precipitation of Ni²⁺ions.Among the commonly used methods for TiO₂preparation,the in-situ oxidation method has low film-forming efficiency and the surface coating method mostly uses toxic chemical reagents,which has potential safety hazards and pollutes the environment.In this paper,a new method named microwave plasma oxidation（MPO）is used to in-situ oxidize Ni Ti alloy.The effect of MPO treatment on the structure,corrosion resistance and biocompatibility of the Ni Ti alloy are investigated.Based on the result that a Ni-rich zone emerged in MPO treated Ni Ti,which may increase the risk of Ni²⁺release,the Ni Ti alloy is also treated by double glow plasma surface Ti-alloying following with MPO treatment.The effects of Ti-alloying and Ti-alloyed/MPO treating on the surface properties of Ni Ti alloy are compared.The main contents and results are summarized as follows:（1）Ni Ti alloy is treated by the MPO method with different oxidation temperatures（500℃,600℃,and 700℃）and oxidation times（10 min,30 min,and 60 min）.The results show that rutile TiO₂ coatings with nanoparticle structure are formed on the surface of Ni Ti alloy under MPO treatment with different temperatures and times.The surface Ni content of all the specimens significantly reduces from 50 at%to 1.2 at%.With the increase of oxidation temperatures and times,the surface roughness of MPO-treated Ni Ti alloy increases,the oxide layer thickens gradually,the water contact angle of specimens decreases,and the corrosion resistance of specimens in DMEM solution raises.In addition to the specimens after MPO-treating at 700℃for 30 min,the other specimens all have improved cell adhesion,proliferation and spreading properties due to reduced surface Ni content and cytotoxicity.Among them,the Ni Ti alloy after MPO-treating at 600℃for 30 min has the least surface Ni content,the smallest contact angle,highest corrosion resistance,and best biocompatibility.（2）After MPO treatment,a Ni-rich zone is formed underneath the TiO₂ coating.For the specimens treated at 500℃,600℃and oxidized for 10 min and 30 min,only one Ni-rich zone exists,which has a Ni content of about 70 at%.However,for the specimens treated at 700℃or oxidized for 60 min,the Ni-rich zone increases,and can be divided into high Ni-rich zone and low Ni-rich zone,the Ni content is about 90 at%and 70 at%,respectively.It is found that the oxide films with thick thickness will rupture due to the existence of residual stress.Moreover,the exposed Ni-rich layer will seriously increase the cytotoxicity.This phenomenon occurs in the specimen after MPO-treating at 700℃for 30 min.（3）To avoid the formation of Ni-rich zone under the TiO₂ film and reduce the risk of Ni²⁺release,the double glow plasma surface alloying technology is used to prefabricate Ti metal coating on the Ni Ti surface for subsequent MPO-treatment.The results show that pure Ti coating can not be formed on the Ni Ti alloy surface by Ti alloying at 800℃for 1 h,2h and 3h.For all the specimens,the Ni content on the surface is reduced to a low value less than 11at%.The Ti alloying layers are composed of alternatively distributed bright and dark regions.The dark regions show a low Ni content of about 5 at%,which indicates a composition ofα-Ti and Ni Ti₂ phases.The bright regions have a high Ni content of about 25 at%,which demonstrates that Ni Ti₂ and Ni Ti are the main phases.In all the Ti alloying layers,Ti and Ni distribute gradually along the depth direction,indicating a diffusion-bonded interface between Ti layer and Ni Ti alloy.The Ti-alloying increase the surface roughness and decrease the water contact angle by nearly 20^°.The corrosion resistance of the Ti alloyed specimens in DMEM solution is significantly improved,besides,the surface protein adsorption and the cell compatibility are enhanced.（4）MPO-treatment is perfprmed on the Ti-alloyed Ni Ti specimens to convert the Ti-alloying layer into oxide film.The results show that the surface of Ni Ti alloy after Ti-alloying/MPO composite treatment shows micro-scale granular structures and nano-scale strips on them.Compared with other methods in this paper,the surface Ni content after Ti-alloying/MPO composite treatment is the lowest,which is about 0.5 at%.In addition to the rutile TiO₂ phase,Ti_xO unsaturated oxide is also found in the oxide film.Due to the prefabricated Ti alloying layer,the Ni-rich zone under the oxide layer disappears.The surface hydrophilicity,corrosion resistance in DMEM solution and biocompatibility of the specimens after composite treatment are better than those after MPO-treatment and Ti-alloying treatment.All in all,the Ti-alloying/MPO composite treatment is an excellent method for reducing the Ni content on Ni Ti alloy and improving its biocompatibility.