Evolution Of The Structure And Phase Composition Of The Bioactive Film On Titanium Prepared By Induction Heat Treatment

Titanium is used as dental implants,artificial bone,and other hard tissue replacements due to the advantages of low elastic modulus,high specific strength,good biocompatibility,and corrosion resistance,but the inherent biological inertness and poor tribological properties limit its further clinical applications.A micro-and nanostructured layer with high bioactivity and stability on the titanium facilitates the enhancement of the mechanical and biological properties of the implant.Titanium oxide and titanium nitride have high hardness,low coefficient of friction,high corrosion resistance,good biocompatibility,and the ability to promote osseointegration,meeting the demands for surface-modified layers of implants.Induction heat treatment(IH)has the advantages of a fast heating rate,controlled heating process,low energy consumption,and environmental friendliness.Therefore,in situ construction of titanium oxide and titanium nitride layers on titanium using induction heat treatment will be beneficial in improving the biological properties and long-term stability of titanium implants.In this study,hierarchical micro-and nanostructured films were prepared on titanium by combining different pretreatment methods with induction heating under air or nitrogen atmosphere.The morphological evolution of the films was analyzed by field emission scanning electron microscopy(FE-SEM)and focused ion beam(FIB)cutting techniques.The composition evolution of different films was analyzed using energy spectroscopy(EDS),Xray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),and transmission electron microscopy(TEM).The roughnesses and wettabilities were analyzed by laser confocal microscopy(LSCM)and contact angle measurement.In addition,the bond strength and surface hardness were assessed using TriboIndenter,and the corrosion resistance was analyzed by electrochemical testing.Also,the ability to induce hydroxyapatite deposition was explored by the simulated body fluid immersion experiment,and the adhesion,proliferation,and differentiation of BMSCs on different films were assessed by the in vitro cell culture experiments.The results show that titanium oxide nanocrystals with a diameter of less than 20 nm are formed on the SLA-IH samples,and the metastable anatase can be formed even at a high temperature(900℃),yielding a composite layer composed of anatase and rutile.The oxide layers on SLA-FH samples are mainly composed of rutile with large blocky grains(above 200 nm)on the surface.When the temperature of the conventional furnace heat treatment is 800℃,the oxide layer exfoliates severely from the substrate,and the binding strength is 713.0μN.Induction heating improves the binding strength between the film and the substrate(3097.4 μN),while small nanopores exist in the film.Hence,further investigation of induction heating on the ODL samples with high content of oxygen vacancy(87.29%)revealed that not only the anatase is maintained at a high temperature,but also the sub-oxide Magneli phase is formed.As a result,a theory for increasing the anatase content by entropy enhancement through increased heating rate,increased lattice distortion,and construction of crystal defects is proposed on the basis of thermodynamics.The oxide film on the ODL-IH800 sample exhibits no obvious demarcation with the substrate and the binding force is enhanced to 3812.0 μN.The alkali-heated layer prepared by induction heating of the SLAA sample consists of sodium titanate,rutile,and anatase.The thickness of the layer on the SLAA sample is 0.98μm,and that of the SLAA-IH700 sample is increased to 1.35 μm.Furthermore,hierarchical micro-and nanoporous structures can be formed even at 900℃ by induction heating of the SLAA sample.The mechanisms for the structure and phase evolution of the alkali-heated layer are discussed.Sodium titanate has TiO6 octahedral structure similar to that of titanium oxide,and the lattice plane(101)promotes the vertical growth of the anatase(101),which plays an important role in improving the content of the anatase and the structural evolution.It is noteworthy that the skin effect of induction heating enables the formation of a gradient structure with increasing crystallinity and density from the outside to the inside of the sample.When the temperature of the heat treatment is 800℃,the binding strength between the alkaliheated layer prepared by conventional furnace heating and the substrate is only 737.0 μN,while it can be increased to 2619.0 μN by induction heating.In addition,the film surface with low crystallinity facilitates the increase of Ti-OH groups as well as the ion exchange activity in SBF,promoting the ability to induce hydroxyapatite deposition.Induction heating of titanium under the nitrogen atmosphere is investigated to obtain films that are more favorable to mechanical and biological properties.Rapid nitriding can be achieved by induction heat treatment and the doping of nitrogen has an inhibiting effect on the transformation of anatase to rutile.In addition,the efficiency of preparing nitrogen-containing films in a short time can be further improved by constructing the oxygen diffusion layer on titanium.The film fabricated by induction heating of the ODL sample in the nitrogen atmosphere at high temperatures(above 800℃)is a pore-like structure composed of nanopillars or nano-massive grains.The composite films of the ONH samples are composed of titanium nitride,anatase,rutile,and oxynitride of titanium.The thickness of the film on the ONH900 sample is about 1 μm Moreover,the formation of the sub-oxide Magneli phase at the interface between the film and the substrate has a positive effect on the toughness of the film.The mechanical properties,corrosion resistance,and the ability to induce apatite deposition of ONH samples increase with the elevated temperature of induction heating.The SLAA-NH samples also yield a composite film composed of titanium oxide and titanium nitride with a thickness of around 1.35 μm.While the oxides are anatase as well as the Magneli phase,with almost no rutile formed.And the coral-like porous structure is maintained even at a high treatment temperature(900℃).Further,we discuss the staged growth process of the composite film based on the layered structural analysis of the ONH and the SLAA-NH samples.It provides a basis for the subsequent adjustment of nitriding parameters and the regulation of phase compositions.In-vitro cell culturing results show that the composite film composed of anatase and rutile prepared by induction heating is more conducive to the biological behavior of BMSCs than the rutile film obtained by conventional furnace heating.Cell adhesion can be promoted by the rough surfaces of SLA-IH and SLAA-IH samples,exhibiting a wide spreading area and elongated pseudopods.However,cell activity and proliferation of BMSCs on rough surfaces are weak in the early stage,while long-term proliferation and differentiation can be enhanced.ODL-IH800 samples with high content of oxygen vacancies allow the formation of abundant Ti-OH groups,which facilitate the recruitment,adhesion,proliferation,and differentiation of BMSCs.Also,the ONH800 sample with low surface roughness and transient hydrophobicity still showed significantly better bioactivity,biocompatibility,and osteogenesis compared with the other groups.The adhesion,proliferation,and differentiation of BMSCs on the ONH800 sample are superior to other samples,showing greater potential for application in the field of surface-modified layers on titanium implants.