Structural Modification And Biological Performance Of Ceramic Coatings On Pure Titanium By Microarc Oxidation

Microarc oxidation（MAO） was used to prepare amorphous ceramic coatings containing Ca, P, Si and Naon pure titanium（Ti）. The modifying techniques, including multi-step microarc oxidation, heat treatment, hydrothermal treatment and steam-hydrothermal treatment, were used to fabricate different scale structural surfaces and to adjust the chemical states of the MAO coatings. The microstructures, mechanical properties and apatite-inducing ability of the MAO coatings before and after modification were investigated by the X-ray diffraction（XRD）, scanning electronic microscope（SEM）, transmission electron microscope（TEM）, fourier transform-infrared spectra（FT-IR）, raman spectroscopy（Raman）, X-ray photoelectronic spectroscopy（XPS）, N2 adsorption-desorption, micrometrics, nano-indentation and mechanics universal testing machines. The in vivo compatibility, osseointegration and mechanical performance of the implants with different surface structures and chemical states were investigated via placing into rabbit tibia for different time by radiographic evaluation, micro-CT test, histological evaluation and biomechanics.An electrolyte containing EDTA-2Na, Ca（CH₃COO）₂·H₂O, Ca（H₂PO₄）₂·H₂O, Na₂ SiO₃·9H₂O and NaOH was used to prepare Ca, P, Si and Naincorporated MAO coatings on pure Ti. The main phase compositions of the MAO coatings were amorphous and little content of anatase, and the main elemental compositions were Ca, P, Si, Na, Ti and O. The MAO coating formed at 300 V showed uniform surface with microporous structure. With the increasing of applied voltage, the thickness of the MAO coatings and the size of micro-pore on the coating surfaces increased, but the density of the micro-pore decreased. Meanwhile, elemental concentrations of Ca, P, Si and Naincreased, while that of Ti decreased. In the MAO coatings, Ti, O, Ca, P and Si atoms showed a graded distribution.An electrolyte containing EDTA-2Na, Ca（CH₃COO）₂·H₂O, Ca（H₂PO₄）₂·H₂O, Na₂ SiO₃·9H₂O, NaNO₃ and NaOH was used as the second step electrolyte. Dense Ti₃O₅ layer coated sub-millimetric scale macro-pores were formed on the MAO coating covered Ti after the second step MAO treatment. With the increasing of NaOH concentration, the density of the macro-pores increased, while the size of the macro-pores decreased. Meanwhile, little amounts of Ca, P, Si and Nawere introduced into the oxide layer on the macro-pore surface. An electrolyte containing EDTA-2Na, Ca（CH₃COO）₂·H₂O, Ca（H₂PO₄）₂·H₂O, Na₂ SiO₃·9H₂O and NaOH was used for the third step MAO to fabricate double-level porous MAO coating in sub-millimetric and micro scales on Ti surface. The density of micro-pore formed in the macro-pore area was smaller than that formed on the flat area. Meanwhile, Ti-OH and Si-OH groups were formed on the surface of the macro-pore.The micro-scale structure of the MAO coating can be adjusted by heat treatment. After heat treatment in air atmosphere at 800°C, anatase, rutile and Ca Ti₄（PO₄）₆ were formed. The coating surface remained microporous structure. Meanwhile, the thickness of the coating increased. However, the trace of C element almost disappeared from the coating. After heat treatment in Ar atmosphere at 800°C, anatase and rutile were formed. The previously microporous coating surface was covered by particle crystals. Meanwhile, the thickness of the coating increased. Besides, a β-Ti layer was generated between the coating and the substrate.Nano-scale structural surface can be formed on the MAO coating via both hydrothermal and steam-hydrothermal treatment. During hydrothermal treatment, the incorporated elements of Ca, P, Si and Nadissolved from the MAO coating into the solution. With the increasing of NaOH concentration, the amount of formed HA crystal first increased and then decreased. Meanwhile, the corrosive ability of OHions increased. The previously microporous surface was covered by H₂Ti5O11·H₂O nanorods with dense structure. During the steam-hydrothermal treatment, the coating surface remained microporous structure, and the surface elemental concentration of Ca, P, Si and Naremained steadily. With the increasing of NaOH concentration, the thickness of the coating decreased, while the amount of HA crystal with lower aspect ratio increased. Besides, Ti-OH groups were formed on the steam-hydrothermally treated coating surface. In addition,（NaOH）₂（H₂O）7 was deposited on the coating surface when 1 mol/L NaOH solution was used during the steam-hydrothermal treatment.During SBF immersion, the ionic exchange between Na+ ions from MAO coating and H₃O+ ions in SBF can result in the formation of Si-OH groups, greatly promoting the apatite formation on the coating surface. The macro-pore area of the multi-step MAO treated Ti exhibits better apatite-inducing ability than that of the MAO coating in the flat area. The reason for this is that the increased aspect area in the macro-pore area can promote the formation of Si-OH groups. Fortunately, the Si-OH and Ti-OH groups were formed on the macro-pore area of the three-step MAO treated Ti, dramatically improving the apatite-inducing ability of the coating. The coating after heat treatment at 800°C in Ar atmosphere shows good apatite-inducing ability due to the formation of rutile particle layer on the surface. The rutile（101） plane has good crystallographic matching with HA（0001） plane probably providing good sites for apatite nucleation by the epitaxial deposition process. The coatings after steam-hydrothermally treated in NaOH solution exhibit excellent apatite-inducing ability attributing to the formation of HA and Ti-OH groups on the surfaces. Apatite induced by all the coatings contains CO₃2- and HPO₄2- groups, exhibiting nanoscale porous network structure.The macro-pore formed on the multi-step MAO treated Ti can mesh with resin, promoting the fracture strength of the coating. After steam-hydrothermal treatment, the hardness and Young’s modulus of the coating remain steadily. Meanwhile, as the growth of anatase nano paticle heals the defects in the coating, the bonding strength between the coating and substrate is significantly enhanced. However, fracture phases are formed in the coating after the heat treatment at 800°C, thus the bonding strength of the coating decreased.After placing into rabbit tibia, the three-step MAO treated Ti implant shows good in vivo biocompatibility without rejection and infection. The bone tissue around the implants shows good absorption, and new bone can grow into the sub-millimetric scale pores on the implants surface. According to the morphology of the pushed out implant, residual bone tissue which meshed with the implant can be observed from the macro-pores area, thus the bonding strength between the bone tissue and implant has been significantly improved. The in vivo experiment also reveals that bone tissue can grow intimately on the surface of the steam-hydrothermally treated implant with MAO coating, exhibiting excellent bone-implant contact. Thus, the bonding strength of such implant with bone tissue is obviously increased. Besides, residual bone tissue also can be observed on the surface of such surface modified implant.In conclusion, various structures can be fabricated on the MAO coating surface via modifying techniques including multi-step MAO, heat treatment, hydrothermal treatment and steam-hydrothermal treatment. Such structures can benefit the apatite-inducing ability for the coatings. The three-step MAO treatment and the steam-hydrothermal treatment can significantly improve mechanical properties and osseointegration of the MAO coating covered Ti implant.