A Study on the Surface Characteristics of Porous Oxide Coatings Incorporating Strontium on Titanium by Micro-arc Oxidation

Abstract

Titanium and its alloys are widely used in dental and orthopedic fields due to their excellent chemical stability and biocompatibility Because they are considered as bio-inert materials in clinical use it is difficult to achieve a chemical bonding with bone tissue The micro-arc oxidation (MAO) technique is a modifiable method to improve surface characteristics of titanium This study focuses on the MAO coatings formed in the electrolytes with different strontium content Several properties of MAO coatings were investigated including physicochemical characteristics bioactivity corrosion resistance and cell behavior Besides an appropriate heat treatment is an effective method to improve the mechanical property The physicochemical characteristics were investigated using scanning electron microscope (SEM) observation energy dispersive X-ray spectrometer (EDS) thin film X-ray diffraction (TF-XRD) analysis electron spectroscopy for chemical analysis (ESCA) and surface roughness test The formation of an apatite phase generated on the surface of materials has been suggested to be indicative of biocompatibility The bioactivity of materials was evaluated based on the ability to induce a bond-like apatite layer on the surface in simulated body fluid (SBF) In clinical application the adhesion between the coating and the substrate is important factor for dental implants and artificial joint prosthesis The adhesion strength was evaluated using scratch test Cell behavior included morphology observation by SEM and number count by methylthiazoletetrazolium assay of the cells Using the SEM the surface morphology exhibited uniform porous structure on titanium After heat treatment the TF-XRD results indicate that the tricalcium phosphate (Ca3(PO4)2) phase is formed at a temperature of 800°C and this could be attributed to crystallization of amorphous phase SEM results show that the surface morphology does not change However the peak of strontium-substituted hydroxyapatite was not observed After soaking in simulated body fluid (SBF) the apatite layer formed on the surface of 400°C-treated MAO coatings containing strontium but the formation of apatite layer was not found in all kinds of 800°C-treated specimens The crystallization of coatings would inhibit the ability of apatite formation Furthermore the scratch test results reveal that the adhesion strength between the coatings and the substrate increases with increasing heat treatment temperature The adhesion strength of MAO coatings without heat treatment is sufficient to use in clinical applications The average thickness of all oxide coatings is from 3 70 to 3 79 μm and the surface roughness of all specimens is from 0 31 to 0 34 μm The TF-XRD results indicated that phase of MAO coatings was anatase and rutile The calcium phosphorus and strontium were detected in the oxide coatings by EDS and ESCA It indicated that calcium phosphorus and strontium were amorphous phase in the oxide coatings After the materials were soaked in SBF for 1 day precipitates formed on the surface of MAO coating The surface of MAO coatings was completely covered with precipitates after 7 days The precipitates which were found to be composed of fiber structures were identified as the apatite phase using TF-XRD The results show that MAO coatings incorporating strontium can induce the formation of an apatite layer on their surface In the potentiodynamic test oxide coatings exhibited a more noble corrosion potential (Ecorr) than that of titanium in SBF In the passive region the current density of oxide coatings was lower than that of titanium The phenomenon showed that MAO coatings provided for a layer of corrosion resistance Owing to forming TiO2 on the titanium surface the polarization resistance of titanium is higher than oxide coatings Besides the polarization resistance of oxide coatings decreased with increasing strontium contents because strontium conductivity is higher than calcium Cell culture experiments demonstrate that MAO coating formed in the electrolytes with different strontium content would not alter initial cell morphology and 1-day and 7-day cell numbers The cell proliferation of coatings containing 1% or 5% strontium content at 14-day culture was higher than coatings without strontium content at 14-day culture but the higher strontium content (10%) could not be beneficial to cell growth All findings in this study indicated that (1) strontium incorporated into MAO coatings did not change the physicochemical characteristics but exhibited an effect on biological responses (2) MAO coatings containing strontium have good bioactivity and corrosion resistance for clinical applications (3) MAO coatings containing strontium with optimal heat treatment could improve mechanical properties and maintain the bioactivity and cell responses

Date of Award	2014 Jan 23
Original language	English
Supervisor	Truan-Sheng Lui (Supervisor)