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Formation of Two-Step Plasma Electrolytic Oxidation Coating On Commercially Pure Titanium and Study its Properties

Ahmadnia, Samira | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52275 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Ghorbani, Mohammad
  7. Abstract:
  8. Improvement of biological properties by modifying the composition and morphology of the surface layer is one of the main goals of using plasma electrolytic oxidation (PEO) for coating titanium implants. The purpose of this research is formation of biocompatible coating containing calcium, phosphorus and silicon on titanium by two-step PEO method in DC, constant voltage mode. In addition to good corrosion properties of the resultant coatings in simulated body environment (SBF), Having good adhesion due to the use of electrolytes with useful elements for biological function in both step of coating process rather than acidic electrolytes which are incompatible with the body environment, this coating is biocompatible. In this study, three electrolytes; silicate, calcium phosphate and mixture of calcium phosphate and silicate were used. In silicate electrolyte, by applying different voltages and studying corrosion properties of coatings in SBF, voltage of 270 V, and considering the current-time changes, time of 15 minutes was considered as the optimal conditions of coating in silicate electrolyte and voltage of 300 V in 7 minutes was applied in calcium phosphate electrolyte. Studying growth kinetic of two-step method, first in silicate electrolyte then in calcium phosphate electrolyte, showed the highest rate of growth, 0.02 μm/s, within 15 minutes. Finally, the effect of coating steps on the morphology, corrosion and biocompatibility properties of the coatings was evaluated by using electron microscopy, X-ray diffraction, polarization test, electrochemical impedance spectroscopy and immersion test in SBF at 37 ° C and pH 7.4. The phase studies indicated the presence of anatase, rutile and calcium phosphates phases in the coatings. EDS analysis indicated the presence of calcium, phosphorus and silicon elements in the coatings. According to the results of polarization test after two weeks immersion in SBF, compared to other coatings, two-step calcium phosphate-silicate (CpS) coating, has the corrosion rate of 0.43 mpy and the highest protection efficiency of 76%. Electrochemical impedance studies, represent the presence of two layers, the inner barrier layers, and the outer porous layer that is also visible in the SEM images. CpS coating has the highest inner layer resistance of 5055000 Ω, and the highest diffusion resistance of 0.12E + 6 Ω, as a result, the highest corrosion resistant. Immersion test in SBF for 30 days, results in increasing the Ca/P ratio to 1.7 to 1.5 and formation of apatite on the surface of a Si-incorporated samples in both two-step and one-step conditons, thus suitable biocompatible coatings showed. According to the results of ICP test of SBF solution after 30 days of immersion of CpS, the amount of titanium ions released is about 0.72 ppm; less than cytotoxicity limit (5 ppm). pH also had not changed and do not cause alkaline poisoning
  9. Keywords:
  10. Plasma Electrolytic Oxidation ; Two Stage Plasma Electrolytic Oxidaition (PEO) ; Titanium ; Biocompatibility ; Silicates

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