| 其他摘要 | Preparation and performance of bioactive and corrosion-resistant ceramic coatings based on PEO technology
Wei Zhang (Materials Science)
Supervised by: Prof. Fuhui Wang and Prof. Chuanwei Yan
Due to low density, high strength and good machinability, titanium and magnesium alloys have become the most attractive candidate as functional materials. However, poor corrosion resistance for Mg alloys and low bioactivity for Ti alloys have restricted their wide application in practical environment. So, in the present work, the bioactive and corrosion-resistant coatings have been prepared respectively for titanium and magnesium alloys by plasma electrolytic oxidation for improving their surface performance. Moreover, deposition process of apatite on bioactive functional coatings on Ti-alloys and the treatment process for densification of PEO coatings on Mg alloys were studied by means of analyzing the microstructure and composition of PEO coatings.
PEO electrolytes containing Ca, P and Si have been prepared for fabrication of Si-incorporated PEO coating on pure titanium. The results showed that the catenulate molecule structure and chelation of P3O105- in solution have decreased conductivity of the solution and thus effect the growth rate of PEO coating. At the same time, the lower diffusion coefficient of P3O105- than P3O105- led to the decreasing of P in the coating. In addition, the abundant adsorptions of Ac- ion at the interface of coating/electrolyte hindered the incorporation of Ca and Si into the coating. Therefore, solution D without Ac- and P3O105- was considered to be an optimal electrolyte for PEO process.
In order to optimize the microstructure of PEO coating and to increase the content of bioactive elements in PEO coating, the effect of technical parameters (voltage, frequency and time) on the microstructure and composition of PEO coating were then investigated. The results indicated that (1) with the increasing of pulse voltage; the porosity and pore density of PEO coating gradually increase. However, too high voltage may result in some micro cracks on the surface of PEO coating. The content of Ca and Si in the coating gradually increases with the increasing of pulse voltage, whereas, the content of P in the coating first decreases and subsequently almost keeps invariant (2) with the increasing of pulse frequency, the porosity and its homogenization of PEO coating gradually increases, however, both begin to decrease after 800Hz. The content of Ca and Si in the coating firstly increases and subsequently decreases, however, the P element content in the coating almost keeps invariant (3) the pore size and the porosity of PEO coating gradually increases with the increasing of oxidation time, but ,the pore density of PEO coating decreases. Also, the content of Ca, P, Si in the coating gradually decreases. Eventually, the optimal technical parameter for PEO process is as follows: pulse voltage：300~350V，pulse frequency：200~400Hz，oxidation time：5~10min。
The nuclei and growth process of apatite on the surface of bioactive PEO coating in SBF solution was monitored in-situ by electrochemical impedance spectra (EIS). The results showed that after 7 day immersion in SBF solution, a new time constant appeared in high frequencies of EIS of PEO coating with Si, which implied the initiation of apatite nuclei on the surface of PEO coating with Si. However, after 18 day immersion in SBF solution, the value of Rp for the PEO coating without Si just begins to increase. SEM observation revealed that after 28 day immersion, the PEO coating with Si was completely covered by an apatite crystalline layer whereas the surface of benchmark PEO coating just show a few of apatite deposition. The incorporation of Si into film have increased the solubility product of apatite and decreased the contact angle between crystal and substrate, which led to low interface energy for apatite deposition and therefore increased the growth rate of apatite crystals.
The effect of pulse power supply mode on the microstructure, composition and corrosion resistance of PEO coating on magnesium alloy in silicate solution have been studied, the results show that with bipolar pulse mode for PEO process, the growth rate of PEO coating on magnesium alloy firstly decreased and some big micropores present inside the PEO coating, at the same time, the content of Ca and Si element in the PEO coating was low. However, with the increasing of the oxidation time, the PEO coating became much homogeneous and the porosity of the coating decreased largely. Moreover, more stable phases of Mg2SiO4、Mg7F2[SiO4]3 have been incorporated into the coating. Electrochemical impedance spectroscopy (EIS) and polarization measurement showed that with the increase of oxidation time, the corrosion resistance of PEO coating was enhanced and its corrosion current density was decreased. The corrosion resistance of PEO film prepared with bipolar pulse mode is higher than that with single pulse mode.
A compact fluoride containing PEO ceramic coating (named D-KF-A coating) was successful fabricated on magnesium alloy AZ91D in KF solution with addition of dihydric phosphate by controlling the bio-polar pulse voltage. The results showed that the additives in the KF electrolyte help to decrease the concentration of OH- at the interface of coating/electrolyte, and therefore to incorporate more MgF2 rather than MgO into the coating, producing a PEO coating with a compact inner layer on the magnesium alloy. Whereas, the use of bipolar pulse voltage can change the size of sparks appeared on the surface of the coating and promote the incorporation of K into the coating, so the KMgF3 became the dominate phase constitute of the coating, therefore, the outer-layer compactness and uniformity of PEO coating on magnesium alloy are realized. The results of potentiodynamic polarization and EIS show that D-KF-A coating formed in the modified KF electrolyte by polar pulse mode can provide superior corrosion resistance for magnesium alloy. Its corrosion current density (icorr) attain to 2.2969E-9A/cm-2 and after immersion in 3.5% NaCl solution for 200 hours, the first pitting corrosion just appeared on the surface of D-KF-A coatings.
Keywords: plasma electrolytic oxidation，pure titanium，magnesium alloy，
ceramic coating，bioactivity，compactness |
修改评论