|Ph.D Student||Larianovsky Natalya|
|Subject||Coating Magnesium Implants for Slow biodegradation Kinetics|
|Department||Department of Materials Science and Engineering||Supervisor||Research Professor E Dan Shechtman|
|Full Thesis text|
Magnesium and its alloys are potential candidate materials for degradable metallic implants due to their degradability, biocompatibility, and mechanical properties. However, the high corrosion rate and accumulation of hydrogen gas upon degradation hinders its clinical application.
It is apparent that reducing the corrosion rate of Mg in body fluids to a sufficiently low value is a key issue to be addressed before Mg can be employed as a degradable and yet functional implant material. Only a few preliminary trials for improving the corrosion resistance of Mg implants have been reported in the literature with varying degrees of success.
The purpose of this proposed study is to improve the corrosion performance of magnesium alloys by a coating treatment that will help maintain optimal mechanical integrity of the magnesium implants in the early postoperative period.
The protective coating must be non-toxic, biocompatible and biodegradable. Ideally, the coating formed on a magnesium implant should also be wear resistant, in order to avoid damage during implanting.
Two possible coatings are proposed for the Mg alloy RS66 (Mg-6Zn-1Y-0.6Ce-0.6Zr (wt.%)) developed in our Laboratory. Fluoride conversion coatings and high-purity magnesium coatings were developed and studied.
In the present work, all the coatings were applied at low temperatures to prevent any changes to the microstructure and properties of the rapid-solidified material.
The surface morphology of the coated samples and the degradation of the coatings were determined by SEM and HRSEM techniques. X-ray diffraction technique was used for analyzing the intermetallic compounds that formed. The corrosion behavior of the coated samples in Hank’s solution was studied by immersion tests and by using electrochemical methods. The mechanical properties were evaluated by microhardness and tension tests. Since stress corrosion cracking is a serious concern for implant devices of magnesium alloys, the SCC susceptibility of RS66 alloy was investigated using in slow stress rate tensile testing in physiological environment.
During the study, the following concluding observations were made:
- The significant improvement (up to 35%) in the corrosion resistance was obtained for the composite prepared from 24% HF solution-treated chips.
- The additional external fluoride conversion coating of about 1.5 μm thick was formed on extruded RS66 alloys by immersion in 48%HF for 24 h. This coating degraded gradually at corrosion rate of 0.007 mm/year while the uncoated samples degraded fast at corrosion rate of 1.24 mm/year.
- The uncoated and fluoride coated RS66 specimens were found to be susceptible to the SCC in Hanks solution. A decrease in their mechanical integrity up to 37% was observed in SSRT testing.
- The vacuum distillation method led to successful reduction of the Fe concentration in raw magnesium, going down to 6 ppm. This high-pure magnesium was deposited on the extruded RS66 alloy and showed increased corrosion resistance approximately one order of magnitude.