Preparation And Biodegradation Behavior Of Calcium Phosphate Coatings On Magnesium Alloy And Composites

Magnesium（Mg） and its alloys have attracted considerable attention as potential implant materials in recent years. Compared with traditional metal implants, such as stainless steel, titanium alloy and cobalt-chromium alloy, Mg and its alloys exhibit a combination of good mechanical properties, attractive biocompatibilities and biodegradation properties. However, they corrode quickly in the human body when used as orthopaedic implants, the over-rapid corrosion rate would make the implants not maintain mechanical integrity before the sufficient tissue healing. Several possibilities exist to tailor the corrosion rate of Mg by using high purity Mg, alloying elements, composite structure and surface modification. Compared to the other routes, appropriate surface treatment can not only better match the tissue healing requirement of providing adequate mechanical support in the initial period of implantation and faster degradation when healing is near completion, but also produce an improved bone/implant interface in the early stage of operation to help the healing of the surgical region. Calcium phosphate（Ca P）, such as dicalcium phosphate dihydrate（DCPD）, and hydroxyapatite（HA）, all have intrinsic bioactivity and biocompatibility because Ca and P are the main elements in bone minerals. Therefore, simple and easy-controlled coating technologies, including conversion coating method and sol-gel spin coating method, were used to produce different Ca P coatings with various microstructures and morphologies in the present study. The primary results are summarized as follows:1. A Ca P chemical conversion（CPCC） coating with a layered composite structure was obtained on the surface of an Mg alloy through the conversion coating method. Because of the continuously changing Mg²⁺ concentration at the solution/coating interface, three layers were progressively produced to form the conversion coating, as evidenced by the experimental results and the modified log[Mg²⁺] vs. p H thermodynamic stability diagram. The inner layer was composed of MHPT and was 1.2 μm thick. The middle layer consisted of DCPD, MWH, and MHPT and had a thickness of 1.5 μm. The top layer consisted of DCPD and a lower content of MWH, and was 2.5 μm thick. The effect of coating time on the electrochemical behavior of CPCC coating in SBF was analyzed, and the coating formed at 20 min could supply the optimum corrosion protection for Mg alloy. The in vitro immersion and cell culture tests and in vivo tests show that CPCC coating could provide the Mg alloy with much lower in vitro and in vivo biodegradation rates, and significantly improved surface bioactivity and cytocompatibility.2. Since DCPD in the CPCC coating is easily transformed into HA in alkali environments, the CPCC coating was transformed to HA coating via a simple alkali-heat treatment. The effect of alkali-heat treatment time on the coating morphology and corrosion resistance was investigated. The results show that the HA coating obtained through treating in the alkali solution for 1 h exhibited a dense and uniform coating structure, improved corrosion resistance and biomineralization behavior in SBF.3. In order to get a further enhancement to CPCC coating and HA coating performance, a simple fluorine post-treatment was applied on CPCC coating to form an F-CPCC coating. The effects of solution p H and time of the post-treatment on the coating morphology and electrochemical behavior were analyzed and the optimum corrosion resistance was obtained at p H 12 for 2 h. The F-CPCC coating was consisted mainly of fluor-hydroxyapatite（FHA）, MWH and magnesium fluoride. Electrochemical and immersion tests in SBF reveal that the F-CPCC coating exhibited an improved corrosion resistance with more active biomineralization than the CPCC coating and HA coating, because of its denser coating structure and smoother surface.4. Sol-gel spin coating method and three sol-gel coatings including HA, FHA and fluorapatite（FA） were applied on CPCC coating in order to seal its porous coating surface. The effects of F contents and sol-gel layers on the coating morphology and electrochemical behavior were analyzed and the optimum coating was deter mined to be 5 layers of FA sol-gel coating. Electrochemical and immersion tests in SBF reveal that the dense and stable FA coating could serve as a physical barrier between the Mg substrate and the aggressive environment, resulting in an improved corrosion resistance.5. Mg matrix composites reinforced with different percentages of HA particles were fabricated by the powder metallurgy method. The corrosion current density decreased for the Mg-20%HA composites but increased for the Mg-10%HA composites. Therefore, in order to enhance the corrosion resistance and further develop the surface bioactivity of the Mg/HA composites, CPCC coating and HA coating were deposited on the Mg-10%HA composites by the conversion coating method and the subsequent alkali post-treatment, respectively. The conversion coating mechanism was studied by comparing the conversion coating processes on the composites and an AZ60 Mg alloy and found that the composites substrate showed quicker activation and coating nucleation during the conversion coating process. The electrochemical and immersion tests in SBF reveal that both coatings, especially the HA coating, could supply improved corrosion resistance and more active biomineralization ability for the HA/Mg composites.