Fabrication And Performance Of Functional Coatings On Magnesium Alloys

Magnesium and its alloys are attracting tremendous attentions to aerospace,electronic and biomedical industries owing to their low density,high electrical conductivity,excellent electromagnetic shielding characteristics,high specific strength and fine biodegradability.Nevertheless,the poor resistance to surface degradation,especially in aggressive environments,seriously restricts its widespread application.Surface modification using functional coating has been proved to be an effective method for preventing corrosion and also improving other properties such as wear and biocompatibility.In contrast to the triditional 'wet' plating techniques,physical vapour deposition(PVD),a 'dry' and clean method for the fabrication of the protective coatings with denser microstructure on the Mg alloys surfaces,has been extensively investigated.Herein,we focus on the fabrication of advanced bifunctional coatings via magnetron sputtering and other methods to fulfill the special requirements of anti-corrosion yet conductive,anti-corrosion yet biocompatible as Mg alloys used in electronics and in biomedical application.The main conclusion and achievements are summarized as followings:1.Metallic Mg,Y,Al,Ti,Zr and Hf coatings were fabricated on Mg alloys surface via magnetron sputtering.The Hf-coated Mg alloy exhibited a superior anti-corrosion performance both in short-and long-term corrosion tests.Corrosion analysis revealed that the low differences in potentials,excellent corrosion resistance and good adhesion to the substrate were considered probable causes for the anti-corrosion performance of the Hf coating.2.The influence of substrate bias voltage on micro structure and corrosion performance of Hf coatings was investigated.The Hf coating deposited at-100 V exhibits the best protective performance.This perfect anticorrosion property was due to the dense structure and low porosity lead by applying appropriate substrate bias voltage.The corrosion performances further indicated that the coating failure is depended on the coating defects and the random phase distribution in substrate.3.Hf/Si3N4 multilayer coatings were deposited on Mg alloys via alternate DC and RF magnetron sputtering.It was found that the columnar growth of the Hf sublayer was suppressed by Si3N4 interlayer.This multilayered structure resulted in the relief of residual stress and the decrease in porosity.These changes in microstructure inhibited the permeation of corrosion media into the substrate and decreased the diffusion rates of corrosion products.The Hf/Si3N4 multilayer coated sample exhibited fine anticorrosion performance.Additionally,the introduction of Si3N4 sublayer has little impact on the conductivity of the multilayer coating due to the shadowing effect and high roughness of Hf sublayer during the deposition.4.We further fabricated anticorrosive yet conductive Hf/HfN multilayer coatings on Mg alloys to optimize the multilayered structure.The growth of nano-scale pinholes along with the columnar structure of Hf sublayers have been completely suppressed by introducing the nanocrystalline HfN sublayers.The optimized Hf/HfNN multilayer exhibited superior anticorrosion performance,which is due to the decreased coating porosity,confined pathway and increased energy barrier for corrosion media induced by nano crystalline HfN interlayer.It also should be noted that the thickness of the HfN sublayer must be restricted in a right scope due to its high hardness.Furthermore,the Hf/HfN multilayer coatings exhibited the fine electrical conductivity to magnesium alloy.5.Hf/HfO2 coatings are fabricated on Mg alloys by magnetron sputtering coupled with the thermal oxidation treatment.Thin hafnium oxide film and new grain boundaries are observed on the hafnium coatings during the appropriate treatment temperature.These changes in micro structure resulted in surface densification,oxidation and low porosity of the treated coating that significantly decreased its susceptibility to corrosion.Consequently,the thermal oxidation treatment hafnium coating exhibits much higher corrosion resistance as well as cytocompatibility than that of the as-deposited coating.However,overly high treatment temperature resulted in a poor adhesion and led to coating failure during the long-term corrosion test.6.We combined the merits of magnetron sputtering and dip-coating method to fabricate a novel and bifunctional hafnium/poly(1,1-lactide)duplex coating(Hf/PLLA)on Mg alloy.The in vitro electrochemical tests revealed that the Hf/PLLA duplex coating has significantly suppressed the degradation of Mg alloy.Simultaneously,the cytocompatibility of Mg alloy was further improved,as observed by the superior cell adhesion and proliferation of human osteoblast-like MG63 cells on the duplex coating.