Microstructure And Properties Of Mg（OH）₂/Organosilane Layer-By-Layer Coatings Drived By Hydrogen Bonding On Pure Magnesium

Compared with the traditional medical implant metallic materials such as stainless steel,titanium alloys,magnesium and magnesium alloy have the closest density,elastic modulus,specific strength to human bone and show good biocompatibility and biodegradability.Therefore,magnesium and magnesium alloy are considered as new orthopedic implant materials.However,due to the low standard potential(-2.36V)of pure magnesium,Mg and its alloys would take place rapid corrosion,especially in the environment containing aggressive ions(such as Cl-),and result in a large amount of hydrogen,which greatly limits their applications.Surface modification is an effective way to improve the corrosion resistance of magnesium alloys.Mg(OH)2 has good biocompatibility and bioactivity.In this paper,the dense packed Mg(OH)2 coating on pure magnesium is obtained by adjusting hydrothermal condition with the in-situ hydrothermal method.On the basis,Mg(OH)2/organosilane composite coating is prepared on the surface of pure magnesium by the sol-gel method with the help of organosilane with multiple hydrogen bonds synthesized in this paper to achieve a good combination of silane coating and Mg(OH)2 coating.The effects of silane concentration and coating times on the microstructure and properties of the composite coatings are studied.The results are as follows.(1)The effects of hydrothermal temperature,time and pH values on the microstructure and properties of Mg(OH)2 coating are studied.The Mg(OH)2 coating is composed of hexagonal sheet particles with simple packing when the hydrothermal condition is 140℃,3h and pH=11.The microstructure of the coating takes self-assembled cage-liked braided structure with dense packing when the hydrothermal condition is 150℃,5h and pH=10.The coating does not completely cover the substrate when the hydrothermal condition is 170℃,2h and pH=9,and the microstructure is composed of dispersively irregular spherical Mg(OH)2 particles.The results of the electrochemical test show that the sample prepared at 150℃,5h,pH=10 has the smallest corrosion current density,the most positive corrosion potential and the best corrosion resistance.The effects of hydrothermal conditions on the microstructure and properties of the coating are discussed from the perspective of the growth kinetics of the coating by analyzing the hydrothermal temperature,dwelling time and pH values.The Mg(OH)2 particle in the medium temperature,long-term and moderately alkaline environment has the best growth power to self-assemble and form the uniform and compact microstructure.(2)On the basis of Mg(OH)2 coating prepared by the best hydrothermal method,Mg(OH)2/organosilane composite coating is fabricated on the surface of pure magnesium by sol-gel method.The effects of silane sol concentration and coating times on the microstructure and properties of the composite coating is studied by using a single silane coating as the control.The microstructure characterizations show that the silane coating is colorless and transparent,and has no effect on the microstructure of Mg(OH)2 coating.The self-assembled cage-like braiding structure Mg(OH)2 coating can still be observed when characterizing the microstructure morphology of the composite coating.The results show that the bonding force,contact angle and corrosion potential of the composite coating are 65.6N,113.99° and-1.34VSCE,respectly,when the sol concentration is 20%and the coating is coated three times.When the sol concentration is reduced to 10%,the composite coating with three times coating has better comprehensive properties,the bonding force is 71.4N,the contact angle is 135.49°,and the corrosion potential is-1.27VSCE.