Preparation,Modification And Performance Study Of The CaP/PLA Composite Coating On The Magnesium Alloy

Magnesium and its alloys are light metallic materials with excellent biological properties.Magnesium alloys also have very high specific stiffness,specific strength,excellent biocompatibility,biodegradability and bioactivity.There are no toxic side effects on human body in its degradation process.Its elastic modulus and density is close to human bones,thus it can be used as scaffold material for bone tissue engineering,without the stress shielding effect easily produced by traditional bone scaffold materials.Mg alloy based bone scaffolds have become one of the most promising biomaterials in bone tissue engineering field.However,the corrosion resistance of Mg alloys in body fluid is poor,and its overfast corrosion rate of doesn’t match the healing rate of bone tissue,which has become a major factor limiting the clinical application of Mg alloy bone scaffolds.Researchers have developed many methods in order to overcome this problem.Among them,calcium phosphate film（CaP）treatment is one of the most promising methods.CaP coatings present not only excellent biocompatibility and biodegradability,but also outstanding osteoinductivity.But its drawback is that there are usually many defects on the surface,which greatly limits its corrosion resistance in body fluid.In this paper,with the aim of reducing the surface defects,a polylactic acid（PLA）organic film was deposited on the CaP coating.Since polylactic acid is a material with high biocompatibility and biodegradability,while the surface defects of PLA film are few as there are no obvious cracks and holes,PLA was chosen as the sealing material of CaP coating.In this experiment,a layer of CaP coating was first prepared on AZ60 Mg alloy surface by chemical conversion method with the best processing parameters,and then PLA film was coated on the surface of CaP by dip coating method to form a CaP/PLA composite coating.The optimum preparing parameters for the composite coating were explored by controlling variable method.After that,a novel KH550 modified CaP/PLA composite coating was prepared by coating PLA film on the surface of KH550 modified CaP coating to improve its weak inorganic/organic interfacial adhesive force.The surface topographies of different kinds of coatings was observed by using SEM.Their corrosion resistance was investigated by electrochemical and immersion tests.The internal composition and phase structure of the coatings were analyzed with XRD test,and their functional groups were characterized by FTIR spectra.Additionally the interfacial adhesion of KH550 modified CaP/PLA composite coating was measured by scratch method.1.The SEM results show that the CaP/PLA composite coating exhibites more uniform and compact surface morphology than the pure CaP coating,and the surface defects such as cracks and voids are obviously reduced.However,the most complete and non-destructive surface morphology is attributed to the CaP/PLA composite coating modified by KH550.The surface of the composite coating has no apparent defects and looks more glossy and uniform.In particular,the cracks at the junction of the two phases are greatly reduced.2.XRD results suggests that the CaP coating is mainly composed of brushite DCPD phase（CaHPO₄·2H₂O）.PLA film is mainly composed of amorphous polylactic acid macromolecules.The combination of the two components and the modification on that don’t produce any new phase.FTIR spectra of the modified composite coating shows that some KH550 molecules can interact with DCPD to form functional groups of-Si-O-P,which provides preliminary conditions for enhancing the binding force between the two phases.3.The electrochemical tests results indicate that the corrosion resistance of CaP/PLA composite coating is much better than that of CaP coating,but it is still lower than that of KH550 modified composite coating.This suggests that PLA layer has a significant effect on sealing the surface of pure CaP,and reduces the probability of corrosive solution contacting the Mg alloy matrix.The internal structure of the modified composite coating is much more compact and the voids and crevices are further reduced.4.The immersion test shows that the corrosion resistance of the CaP/PLA composite film is also enhanced significantly in long-term service,but their SEM photos after the immersion showed that most of the PLA layer on the surface had fallen off,which indicated that the poor interface bonding strength of CaP/PLA.Through the modification with KH550,the detachment phenomenon of PLA film is obviously inhibited,and most of the film remained intact during the immersion process.5.Cross-cut test results directly show that the interfacial bonding strength of CaP/PLA composite coating modified by KH550 is greatly improved comparing to that of unmodified composite coating.