Laser Surface Regulation Of Bone Implant Magnesium And Magnesium Alloys And Corrosion Resistance Property Study

Due to low density,high specific strength,good biosafety and Young’s modulus closing to the human dense bone,which offers sufficient support strength and avoids to stress shielding,magnesium（Mg）and its alloys are considered as appropriate candidates for biomedical implant applications.Mg and its alloys can be corroded and gradually degraded in the body fluid environment.During the degradation,Mg ions are released,which promotes the regeneration of osteoblasts,proliferation,and repair.Therefore,Mg and its alloys,as ideal degradable materials,get much attention in recent years.However,the degradation rate of Mg alloy is too fast and not match the growth rate of biological tissue.Too fast degradation rate results in excessive hydrogen release,and local solution p H rise in the implant,which affects bone tissue repair or causes premature failure of the implant.Hence,regulating the corrosion rate of Mg alloy in body fluid environment has become one of the key issues in the clinical application of degradable medical Mg alloy.Degeneration corrosion usually initiates at solid-liquid interfaces,so tailoring surface microstructures and creating corrosion-resistant coatings are effective methods to enhance the corrosion resistance of Mg and its alloys.Since the laser energy can rapidly change the state of the material surface,the investigation on mechanisms of interaction between laser and Mg alloys has been carried out in this work.Specifically,the interaction mechanism between laser oscillating modes（continuous and pulse）and Mg alloys has been separately studied.Furthermore,based on characteristics of thermal fields produced by laser,corrosion resistant layers with different chemical compositions and thicknesses on the material surfaces are achieved to enhance the service performance of Mg and its alloys.In this research work,initially,a small beam spot,fast moving continuous infrared laser was chosen as the heat source to study the surface modification of the two-type Mg alloys,i.e.,Mg-0.6Ca and Mg-0.5Zn-0.3Ca.The results showed that the corrosion resistance of the materials was substantially improved after laser surface modification,and the corrosion rate was reduced to～1/2 of that of the original matrix.The surface hardness of both Mg alloys was also significantly increased after laser surface modification,～20%and～17%increases of surface hardness were shown in the Mg-0.6Ca alloy and Mg-0.5Zn-0.3Ca respectively.Surface morphologies,microstructures and residual stresses of the investigated materials were analyzed by SEM,TEM,XRD and XPS techniques.The main reasons for the corrosion resistance improvement are attributed to the formation of basal texture and the reduced size of dispersed second-phase particles,which can enhance the surface corrosion resistance and weaken the tendency of galvanic coupling corrosion.The formed basal texture results from the laser-induced non-equilibrium solidification.The rapid solidification induced by laser changes stain fields of the Mg alloys surfaces and enhances their hardness.The corrosion resistant metallic glass layer was coated on the surface of pure Mg by continuous infrared laser through layer-by-layer printing.The formation process of the metallic glass layer and the diffusion state of elements at the interface between the matrix and the amorphous heterogeneous structure were studied.The results show that under an action of the high-energy-density laser,the original metallic glass powders are melted and solidified to form amorphous protective layers with a certain bond strength（～16 N）between the substrate.The pure Mg experiences melting and solidification simultaneously,but there was no obvious elemental diffusion between Mg and metallic glass phase,and structures with a clear dual-phase interface were formed after solidification.The presented metallic glass layer significantly enhanced the corrosion resistance performance,reducing the corrosion rate from 0.89 mm/y to0.11 mm/y.Meanwhile,the hardness increased from～0.46 GPa for the pure Mg matrix to～14.3 GPa for the metallic glass coating layers.Furthermore,the amorphous alloy layer was not significantly cytotoxic.The water wettability on the metallic glass layer surface is stronger than that of the pure Mg matrix,which facilitates the adhesion and spreading of cells on the investigated metallic glass surfaces.The surface modification of Mg₆₈Zn₂₈Ca₄ Mg alloy and pure Mg were studied separately by using nanosecond UV laser.The results showed that the nanosecond UV laser changed the Mg₆₈Zn₂₈Ca₄ surface crystal structure,achieving phase transition from crystalline to amorphous state.The temperature field induced by nanosecond UV laser was simulated by using COMSOL software,and it was found that the high cooling rate of the laser（～10⁶ K/s）was a key factor for the formation of the amorphous layer.This crystalline→amorphous transition improved the hardness,Young’s modulus,and corrosion resistance of the Mg alloy surface.The hardness of initial matrix is～3.1 GPa,and the hardness is increased to～3.6 GPa in the sample experienced amorphous transformation with～16%increase.Its Young’s modulus increased from～25.3 GPa to～44.7 GPa,with～76.7%increase.The corrosion rate was reduced from 0.61 mm/y to 0.24 mm/y.It was further demonstrated by various characterization techniques including TEM,XRD,and nanoindentation that a single pulse energy can promote the formation of an amorphous layer with a thickness of～2μm on the surface of Mg₆₈Zn₂₈Ca₄ alloy.Applying multiple pulses acted on the same area did not cause the crystallization of the already amorphous surface layer,but also did not increase the amorphous layer thickness.Despite the high cooling rate induced by the UV pulse laser on the sample surface,it is not yet possible to directly transform the crystalline pure Mg into the amorphous state.The Mg O layer generated on the modified surface of pure Mg improves its corrosion resistance,and the corrosion rate decreases from 0.82 mm/y to 0.28 mm/y.The dissertation has systematically investigated the interaction process between laser and Mg and its alloys.Laser with different oscillation modes was adopted to carry out surface treatments.Microstructure evolution,mechanical properties and corrosion performance under non-equilibrium solidification were investigated.Based on achieved results,we also designed and built a dual laser system capable of material forming and in-situ modification.The research content is of practical significance to improve the service performance of Mg and its alloys.