Microstructure And Properties Of HA-TiO₂ Coating Prepared By Sol-Gel Via Laser Cladding On Magnesium Alloy

Magnesium alloys have been widely used in aerospace,electronic products,automotive industry and biomedical materials because of their low density,high specific strength,good casting and cutting performance.But the poor wear resistance and corrosion resistance restrict the full utilization and development of magnesium alloys in various fields.Therefore,it is of great practical significance to improve the surface wear resistance and corrosion resistance of magnesium alloys.In this paper,The HA-TiO₂ composite coating was prepared on the surface of AZ71magnesium alloy via sol-gel method and sol-gel composite laser cladding method.There are two mothods to prepare HA-TiO₂ composite coating by sol-gel composite laser cladding:first,laser directly treats the sol-gel coating surface,the effects of laser power on the microstructure,mechanical properties and corrosion resistance of the composite coating were analyzed.Secondly,the HA and TiO₂ powders prepared by sol-gel method were pretreated on the surface of magnesium alloy substrate by binder,and then the pre-treatment layer was treated by laser.The differences of microstructure,mechanical properties and corrosion resistance of the coatings with different proportions of TiO₂ and HA powders were studied.Metallographic microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD),energy dispersive spectroscopy(EDS)and microhardness test were used to characterize the coatings'micromorphology,phase content,chemical composition and hardness of the coating;Wear tester and electrochemical workstation were used to test the wear and electrochemical behavior of the coatings.The results show that there are obvious cracks in the TiO₂ sol-gel coating.The cracks of TiO₂/HA sol-gel coating are effectively filled by HA power,the hardness is higher and the corrosion resistance is better.The self-corrosion potential of TiO₂/HA sol-gel coating reached to-1.445 V,increased by 81 m V compared with the substrate,the self-corrosion current decreased by 0.579 m A/cm²,and the corrosion resistance was improved significantly.In order to study the influence of different laser power on the properties of TiO₂/HA cladding layer,three power parameters of 200 W,300 W and 400 W were chosen for laser treatment of the coating.The results show that the thickness of cladding layer increases with the increase of power.When the laser power is low,the adhesion between cladding layer and substrate is poor.When the laser power is high,cracks and pores appear in the cladding layer,which leads to the decrease of corrosion resistance and wear resistance.It is found that the microstructure and properties of TiO₂/HA cladding layer are the best when the laser power is 300 W.After laser cladding treatment of TiO₂ and TiO₂/HA sol-gel coatings,the original loose surface morphology changed to a smooth and compact coating.New phases such as CaTiO₃,Mg₃(PO₄)₂ and MgO were formed in the TiO₂/HA composite cladding layer.The average surface hardness of the cladding layer is 3 times of that of the substrate,about186.32 HV0.1,and its friction coefficient is 0.204,which is 45%lower than that of the substrate(0.366).The self-corrosion potential of TiO₂/HA cladding layer is-1.302 V,which is 224 m V higher than that of the substrate,and the self-corrosion current is reduced by 0.725 m A/cm².The mechanical properties and corrosion resistance of TiO₂/HA sol-gel coating were further improved by laser cladding.After laser treatment,the thickness of the HA+TiO₂ cladding layer is about 200?m,and there are different degrees of cracks and pores inside.The pure HA cladding layer is mainly composed of HA,CaO,Ca₃(PO₄)₂ and Mg₃(PO₄)2 phases.With the addition of TiO₂,TiO₂ and CaTiO₃ phases appear in the cladding layer,and Ca₃(PO₄)₂ disappears.The hardness of 30%TiO₂+70%HA composite coating is 816.18 HV0.1,which is about 11 times of that of the substrate,and the friction coefficient is 0.176,which is 52%lower than that of the substrate.The self-corrosion potential of 80%HA+20%TiO₂ composite coating is-1.275 V,which is 251 m V higher than that of the substrate,and the self-corrosion current is reduced by 0.712 m A/cm².The results show that the hardness and wear resistance of70%HA+30%TiO₂ composite coating are the highest,and the corrosion resistance of80%HA+20%TiO₂ composite coating is the best.