| Magnesium alloys are widely used in aerospace,automotive engineering,biomedicine and other fields due to the advantages of low density,high specific strength,and good casting properties.However,the poor corrosion and wear resistance of magnesium alloys have limited their further application prospects.Known as the "space metal" and "marine metal",titanium alloys have the advantages of light weight,high specific strength,corrosion resistance and excellent damping resistance,and are widely used in aerospace and marine engineering.However,the erosion and wear resistance of titanium alloys is unsatisfactory.If titanium alloys are exposed to corrosive and erosive environments,damage will be accelerated.Therefore,solving the above problems is of great significance to the application and protection of magnesium and titanium alloys.In this paper,multifunctional composite coatings were prepared on the surface of magnesium and titanium alloys by micro arc oxidation,electroless plating,physical vapor deposition.By observing the morphology of the surface and cross-section of the composite coatings,the interface bonding characteristics were studied.Then,the hardness and elastic modulus distribution of the composite coatings were studied by nanoindentation test.The bonding force and damping properties were analysed by scratch tests and damping tests.Through erosion experiments,electrochemical tests and immersion corrosion experiments,the erosion and corrosion behaviors of the substrate,physical vapor deposition coating(PVD)and composite coatings were studied,and the protective mechanism of the composite coatings against erosion and corrosion of magnesium and titanium alloys,were also given.The results are as follows:(1)The surface morphologies of the composite coatings on the surface of magnesium and titanium alloys are relatively rough,which are composed of micronsized cellular nodules,and each nodule is distributed with smaller cellular nodules.The composite coatings is mainly divided into three layers,which are micro-arc oxidation ceramic film(MAO),electroless nickel plating coating(EN)and physical vapor deposition coating(PVD).Among them,MAO and the alloy show metallurgical bonding characteristics,and EN is embedded in the pores of the micro-arc oxide film to form a mechanical interlocking structure,which helps to obtain higher bonding strength.The PVD covers the cellular structure of the EN and combines well with the EN.(2)The hardness and elastic modulus of the substrate MAO,EN,and PVD in the composite coatings show a gradual upward trend in turn,which is a "soft and hard" gradient structure.The hardness of the magnesium alloy is only 1.166 GPa,while the hardness of the PVD on its outer surface reaches 21.571 GPa.The hardness of the titanium alloy is 4.129 GPa,and the hardness of the PVD on the outer surface reaches25.045 GPa.(3)The bonding force of the composite coatings of magnesium and titanium alloys is higher than that of the PVD of the alloy.The bonding force of the composite coatings of magnesium alloy is about 1.9 times that of the PVD,The bonding force of composite coatings of the titanium alloy is 2.9 N higher than that of the PVD.The reason for the increase in the bonding force of the composite coatings is that the existence of the EN and MAO reduce the difference in hardness and stiffness between the PVD and the substrate,and a large number of cellular structures on the surface of EN provide more nucleation points for the deposition of the PVD,reduce surface defects,and increase the bonding force.(4)Compared with PVD,the composite coatings has better damping performance.This is mainly due to the damping effect between various interfaces in the composite coatings and the gradient structure of the composite coatings,which increase the energy loss of the composite coatings when it is subjected to shock or vibration.(5)The composite coatings has better erosion resistance than PVD.The main erosion morphologies of different material systems are ploughing and micro-cutting.In the magnesium alloy system,the erosion rate of the composite coatings is 0.25 times that of the substrate and 0.45 times that of the PVD.In the titanium alloy system,the erosion rate of the composite coatings is 0.3 times that of the substrate,and 0.7 times that of the PVD.This can be attribute to the high hardness and high damping properties of the composite coatings.(6)The composite coatings of magnesium alloy can significantly increase the corrosion resistance.Compared with the magnesium alloy,the composite coatings can reduce the corrosion current density and improve the resistance.However,PVD has little effect on corrosion protection of magnesium alloy and cannot provide effective protection.The corrosion potential of the composite coatings and PVD of titanium alloy is lower than that of the alloy.After a certain period of immersion corrosion,corrosion products appeared on the substrate,but the corrosion phenomenon of the composite coatings and PVD was not obvious. |
