Preparation And Study Of High Infrared Emissivity And Electrical Conductivity Coating On Mg-Li Alloy

Magnesium-lithium alloy,as the lightest structural metal material in the world,has significant advantages in specific stiffness and specific strength,and has gradually replaced aluminum alloy for aerospace products.With the growing demands of the functions and performance requirements of aerospace products,the shell of aerospace electronic products must have both high electrical conductivity and high infrared emissivity to meet the requirements of thermal control and electromagnetic shielding performance.A large number of research results show that the high infrared emissivity coating generally has good insulating properties,while the high conductivity metal coating has low infrared emissivity.It is difficult to achieve high infrared emissivity and high electrical conductivity performance at the same time.However,the infrared emissivity is significantly dependent on the surface morphology,and porous N i and Zn-Ni coatings prepared by dynamic hydrogen bubble template method are controlled and designed to obtain high electrical conductivity and infrared emissivity,then the high infrared emissivity and high-conductivity porous coating was prepared on magnesium-lithium alloy via a suitable pre-treatment process in this paper.Deposition mechanism,surface morphology,infrared emissivity and the relationship between the porous structure and the infrared emissivity were studied in detail.The main conclusions were shown as follow.1.A high corrosion resistance underlayer(N i-P film layer)and an interlayer(Cu layer) are prepared on Mg-Li alloy by optimizing the surface pretreatment process.This enables the Mg-Li alloy to have good corrosion resistance and satisfies corrosion grade?9 after 24h salt spray test.It lays a solid foundation for the subsequent porous Ni film layer and Zn-Ni film layer.2.The simulation results of infrared emissivity of porous Ni coating by FDTD (Finite-Difference-Time-Domain)solution software show that the existence of porous structure is beneficial to increasing the emissivity of the coating.With the increase of pore size((?)(8),the hole depth(h)and the hole projection area ratio((?)(8),the infrared emissivity of the coating is significantly increased.The N i coating shows the optimal infrared emissivity when the pore size((?)(8)and hole depth(h)are 20(?)m.The increase of the hole projection area ratio((?)(8)which is the most important factor can significantly increase the infrared emissivity.3.The results of electrodeposition porous N i coating by dynamic hydrogen bubbles template method in aqueous solution with N i C l₂·6H₂O as main salt show that the average pore diameter decreases from 7.4(?)m to 3.5(?)m,the pore projected area ratio decreases from 7.4 to 3.5 and the infrared decrease from 0.75 to 0.13 when the concentration of N i C l₂·6H₂O increases from 0.1 M to 0.3 M.The increased concentration of N i C l₂·6H₂O decreases the average pore diameter,pore projected area ratio and finally the infrared emissivity.4.When the concentration of surfactant SDS increases from 0 to 2m M,the average pore diameter increases from 9.3(?)m to 30.2(?)m,pore projected area ratio increases from 0.2 to 0.38 and infrared emissivity increases from 0.36 to 0.80.The addition of SDS significantly increase the evolving hydrogen bubbles during electrodeposition process, which leads to the increase of pore size((?)(8),pore projected area ratio((?)(8)and infrared emissivity of porous N i coating.The effect of porous structure on the infrared emissivity is consistent with the simulation result by FDTD solution.5.In order to further increase the infrared emissivity,the porous Zn-Ni coating was electrodeposited by alloying of Zn element.The addition of Zn salt can effectively inhibit the reduction of N i²⁺,promote the formation of Ni₂Zn₁₁ compound in the porous coating.Moreover,the addition of Zn salt can significantly increase the pore size((?)(8),the hole depth(h)and the hole projection area ratio((?)(8)of the porous Zn-N i coating,which leads to the improvement of infrared emissivity.The infrared emissivity of porous Zn-Ni can reach a maximum value of 0.89.However,the cohesive force of the porous Zn-Ni alloy layer is so weak,and the coating is easy to be split away off the surface of the substrate.6.The addition of NH₄SCN can increase the cohesive force of the continuous matrix of the porous Zn-Ni coating which shows a good coating-based bonding strength. Moreover,the addition of NH₄SCN promotes the formation of hydroxide on the cathode surface during electrodeposition process,resulting in the formation of Zn O and N i O phases in the coating.At the same time,the addition of NH₄SCN can significantly increase the projected area ratio((?))of the porous Zn-N i coating and the infrared emissivity.The infrared emissivity of porous Zn-N i can reach a maximum value of 0.91.7.Micro ceramic particles(Al₂O₃,Si O₂ and Ti O₂)can effectively prevent the hydrogen bubbles escaping from the cathode and increase their standing time as dynamic template during electrodeposition process.When the current density is decreased from 3A/cm² to 1A/cm²,micro ceramic particles are beneficial to obtaining uniform porous Zn-Ni coatings with high infrared emissivity.Among these three ceramic particles,Ti O₂ particles have a great effect on the improvement of infrared emissivity which can be increased from 0.83 to 0.91.