Preparation And Thermal Control Performance Of Mao Caotings On Magnesium- Lithium Alloy

In order to meet the demand of spacecraft about the light-weighting and thermal protection and improve the application range of magnesium-lithium alloy, this work prepared the thermal control coatings with the properties of high absorptivity(ε) and low emissivity(αs) by micro-arc oxidation(MAO). The composition and structure of the coatings were characterized by X-ray diffraction(XRD), X-ray photo-electron spectroscopy(XPS) and scanning electron microscopy(SEM) equipped with energy dispersive X-ray(EDX), and the influence of those on thermal control properties of coatings were studied. Furthermore, the discharges during the MAO process were investigated by the optical emission spectroscopy(OES), which is to estimate the analysis of the composition and structure and the growth characteristic of MAO coatings.With the typical porous structure, the MAO coatings obtained in silicate system are mainly composed of Mg O and Mg2 Si O4. Extending the reaction time, improving the current density and enhancing the concentration of electrolyte solution can increase the thickness and surface area of MAO coatings. The structure of clusters with particles tends to form on the Mg-4Li coatings while the micro- protuberance structure on the Mg-9Li coatings. Compared with the substrate, MAO coatings have better thermal control ability with higher ε and lower αs, and the ability can be optimized by extending the reaction time, improving the current density and enhancing the concentration of electrolyte solution. The MAO coatings prepared in phosphate system have less and larger pores than that in silicate system. The MAO coating obtained under lower frequencies(less than 100 Hz) is composed of crystallized Mg O and crystallized Mg3(PO4)3 and the coating under higher frequencies is composed of crystallized Mg O and amorphous Mg3(PO4)3. The thicker coatings with complete structure prepared in longer reaction time and lower current density have better thermal control performances. Enhancing the concentration of electrolyte solution, the thermal control property of Mg-4Li coating increases while that of Mg-9Li coating decreases.The OES during the MAO process in silicate system were mainly from Na, Li and Mg discharges. By calculated by spectrum, the electron concentration(Ne) during the MAO process is about 2.689×1016 /cm3, which is corresponding to the breakdown current(Ibreak). Ne has less related to the electrical parameter for a certain electrolyte system and substrate. The plasma temperature(Te) is corresponding to the growth current(Igrowth), and Te increase and the growth rate of coating rise as improving Igrowth. The OES during the MAO process in phosphate system were mainly from Na and Li discharges. Owing to the intensity of energy output per pulse, Te has dramatic changes under higher or lower frequencies, which lead to the micro structure and composite of the MAO coatings under different frequencies are various by the effect of liquid-cooling.