Preparation And Space Ultraviolet Effect Of Compound Metal Oxide Thermal Control Coatings On Magnesium Alloy

Thermal control coatings(TCCs)with low solar absorptivity(?S)and high emissivity(?)are one of effective methods ensuring the working stability of exposed Mg and Al components on the spacecraft.However,due to high chemical activity of Mg alloys,the coating formation using traditional techniques is difficult.Therefore,the preparation of high-performance TCCs with low ?S/? ratio on Mg alloys is of important significance.In the thesis,plasma electrolytic oxidation(PEO)and magnetron sputtering techniques were used to prepare TCCs with anti-ultraviolet(UV)irradiation and low ?S/? ratio on Mg alloy surface.It could provide theoretical and technical support for the thermal-control material selection of the spacecraft.According to thermal radiation laws,Fresnel equation and Maxwell-Gannett theory,the composition and structure were designed for MgO-Mx Oy TCCs with low ?S/? ratio on Mg alloy.When TCCs optimum texture was MgO-MxOy(M: Al,Ca,Zn,Zr or Ti),the ?S was reduced,meanwhile the ? in the range of greater than 11 ?m was increased.Base on the theoretical design results,low ?S/? ratio MgO-MxOy TCCs on Mg alloy were in-situ prepared by the PEO technique.The effect of electrical parameters and electrolyte composition/concentrations on the structure,composition and thermal control performances of TCCs were studied.The microstructure and chemical composition were characterized by SEM,XRD and XPS methods.All the results showed that,when the electrolyte was Na5P3O10,Zr(NO3)4,KOH,Na2 EDTA and Na F,the concentration in turn was 30 g/L,10 g/L,1.5 g/L,1 g/L and 0.5 g/L,and then electrical parameters were 10 A/dm2,50 Hz,45% and 10 min,MgO-ZrO2 TCCs could obtain its best performance.The ? and ?S of the obtained coating were 0.873 and 0.403,respectively;The PEO coating had a typical micro-porous structure,which consisted of MgO,ZrO2 and Mg0.13Zr0.87O1.87.Due to ZrO2 doping,MgO-ZrO2 TCCs on Mg alloys possessed good low ?S/? ratio performance,which were very in agreement with the theoretical design.? increased with the increase of the thickness below 59.2 ?m,and ?S reduced with the roughness decrease.In order to further enhance the anti-UV irradiation ability of MgO-ZrO2 TCCs,TiO2/MgO-ZrO2 composite coatings were fabricated by the magnetron sputtering technique.The effect of technical parameters on the coating composition and thermal control performances were studied.When the parameters were 1.5 Pa,80 W,5%,150 ? for 10 min,the performance of composite coatings reached the optimum,and the ? and ?S were 0.914 and 0.375,respectively.The ?S/? ratio of the composite coating was 0.410 which was 0.052 higher than that of MgO-ZrO2 coatings,showing that thermal control performances of Mg alloy was improved further.Analysis results indicated that new compositions on the composite coating were formed: S-TiO2,Mg Ti O3,S-(Ti,Zr)O2 and Tin O2n-1,in which Ti3+ and oxygen vacancies can enhance the spectral absorption of the coating in the short range between 2.5 ?m and 8 ?m,and meanwhile ? in the long range of higher than 8 ?m was increased due to the existence of Mg-Ti-O and Zr-Ti-O chemical-bonds.As a result,thermal control performances of MgO-ZrO2 TCCs can be further enhanced.UV-irradiation damage effects of TCCs on Mg alloy were studied using space irradiation simulation equipment.The effect of UV-irradiation on the coating color,structure,composition,mass-loss ratio and thermal control performances were discussed.With the extension of the irradiation time,?S and mass-loss ratio of TCCs before and after the composite modification increased firstly and then slow-downed.The surface color turned yellow and then deepened;and ? had a slight increase.When the irradiation time was 5062 ESH,??S of composite coatings was 0.118,which was lower than that of MgO-ZrO2 TCCs(??S=0.149).This indicated that anti-UV irradiation damage effects of TiO2/MgO-ZrO2 composite coatings were more effective.Analysis results of the microstructure and chemical composition showed that the coating microsurface was little damaged.The ?S degeneration was mainly attributed to Mg surface enrichment induced by MgO UV photodecomposition.TiO2 on the composite coating effectively obstructed UV-light to restrain MgO photo-decomposition,which reduced UV-irradiation damage.And then TCCs UV-irradiation damage mechanism was revealed.Lifetime prediction models of obtained TCCs under UV-irradiation were established.