| Thermal control coatings are the most important means to adjust surface thermal radiation performance of aluminum alloy structural parts of spacecraft.Traditional thermal control coatings such as paint-type,electrochemical-type,and secondary surface mirror-type are limited by their preparation process and raw material properties,making it difficult to meet the demands of both low absorption and emission ratio and high wear resistance for complex aerospace environments.Therefore,in this paper,conventional micron Al2O3 powders and granulated nanoAl2O3 powders were used as spraying materials,and the conventional micron Al2O3 coatings and nano-structure Al2O3 coatings were prepared on the surface of 2A12 aluminum alloy by atmospheric plasma spraying technology.Morphology,phase composition and component elements of the powders and coatings were characterized by SEM,XRD and EDS.Mechanical properties,wear and friction properties and thermal control properties of micron and nano Al2O3 coatings were analyzed,and the effects of microstructure on mechanical properties,tribological properties and thermal control properties of the coatings were studied.The main research results are as follows:(1)The microstructure analysis results show that the defects such as pores,cracks and unmelted particles in Al2O3 coatings gradually decrease with increasing spraying current,and the completely melted area mainly composed of γ-Al2O3 phase gradually increases with increasing current.When the current is 580 A,the microstructure of the micron Al2O3 coatings basically composed of a completely melted area,while the morphology of the coatings deposited by loose and porous nano-Al2O3 powder presents a two-state distribution.The three-dimensional morphological results of the coatings show that the surface roughness of the nanoAl2O3 coatings is higher than that of the micron Al2O3 coatings,and the surface roughness of the coatings is positively correlated with the deposition thickness.The phase structure of both micron and nano-Al2O3 coatings consisted of γ-Al2O3 and αAl2O3.The semi-quantitative analysis results showed that the α-Al2O3 phase in nanoAl2O3 coatings was 21.4%,which was higher than that of micron Al2O3 coatings(12.7%).(2)The average microhardness and fracture toughness of conventional micron Al2O3 coatings were 1079.7 HV and 1.76 MPa·m1/2,respectively,while the nanocoatings were 8%and 40%higher,respectively,compared to the micron coatings.The hardness and fracture toughness of nano-Al2O3 coatings inherited a two-state distribution of microstructure.The results of the bond strength tests showed that the nano-Al2O3 coatings fractured from the inside during tensile,and the cohesive bonding strength is 35.48 MPa.(3)In the tribological tests with loads of 5-30 N,the friction coefficients of micron and nano-Al2O3 coatings varied from 0.757 to 0.682 and 0.765 to 0.671,respectively,both decreasing with increasing load.The wear volume of the coatings increased with increasing load.And at loads below 20 N,the spheroidal abrasive particles in the nano-Al2O3 coatings can improve the wear resistance of the coatings,and the wear volume is smaller than that of the micron Al2O3 coatings.As the load continues to increase,cracks caused by pores and debris led to a sharp increase in the wear volume of nano-Al2O3 coatings.The main wear mechanism of micron and nano Al2O3 coatings is abrasive wear.(4)The reflectivity of the nano-Al2O3 coatings is better than that of micron Al2O3 coatings,with a reflectivity of more than 75%in the wavelength range of 446-1586 nm.This is related to the content of α-AlO3 phase and the microstructure of the twostate distribution in the nano-alumina coatings.Increasing the thickness of the coatings as well as the reflectance and infrared emissivity of the coatings.In addition,the reflectance of the coatings increases significantly with increasing test temperature,but the emissivity is less affected by temperature,and the infrared emissivity varies less than 0.01 in the range of room temperature to 120℃. |
PDF Full Text Request