Designs And Investigations On Carbon Nanotubes Doped SiO₂/SiO₂-PbO Double-Layer High Emissivity Coating

When the Transatmospheric Hypersonic Vehicle is working at a high speed, the surface temperature increases and the vehicle structure stiffness declines, then the thermal stress and thermal strain will be caused. High emissivity coatings can improve the surface’s emitting ability in order to reduce vehicle’s surface temperature and provide better thermal protection. The higher the temperature is, the stronger the emissivity to be. The investigations and applications about high emissivity coatings are developing, while the designs and enhanced mechanism about high emissivity coatings need to be improved. Therefore, this paper first designs and prepares carbon nanotubes doped SiO2/SiO2-PbO double layer high emissivity coating to achieve high emissivity, low cost, easy repair and large-size preparation.We select SiO2as the coating’s matrix, and design a new hybrid sol-gel method to prepare SiO2based coatings by orthongonal test. According to the compositions, structures, requirements and the new hybrid sol-gel method, we match the hybrid SiO2coating’s oxidation temperature and the SiO2-PbO’s melting point, in order to obtain a porous structure in SiO2-PbO top layer, and therefore the new carbon nanotubes (CNTs) doped SiO2/SiO2-PbO double layer high emissivity coating is prepared.TG, DTA and FTIR are used to investigate the mechanism of hybrid SiO2sol-gel powders under heat treatment. SEM is used to observe the cross section and investigate the element’s diffusion through the interface. XRD and XPS are used to analyze the element’s distribution and determine whether the mullite phase is formed. Oxidation resistance test and orthogonal test are set to investigate the factors affecting coating’s properties, and then the optimum concentration is obtained.SEM is used to observe the CNTs doped SiO2/SiO2-PbO coating’s surface, cross-section, structure and CNTs. EDS is used to analyze the element’s distribution, and the mechanism of CNTs doped SiO2/SiO2-PbO coating’s structure is summarized. AFM is used to observe the3-D morphology of CNTs doped SiO2/SiO2-PbO coating in order to analyze the CNTs’influence on the surface’s roughness. Hardness test and scratching test are used to investigate doping CNTs and porous structure’s influences on coating’s hardness and toughness. Emissivity test is used to evaluate the CNTs doped SiO2/SiO2-PbO coating’s emissivity at300℃and550℃, and the enhancement mechanism of coating’s emissivity is concluded.The results indicate that, hybrid SiO2sol-gel first dehydrates and then decompounds between400℃and650℃. The oxidation at550℃is drastic. During a long-time high temperature heat treatment, the Al and Mn in substrate will diffuse to the coatings through their interface and then become enriched and oxidized. The optimum concentration of the hybrid sol (TEOS87.5g per standard sample) is:B10mol.%, silane resin15mL, butanol25mL and acetone15mL. The CNTs doped SiO2/SiO2-PbO coating can be prepared by hybrid sol-gel method.A porous-structured top layer is formed because SiO2base layer oxides and SiO2-PbO softens at the same time. The doped CNTs enrich towards the coating’s surface, but they do not obviously aggregate.The doping CNTs not only increase the surface’s roughness but also elevate the coating’s hardness and toughness.CNTs doped SiO2/SiO2-PbO coating shows strong blackbody character at300℃and550℃. CNTs doped SiO2/SiO2-PbO coating’s emissivity at550℃(compared by artificial blackbody) can reach as high as0.94.It is demonstrated that doping CNTs and porous structure can obviously increase the coating’s emissivity.