Preparation And Properties Of Hydrothermally Synthesized CeO_ Based Low Emissivity Coatings At High Temperatures

In modern warfare,low emissivity coatings at high temperatures have attracted much attention because of their protective effect on military targets.At this stage,low emissivity coatings at high temperatures cannot be used in engineering applications because the emissivity and thermal shock resistance at high temperatures cannot meet the requirements.According to the infrared stealth requirements of the hot-end parts of the aircraft and the shortcomings of low-emissivity coatings at high temperatures,two preparation schemes of low-emissivity coatings at high temperatures are designed in this thesis.Experiments were carried out around the design schemes of air spraying method and plasma thermal spraying method.Two kinds of low-emissivity coatings at high temperatures were prepared.The specific research results are as follows:1.In order to reduce the emissivity of CeO₂powder,the doping method is adopted to improve the conductivity of the powder.The results show that proper doping can reduce the emissivity of the powder.The conductivity,lattice distortion and lattice vibration affect the change of the emissivity of the powder.When the doping concentration is small,the conductivity plays a leading role;when the doping amount is increased,lattice distortion and lattice vibration have a greater effect.With Zr⁴⁺and Cu²⁺doped powders,the abscissa of the lowest point of the emissivity shifts to the left as the doping amount increases.In addition,low-cost doping has a better effect than parity doping,because low-cost doping makes CeO₂powder have more oxygen vacancies,and its conductivity improves faster.Among them,1%Cu²⁺doped CeO₂powder has the lowest emissivity at 800°C of 0.200.2.For the air spray method,the changes in heat resistance and adhesion of the coatings prepared after different substrate treatments were studied.The processing method of the selected substrate is to add a thermal barrier coating as an intermediate layer.CC-1 powder(1%Cu²⁺-doped CeO₂powder heat-treated at 1300°C for 3 hours)was used as filler,and inorganic silicate was used as binder.The filler content,curing temperature and curing time are optimized.The optimal process conditions for preparing the coating with the lowest emissivity in the 3?5?m waveband at room temperature to800°C are 75%(mass percentage)filler content,600°C curing temperature and 2 h curing time.The emissivity of the coating prepared under the process is 0.241 at 800°C.3.For the plasma spraying method,in order to improve the fluidity of the powder,three kinds of surfactants were added in the process of hydrothermal synthesis of CeO₂powder,namely the cationic surfactant cetyltrimethylammonium bromide,anion surfactant sodium dodecylbenzenesulfonate and non-ionic surfactant polyethylene glycol.Investigate the effect of surfactant types and additions on CeO₂flow.The results show that when 1 g of polyethylene glycol is added,the hydrothermally synthesized CeO₂powder has the best fluidity,the flow rate is 38.7 s/50g.At this time,the emissivity of the powder is also the lowest,which is 0.140 at 800°C.According to the common plasma spraying technical parameters,plasma thermal spraying is performed.The plasma sprayed coating has a high bonding strength,the adhesion level reaches level 1,and it can withstand more than 300 thermal shock experiments.The emissivity of the coating at 800°C is as low as 0.223 and the downward trend still exists.Finally,the comprehensive properties of the two coatings prepared by plasma spraying and air spraying are compared and analyzed.The results show that both coatings have good temperature resistance and oxidation resistance,high hardness and poor impact resistance.