Research On Toughening Behavior Of ZrO₂/HfO₂ Ceramic Fiber Doped Ultra-high Temperature Anti-oxidation Coating

As the service temperature of aerospace vehicles continues to increase,the ultra-high temperature measurement technology is also facing higher challenges.Compared with traditional precious metal thermocouples,tungsten rhenium（W-Re）thermocouples are widely used in the field of ultra-high temperature measurement due to their high temperature measurement upper limit,large thermoelectromotive force,good mechanical strength,affordable price and other advantages.However,tungsten-rhenium thermocouples can only be used for high-temperature measurement in reducing,inert,and vacuum environments,and cannot be used in high-temperature oxidizing atmospheres,because they begin to oxidize at around 300°C in an oxygen environment.Therefore,how to improve the oxidation resistance of tungsten-rhenium thermocouples has always been a subject of high concern in the field of high temperature measurement at home and abroad.In the previous study,the borides/carbides/oxides of refractory transition metal elements of theⅣB～VB group were selected and targeted for doping modification.Through the optimized coating preparation method,they were used to prepare an oxidation-resistant thick film coating with extremely low high-temperature ablation rate on the surface of the special-shaped W-Re thermocouple substrate.The coating can greatly improves the high-temperature oxidation resistance of the thermocouples.However,the internal stress generated during the preparation process and the induced stress generated during the use process（especially multiple use）usually lead to the failure of the coating in the forms of cracking and peeling.In response to the stress failure of the high-temperature anti-oxidation coating on the surface of the W-Re alloy substrate in a high-temperature oxidation environment,HfO₂or HfO₂/ZrO₂（HZ）composite fibers with high thermal stability are doped into the Zr B₂-HfO₂-Si C（ZHS）anti-oxidation coatings.And the toughening mechanism of the fibers and the ability to resist several severe temperature changes（thermal shock resistance）are discussed in this article.So as to further increase the upper limit of the working temperature and extend the service time of W-Re alloy in high temperature air or other high temperature oxidizing atmospheres.First,on the basis of the previous research,aiming at the mismatch of the thermophysical properties between the coating and the substrate,a simulation model of the multilayer film structure of the W-Re substrate and the coatings is constructed.The structure of the high-temperature oxidation-resistant coating on the surface of the W-Re alloy substrate is optimized,by analyzing the stress distribution in the multilayer film system caused by the mismatch of thermophysical properties,and adding suitable transition layers.The stress concentration phenomenon of the coating structure is effectively improved,and the stress difference between the inner and outer layer is significantly reduced.Secondly,HfO₂fibers and HZ composite fibers are prepared through the process method of electrostatic spinning combined with high temperature calcination.Different from conventional fiber reinforcement（Si C fibers and CNTs,etc.）,HfO₂fibers and HZ composite fibers are closer to the physical and chemical properties of the ZHS coating and with a higher melting point.The HfO₂and HZ composite fibers are characterized and studied for high-temperature thermal stability.The study found that with the increase of the calcination temperature,the phase in the HfO₂fibers are changed from the monoclinic phase to the tetragonal and cubic phase,and the phase transition temperature is lower than the theoretical value;however,the HZ composite fibers are composed of monoclinic phase HfO₂and cubic phase ZrO₂,and tetragonal and cubic phase HfO₂are not exisited,indicating that the addition of ZrO₂inhibits the phase change of HfO₂.The results of the cycle thermal stability test at"room temperature～1500℃"showed that all HfO₂fibers are transformed into a more stable monoclinic phase HfO₂,and HZ composite fibers form a monoclinic phase Hf_0.5Zr_0.5O₂solid solution.In addition,the thermal stability of the fiber is directly related to the calcination temperature.When the calcination temperature is lower than 1500°C,the thermal stability of the fibers is better;when the calcination temperature is higher than 1500°C,the fibers will undergo secondary crystallization,and their thermal stability is poor.The critical temperature of secondary crystallization of the HfO₂and HZ composite fibers is 1500℃.Thirdly,the HfO₂fiber and HZ composite fibers with high thermal stability are doped into ultra-high temperature anti-oxidation coatings.On the basis of the optimized design of the multi-layer film structure,the WSi₂transition is grown first on the substrate through the embedded silicon infiltration method to relieve the mismatched thermal and physical properties between the substrate and ZHS coating.Afterwards,the ultra-high temperature coatings are doped with HfO₂fibers or HZ composite fibers with high thermal stability,and the surface morphology and microstructure of the ZHS coating are studied.The analysis results showed that the HfO₂and HZ composite fibers can both reduce the coating crack width（from 1.45μm to 0.22μm,in a local area）,through toughening mechanisms such as crack deflection,fiber bridging,and interface debonding.So,the fibers has a significant inhibitory effect on crack propagation.Finally,in view of the destructive failure of the coating sample repeatedly subjected to rapid environmental temperature changes,the thermal shock resistance and mechanism of the ZHS coating sample doped with HfO₂fibers and HZ composite fibers are studied.The results of the“800～1500℃”thermal shock resistance test showed that the doping of HfO₂fibers and HZ composite fibers can significantly improve the thermal shock resistance of the coating samples.The thermal shock resistance of the coating sample doped with HZ fibers is obviously better than that of the sample doped with HfO₂fiber.When the HZ fiber doping content is 9 wt.%,the effective numbers of thermal shock resistance of the coated sample is509.2%and 289.1%higher than that of the W-Re matrix and the undoped ZHS coating sample,respectively.