High Temperature Oxidation Resistance Of Ultra-high Temperature ZrB₂-based Ceramic Materials

The radar stealth of high-temperature components is a key technology that restricts the omnidirectional stealth of the aircrafts.Due to the limitation of the curie temperature of magnetic materials and the narrow absorbing frequency band of electrically lossy materials,the traditional stealth materials are difficult to meet the application requirements of radar stealth of high-temperature components.The resonance mechanism of metamaterials is expected to break through the limitations of traditional high-temperature stealth materials and realize radar stealth of high-temperature components.Stable high-temperature conductivity is the prerequisite for metamaterials to obtain stable high-temperature electromagnetic properties.However,the oxidation of conductive materials will seriously deteriorate the high-temperature electrical conductivity.Therefore,it is of great significance to develop the high-temperature materials with excellent oxidation resistance under high temperature.The poor high-temperature oxidation resistance of the metal materials greatly limits their high-temperature applications.In this dissertation,ZrB₂ ceramic is used as the matrix and LaF₃ is used as a second phase to improve the high-temperature oxidation resistance of ZrB₂-based ceramics.Combined with the crystal structure,microscopic morphology and structure,and thermal stability analysis,the high-temperature oxidation behavior of in-situ coated modified ZrB₂@LaF₃ composite ceramic and doped modified ZrB₂+LaF₃ composite ceramic was systematically studied,and the high-temperature oxidation resistance mechanism of the modified composite ceramics was studied combining with the oxidation kinetic analysis,which lays the material support and theoretical foundation for the improvement of the oxidation resistance of ZrB₂-LaF₃ composite coating.The ZrB₂-based coatings were prepared by high-enthalpy atmospheric plasma spraying technology.The influence of spraying power on the high-temperature oxidation resistance of ZrB₂-LaF₃ composite coating was studied.The high-temperature oxidation behavior and oxidation kinetics of ZrB₂-based coatings were discussed combining with the thermal stability analysis,confirming that the introduction of LaF₃ contributes to the improvement of the high-temperature oxidation resistance of ZrB₂-based coatings.In addition,the influence of the modification treatment on the electrical properties of ZrB₂-based bulks/coatings was studied,revealing that the ZrB₂-LaF₃ composite coating has the potential to be used as a periodic structural material for the RCS reduction layer due to the good and stable high-temperature electrical conductivity.The main research contents and innovations of this dissertation are as follows:（1）Using the in-situ coating method to protect ZrB₂ ceramic from oxidation,which is named as“prevention type”,the related researches on the high temperature oxidation of coated modified ZrB₂@LaF₃ composite ceramic was launched.The designed verification experiments between LaF₃ and B₂O₃ in the temperature range of800～1400℃ revealed the evolution of the reaction products of ZrB₂@LaF₃ composite ceramic.The results of XRD,SEM,EDS and TG-DSC revealed that LaF₃ coating layer slowed down the oxidation rate of ZrB₂@LaF₃ composite ceramic,the high-temperature oxidation evolution model and in-situ reaction coated oxidation resistance mechanism of ZrB₂@LaF₃ composite ceramics were explored combining with the oxidation kinetics,confirming that LaF₃ has the potential to improve the oxidation resistance of ZrB₂-based ceramic materials.（2）Using the doping method to protect ZrB₂ ceramic from oxidation,which is named as“response type”,the related researches on the high temperature oxidation of doped modified ZrB₂+LaF₃ composite ceramic was launched.The influence of the volume ratio of second phase,oxidation time and oxidation temperature on the oxidation resistance of ZrB₂+LaF₃ composite ceramic was studied through the oxidation in the temperature range of 1000～1400℃ for 1 h,4 h and 8 h,respectively,the high-temperature oxidation evolution model and the oxidation-induced self-healing oxidation resistance mechanism of ZrB₂+LaF₃ composite ceramic were explored combineing with the oxidation kinetics,revealing that the doping modification treatment significantly improved the oxidation resistance of ZrB₂-based ceramic materials,which has important guiding significance for the improvement of high-temperature oxidation resistance of ZrB₂-LaF₃ composite coating.（3）ZrB₂-LaF₃ composite powder was prepared based on the influence of the modification treatment method and volume ratio of LaF₃ on the oxidation resistance of ZrB₂-based ceramic materials.The ZrB₂-based ceramic coating was prepared by high-enthalpy atmospheric plasma spraying technology.It focused on the influence of spraying power on the oxidation resistance of ZrB₂-LaF₃ composite coating combining with the results of XRD,SEM,EDS,density,and macro characteristics before and after oxidation.According to the law of influence,the optimal spraying power of the coating is determined to be 75 k W.After being oxidized at 1000℃ for 30 min,the oxide layer thickness of ZrB₂ ceramic coating and ZrB₂-LaF₃ composite coating are about 100μm and 10μm,respectively,and the weight gain of ZrB₂-LaF₃ composite coating is lower.The addition of LaF₃ significantly reduced the oxide layer thickness of ZrB₂-based ceramic coating and reduced the oxidation weight gain,improving the high-temperature oxidation resistance.The activation energy and the most probable kinetic mechanism function during the oxidation process of ZrB₂-based ceramic coatings were determined by the Flynn-Wall-Ozawa and Malek methods.The oxidation kinetics of ZrB₂ ceramic coating and ZrB₂-LaF₃ composite coating was compared and analyzed.The influence of the plasma spraying process on the crystal structure of ZrB₂ was clarified,and the high-temperature oxidation resistance mechanism of ZrB₂-LaF₃ composite coating was revealed combining the experimental results and theoretical analysis.（4）The influence of modification treatment on the electrical properties of ZrB₂-based ceramic bulks/coatings was studied.The resistivity characteristics of the coated modified ZrB₂@LaF₃ composite ceramic and the doped modified ZrB₂+LaF₃composite ceramic were discussed in combination with the component content,relative density,and crystal grain size,component distribution,the sinterability of particles and pore characteristics,and the influence of temperature changes on the resistivity of ZrB₂-based ceramic bulks,which provide the theoretical support for the research on the high-temperature electrical properties of ZrB₂-based coatings.During the high temperature oxidation process,the resistivities of ZrB₂ coating and ZrB₂-LaF₃composite coating are 2.0×10^-3Ω·m～7.0Ω·m and 3×10^-4～1.4×10^-3Ω·m,respectively.After isothermal oxidation,the room temperature resistivities of ZrB₂ coating and ZrB₂-LaF₃ composite coating are 1.3×10^-1Ω·m and 6.7×10^-3Ω·m,respectively.The addition of LaF₃ not only reduced the high-temperature resistivity of ZrB₂-based ceramic coatings,but also improved the stability of the high-temperature conductivity of the coatings.Therefore,ZrB₂-LaF₃ composite coating has the potential to be used as a high-temperature conductive material for the RCS reduction layer.This dissertation has a guiding significance on the improvement of the high-temperature oxidation resistance of ZrB₂-based ceramic materials,enriches the research of the electrical properties of ZrB₂-based ceramic materials,provides the theoretical basis and experimental support for the application of ZrB₂-based ceramic coatings in RCS reduction layer of the exhaust nozzle,and explored a new material system for the radar stealth application of high-temperature parts of the aircrafts.