Microstructure Evolution And High Temperature Oxidation And Ablation Mechanism Of Mechanical Alloying Nano-Ta₄HfC_5p/SiBCN Powder And Ceramics

With the rapid development of aerospace technology,more stringent requirements are put forward for high temperature thermal protection materials.The research and development of new high temperature materials with high temperature oxidation resistance and ablation resistance has become an urgent need in the aviation industry.SiBCN ceramics have attracted extensive attention due to their unique structural characteristics and excellent high-temperature stability.However,the oxidation products（such as Si O₂）of SiBCN ceramics have relatively poor stability at high temperature and relatively weak resistance to high temperature gas flow erosion,resulting in the continuous damage of the ceramic substrate in the extreme environment.Therefore,the high temperature oxidation resistance and ablation resistance of SiBCN ceramics need to be further improved.In this work,nano-Ta₄HfC_5p/SiBCN multiphase ceramics were prepared by mechanical alloying combined with hot-pressing sintering.The microstructure evolution behavior during mechanical alloying and the high temperature crystallization and oxidation behavior of nano-Ta₄HfC_5p/SiBCN powders were investigated.The microstructure and mechanical properties of nano-Ta₄HfC_5p/SiBCN multiphase ceramics sintered by hot pressing were also investigated.On this basis,the high temperature oxidation resistance and oxidation damage mechanism of nano-Ta₄HfC_5p/SiBCN ceramics were investigated by means of TG-DSC,XRD,Raman,XPS,SEM,FIB-TEM and EDS.The ablation resistance of nano-Ta₄HfC_5p/SiBCN ceramics was evaluated by oxyacetylene flame,and the microstructure of the ablation layer and ablation resistance mechanism were studied.nano-Ta₄HfC₅powder was prepared by mechanical alloying with TaC and HfC as raw materials.On this basis,c-Si,graphite,h-BN and nano-Ta₄HfC₅were mechanically alloyed again to prepare nano-Ta₄HfC_5p/SiBCN powder.Due to the weak Van der Waals force between h-BN and graphite,they transform into the amorphous structure after mechanical alloying for 100 min,while the diffraction peak of c-Si subsequently disappears.It is difficult to achieve complete amorphization of Ta₄HfC₅due to its strong bond energy.After mechanical alloying for 1200 min,nano-Ta₄HfC₅is still distributed in the amorphous powder with a grain size of 3–5 nm.The introduction of nano-Ta₄HfC₅is helpful to promote the formation of amorphous SiBCN.In high temperature/inert atmosphere,amorphous carbon in nano-Ta₄HfC_5p/SiBCN powder undergoes carbothermic reduction with the oxygen introduced during the preparation process,accompanied by sample weight loss and the generation of CO₂.Resulting in partial conversion of Si-O and other chemical bonds to Si-C bonds.The heat treatment process is also accompanied by the grain growth of 3C-Si C,BN（C）and Ta₄HfC₅.The growth process of 3C-Si C is slower than that of Ta₄HfC₅.The tantalum hafnium silicate glass phase formed in the oxidation process of nano-Ta₄HfC_5p/SiBCN powder is coated on the surface of the inner powder to isolate oxygen.The oxidation activation energy of 10 wt%nano-Ta₄HfC_5p/SiBCN powder is263.5 k J/mol,which is higher than that of pure SiBCN powder（205.9 k J/mol）.However,the morphology of pure nano-Ta₄HfC₅powder after oxidation is still loose and porous,and the powder is completely oxidized to Ta₂O₅and Hf₆Ta₂O₁₇.The phases of nano-Ta₄HfC_5p/SiBCN ceramics sintered by hot pressing mainly include 3C-Si C,6H-Si C,BN（C）and Ta₄HfC₅.Ta₄HfC₅is evenly distributed in the SiBCN matrix as nanocrystals with a grain size of about 30 nm-50 nm.The nano-Ta₄HfC₅help to improve the mechanical properties of the multiphase ceramics.The maximum flexural strength and fracture toughness of nano-Ta₄HfC_5p/SiBCN multiphase ceramics are 344.1 MPa and 4.52 MPa·m^1/2,respectively.While the pure SiBCN ceramics are 156.1 MPa and 1.82 MPa·m^1/2,respectively.The oxidation behavior of nano-Ta₄HfC_5p/SiBCN ceramics shows obvious difference due to the difference of composition and oxidation temperature.When the oxidation temperature is 1400 ^oC,the oxidation of nano-Ta₄HfC₅resulted in the formation of more holes in the oxide layer.The oxidation resistance of nano-Ta₄HfC_5p/SiBCN composite ceramics decreases with the increase of nano-Ta₄HfC₅content.When the temperature increases to 1650 ^oC,the oxidation product of nano-Ta₄HfC₅reacts with Si O₂to form tantalum-hafnium silicate glass,which improves the high-temperature stability of the oxide layer and inhibits the precipitation of cristobalite and the generation of cracks.The oxidation resistance of ceramics increases with the increase of nano-Ta₄HfC₅content.The oxide layer thickness of 10 wt%nano-Ta₄HfC_5p/SiBCN ceramics is only 14.5μm after oxidation at 1650 ^oC for 5h,which is much lower than 91.9μm of pure SiBCN ceramics.The main components at the interface between the substrate and oxide layer of nano-Ta₄HfC_5p/SiBCN ceramics are amorphous glass phase,incompletely oxidized ceramic substrate and Hf₆Ta₂O₁₇,etc.Because the oxidation resistance of BN（C）phase is the weakest,oxygen mainly diffuses into the ceramic substrate along BN（C）phase.The microstructure of nano-Ta₄HfC_5p/SiBCN ceramic oxide layer evolves with the increase of oxidation time.After oxidation at 1600 ^oC for 0.5 h,Ta and Hf elements in the oxide layer of 5 wt%nano-Ta₄HfC_5p/SiBCN ceramics are distributed in spherical particles and enriched at the grain boundary of cristobalite.When the oxidation time is 3h,Ta and Hf elements are distributed annularly along the grain boundary of cristobalite in the form of Hf₆Ta₂O₁₇and Ta₂O₅on the surface of the oxide layer.With the further extension of oxidation time,the content of Ta and Hf enriched at the grain boundary of cristobalite decreases gradually.The acetylene flame ablation experiment shows that the ablation resistance of nano-Ta₄HfC_5p/SiBCN ceramics increases with the increase of nano-Ta₄HfC₅content.The linear ablation rate of 15 wt%nano-Ta₄HfC_5p/SiBCN ceramics is 0.123 mm/s after the ablation process lasted for 10 s,which is lower than those of pure SiBCN ceramics by 30.1%.The main components of nano-Ta₄HfC_5p/SiBCN ceramic ablation layer are Ta₂O₅,Hf₆Ta₂O₁₇and tantalum hafnium silicate glass.The structure of Ta₂O₅is mainly interlaced rod-shaped grains,while that of Hf₆Ta₂O₁₇is mainly interconnected spherical grains.Ta₂O₅and Hf₆Ta₂O₁₇form a three-dimensional skeleton structure,and tantalum hafnium silicate glass fills the inside of the skeleton.This structure slows down the volatilization of tantalum hafnium silicate glass and strengthens the structural stability of ablation layer.