Preparation And Characterization Of Rod-shaped Si₃N₄ Toughened ZrB₂ Ultra High Temperature Ceramics

In ultra-high temperature ceramics represented by borides and carbides ofⅣB-ⅤB transition metals,ZrB₂based ceramics have become candidates materials for key parts of reentry vehicles,because of high melting point,low density and excellent high-temperature chemical stability.However,the low toughness and poor thermal shock resistance limit its application in aircraft.In this experiment,Si₃N₄fiber precursor were prepared by wet spinning extrusion method,and the rod-shaped Si₃N₄was obtained by impregnation and cutting.The forming process factors of Si₃N₄fiber precursor were studied.At the same time,the mechanism of mechanical properties,oxidation resistance and ablation resistance of rod-shaped Si₃N₄toughened ZrB₂ceramics was analyzed and discussed,and the synergistic improvement of toughening and oxidation and ablation resistance of ZrB₂ceramics was achieved.In the experiment,Si₃N₄fiber precursor was prepared by wet spinning and extrusion.rod-shaped Si₃N₄was obtained by impregnating and cutting the precursor.The results show that the Si₃N₄fiber precursor with smooth surface and no defects can be prepared when the content of binder and plasticizer conten is 15wt%and the solid content of extrusion slurry is 60%.The minimum diameter of rod-shaped Si₃N₄is about 320μm,and the length is 3 mm.The rod-shaped Si₃N₄is mixed into ZrB₂powder to obtain the composite green body by preloading.After degreasing at 550℃,the binder and plasticizer can be removed reached more than 90%,and the rod-shaped Si₃N₄structure remains intact after degreasing.In the experiment,different content rod-shaped Si₃N₄toughened ZrB₂based ceramics were prepared by hot pressing,which were named as ZSN1（4wt%rod-shaped Si₃N₄）and ZSN2（8wt%rod-shaped Si₃N₄）.The sintering temperature was 1900℃and the pressure was 30 MPa.At room temperature,the fracture toughness of ZSN1 and ZSN2 was 5.6MPa·m^1/2and 5.1 MPa·m^1/2,respectively,which are 19.1%and 8.5%higher than that of monolithic ZS ceramics(4.7 MPa·m^1/2).The main reason for the improved toughness is that rod-shaped Si₃N₄promotes crack deflection and bifurcation during material fracture,which greatly increases the consumption of fracture work.The oxidation behavior of rod-shaped Si₃N₄toughened ZrB₂ceramics was studied at1300℃for different times.The mechanical properties of rod-shaped Si₃N₄toughened ZrB₂ceramics were improved after being oxidized at 1300℃for 5h.The flexural strengths and fracture toughness of ZSN1 were 488 MPa and 7.01 MPa,which were22.4%and 25.2%higher than that at room temperature,respectively.However,the strengths and toughness of ZS were 611 MPa·m^1/2and 5.06 MPa·m^1/2,respectively,which were only increased 3.5%and 7.6%.The improvement of toughness of ZSN1 is greater than that of ZS,because more Si O₂glass phase infiltrates in ZSN1.The Si O₂glass phase can repair the defects of the material and reduce the number and size of the defects in the material.After the rod-shaped Si₃N₄toughened ZrB₂ceramics were ablated by air discharge plasma for 120s,ZSN1 ceramics showed better ablation resistance.The mass ablation rates of ZSN1 and ZS were-0.19 mg/s and-0.26 mg/s,and the linear ablation rates were-0.25μm/s and-1.25μm/s,respectively.This indicated that both ZSN1 and ZS exhibit non ablative properties in the ablation process.After ablation,ZSN1 was complete and free of defects,while ZS cracked into two halves.The reason for the more excellent ablation resistance of ZSN1 ceramics is the synergistic effect of rod-shaped Si₃N₄reducing thermal stress in the material and promoting the crack deflection in the material.