Preparation And Characterization Of Fiber Monolithic ZrB₂ Ultra-high Temperature Ceramics

Because of its high melting point,high strength and excellent oxidation resistance,ZrB₂ceramics had attracted extensive attention in the fields of high-temperature structural ceramics,electrode materials and nuclear control materials.However,ZrB₂ceramics were prone to catastrophic damage during service due to intrinsic brittleness,so its toughness must be improved.Inspired by the bionic structure of bamboo,the design of fibrous monolith structure could greatly improve the fracture performance of ZrB₂ceramics,but the interface separation layer mostly adopted poor oxidation resistance weak interface,which reduced the high temperature performance of the material.In order to solve the problem of poor oxidation resistance of weak materials as the interface of ZrB₂cells,in this experiment,the fiber monolithic ZrB₂ceramics with double interface were prepared by extrusion-molding and hot-pressing sintering,and its mechanical properties,thermal properties,oxidation and ablation properties were studied.The preparation technology of fiber monolith ZrB₂ceramics was studied.First,the fiber monolith precursor with smooth surface and defect-free was prepared by extrusion process with PVB,PEG,DBP and solid content of 20 wt.%,10 wt.%,4.5 wt.%and 57.5%,respectively.Secondly,the optimal impregnation conditions of BN-based inner interface and ZrB₂-based outer fabric pulp were determined by sedimentation method:the solid contents were 15%and 20%,and the CMC contents were 2 wt.%and 4 wt.%,respectively.Finally,fiber monolith ZrB₂/BN/ZrB₂ceramics with double-interface and fiber monolith ZrB₂/BN ceramics with single-interface were prepared by controlling the number of interface layers,which were labeled as FZD and FZS ceramics,respectively.The microstructure and mechanical properties of fiber monolithic ZrB₂ceramics were studied.The thicknesses of ZrB₂cell body,BN interface and ZrB₂interface in double-interface FZD ceramics were 150μm,10μm and 10μm,respectively.The flexural strength and fracture toughness of double-interface FZD ceramics were 250 MPa and 5.2MPa·m^1/2,respectively.Compared with single-interface FZS ceramics,the flexural strength was increased by 14.7%,but the fracture toughness was decreased by 23.5%,due to the low content of BN weak phase in FZD ceramics,which promoted the strength of the material,but the toughness of the material decreased due to the reduction of crack deflection toughening mechanism.The thermal shock resistance of double-interface FZD and single-interface FZS ceramics were evaluated by water quenching method.The residual strengths of FZD and FZS ceramics decreased with the increase of thermal shock temperature difference,and the critical thermal shock temperature difference of them were 652℃ and 644℃,respectively.The excellent thermal shock resistance of FZD ceramics was attributed to the higher thermal conductivity and lower thermal expansion coefficient of FZD ceramics,which reduced thermal stress gradients.The finite element simulation showed that the thermal stress gradient of FZD ceramics was 43.84 MPa/mm at 800℃ thermal shock,which was13.8%lower than that of FZS ceramic（50.88 MPa/mm）.The ablation properties of double-interface FZD and single-interface FZS ceramics were evaluated by plasma flame with a flow rate of 8.2 MW/m².After ablation for 120 s,both FZD and FZS ceramics exhibited excellent non-ablative properties,with mass ablation rates of-0.26 mg/s and-0.17 mg/s,respectively,and linear ablation rates of-0.17μm/s and-0.42μm/s,respectively.In addition,the structure of FZD ceramics remained intact,while the delamination phenomenon appeared in FZS ceramics.The excellent ablation performance of FZD ceramics was attributed to the excellent thermal conductivity of FZD ceramics,which reduced the surface temperature and the introduction of double interfaces reduced the oxidation and volatilization of BN,thereby slowing down the degree of material ablation.