Study On The Preparation And Properties Of The Laminated HfB₂-SiC Based Ceramic Composites

HfB₂ ceramics have high melting point,high hardness,high thermal conductivity and good corrosion resistance.Currently,they are the candidate materials for high-temperature aerospace research.Pure HfB₂ powder is difficult to sintering densification,and the inherent brittleness limits their wide applications in high temperature extreme environments.Based on the inspiration of bionic structure,laminated HfB₂-SiC/SiC ceramics were successfully fabricated by tape-casting and hot-press sintering with HfB₂and SiC powders as matrix layer materials and rod-like SiC as the interface layer materials at 1900? under 30 MPa.The preparation,mechanical properties,thermal shock resistance,oxidation resistance and ablation resistance of laminated ceramics were also studied.The laminated cast sheets were prepared by tape casting.When the binder content of the matrix layer is 15 wt.%,the interface layer is 40 wt.%,and the stirring time is 12 h,the continuous and homogeneous sheets were obtained in adapt circumstance.The degreasing process was performed at 650? for 1 h with a degreasing rate was 0.2?/min.Two kinds of laminated HfB₂-SiC/SiC ceramics were prepared by controlling the distribution ratio of interface layer,which were denoted LHS1?interface:80 vol.%SiC?and LHS2?interface:90 vol.%SiC?.The flexural strength of LHS1 in the parallel and perpendicular directions were 413 MPa and 371 MPa,and the LHS2 is 385 MPa and 305MPa,respectively.The fracture toughness of LHS1 and LHS2 in the parallel direction is10.2 MPa·m¹/2/2 and 7.4 MPa·m^1/2,respectively.The high toughness in parallel direction is mainly due to the existence of the interface layer,which prompted crack deflection and branching,increased crack propagation path and consumed more fracture energy.The excessive content of SiC in the interface layer made the material was difficult to be fully densified,resulting in lower mechanical properties of LHS2 than LHS1.The residual strength decreased of the laminated LHS1 and monolithic HS with the change of the thermal shock temperature.The critical thermal shock temperature difference of LHS1?parallel direction:554?,perpendicular direction:526??is higher than that of HS ceramic?459??.After 1020? water quenching,the monolithic HS showed a penetrating fracture,while the laminated LHS1 has only micro-crack formation.The showed that the laminated structure has a better effect on the crack deflection,inhibited the formation of the penetrating crack and improved the thermal shock resistance of laminated LHS1 ceramics.The oxidation of laminated LHS1 and monolithic HS increased with the increase of oxidation time at 1300?.After oxidation for 10 h,the weight gains of LHS1 and HS were7.05 mg/cm² and 7.35 mg/cm²,respectively.The rate of oxidation decreased with the increase of oxidation time,which was mainly duo to the SiO₂ glass phase filled pores of the material and effectively inhibited oxygen from entering the interior of ceramic materials.After oxidation for 10 h,the flexural strength and fracture toughness of LHS1were both reduced.The temperature of the surface ablation zone of laminated LHS1 and monolithic HS was 2448? and 2285? after at 2000? for 200 s,and the high thermal conductivity results in lower surface temperature of monolith HS.The mass ablation rates of LHS1 and HS were 20 mg·s^-11 and 0.2 mg·s^-1,and the linear ablation rates were 0.90?m·s^-11 and 0.45?m·s^-1,respectively.The ablation rate of laminated ceramics is significantly higher than that of monolithic ceramics,which is consistent with the severe ablation morphology of laminated ceramics.