Studies Of Laminated ZrB₂-SiC Composites With Si₃N₄ As Strong Interface

In recent years,with the rapid development of the aerospace industry,ZrB₂-based ultra-high temperature materials have been highly valued by developed countries due to their advantages such as high melting point,low density,high thermal conductivity and high electrical conductivity.However,the low toughness,low thermal shock resistance and low oxidation resistance of ZrB₂ based ceramic materials have been the key factors which limit their wide application.In this study,the biomimetic laminated idea was applied to ZrB₂-based ultrahigh-temperature materials.The laminated ZrB₂-SiC composites were prepared by tape-casting,stacking and hot-pressing.The modern analysis methods and testing were used to study the layered composites.Physical properties,mechanical properties,thermal shock resistance and oxidation resistance were studied.In this experiment,laminated ZrB₂-SiC composites with SiC particles and Si₃N₄powder as interface components were prepared.The thicknesses of the substrates and interface layer of the laminated ZrB₂-SiC-Si₃N₄ material were controlled to 300?m and30?m,and the relative densities were over 90%.The matrix layers and interface layers were arranged alternately without any faults.Through the study of the mechanical properties of the interfacial layer of the laminated material,it is found that the directionality of the layer structure has little effect on the bending strength of the material.The strength of the laminated ZrB₂-SiC composites is similar to that of the monolithic ZrB₂-SiC ceramics;But the fracture toughness in the parallel direction is as high as 15.3MPa·m^1/2,which is 2.1 times and 3.2 times than that of the laminated ZrB₂-SiC material,monolithic ZrB₂-SiC ceramics,respectively.The toughening mechanism is mainly the mismatch of thermal expansion coefficients between the layers.The large compressive stress causes the crack to deflect and bifurcate at the interface layer,and the effective propagation path of crack increases significantly,which increases the work of fracture.The bending strength of laminated ZrB₂-SiC-Si₃N₄ materials and laminated ZrB₂-SiC materials at high temperature first decreases and then increases with the increase of temperature.The strength of the laminated ZrB₂-SiC-Si₃N₄ sample at 1500?is 360 MPa,which is 27%higher than that at 1400?.This mainly due to the formation of more glass phase which inprove the creep resistance of the material at 1500?.The protective film weakens the interior oxidation of the material and maintains the strength.The thermal shock resistance of laminated ZrB₂-SiC-Si₃N₄ materials and monolithic ZrB₂-SiC ceramics was tested by water quenching.The critical temperature differences were 572?and 521?,respectively.The strength of the laminated materials after thermal shock was higher than that of the monolithic ZrB₂-SiC ceramics.When the temperature difference is 1000?,the monolithic ZrB₂-SiC sample is directly fractured,while the laminated material is intact and the bending strength is 148 MPa.The improvement of thermal shock resistance is mainly attributed to the introduction of the layered structure,which disperses the thermal stress generated by the thermal shock inside the material,weakenings the stress concentration,and improving the ability of the material to resist crack propagation.After being oxidized at 1300?for 10 h,the laminated ZrB₂-SiC-Si₃N₄ material has an oxidation weight gain of 2.36 mg/cm² and a bending strength of 430 MPa,which is92%of that at room temperature.The retention of strength is mainly attributed to the healing effects to cracks and isolation to air of the glass.The sharp decrease of the fracture toughness of the laminated material after oxidation is mainly due to the oxidative destruction of the interfacial layer structure of the material and the weakening mechanism of the layered structure.The toughness is 5.3 MPa·m^1/2after 10 h oxidation,which is still higher than that of monolithic ZrB₂-SiC ceramics at room temperature.It can be seen that although the layered structure is destroyed by oxidation at high temperature,it still exerts the toughening effect of the layered structure.