Preparation And Performance Studies Of Laminated ZrB₂-SiC/SiC Composites

ZrB₂-based ceramics,as one kind of ultra-high temperature ceramics（UHTCs）,has attracted more attentions because of their low density,high melting point and excellent thermal shock resistance.However,the inherent brittleness of ceramic materials limits their wide applications.In order to improve the fracture toughness,laminated ZrB₂-Si C/SiC composites were successfully fabricated by tape-casting,stacking and hot-press sintering with ZrB₂ and SiC powders as raw materials in this paper.The final sintering was performed at 1900°C under a uniaxial pressure of 30 MPa for 1 hour.The microstructures and mechanical properties in different directions were investigated.The oxidation resistance,high temperature mechanical properties and thermal shock resistance were also analyzed.In this study,two kinds of laminated ZrB₂-SiC/SiC composites were prepared with different SiC content in the interface layers.The matrix layers and interface layers were arranged alternatively and the thickness of matrix layers was about 300μm.The thicknesses of interface layers of two kinds of composites were about 30μm and 50μm,respectively.The flexural strength in parallel direction of LZS1（laminated ZrB₂-SiC/SiC composites which the content of SiC in the interface layers was 80 vol.%）was 607 MPa and the fracture toughness was 12.5 MPa·m^1/2at room temperature.The flexural strength and fracture toughness in perpendicular direction of LZS1 were 574 MPa and 5.1MPa·m^1/2,respectively.The mechanical properties of LZS2（the content of SiC in the interface layers was 90 vol.%）is lower than LZS1 at room temperature because of the low density and interfacial bonding strength of LZS2 caused by the too high content of SiC in the interface layers.The improvement of fracture toughness in parallel direction was mainly attributed to the formation of residual stress caused by the mismatch of thermal expansion coefficients between matrix layers and interface layers.The residual stress could induce repeated crack deflection,increase the path of crack propagation and consume more fracture energy to improve the fracture toughness.The oxidation weight gain of laminated ZrB₂-SiC/SiC composites increased with the increase of oxidation time at 1500°C.When the oxidation time was 10 h,the mass gain achieved to 10 mg/cm².The increased rate of oxidation weight gain decreased gradually with the increase of oxidation time because of the formation of dense SiO₂ layer on the surface,which could prohibit the oxygen from entering the interior of the material.The flexural strength of laminated ZrB₂-SiC/SiC composites decreased with the increase of oxidation time.The strength and toughness were 426 MPa and 5.4 MPa·m¹/2/2 after oxidation at 1500°C for 10 h,respectively.The reduction of mechanical properties was mainly attributed to the serious oxidation and the formation of porous and loose ZrO₂layer on the sub-surface of material.The mechanical properties under high temperature of laminated ZrB₂-SiC/SiC composites decreased with the increase of testing temperature.Compared to the strength at room temperature,the flexural strengths at 1500°C in parallel and perpendicular direction were 314 MPa and 330 MPa,which decreased 48.3%and 42.5%,respectively.The decrease of bending strength was mainly due to the increase of the content of glass phase produced by oxidation.The results of thermal shock experiments showed that the thermal stress could form on the surface of samples when the material was subjected to thermal shock.The thermal stress could induce the formation of cracks on the surface of specimens which led to the decrease of bending strength.The critical thermal shock temperature difference of LZS1was 625°C,which was higher than that of 468°C of monolithic Zr B₂-SiC ceramics.The thermal shock resistance of laminated ZrB₂-SiC/SiC composites was better than monolithic ZrB₂-SiC ceramics.The improvement of thermal shock resistance was mainly due to the introduction of laminated structure,which improved the fracture toughness and the ability to resist the crack propagation.