Performance Studies Of Biomimetic Laminated?ZrB₂-SiC?/BN Ultra-high Temperature Ceramics

Modern aircraft are developing towards higher-speed,higher-altitude,higher-thrust,longer-distance and safer,so the demand for ultra high temperature materials has become more and more urgent.Many countries in the world started concentrating on the reaserch of ultra-high temperature ceramics,especially the ZrB2-based ultra-high temperature ceramics,from the end of 20th century,but its toughness has always restricted its wide application.A biomimetic laminated structure design was adopted in this paper to improve the fracture toughness.Laminated(ZrB2-SiC)/BN ultra-high temperature ceramics with ZrB2-SiC as matrix layers and BN as interface layers were successfully fabricated by hot-pressing at 1950? under 30MPa for 60minites with ZrB2,SiC and h-BN powders as raw materials.The microstructure,mechanical properties,thermal shock resistance,oxidation resistance were systematically investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM),three-poing bending test,etc.In addition,the toughening mechanism,the thermal shock resistance mechanism and the oxidation resistance mechanism were revealed.The average thicknesses of matrix layers and interface layers of laminated(ZrB2-SiC)/BN ceramics were about 270?m and 22?m,respectively,and the composition was accurately controlled to ZrB2-19vol%SiC-7.5vol%BN.Bulk density of the(ZrB2-SiC)/BN ceramic was 4.89g/cm3,and relative density was 92.96%.The mechanical property of the composites was anisotropic between the directions perpendicular and parallel to the interface.In the two directions,Vickers hardness were 18.87GPa and 14.02GPa,respectively,and both were less than 22.58GPa for ZrB2-SiC ceramics;Rockwell hardness were 82.88 and 78.83,repectively,and both were less than 92.63 for ZrB2-SiC ceramics;flexural strength were 427MPa and 325MPa repectively,and both were less than 494MPa for for ZrB2-SiC ceramics;fracture toughness were 15.5MPa·m1/2(about 3 times to 4.8MPa·m1/2 for ZrB2-SiC ceramics)and 4.5MPa·m1/2.Compared with ZrB2-SiC ceramics,hardness and flexural strength of laminated(ZrB2-SiC)/BN ceramics reduced a little,but the fracture toughness increased a lot.Toughening mechanism mainly were the following two points:a)the weak interface layer relaxed the stress at crack tip,and weakened the crack growth driving force;b)the deflection and branch of the crack and the new microcracks,which would increase the propagation path and fracture work.Thermal shock resistance test showed that the retained strength of laminated(ZrB2-SiC)/BN ceramic decreased as thermal shock temperature difference increased,and the critical thermal shock temperature difference ?Tc was 364?,which was reduced by about 100?,compared with ZrB2-SiC ceramics.Mechanism of thermal shock analysis showed that due to the big differences on thermal conductivity and thermal expansion coefficient between matrix layers and interface layers,the critical temperature difference of laminated(ZrB2-SiC)/BN ceramics decreased,but the thermal stress damage resistance increased notably.Oxidation test showed us that thickness of oxidation layer and oxidation mass gain per unit area were positively related to the oxidation temperature and oxidation time.When oxidation at 1500?,mass gain curve of laminated(ZrB2-SiC)/BN ceramics was very similar to ZrB2-SiC ceramics,and flexural strength of laminated(ZrB2-SiC)/BN ceramics increased to 613MPa,and increased by 43.6%compared with room temperature strength,after a long time(10h)oxidation.The surface and the area in the matrix layers closed to the BN layers oxidated more seriously in laminated specimen,and the oxidation layer can be devided into three parts:(1)a SiO2-rich glassy layer;(2)a thin ZrO2-SiO2 layer;(3)a SiC-depleted layer.Mechanism of oxidation analysis showed that laminated(ZrB2-SiC)/BN ceramics can be oxidated at room temperature theoretically,but extremely weak;when oxidated at high temperature,temperature and time were the main influencing factors.At the initial stage,oxidation was mainly controlled by interface reaction,and then jointly controlled by diffusion and interface reaction when the glassy layer covering the specimen surface reached a certain thickness,and in this stage,because of lower oxygen partial pressure needed in the oxidation reaction of SiC than BN,oxidation in matrix layers was much more serious than that in interface layers.