Study On Processing And Properties Of ZrB₂-based Ultra-high Temperature Ceramics Fabricated By Reactive Sintered

Zirconium diboride (ZrB2) has high melting point, high hardness, high strength, good electric and heat conductivity, and excellent corrosion resistance, which is one of the most potential ultra high temperature ceramics in the aviation and aerospace fields. However, due to its high melting point, strong covalent bonding and low-volume and grain boundary diffusion rates characteristics, densification of ZrB2is extremely difficult and often needs a very high temperature and pressure. Moreover, the poor toughness and poor high temperature oxidation resistance also limits its applications.In this paper, ZrB2, ZrC and Si powder were selected as raw materials to fabricate ZrB2-SiC-ZrSi2composites. ZrB2-SiC-ZrSi2composites have been prepared by the reactive spark plasma sintering and reactive hot pressing, respectively. SiC and ZrSi2were formed by reaction of ZrC and Si, which would improve both sinterability and mechanical properties of the ZrB2ceramics. XRD and SEM were used to characterize the phase and microstructure of the ZrB2-SiC-ZrSi2composites, respectively. Relative density, microhardness, bending strength and fracture toughness of the ZrB2-SiC-ZrSi2composites were characterized. The oxidation resistance of the ZrB2-SiC-ZrSi2composites has been studied using high temperature static oxidation test. Moreover, oxidation resistant mechanism of the ZrB2-SiC-ZrSi2composites was discussed.The nearly fully dense ZrB2-SiC-ZrSi2composites have successfully prepared by the reactive spark plasma sintering at1500℃for10min under a pressure of40MPa. With the amount of SiC and ZrSi2increasing, the relative density and mechanical property of the ZrB2-SiC-ZrSi2composites firstly increased and then descend. The relative density of the ZrB2-30wt.%(SiC-ZrSi2) composites reached to100%. The hardness, bending strength and fracture toughness of the ZrB2-30wt.%(SiC-ZrSi2) composites increased to19.1±2.3GPa,471±15MPa and7.33±0.24MPa-m1/2, respectively. The mechanical property of the ZrB2-SiC-ZrSi2composites decreased when the amount of SiC and ZrSi2exceeded30wt.%due to the heterogeneous phase distribution of material caused by agglomeration of SiC and ZrSi2.Nearly fully dense ZrB2-SiC-ZrSi2composites were fabricated by reactive hot pressing (RHP) at1600℃for1h under a pressure of20MPa. Both relative density and mechanical property of the ZrB2-SiC-ZrSi2composites firstly increased and then decreased with the amount of SiC and ZrSi2increasing. The bending strength and fracture toughness of the ZrB2-30wt.%(SiC-ZrSi2) composites reached a maximum of279±32MPa and5.22±0.18MPa-m1/2, respectively.The oxidation resistance of the ZrB2-SiC-ZrSi2composites has been studied at1200℃,1300℃,1400℃and1500℃in air for30min using high temperature static oxidation test. After oxided, the mass gain of specimens declined significantly with the amount of SiC and ZrSi2increasing. When the amount of SiC and ZrSi2increased to30wt.%, the mass gain of specimens reached a minimum. The thickness of SiO2-glass phase increased with the amount of SiC and ZrSi2increasing during high temperature oxidization of ZrB2-SiC-ZrSi2composites, which has effectively prevented the diffusion of oxygen element and improved the oxidation resistance of the ZrB2-SiC-ZrSi2composites.