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Two hafnium diboride based ceramic matrix composites containing 20％ (volume fraction) SiC particle and with or without AlN as sintering additives were fabricated by hot-pressed sintering.The mechanical properties and microstructures of these two composites were tested and the thermal shock resistances were evaluated by plasma arc heater.The results indicate that the composite with AlN as sintering additive has a denser and finer microstructure than composite without sintering additive,and the mechanical properties,thermal shock resistance of the composite with AlN as sintering additive are also higher than those of the composite without AlN.Microstructure analysis on the cross-section of two composites after thermal shock tests indicates that a compact oxidation scale contains HfO2 and Al2O3 liquid phase is found on the surface of composite with AlN,which could fill the voids and cracks of surface and improve the thermal shock resistance of composite.