| Hafnium diboride(HfB2)is the most potential material in the ultra-high temperature ceramic family due to its advantages of high melting point,high strength and high hardness,good electrical/thermal conductivity and good thermal shock resistance.HfB2 is expected to be used as the surface thermal protection materials for hypersonic vehicles such as re-entry aircraft nose cones,wing leading edges,hypersonic aircraft bodies,nose cones and engine ports.However,due to its high melting point,strong covalent bond and low volume diffusion rate,the effect of grain boundary diffusion and volume diffusion during sintering is not obvious,which cause the poor sinterability of HfB2.In addition,the low toughness and poor oxidation resistance of HfB2 limit its application and development.In this paper,ZrC,Si and C powders were used as the sintering aids to enhance the densification and the mechanical properties of HfB2 ceramic.The in-situ reaction between ZrC-Si-C can occur at sintering temperature to form ZrSi2 and SiC.Then the dense HfB2-ZrSi2-SiC ceramic composites have been fabricated by reactive spark plasma sintering and reactive hot pressing,respectively.The composition phase and microscopic morphology of the sintered samples were analyzed by means of X-ray diffractometry(XRD)and scanning electron microscopy(SEM).The relative density,Vickers hardness,flexural strength and fracture toughness of the sintered samples were charactered.The oxidation resistance of sintered samples at high temperature were determined via high temperature static oxidation method.And the oxidation mechanism was explored.The thermal shock resistance of sintered samples was determined via water quenching method.And the mechanism of thermal shock resistance was revealed by combining crack and fracture observation.The results were showed as follows:Nearly full dense HfB2-ZrSi2-SiC ceramic composites have been fabricated by reactive spark plasma sintering at 1600? for 10 min under a pressure of 40 MPa.With the amount of ZrC-Si-C increasing,the Vickers hardness,flexural strength and fracture toughness of the sintered samples increased first,then decreased.HfB2-10wt.%ZrC-Si-C composites showed the best comprehensive mechanical properties.The Vickers hardness,flexural strength and fracture toughness of HfB2-10wt.%ZrC-Si-C composites were 26.80±1.2 GPa,504±40 MPa and 4.66±0.21 MPa·m1/2,respectively.HfB2-ZrSi2-SiC ceramic composites have been fabricated by reactive hot pressing at 1800?,1850?and 1900? for 120 min under a pressure of 30 MPa.With the amount of ZrC-Si-C increasing,the Vickers hardness,flexural strength and fracture toughness of the sintered samples increased first,then decreased.HfB2-30wt.%ZrC-Si-C composites fabricated at 1900? showed the best comprehensive mechanical properties.The relative density,Vickers hardness,flexural strength and fracture toughness of HfB2-30wt.%ZrC-Si-C composites were 96.40%,19.27±1.01 GPa,602±47 MPa and 6.18±0.12 MPa·m1/2,respectively.The oxidation resistance of HfB2-ZrSi2-SiC composites fabricated by reactive hot pressing sintering was determined via high temperature static oxidation method.The mass change and flexural strength of the samples after oxidation at different temperatures(1000?,1250? and 1500?)for 60 min were charactered.The surface and oxide cross section of the sample after oxidation were observed using SEM-EDS.With the oxidation temperature increasing,the mass gain of the sample after oxidation increased.HfB2-30wt.%ZrC-Si-C composites had the smallest oxidation weight gain and the highest residual strength after oxidation.The oxidation weight gain and the residual strength after oxidation at 1000? for 60 min were 0.1612 mg/cm2,790 MPa,respectively.It is shown that HfB2-30wt.%ZrC-Si-C composites have the most excellent oxidation resistance.The thermal shock resistance of HfB2-ZrSi2-SiC composites fabricated by reactive hot pressing sintering was determined via water quenching method.The thermal shock resistance mechanism was revealed by crack and fracture morphology.The thermal shock temperatures(300?,400?,500?,600? and 700?)were selected to determine the residual strength of samples after thermal shock at different temperature.The fracture morphology of the samples after thermal shock was analyzed via SEM.It is shown that HfB2-30wt.%ZrC-Si-C composites have the most excellent thermal shock resistance.Its critical thermal shock temperature difference was 515?. |
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