Effects Of Nano Powders On The Fabrication And Properties Of ZrB₂-based Ultra High Temperature Ceramics

Transition metal diborides based ultra-high temperature ceramics （UHTCs） arethought to be one kind novel structure material for use at extreme conditions due totheir refractory, good scouring resistance and superb ablation resistance, etc. In thisstudy, based on the low strength of ZrB₂-SiC-Graphite （ZSG） composites, thenano-SiC was added to ZrB₂-based composites that contanting graphite to producethe ZrB₂-SiC_np-G (ZS_npG) composite. The ZS_npG composite was prepaered byhot-pressed and achieved near full density. After that, the nano ZrB₂(ZrB_2np) wasintroduced as the base and sintered with nano SiC (SiC_np) to produce theZrB_2np-SiC_npcomposite. The microstructural features of the hot-pressed compositewere observed by scanning electron microscopy （SEM） with simultaneous chemicalanalysis by energy dispersive spectroscopy （EDS）. The phase composition wasdetermined by X-ray diffraction （XRD）.The dispersion and co-dispersion behavior of ZrB₂and SiC nanopowders inethanol solution was studied by sedimentation test, particle size measurement andtransmission electron microscope （TEM） analysis. The dispersion behavior of ZrB₂and SiC nanopowders in ethanol solution was strongly dependent on the pH values,types and amounts of dispersant. The well co-dispersed ZrB₂and SiCnanocomposite powders can be achieved by using1wt.%or more PEI below pH10.The effect of sintering temperature on microstructure and properties ofZrB₂-SiC_np-G has been studied. The results showed that when the sinteringtemperature is1800℃，the ZS_npG ceramic can not be fully dense and the relativedensity decreased with the increasing of graphite content. When the sinteringtemperature increased to1840℃, the ZS_npG ceramic achieved near full density andthe relative density did not changed with the changing of graphite content. Theflexural strength and fracture toughness of ZS_npG ceramic decreased with theincrease of sintering temperature before1880℃.The effect of graphite content on microstructure and properties ofZrB₂-SiC_np-G have been investigated. The results showed that when the sinteringtemperature was1880℃, the flexural strength and fracture toughness of ZS_npGceramic generally decreased as the graphite volume fractions increased, but therelative density values kept100%. The increase of graphite content can improve theability of thermal shock resistance and residual strength of ZS_npG ceramic. The ΔTcof ZS_npG210, ZS_npG220and ZS_npG230was about375℃,425℃and430℃,respectively. The residual strength of ZS_npG210, ZS_npG220and ZS_npG230was about 90MPa,120MPa and150MPa, respectively.The effect of graphite diameter on microstructure and properties ofZrB₂-SiC_np-G have been studied. The results showed that the ZS_npG ceramicsachieved near full density when the graphite diameter increased from5to10μm andthe further increasing of graphite diameter to20μm resulted in a reduction of therelative density （98.7%）. The flexural strength of ZS_npG ceramics firstly increasedand then decreased with increasing the diameter of graphite, whereas the fracturetoughness did not change significantly. The critical temperature differences ofZS_npG ceramics containing graphite with different diameters were almost the same,but the thermal shock behavior was different. The calculated thermal shockparameters confirmed that the graphite with finer starting diameter had beneficialeffects on the blocking of crack initiation and the graphite with larger diameter canrestrain crack propagation of ZS_npG ceramics.The strengthening and toughening mechanisms of ZrB₂-SiC_np-G ceramic havebeen discussed. The results showed that the higher rising R-curves exhibited thehigh resistance to crack growth and damage tolerance of ZS_npG than ZSG. Thereason for the high performance of ZS_npG ceramic can be summarized as thecombination of the strengthening and toughening mechanism of intragranular andlayer structure. The formation of the intragranular nano-SiC will causetransgranular fracture, which caused the high flexural strength of ZS_npG. Theimprovement in toughness was resulted from the multiple toughening mechanismssuch as crack deflection, crack bridging and relaxation type absorption of crack tipowing to the addition of graphite.The ablation behavior of ZrB₂-SiC_np-G ceramic has been studied. The resultsshowed that the wind tunnel was used to evaluate the oxidation resistance and thethermal shock resistance of the ZrB₂-SiC-G composite. In the high-frequencyplasma wind tunnel experiments, after the ablation of425s at low state, the surfacetemperature of the specimen maintained at a temperature of about1700℃, the oxideon the surface of the specimen is mainly SiO2glass. After ablation the integrity ofthe sample is still good, no cracks taked place.The present study showed that the traditional single-step sintering was notsuitable for the preparation of nano ZrB₂ceramic. The controlling of nano ZrB₂grain growth can be achieved by multi-step sintering. The onset temperature ofdensification of nano ZrB₂was at about1300℃, which is much lower than microsized ZrB₂. The application of MSS2led to remarkable small grain sizes. Therelative density of nano ZrB₂sintered in the MSS2regime is about80%, while theflexural strength and fracture toughness were599.45MPa and4.1MPa·m1/2, respectively, which was resulted from grain refinement.The sintring of nano-ZrB₂powders has been studied. The results showed thatthe activation energy for sintering of nano-ZrB₂particle was studied by the mastersintering curve theory. The result showed that the activation energy for sintering ofnano-ZrB₂particle was863kJ/mol, which was much lower than that of micro-ZrB₂particle.The in situ observation of nano-ZrB₂sintering showed that the grain did notgrow under1000℃. When the sintering temperature increased to1100℃, the masstransfer begun, which followed the orientation of hexagonal-structure. The grainboundary became disappear and the neck format, when the temperature increased to1350℃. After dwell for500s, the grain boundary and the neck format was almostdisappeared.ZrB_2np-SiC_npceramic was prepared by the MSS2method. The flexural strength,fracture toughness and relative density of ZrB_2np-SiC_npceramic was521.20MPa,3.92MPa·m1/2and80％, respectively. The performance of ZrB_2np-SiC_npceramicneed be further improved by the improvement of sintering method and theapplication of third phase.