【作者】 侯杨；

【导师】 张幸红；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2013， 博士

【摘要】 过渡族金属硼化物基超高温陶瓷复合材料具有耐高温、耐烧蚀、抗冲刷等优良特性，被认为是适用于极端环境下使用的新型耐高温结构材料。本研究针对目前超高温陶瓷材料强韧化以及抗热冲击方面的缺点，首先以微米ZrB₂为基体，纳米SiC(SiC_np)为第二相，石墨（G）为第三相，利用热压烧结制备出致密的ZrB₂-SiC_np-G(ZS_npG)复合材料。之后将纳米ZrB₂(ZrB_2np)作为基体引入超高温陶瓷，与纳米SiC粉体混合后制备了ZrB_2np-SiC_np复合材料。利用X射线衍射（XRD）、扫描电镜（SEM）及能谱分析（EDS）等对添加纳米粉体的超高温陶瓷复合材料的组织结构和性能进行了研究。利用沉降实验、激光粒度测试和TEM等方法，研究了纳米ZrB₂和SiC粉体的分散性，并在单相粉体的分散体研究的基础上进一步研究了两种粉体的共分散性。实验发现纳米粉体的分散行为强烈依赖于溶液的pH值、分散剂的类型和用量。通过添加1wt.%或更多的PEI，同时调节pH值低于10，可以获得均匀分散的ZrB₂和SiC纳米复合粉体。考察了烧结温度对ZrB₂-SiC_np-G复相陶瓷材料组织性能的影响。结果表明，当烧结温度为1800℃时，材料无法完全致密，并且随着石墨含量的增加材料的致密度降低；在高于1840℃下烧结后的ZS_npG复相陶瓷的相对密度接近完全致密，并且由石墨含量不同导致致密度不同的现象也随之消失。当石墨含量相同时，随着烧结温度的升高，材料的弯曲强度逐渐上升，尤其是1880℃之前，上升较为明显，之后材料强度的增加相对缓慢，同时材料的韧性也会略有下降。研究了石墨含量对ZrB₂-SiC_np-G复相陶瓷材料组织性能的影响。结果表明，在相同的烧结温度下，随着石墨含量的增加，ZS_npG复合材料的弯曲强度和弹性模量逐渐降低，材料的断裂韧性也略有下降。随着石墨含量的增加导致了材料抗热冲击能力以及热冲击后的残余强度提高。含10vol.%石墨材料的临界热震温差ΔTc=370℃，含20vol.%石墨材料的ΔTc=420℃，含30vol.%石墨材料的ΔTc=435℃。含10vol.%石墨材料在经过热冲击后的残余强度约为90MPa，含20vol.%石墨材料的残余强度约为120MPa，而含30vol.%石墨材料的残余强度大于150MPa。探讨了石墨粒径对ZrB₂-SiC_np-G复相陶瓷材料组织性能的影响。结果表明，在相同的烧结工艺和材料组分条件下，对于ZrB₂-20vol.%SiC_np-20vol.%G复相陶瓷材料，当添加的石墨直径由5μm增加到10μm时，材料的弯曲强度随之增大。但当石墨直径增加到20μm时，材料的强度却发生明显降低。在石墨粒径增加的过程中，材料的断裂韧性却没有发生十分明显的变化。当石墨直径为5μm和10μm时，材料接近完全致密。但当石墨直径增加到20μm时，材料的致密度却略有降低至98.7%。随着石墨粒径的增加，各组分材料的临界热震温差基本相同（约425℃），但热冲击后的残余强度却不同。较小直径的石墨有助于提高陶瓷材料的强度和阻碍热冲击过程中裂纹的萌生（提高R值），而较大直径的石墨则会降低材料的强度能但会阻碍热冲击过程中裂纹的扩展，并保持较高的残余强度（提高R’’’’值）。考察了ZrB₂-SiC_np-G复相陶瓷材料的强韧化机理。ZS_npG复合陶瓷材料实现强韧化的主要原因在于内晶型结构和层状结构二者的强韧化机制的有效结合所致。纳米SiC形成内晶型结构会诱发穿晶断裂，使材料具有了较高的强度并降低了由于添加石墨而导致材料强度降低的负面效果；另一方面，材料韧性的提高主要是由于石墨引发了裂纹的偏转，而这种效应由于纳米SiC的加入而得到了增强，因为裂纹在穿晶后已经消耗了大量的能量，这样更有利于石墨对于裂纹偏转的诱导。ZS_npG复相陶瓷表现出很明显的R-曲线效应即是材料强韧化的有力体现。研究了ZrB₂-SiC_np-G复相陶瓷材料的抗烧蚀性能。结果表明，对ZrB₂-20vol.％SiC_np-20vol.％G组分材料的氧化烧蚀行为的实验结果表明高频等离子风洞烧蚀实验中，烧蚀425s后，试样表面温度维持在1700℃左右，接近零烧蚀率，烧蚀后仍保持良好的完整性，没有出现裂纹；同时，冷却后的试样表面的氧化层也没有发生脱落现象，这说明氧化层较薄并与试样基体有较好的结合强度，试样表面的氧化物主要是SiO2玻璃。对纳米ZrB₂陶瓷的烧结工艺过程进行了研究。结果表明，传统的单步烧结不适合纳米的ZrB₂陶瓷的制备。纳米的ZrB₂晶粒生长的控制，可以由多步骤的烧结实现。纳米ZrB₂的致密化起始温度是在约1300℃。纳米ZrB₂的相对密度在利用优化后的多步烧结工艺烧结后达到80％左右，而弯曲强度和断裂韧性分别为599.45MPa和4.17MPa·m^1/2。利用主烧结曲线理论对纳米ZrB₂粉体的烧结激活能进行研究，结果表明纳米ZrB₂粉体的烧结激活能为863kJ/mol，远远低于微米ZrB₂粉体。对纳米ZrB₂粉体烧结过程进行了原位观察。结果表明，在温度较低时，晶粒自身和晶粒之间在升温的过程中都没有出现明显的变化；纳米ZrB₂粉体的传质在1000℃左右开始进行，而且在烧结的过程中是按照ZrB₂的密排六方结构进行传质；当温度升高到1350℃左右的时候，两晶粒之间的晶界才出现要消失的迹象，而晶粒边缘接触处也有出现烧结颈的趋势；在此温度保温500s后，晶粒出现明显的烧结迹象，晶粒之间的晶界变得模糊。利用单相粉体烧结实验优化出的烧结工艺MSS2对复相粉体进行烧结，最终获得的ZrB_2np-SiC_np材料弯曲强度为521.20MPa，断裂韧性为3.92MPa·m^1/2，致密度为80％。ZrB_2np-SiC_np复相陶瓷的性能还有较大的提升空间，这不仅需要对制备工艺进行相应的优化，还需要对组分进行相应的调整，即添加第三项来改善ZrB_2np-SiC_np复相陶瓷的性能。更多还原

【Abstract】 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.更多还原