【作者】 任帅；

【导师】 孙康宁；

【作者基本信息】 山东大学 ， 材料学， 2007， 博士

【摘要】 碳化钛具有高的熔点、优异的高温强度、热稳定性,同时还具有高的弹性模量、高的硬度和好的耐磨性,已被广泛应用于刀具、模具等金属陶瓷材料领域。金属陶瓷中粘结相对于金属陶瓷的整体性能有重要的影响,TiC基金属陶瓷中的粘结相一般采用金属Co和Ni,其熔点和高温强度一般较低、高温抗氧化性较差,这导致硬质合金的高温性能和抗氧化性能较差,从而限制了其应用,特别是高温应用。具有较高熔点、较好高温强度的金属间化合物的出现,为TiC基金属陶瓷提供了新的粘结剂。其中,Ti-Al系金属间化合物具有密度低,高温强度高,抗蠕变和抗氧化能力强等优点,成为研究的热点。ZrO₂熔点高,莫氏硬度8～9,化学稳定性好,且其相变时伴有体积效应和剪切应变,常利用其相变特性作为陶瓷材料的增强相。基于以上分析,本文以TiC为基体设计和制备了一种新型的TiC/（Ti-Al+ZrO₂）多元复合材料,对复合材料的成份优化、制备工艺、力学性能、烧结机理、微观结构及增韧机制进行了系统研究。主要研究内容如下;通过将Ti/Al混合粉体进行高能球磨,对其机械合金化进行了理论和实验研究,并进行热处理使其转化为金属间化合物。通过对行星式高能球磨机进行运动学分析更深入认识了球磨过程;进一步通过XRD和TEM对球磨粉体的结构、形貌进行了检测和观察,结果发现;Ti/Al粉体在高能球磨过程中结构、形貌发生很大变化,相继出现过饱和固溶体、非晶、金属间化合物等物相;球磨10 h的粉体经过600℃真空热处理30 min后将完全转变为Ti₃Al和金属间化合物。通过一系列实验对复合材料中Ti-Al金属间化合物的添加方式、ZrO₂的用量和烧结工艺进行研究,根据烧结体性能,最终确定Ti-Al金属间化合物的添加方式为;球磨Ti-Al粉体不经热处理直接与TiC、ZrO₂粉混合,在随后的烧结过程中于600℃保温30 min转变为Ti-Al金属间化合物;ZrO₂的最佳用量确定为15wt%（质量分数,下同）;真空热压烧结工艺最终确定为烧结温度1550℃,于600℃、1100℃和1550℃分别保温30 min,升温速度为20℃/min,压力为30 MPa。分别使用不同含量的Ti₃Al和TiAl作为粘结剂加入复合材料中进行烧结,通过烧结体力学性能的比较,确定最适合的粘结剂及其用量。结果表明;随着金属间化合物用量的增加,烧结体趋于致密化,性能相应提高,但当金属间化合物含量超过一定量时性能反而下降;Ti₃Al最佳用量为20 wt%,而TiM为25wt%。综合性能最佳的复合材料成分为TiAl 25 wt%,ZrO₂ 15 wt%,TiC 60 wt%,抗弯强度653.5MPa,断裂韧性7.11 MPa·m^1/2,硬度HRA 92.0。为了确定ZrO₂在复合材料中所发挥的作用,本文还制备了一系列的TiC/（Ti₃Al+Al₂O₃）进行对比研究。虽然其具有较好的抗弯强度,但内部存在大量气孔和层状结构,同时缺少ZrO₂的增韧作用,断裂韧性为6.12 MPa·m^1/2,低于TiC/（Ti-Al+ZrO₂）复合材料。对复合材料烧结过程中存在的机制进行分析,详细讨论了液相烧结、热压烧结和真空烧结机制对TiC/（Ti-Al+ZrO₂）复合材料烧结的影响。然后分别烧结不含ZrO₂和Ti-Al金属间化合物的试样,通过断口形貌SEM图片观察其烧结情况,结果发现不含TiAl的试样中,虽然有些TiC颗粒长大,但总体未烧结致密;而含有TiAl的试样已经致密化,颗粒也比较细小均匀,得到了较好的烧结,充分说明Ti-Al金属间化合物在烧结中起到了粘结相的作用使烧结得以顺利进行。对成分为TiM 25 wt%,ZrO₂ 15 wt%,TiC 60 wt%的复合材料进行了不同时间（5,15,30,60 min）的烧结,结合其微观结构和力学性能对烧结过程进行分析。结果发现复合材料的烧结过程与硬质合金的烧结相似,液相烧结过程是烧结中的主要过程,烧结过程中TiC与Ti-Al液相优先形成大的聚集体。两次颗粒重排,尤其是二次重排对烧结起到了较大的促进作用,15—30 min聚集的颗粒被打碎,从而使复合材料获得了细化结构。并接下来通过溶解—析出和固相烧结实现了颗粒的合理分布和结合,获得较好的结构和性能。通过对不同Ti-Al含量复合材料的力学性能及断口形貌的分析,确认了液相的量对烧结起决定性作用,Ti₃Al用量需要20 wt%,而TiAl用量需要25 wt%才能获得足够的液相使烧结顺利进行,获得烧结致密、性能较好的材料,而过多的液相反而会使烧结恶化。且Ti在烧结中起到的作用比Al的作用大,在混合熔液中的含量比较重要。液相的粘度与数量随温度的变化而剧烈变化,所以温度的控制是液相烧结的重要因素。当复合材料的烧结温度进一步提高至1600℃时,烧结中出现过烧现象,晶粒异常长大且发生流渗,本来应该存在于TiC颗粒周围的Ti-Al流失,使烧结体内部出现大量孔洞,只剩TiC颗粒直接的少量的脆弱结合,结构和性能都变差。为了进一步了解TiC/（Ti-Al+ZrO₂）复合材料的烧结规律并改善材料性能,以及改善TiC的脱碳情况,向复合材料中添加少量对烧结有较大影响的C元素,对复合材料烧结的变化和性能的改变进行分析。结果表明,通过少量C的添加,使复合材料的液相烧结发生重大变化;少量的C（1 wt%）使液相粘度增加,导致二次重排和溶解—析出被抑制;添加2 wt%时,由于Ti、Al混合液产生的毛细管力足够破碎大TiC颗粒促进了烧结体的晶粒细化;而随C含量继续增加（5 wt%）由于放热反应而使烧结体过烧。复合材料的性能与烧结情况相对应,当C含量为2 wt%时,烧结合理进行且TiC与金属间化合物结合紧密,材料的性能最佳,抗弯强度712.6 MPa,韧性9.11 MPa·m^1/2,超过了未添加C的复合材料。通过SEM和TEM对TiC/（Ti-Al+ZrO₂）复合材料的微观结构进行了观察,对相的分布及作用进行讨论并进一步揭示其中的增韧补强机制。发现添加Ti-Al金属间化合物和ZrO₂对复合材料的微观组织结构有很大的影响,TiAl较为均匀地分布TiC颗粒间,而细小的ZrO₂则分布在TiC、TiAl晶间和晶内,有利于性能的提高。Ti-Al金属间化合物的增韧补强作用主要表现在;烧结过程中通过二次重排和溶解—析出使晶粒细化均匀,减小了原始裂纹尺寸;塑性比TiC好,断裂时能够起到桥联作用阻碍裂纹扩展;此外,Ti-Al存在于TiC晶问,改善了TiC之间的脆性结合,提高了材料的抗冲击能力。ZrO₂的强韧化作用是;一部分ZrO₂颗粒被固定在晶界形成晶间型结构,在烧结过程中不断粗化;一部分得到细化的纳米颗粒被其它生长晶粒包裹形成晶内型结构,利用其相变增韧机制,在晶问、晶内充分发挥作用,阻碍裂纹扩展,起到了一定程度的协同增韧作用。C的添加一方面可以调整基体TiC中C/Ti原子比,保持其颗粒的性能;另一方面通过对烧结的影响,使复合材料中的各相结合增强,有利于复合材料性能的提高。总之,本论文较系统地研究了TiC/（Ti-Al+ZrO₂）复合材料设计与制备工艺,以及烧结机理、微观结构及性能的改善方法,尤其在添加2 wt%C后,获得了抗弯强度和断裂韧性分别为712.6 MPa,9.11 MPa·m^1/2的复合材料,为该材料的发展和应用提供了一定的实验和理论基础。更多还原

【Abstract】 Titanium carbide has been extensively applied in cutting tools,die tools owing to their high melting point and Young’s modulus,excellent high-temperature strength, thermal stability,hardness and wear-resisting property.But the metals,Co and Ni, which are used as binders in cemented carbide have lower melting points and thermal strength.The application,especially the high temperature application of TiC is greatly limited.With the invention of intermetallics which have high melting points and excellent thermal strength,cermets with TiC matrix got a new kind of binder.Ti-Al intermetallics exhibit some excellent properties such as low density,high specific strength and high melting temperature and environmental resistance at elevated temperature,which makes it a suitable binder.ZrO₂ has high melting point and good chemical stability and its Mohs hardness is 8-9.As it’s phase transformations occur, there are volume effect and shearing strain,which can toughen many composites. According to the philosophy mentioned above,the application of Ti-Al intermetallics and ZrO₂ as reinforcement in TiC matrix with the aim of preparing one new TiC/（Ti-Al+ZrO₂）composite is conducted.We systematically study the optimization of components,preparation process,mechanical properties,microstructure and strengthening mechanism of this composite.The mechanical alloying of Ti/Al powder mixture was studied firstly.According to the analyzing of planetary high-energy ball mill’s kinematics,we preliminary learned about the deforming of powder in ball-milling process.Then the milled powder is heat-treated after its microstructure and morphology are detected and observed.The results showed that structure and morphology of milled Ti/Al powder mixture will greatly change,and supersaturated solid solution,amorphous phases and intermetallics with lots of defects and stress will form in consequence during ball milling.The powder milled for 10 h will completely transform into intermetallics after 30 min heat treatment at 600℃in vacuum.The sintering process,adding type of Ti-Al and the content of ZrO₂ are studied by a series of experimental.At last,it was confirmed that the adding type of Ti-Al intermetallics is by directly blending the untreated milled powder with TiC and ZrO₂ powder,and then heating at 600℃for 30 min in sintering process.The optimal content of ZrO₂ is 15 wt%（mass fraction）,and the sintering progress have been established as that the sintering temperature is 1550℃,holding temperature at 600℃,1100℃and 1550℃for 30min,heating rate is 20℃/min,pressure is 30 MPa.Composite samples with different dosages of Ti₃Al or TiAl as binder were sintered.According to their mechanical properties,we can find out the best binder and its suitable dosage.The results showed that with the increasing of intermetallics, the sintered samples tend to have more compact structure and improved properties. But when the dosages of intermetallics exceed a certain extent the properties of composite will deteriorate,and the best dosage of Ti₃Al and TiAl is 20 wt%and 25 wt%respectively.As a whole,the best content of composite is 25 wt%TiAl,15 wt% ZrO₂ 60 wt%TiC with respective bending strength 653.5 MPa,fracture toughness 7.11 MPa.m^1/2and hardness HRA 92.0.A series of TiC/（Ti₃Al+Al₂O₃）composites have been prepared as contrast.Though they have good bending strength,there are many pores and lamellar structures in them,which made them have poorer fracture toughness,6.12 MPa.m^1/2.The sintering mechanisms in sintering process of composite were analyzed, which conclude liquid phase sintering,hot-pressing sintering and vacuum sintering mechanisms.Samples respectively without ZrO₂ and Ti-Al are prepared to study their effects on the sintering process.According to the SEM figures of fracture morphology, we can find out that the sample without TiAl have not densified only with very few grown grains,while the sample without ZrO₂ are quite compact with fine and uniform grains.So we concluded that Ti-Al intermetallics play an important role in sintering process. The composites with same content,TiAl 25 wt%,ZrO₂ 15 wt%,TiC 60 wt%,are sintered for different time（5,15,30,60 min）.According to their microstructures and mechanical properties,we can find out that sintering process of this kind of composites is similar to that of cemented carbide,in which the liquid phase sintering is the main process,and the second-arranged and dissolution-precipitation mechanism play an important role.TiC grains undergo particular transition from 15 to 30 rain.The sintering statuses of composites with different Ti-Al dosage are analyzed base on their properties and fracture morphologies.It was confirmed that the quantity of liquid play an most important role,and the corresponding best dosages of Ti₃Al and TiAl are 20 wt%and 25wt%respectively.And when the dosages increase sequentially,the sintering process will deteriorate.Further analyze showed that the effect of Ti liquid is more significant and its dosage is the key factor for sintering.When sintering temperature was increased to 1600℃,the composite overburned with abnormity grown grains and metal losing.Lots of porosities occurred around the grown TiC grains,which made the combination between TiC grains very friability and the structures,properties deteriorate.In order to ulteriorly know about the sintering rules of the composite,a small quantity of carbon was added.The varieties of sintering and properties showed that with the addition of carbon,the sintering process greatly changed with the second-arrange and dissolution-precipitation mechanism restrained.But when the dosage of carbon is 2 wt%,the mixture melt liquor of Ti and Al has a lower viscosity and large capillary force which may promote sintering and the refinement of TiC grains.With such content,the composite has quite excellent chemical properties,bending strength 712.6 MPa,fracture toughness 9.11 MPa.m^1/2,which are higher than the composites without carbon.According to SEM and TEM photos,the microstructures of TiC/（Ti-Al+ZrO₂） composites were observed,and the phase distribution is discussed to fred out the toughening mechanism in these composites.It was discovered that with the addition of Ti-Al and ZrO₂ the microstructures of composites will greatly change and become more compatible,which may improve the properties.The effects of Ti-Al intermetallics exhibit in followed aspect.First,Ti-Al promote sintering and refine the grains;secondly,it have better plasticity which may toughening composite by crack-bridging;in addition Ti-Al grains locate between TiC grains and strengthen their combination,which may improve the shock-resistance property and increase the possibility of transgranular fracture.Some refined ZrO₂ particles were restricted on grain boundaries to form intergranular structure,and other particles were incorporated by grown TiC or Ti-Al grains to form intragranular structure.In a word,by its phase transformation characteristic,ZrO₂ operate in grains or at grain boundaries to toughen the composites cooperating with Ti-Al.Based on the results mentioned above,the sintering mechanism,the improvement of microstructures and properties of TiC/（Ti-Al+ZrO₂）composites are investigated,which may provide an experimental and theoretical foundation for the practical application.The best bending strength and fracture toughness of TiC/（Ti-Al+ZrO₂）composites are improved to712.6 MPa and 9.11 MPa.m^1/2.更多还原