【作者】 荣守范；

【导师】 吉泽升；

【作者基本信息】 哈尔滨理工大学 ， 材料学， 2008， 博士

【摘要】 从陶瓷材料结构组成出发,针对影响晶粒形态的主要因素,通过显微结构的设计和制备新途径,促使增韧元在基体中原位生成。采用工业γ-Al₂O₃粉为主要原料,分别加入了玻璃粉Al₂O₃-CaO-SiO₂（简称CAS）、Nb₂O₅、MgO和3Y-TZP粉做添加剂,通过无压烧结制备出了具有柱状晶的复合增韧Al₂O₃陶瓷材料,利用三点弯曲等力学方法以及TEM、SEM、XRD、EDAX等检测手段,系统研究了制备工艺、化学组成、显微结构与力学性能之间的关系,同时研究了添加剂CAS、Nb₂O₅、MgO、和3Y-TZP对Al₂O₃陶瓷柱状晶的影响规律,在此基础上探索了柱状晶生长机理及复合增韧机制。通过原位生长工艺,采用CAS、Nb₂O₅、3Y-TZP作为多相复合添加剂,利用Nb₂O₅配合CAS,使陶瓷材料在烧结过程中促进了氧化铝晶粒异向生长的动力学条件,诱导了氧化铝晶粒在某些方向上优先生长成柱状晶,并建立了柱状晶生长模型。研究证明了液相烧结时其驱动力来源于液相两边晶界曲率不同所造成的化学位的差异,液相的流动和扩散使晶粒与晶粒粘结并促进柱状晶生长。柱状晶生长过程中,晶界的移动包括如下途径:1)气孔靠晶格扩散迁移;2)气孔靠表面扩散迁移;3)气孔靠物质的气相传递;4)气孔聚合靠晶格扩散;5)气孔聚合靠晶界扩散及杂质牵制的晶界移动等多项机理的同时作用。研究结果表明:纯Al₂O₃陶瓷形成等轴晶,是因为烧结过程中交叉生长的晶粒不断长大,互相紧密接触,制约了晶体结构各向异性,阻碍了晶粒的异向生长。CAS、CAS/Nb₂O₅等添加剂的加入,消除了束缚异向生长阻力,增加了晶粒间活度,使其按着各向异性的方式自由长大,当生长动力足够时就会成长为柱状晶。研究发现,当CAS加入量为0.6wt%时,能够得到晶粒发育完全的柱状晶,长径比达到8:1,相对密度97.2%。Nb₂O₅单独使用时,提高了材料的烧结性能,获得的组织主要为等轴晶,并有少量的板块状晶粒;Nb₂O₅配合CAS使用时,会有助于柱状晶的形成,长径比在8:1以上,材料的相对密度可达97.8%。烧结性能明显的优越于相同含量CAS粉单独作用的结果。MgO对溶质拖拽的作用,会影响柱状晶的生长发育,但有利于陶瓷烧结过程中的致密化。ZrO₂在晶界溶质成分扩散过程中,组分相对独立存在,因而在空间位置上对柱状晶生长发育会有一定影响。利用动力学原理,对烧结过程进行了研究,添加剂可以明显的降低Al₂O₃陶瓷烧结时扩散激活能,CAS/Nb₂O₅/3Y-TZP复合添加剂,比各自单独加入时效果更佳,CAS/Nb₂O₅与3Y-TZP配合使用时,相对减弱了第二相对Al₂O₃晶粒生长的干预,促进了Al₂O₃柱状晶的形成。显微组织结构及裂纹扩展研究表明,原位生长柱状晶Al₂O₃陶瓷由于没有复合界面影响,所以增韧效果好于外加纤维的复合增韧。多相复合Al₂O₃陶瓷的增韧效果并不是多相简单的叠加,柱状晶在起到裂纹偏转、分叉增韧作用的同时,还延长了裂纹扩展路径,使马氏体相变量增加,强化了相变增韧。制备出的柱状晶多相复合增韧Al₂O₃陶瓷,最高抗弯强度为570MPa,断裂韧性为7.4MPa·m^1/2。从裂纹扩展过程分析可以看出,除第二相Nb₂O₅引起裂纹偏转外,ZrO₂相变增韧和柱状晶引起的裂纹桥接和拔出、裂纹分叉等多种增韧机制之间相互协同作用,大幅度提高Al₂O₃陶瓷材料的力学性能,其效果大于各种增韧机理单独作用的总和。更多还原

【Abstract】 From the structures of ceramic materials and the main factors influencing the grain shapes, the toughening element was in-situ synthesized in Al₂O₃ ceramic by the design of microstructure and the new processing way. In-situ columnar crystal toughening Al₂O₃ composite ceramic materials were prepared by pressureless sintering by adding CaO-SiO₂-Al₂O₃ （CAS） glass powder, Nb₂O₅, MgO and 3Y-TZP powder in commercialγ-Al₂O₃ powder as additives, respectively, The relationship between the preparation technology, the chemical composition, the microstructure and the mechanical properties has been studied systematically by a three-point bending testing and other measurements, such as TEM, SEM, XRD and EDAS. Also, the effect of CAS, Nb₂O₅, MgO, and 3Y-TZP additives on the Al₂O₃ ceramic columnar crystals was studied. Further, the columnar crystal growth mechanism and the toughening mechanism were analyzed.In the in-situ preparation process of Al₂O₃ ceramic materials by using CAS, Nb₂O₅ as well as 3Y-TZP as multiphase composite additives, the using of Nb₂O₅ and CAS promoted the formation kinetic condition of abnormal growth of Al₂O₃ grain, and then induced the formation of columnar Al₂O₃, preferably along some gain direction. A grain growth model of the columnar grain was discussed.The results have shown that the driving force in the liquid phase sintering came from the difference in the chemical potential caused by the grain boundary curvature. The flow of liquid phase and diffusion made some grains contact with other grains, which promoted the growth of the columnar grain. During the growth of the columnar grain, the movement of grain boundary may include followings: 1) the migration of hole by lattice diffusion;2) the migration of hole by surface diffusion;3) the transmission of hole by gaseous substance ;4) the aggregation of hole by lattice diffusion;5) the aggregation of hole by lattice diffusion and grain boundary movement caused by inclusion.The results have also shown that the pure Al₂O₃ ceramic formed equiaxed crystal due to the fact that the continue growth of Al₂O₃ in a resisted the inhomogenity of crystal structure, prohibited the grain growth in an abnormal growth model. The addition of CAS and CAS/Nb₂O₅ eliminated the resistance to the abnormal growth and increased the activity of grains, thus, the grains can freely grow in an anisotropy. It was found that the grains grew to a columnar crystal as the addition content of CAS was 0.6 wt%. The aspect ratio was 8:1, and the relative density was 97.2%.The additive of Nb₂O₅ can improve the sintering properties of material, and result in the formation of equiaxed grains with small amount of plate-like grains; The additive of Nb₂O₅ combined with CAS was helpful to the formation of columnar crystals with an aspect ratio of over 8:1. The relative density of the ceramic materials was up to 97.8%. The sintering property of the ceramic materials with Nb₂O₅ or Nb₂O₅/ CAS was obviously superior to that of the ceramic materials with only CAS additive. MgO showed a solute-drag function, which affected the growth of columnar crystals, but was beneficial to the densification of the ceramic in the sintering process. ZrO₂ has no effect on the solute diffusion by the grain boundary, but has a certain impact on the growth and development of columnar crystals in spatial position.The sintering process was investigated according to dynamic theory. The additives can significantly reduce the diffusion activation energy of Al₂O₃ ceramic. The additive of mixed CAS/Nb₂O₅/3Y-TZP has better function that any single additive. The using of mixed CAS, Nb₂O₅ and 3Y-TZP can relatively decrease the influence of second phase on the growth of Al₂O₃ grains and promote the formation of Al₂O₃ columnar crystals.Studies on microstructure and crack propagation showed that toughening efficiency of the columnar Al₂O₃ grain was much better than that of fiber due to that no interface existed in in-situ columnar Al₂O₃ grain ceramic. The toughening mechanism in the multiphase composite toughening Al₂O₃ ceramic was not a simple superposition of phases. The columnar crystals played a role in toughening with crack deflection and bifurcation, but also extended the crack propagation path, which resulted in the increase of martensitic transformation amount and intensified the phase transformation toughening. The maximum bending strength of the columnar crystal multiphase toughening Al₂O₃ ceramic was 570MPa and the fracture toughness was 7.4MPa·m^1/2.From the crack propagation process, it can be found that in addition to the crack deflection caused by the second phase Nb₂O₅, ZrO₂ phase transformation toughening, crack bridging and unplugging caused by columnar crystals, crack bifurcation, together with other toughening mechanisms function together. This toughening efficiency was greater than the sum of separate role of each toughening mechanism. As a result, the mechanical properties of Al₂O₃ ceramic were significantly increased.更多还原