【作者】 杜大明；

【导师】 杨海涛；

【作者基本信息】 武汉理工大学 ， 材料学， 2004， 硕士

【摘要】 本论文详细地介绍了氮化硅的结构、性质及应用和常见的几种制备方法，并着重介绍了氮化硅的常压烧结方面的研究进展。 与其他方法相比较，常压烧结法的实用价值最大，应用范围最广，极具有研究价值。选择既能显著提高制品致密化，又不明显降低其高温强度的烧结助剂是氮化硅常压烧结的关键问题。本论文将烧结助剂的选择和常压烧结作为研究的重点，选用MgO、Y₂O₃、La₂O₃、CeO₂、TiC组成的复合助烧剂作为Si₃N₄的烧结助剂，采用常压烧结工艺，在1720℃制备Si₃N₄陶瓷。通过密度、抗弯强度、维氏硬度和断裂韧性等力学性能测试、X射线物相分析（XRD）、扫描电镜（SEM）及透射电镜（TEM）的观察，对氮化硅的烧结过程、组织结构进行了研究。 研究结果表明，常压烧结氮化硅陶瓷的致密化主要是通过液相烧结实现的，由烧结助剂和Si₃N₄表面的SiO₂反应形成低熔点的硅酸盐液相，促进烧结致密化，冷却后，在晶界形成玻璃体。由MgO和稀土氧化物组成的复合助烧剂是Si₃N₄有效的烧结助剂，能够显著降低烧结温度，在1720℃烧结可以获得较高致密度和优良力学性能的Si₃N₄制品。所有试样的力学性能都随相对密度的增大而提高，添加少量的TiC能够提高烧结制品的抗弯强度和硬度，当未添加TiC，相对密度为97.8％时，抗弯强度、硬度和断裂韧性分别为903MPa、13.9GPa和7.50MPa.m^1/2；当添加1％的TiC，相对密度达到97.4％时，抗弯强度、硬度和断裂韧性分别达到923MPa、14.5GPa和7.02MPa.m^1/2。 微观分析结果表明，氮化硅烧结体的显微结构为等轴状的α-Si₃N₄和长柱状的β-Si₃N₄相互交织，与它们之间连续分布的玻璃相，形成了结构致密的材料。对于粗大的杆状β-Si₃N₄的形成主要与起始的物料有关，从α-Si₃N₄粉末制备的材料包含有异常长大的柱状晶粒，这种结构有利于提高烧结体的强度和韧性。在扫描电镜的分析中发现，裂纹的扩展既有沿晶断裂，又有穿晶断裂，说明烧结体既具有较高的晶界强度，在断裂时又表现出较好的韧性。透射电镜的观察发现，在异常长大的α-Si₃N₄晶粒中存在显微缺陷，周围存在裂纹，甚至有裂纹穿过异常长大的晶粒。结果表明，烧结体中存在的较大的孔洞、晶粒的异常长大、裂纹等微观缺陷是513从陶瓷发生力学破坏的常见原因，改善微观组织，减小缺陷尺寸，是提高a一513从陶瓷性能的有效途径。更多还原

【Abstract】 The construction, the property, the application and the preparative methods of silicon nitride ceramics were introduced hi this paper, and the pressureless sintering progress of Silicon Nitride ceramics was emphatically introduced.The pressureless sintering Si3N4 ceramics exhibits good use value and good performances in comparision with other sintered Si3N4 ceramics. The election of additives to increase density and to keep high-temperature strength is a key process of the pressureless sintering Si3N4 ceramics. The election of additives and pressureless sintering is the primary coverage of the paper. Si3N4 ceramics have been fabricated by pressureless sintering with rare-earth oxide composite additives at 1720 C , and the mechanical properties have been measured. The sintering process , composition of phase in Si3N4 ceramics and the microstructure have been studied by means of X-ray diffraction (XRD), Scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The results show that the densification process was controlled by the mechanism of liquid phase sintering. The additives would reacted with the SiO2 which was on the surface of the Si3N4 particles to from silicate liquid phase at high temperature, and silicate liquid phase would from glassy phase at grain boundary which was harmful to the high temperature properties. The additives with MgO and rare-earth oxide was found to be much effective as a sintering aid for Si3N4 because it can effectively acceleration of sintering and reduce suiter point. The results indicated that the higher the relative density, the higher the mechanical properties, and 1% TiC can increases the bending strength and hardnessl. The pressureless sintered silicon nitride achieved a relative density of 97.8%, a bending strength of 903 MPa, a Vickers diamondhardness of 13.9 GPa and a fracture toughness of 7.5MPa.M1/2. Corresponding, the pressureless sintered silicon nitride achieved a relative density of 97.4%, a bending strength of 923 MPa, a Vickers diamond hardness of 15.0 GPa and a fracture toughness of 7.02MPa.M1/2.Micro-analysis indicated that the typical microstructure. of sintered SisN4 is made of equiaxed a-Si3N4 crystalline grains and cylindrical B- Si3N4 crystalline grains. The amount and shape of B- Si3N4 crystalline grains is relative of the in raw stuff power. Si3/be made ofa-Si3N4 raw stuff power contains abnormal growth cylindrical - Si3N4 crystalline grains which was helpful to the mechanical properties, intergranular cracks and transgranular cracks was found in sintered Si3N4 ceramics . The blow holes , abnormal grain growth and crack is the chief cause of ceramics damage.更多还原