High-Temperature Properties of Si_3N_4 Ceramics
High-Temperature Properties of Si_3N_4 Ceramics
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DOI：
10.1557/s0883769400049186
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发表时间：
1995-02
期刊：
影响因子：
5
通讯作者：
M. Hoffmann
中科院分区：
文献类型：
--
作者：
M. Hoffmann
氮化硅是一种高度共价键合的化合物，在1877 ° C下分解。因此，在不添加烧结助剂的情况下，氮化硅的致密化是不可能的.致密化是通过液相烧结来实现的，通常使用金属氧化物如MgO、Y2 O3、Al2 O3和大多数稀土氧化物作为烧结添加剂。氧化物与SiO2反应-总是存在于Si3N4颗粒的表面-形成氧化物熔体，随着温度的升高，通过Si3N4的溶解形成氧氮化物熔体。所得到的显微组织由细长的Si 3 N 4针状物和晶界相组成，针状物嵌在较小的等轴Si 3 N 4晶粒的基体中，如图1所示。烧结助剂的量和化学性质决定了晶界相的体积分数。完全致密化所需的含量取决于烧结技术：如果致密化由高外部压力（热压[HP]或热等静压[HIP]）支持，则2 - 5体积%的添加剂就足够了;无压烧结和气压烧结（10 MPa氮气压力）材料的添加剂含量高达15体积%。如今，氮化硅陶瓷被认为是可与钢相媲美的一类材料。不同的质量取决于氮化硅晶粒的尺寸和形状以及晶界相的量和化学性质。具有高室温强度的材料表现出细晶粒、细长的微观结构，而具有高断裂韧性的材料则更粗晶粒。在这两种情况下，一个弱的界面，需要诱导穿晶断裂。(See Becher et al.在这个问题上。）由于所有Si 3 N 4晶粒都被晶界相完全润湿，因此界面强度由添加剂成分决定。然而，由于晶界相在低温下具有优异的性能，但在高于其软化点的温度下却限制了其性能，因此，高强高韧Si3N4陶瓷的发展与耐高温材料的发展之间出现了矛盾。
Silicon nitride is a highly covalent bonded compound which decomposes at 1877°C. Therefore, it is impossible to densify Si 3 N 4 without sintering additives. Densification is achieved by liquid-phase sintering usually using metal oxides such as MgO, Y 2 O 3 , A1 2 O 3 , and most of the rare-earth oxides as sintering additives. The oxides react with SiO 2 —always present at the surface of Si 3 N 4 particles—to form an oxide melt and, with increasing temperature, an oxynitride melt by dissolution of Si 3 N 4 . The resulting microstructure consists of elongated Si 3 N 4 needles embedded in a matrix of smaller equiaxed Si 3 N 4 grains and a grain boundary phase, as shown in Figure 1. The amount and chemistry of the sintering aids determine the volume fraction of the grain boundary phase. The content required for complete densification depends on the sintering techniques: 2–5 vol% additives are sufficient if densification is supported by a high external pressure (hot pressing [HP] or hot isostatic pressing [HIP]); pressureless-sintered and gas-pressure-sintered (10-MPa nitrogen pressure) materials have additive contents of up to 15 vol%. Today, silicon nitride ceramics are regarded as a class of material comparable to steel. Different qualities depend on the size and shape of the silicon nitride grains and the amount and chemistry of the grain boundary phase. Materials with a high room-temperature strength exhibit a finegrained, elongated microstructure, while materials with a high fracture toughness are more coarse-grained. In both cases, a weak interface is required to induce transgranular fracture. (See Becher et al. in this issue.) Since all Si 3 N 4 grains are completely wetted by the grain boundary phase, the interface strength is determined by the additive composition. Nevertheless, a contradiction arises between the development of high-strength and high-toughness Si 3 N 4 ceramics and high-temperature resistant materials because the grain boundary phase is responsible for the excellent properties at low temperatures, but limits the properties at temperatures above its softening point.