Effects of rare-earth (RE) intergranular adsorption on the phase transformation, microstructure evolution, and mechanical properties in silicon nitride with RE2O3 + MgO additives:: RE=La, Gd, and Lu
Effects of rare-earth (RE) intergranular adsorption on the phase transformation, microstructure evolution, and mechanical properties in silicon nitride with RE2O3 + MgO additives:: RE=La, Gd, and Lu
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DOI：
10.1111/j.1551-2916.2008.02448.x
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发表时间：
2008-07-01
影响因子：
3.9
通讯作者：
Lin, Hua-Tay
中科院分区：
文献类型：
--
作者：
Becher, Paul F.;Painter, Gayle S.;Lin, Hua-Tay
氮化硅粉末通常主要由α相组成，在Si3N4陶瓷的致密化和显微组织演化过程中转变为β相。在MgO和RE2O3致密化添加剂的共同作用下，随着稀土原子序数的增加，相变开始温度逐渐降低。这一趋势与预测和观察到的随着稀土原子序数的增加，稀土对六方棱柱状β颗粒棱柱面上的偏析和吸收的亲和力降低相吻合。当稀土吸附减少时，Si（和N）在光滑棱柱面上的附着增强，从而增加了直径生长速率，通常是受附着机制限制的反应速率。结合典型的快速生长[0001]，正是这种增强的晶粒生长有助于在较低温度下开始α - β转变。随着相变的增强，观察显示在致密化早期阶段生长的β晶粒数量增加。另一方面，稀土吸附增加导致生长各向异性增大，从而形成高长径比的晶粒。因此，Lu2O3生成的增强晶粒直径较大且拉长，而La2O3生成的增强晶粒长径比较大。Gd2O3的添加性转变和显微组织特征介于镧系端元元素之间。尽管存在这些差异，但在每种添加剂组成中都发现了大量的大增强晶粒。结果表明，所制陶瓷的力学性能基本一致，抗弯强度超过1 GPa，断裂韧性达到10 ~ 10 MPa。在室温下m(1/2)，在1200℃时具有优异的强度保持性（> 800mpa）。
Silicon nitride powders most often consist primarily of the alpha phase, which transforms into the beta phase during the densification and microstructural evolution of Si3N4 ceramics. The temperature at which the transformation initiates in the presence of a combination of MgO and RE2O3 densification additives is found to decrease with increasing atomic number of the rare earth (RE). This trend coincides with the predicted and observed decrease in the affinity of the RE to segregate to and absorb on the prism planes of hexagonal prism-shaped beta grains with an increase in the atomic number of the RE. When RE adsorption is diminished, Si (and N) attachment on the smooth prism planes is enhanced, which increases diametrical growth rates, normally reaction-rate limited by an attachment mechanism. Combined with the typically fast [0001] growth, it is this augmented grain growth that contributes toward the initiation of the alpha-beta transformation at lower temperatures. With the enhanced transformation, observations reveal an increase in the number of beta grains growing in the early stages of densification. On the other hand, increased RE adsorption leads to greater growth anisotropy, resulting in the formation of higher aspect ratio grains. Thus, Lu2O3 generates larger diameter, yet elongated, reinforcing grains, while La2O3 results in reinforcing grains of a higher aspect ratio. The Gd2O3 additive transformation and microstructural characteristics lie intermediate to those of the lanthanide end-member elements. Despite these differences, a substantial fraction of large reinforcing grains were found for each additive composition. As a result, the mechanical properties of the resultant ceramics are similar with flexure strengths in excess of 1 GPa, fracture toughness values > 10 MPa.m(1/2) at room temperature, and excellent strength retention (> 800 MPa) at 1200 degrees C.