Silicon Nitride - Improving the Properties of One of the World’s Most Important Structural Materials

With excellent high temperature strength, good resistance to oxidation and low coefficient of thermal expansion Si₃N₄ ceramic is one of the most important structural materials.  However, the pure silicon nitride ceramics are difficult to densify and the addition of various oxides is used to improve sinterability.

Previous work has shown Lu₂O₃-SiO₂ additives are useful in improving the bending strength of Si₃N₄ at elevated temperature. The bending strength of the tape-cast Si₃N₄ with rod-like β-Si₃N₄ seed can be further improved because the elongated grains from seeds were preferentially oriented parallel to the casting direction. In this study from the National Institute of Advanced Industrial Science and Technology (AIST) and published in AZojomo*, Lu₂O₃-SiO₂ additives were selected as sintering additives, and tape casting was used to prepare the unidirectional aligned β-Si₃N₄ elongated grain Si₃N₄ ceramics.

The researchers Yu-Ping Zeng, Jian-Feng Yang, Naoki Kondo, Kiyoshi Hirao, Tatsuki Ohji, Hideki Kita and Shuzo Kanzaki found that the seeded and tape-cast Si₃N₄ materials showed anisotropic microstructures and bending strength with the thickness of green tape having some effect on the bending strength. Due to the formation of a high melting point grain boundary phase Lu₂Si₂O₇N₂ and the unidirectionally aligned β-Si₃N₄ elongated grains, the seeded and tape-cast Si₃N₄ shows good bending strength at elevated temperature.

The article is available to view at https://www.azom.com/Details.asp?ArticleID=3078