Impurities in Silicon Nitride Raw Materials and Correlation with Mechanical Strength of the Hot-pressed Body

Abstract

Spark-source mass spectrometry was used in order to determine the impurity elements from Li to La in nine commercial samples of Si₃N₄ raw materials. Total contents of thirty three impurity elements detected from Si₃N₄ raw powders which were prepared by a reaction of Si and N₂ were 1.0 to 2.2wt%. Otherwhile there were few kinds of impurities in samples which were prepared by reactions of pure SiCl₄-NH₃ and SiO₂-C-N₂, and total impurity contents was 0.07-0.14%. Three-point bending strength at 1200°C for the hot-pressed body from α-rich powder with 2wt% Al₂O₃ and 5wt% Y₂O₃ was decreased with the total impurity contents. Similar correlations were observed between the 1200°C-strengths and respective contents of Ca, Ti or Fe. However the room temperature strength was independent on the total impurity contents. Hot-pressed bodies from β-, α≅β- and amorphous powder showed lower strengths than those expected from the correlation curve for α-rich powder.