Processing, Phase Formation and Creep Behavior of Si₃N₄ with Y₂O₃ and Al₂O₃

Abstract

The formation of secondary crystalline phases in Si₃N₄ with Y₂O₃ / Al₂O₃ additions was investigated by x-ray diffractometry on annealed specimens with different additive levels and silica content. With increasing silica content the expected phases according to the Si₃N₄-SiO₂-Y₂O₃-diagram were observed with the exception that Y₁₀(SiO₄)₆N₂ (N-apatite) did not appear. This is attributed to the kinetic hindrance of its crystallization. Based on this investigation two compositions have been chosen which were then prepared by continuous milling and subsequent spray drying. The isostatically pressed bodies were gas pressure sintered (BN-crucible and nitrogen atmosphere) and finally devitrified at 1250°C. The creep measurement was carried out in four point bending geometry at 1250°C in air. Measurement for an as-annealed sample showed that compositions containing YSiO₂N had better creep behavior than those containing Y₂SiO₅. This is attributed to the faster precipitation of the YSiO₂N phase during precipitation. However, if the samples were devitrified prior to creep measurement, the tendency was reversed. Generally, the creep rate was lower for both types of samples but the Y₂SiO₅-containing material showed the best creep behavior of all samples. As a possible explanation, it is hypothesized, the nitrogen content in the remaining glass is enriched during Y₂SiO₅ precipitation. Precipitation of YSiO₂N would create a more oxygen-rich glass.