AbstractThe mechanical properties and the thermal-shock and thermal-fatigue characteristics of three reaction-sintered (density 1·9-2·6 g/ml) and one hot-pressed (density 3·00-3·18 g/ml) samples of silicon nitride showed significant differences according to composition and method of manufacture. The resistance to creep (at 1000-1200°C) and to thermal fatigue (at 1000°C), particularly of the dense grades, can be considerably superior to that of creep-resistant alloys. However, the appreciable variation in performance of all grades indicates that further development of the manufacturing technique is necessary to ensure consistent behaviour.