Preparation and Formation Mechanism of Elongated (Ca,Dy)‐α‐Sialon Powder via Carbothermal Reduction and Nitridation

Abstract

(Ca,Dy) co‐stabilized α‐Sialon with elongated grains was prepared from SiO2–AlN–CaF2–Dy2O3 powders by carbothermal reduction‐nitridation (CRN) at 1700°C. Detailed investigations on the formation process of (Ca,Dy)‐α‐Sialon elongated grains performed under various firing conditions revealed that the big quartz particles were initially reduced and decomposed gradually by carbon, and a large number of ball‐shaped particles consisting of amorphous (Si,Al,Ca,Dy)x(O,N)y were formed. With increasing temperature, nitrogen continuously diffused into the liquid to form nitrogen‐rich solid balls. Further increasing the temperature to 1700°C and held for 2 h, hollow α‐Sialon spheres with nanometric particles were obtained from nitrogen‐rich solid balls. Subsequently, the framework structure of hollow spheres was destroyed at 1700°C for 3 h, and (Ca,Dy)‐α‐Sialon grains grew up continuously by consuming nanometric particles in the rough interface via Ostwald ripening and growth stages moving on the smooth interface. Finally, (Ca,Dy)‐α‐Sialon developed well‐defined crystalline elongated grains because the growth rate of rough surfaces is more rapid than that of smooth surfaces.