Preparation and microstructure characterization of crack-free zirconium nitride microspheres by internal gelation combined with two-step nitridation

Zirconium nitride microspheres with uniform size of ~680 μm, good sphericity, and good integrity were prepared by internal gelation process and two-step nitridation. The effects of carbon black and temperature on stability of sol were investigated. Induction of carbon black decreased stability of sol but the amount of carbon black added has little effect on the sol stability. Increasing temperature linearly decreased the stable time of the sol. Sol with stable time of up to 1 h, which was adequate for casting of liquid spheres was achieved at 11.0 °C, a significant increase from conventional 4.0 °C. Two-step nitridation was developed to prepare crack-free ceramic microspheres with high relative nitride phase content. The special microstructure on surface and cross section of microspheres reveals stepped-growth of nitride crystals.