Effect of Si₂N₂O content on the microstructure, properties, and erosion of silicon nitride-Si₂N₂O in situ composites

Silicon nitride-Si₂N₂O in situ composites were prepared by hot pressing powder mixtures of α-Si₃N₄, 6 wt% Y₂O₃, 1 wt% Al₂O₃, and 0-12 wt% SiO₂ X-ray diffraction (XRD) analysis indicated that the volume percents of Si₂N₂O, 13, 31, and 54 for the composites prepared with 0, 4, 8, and 12 wt% SiO₂, respectively. XRD results also indicated that both silicon nitride grains and Si₂N₂O grains were laid down perpendicular to hot pressing direction. As the volume percent of Si₂N₂O increased, the width and the amount of elongated silicon nitride grains decreased, but the fracture toughness increased. Young's modulus of the in situ composites decreased as the Si₂N₂O content was increased. The erosion rate decreased as the Si₂N₂O content was increased, in part, due to both the increased fracture toughness and the reduced grain size. Erosion of the composites occurred primarily due to the grain dislodgment. The sample without Si₂N₂O experienced micro-chipping due to transgranular fracture.