Fracture Strength and Microstructure of β‐SiAlON with Hafnia Addition

The influence of HfO ₂ addition on the fracture strength and microstructure of ß‐SiAlON ceramics sintered from Si ₃ N ₄ and Al ₂ O ₃ powders was investigated. The strength was increased by the addition of HfO ₂ , from ∼500 MPa to 700 MPa, and was almost constant from ambient temperature to 1300°C. Monoclinic HfO ₂ grains that were distributed in the SiAlON grain boundaries had a flat shape (∼20 nm thick) and were surrounded by an amorphous phase. The aluminum concentration in ß‐SiAlON in the samples with an Al ₂ O ₃ starting composition of 15 wt% was decreased by the addition of HfO ₂ . The amount of secondary phases was very small at grain boundaries between the SiAlON grains; amorphous phases were observed infrequently at the triple points but were very small (∼20 nm). The effects of HfO ₂ addition on the mechanism of the microstructure development and fracture strength are discussed.