Relationship between Microstructure, Toughening Mechanisms, and Fracture Toughness of Reinforced Silicon Nitride Ceramics

Different microstructures in Si 3 N 4 ceramics containing Y 2 O 3 and Al 2 O 3 as sintering additives were prepared by two‐step sintering. Pull‐out and elastic bridging were most frequently observed as the toughening mechanisms in samples with fine‐grained microstructures having needlelike β‐Si 3 N 4 grains with diameters of <1 μm. Crack deflection was the main toughening mechanism observed in samples with coarse‐grained microstructures having grains with diameters of >1 μm. The values of fracture toughness were varied from 6.1 to 8.2 MPa·m 1/2 with respect to the microstructural characteristics, characterized by the volume fraction of needlelike grains and their diameter.