RELATIONSHIP BETWEEN MICROSTRUCTURE, TOUGHENING MECHANISMS, AND FRACTURE-TOUGHNESS OF REINFORCED SILICON-NITRIDE CERAMICS

Different microstructures in Si3N4 ceramics containing Y2O3 and Al2O3 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 beta-Si3N4 grains with diameters of <1 mu m. Crack deflection was the main toughening mechanism observed in samples with coarse-grained microstructures having grains with diameters of >1 mu m. The values of fracture toughness mere 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.