A MICROSTRUCTURAL STUDY OF MECHANICAL-PROPERTIES IN SI3N4 CERAMICS

To clarify the effect of microstructure on crack growth behavior in Si3N4 ceramics, fracture toughness and three-point bending tests have been carried out for pressureless sintered (PLS) and hot-pressed (HP) Si3N4 specimens. As to the hot-pressed Si3N4 ceramics, the specimens were cut out parallel (HP-TL) and perpendicular (HP-SL) to the hot-pressing direction respectively for studying the anisotropy of mechanical properties. Microstructures subsequent to crack growing were characterized on the fracture surface and in the process zone wake by optical and scanning electron microscopies. The anisotropy was discernible in fracture toughness and bending strength, but hardly in microhardness. The process zone wake was damaged more severely in the HP-TL specimen than in the HP-SL one. The X-ray diffraction line profiles and pole figure analyses showed a preferred orientation of the beta-phase in hot-pressed Si3N4. The HP-TL specimen had a fracture toughness and a bending strength about 10 percent greater than the HP-SL specimen. Pulling-out of elongated grains was more frequently observed on the fracture surface of the PLS specimen. The results obtained were discussed in connection with the preferred orientation of elongated beta-phase grains from the microstructural points of view.