Crack Growth Mechanism in Structural Ceramics and Microstructural Analysis

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Microstructures of the process zone and the process zone wake which are developed around growing cracks were studied by scanning and transmission electron microscopy. The results obtained are as follows.1) The anisotropy of bending strength (sigma_b) and fracture toughness (K_<IC>) in hot-pressed (HP) Si_3N_4 ceramics has been studied at room temperature to 1573 K.Specimens were cut out parallel (HP-LS,HP-LT) and perpendicular (HP-SL,HP-ST) to the hot-pressing direction. The chevron notched beam method was employed for the K_<IC> measurement and the three-point bending test for obtaining the sigma_b. The anisotropy was found in the K_<IC> at the testing temperature up to 1073 K,but hardly at 1473 K.However, it reappeared at 1573 K remarkably. In the sigma_b this was discernible up to 1473K,although differences between the HP-LT and the HP-ST specimens decreased with increasing temperature and diminished at 1573 K.The results obtained were discussed in connection with the orientation of elongated beta-phase grains, comparing with those of pressureless sintered (PLS) specimens.2) The effects of temperature on bending strength and fracture toughness of zirconia ceramics have been studied in connection with microstructural changes in the process zone wake of a crack. Three point bending and fracture toughness measurement tests have been carried out at room temperature, 873 and 1273K.Microstructures in the vicinity of cracks and on fracture surfaces were examined by SEM,TEM and X-ray diffraction techniques. At the higher temperatures the three-point bending strength and the K_<IC> values decreased to about one third of that at room temperature. The stress induced phase transformation from tetragonal structure to monoclinic one has occurred more greatly at room temperature than at the higher temperatures. In the case of cyclically fatigued specimens, microstructures developed around fatigue cracks have also been studied by SEM and TEM techniques.