Crack Growth Mechanism in Structural Ceramics and Microstructural Analysis

用扫描和透射电子显微镜研究了扩展裂纹周围形成的过程区和过程区尾迹的微观结构。1)研究了室温至1573K热压(HP)Si_3N_4陶瓷的抗弯强度(Sigma_B)和断裂韧性(K_&lt；IC&gt；)的各向异性。K&lt；IC&gt；采用人字形缺口梁法测量和三点弯曲试验获得了σb。在测试温度高达1073K时，K&lt；IC&gt；出现了各向异性，但在1473K几乎没有发现各向异性，但在1573K时又出现了显著的各向异性。尽管HP-LT和HP-ST之间的差异随着温度的升高而减小，并在1573K时减小，但在Sigma_b中，这一点在1473K之前是可察觉的。与无压烧结(PLS)样品相比，所得到的结果与细长的β相的取向有关。2)结合裂纹后加工区微观结构的变化，研究了温度对氧化锆陶瓷弯曲强度和断裂韧性的影响。在室温、873和1273K下进行了三点弯曲和断裂韧性测试，并用扫描电子显微镜、透射电子显微镜和X射线衍射仪分析了裂纹附近和断口的显微组织。在较高温度下，三点弯曲强度和K&lt；IC&gt；值下降到室温下的三分之一左右。由四方结构向单斜相的应力诱导相变在室温下比在较高温度下发生得更大。在循环疲劳的情况下，还用扫描电子显微镜和透射电子显微镜研究了疲劳裂纹周围的微观组织。

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.