Rißzähigkeitsermittlung mit Härteeindruckrissen an einer grobkörnigen Aluminiumoxid-Keramik

In spite of the high temperature strength, the wear resistance, the oxidation and hot corrosion resistance, and the low density, the potential applications for ceramic materials are limited by their brittleness. Intensive efforts in materials research and development are therefore being undertaken with the aim of producing a 'tough' ceramic material. Progress in this direction has been made through the combination of materials development and microstructural fracture characteristics; further developments are expected. The present work is concerned with the testing of short cracks for the fracture characterization of a coarse grained alumina. In the literature, the clear R-curve behavior of the examined alumina has been confirmed, that is, the crack resistance increases with increasing crack length. To investigate the crack initiation and crack growth, the indentation method was used. With indentation cracks and complementary crack opening tests, it was possible to achieve a consistent fracture mechanics result. At room temperature a constant crack resistance of K$_{R}$=4MPam$^{1/2}$ was measured in air and K$_{R}$=5.5MPam$^{1/2}$ in vacuum. At 1000°C in air a crack resistance of K$_{R}$«3.3MPam$^{1/2}$ was determined. The results are in qualitativeagreement with the reduction of crack resistance due to environmental effects (for example, humidity) and thermally activated crack growth. It was interesting that only constant crack resistance values for controlled, stable crack extension of the indentation cracks were measured for the alumina, in spite of the knownR-curve behavior. This result clearly contradicts the published literature, both for the indentation cracks and for the Jong cracks. It must, however, be regarded as correct because of the accurate analysis of the internal stresses, which are responsible for the propagation of the indentation cracks. An attempt was made to investigate the observed difference in crack resistance of indentation cracks and long cracks using crack opening tests. It was indeed found that the ratio of crack opening to crack length was significantly higher for indentation cracks than for long cracks. Obviously the toughening mechanisms that are the basis for R-curve behavior are almost saturated in the case of indentation cracks. In addition, crack propagation experiments with indentation cracks are required in order toconfirm generally these considerations for ceramics that show R- curve behavior