Search

Owing to its excellent mechanical and thermal properties such as high strength, high hardness, and good thermal shock resistance, silicon nitride is a material suitable for forming tools. This work analyses the failure of a compaction roll made of Si3N4. One of the two ceramic rings broke during the compaction rolls' operation. Fractographic examinations have shown that very high tensile stresses occurred in the ceramic material during operation. These stresses first led to the formation of cracks and then to the total failure of the roll ring. Failure analysis has shown that roll design and clamping are of major importance. Causes for the generation of tensile stresses are shown and suggestions are made on how to ensure a safe operation of the rolls.

This study examines ceramic varistors made from zinc oxide whose electrical resistance drops by many orders of magnitude within a few nanoseconds when a critical voltage threshold is exceeded. These unusual properties are caused by charges which are build into grain boundaries or accumulate near the grain boundaries. Zinc oxide crystals are polar and piezoelectric. Therefore, the charges in the grain boundary area and thus the electrical properties of the varistor depend on the orientation of the crystals towards the grain boundaries and on the mechanical tensions in the individual crystals. In this study, the microstructures of ZnO varistors, particularly the size and distribution of the grains and their orientation towards the grain boundaries will be examined. Among other things, lock-in thermography with a target preparation of strongly active grain boundaries and EBSD analyses in combination with SEM examinations, ion-etching and wet-chemical etching for the determination of crystal orientations are used. For selected grain boundaries, the electrical resistance in dependence of the applied voltage is determined via Micro-4 pole measurements.

Owing to its excellent mechanical and thermal properties, silicon nitride is an increasingly used tool material in the metallurgical industry. Some of its applications involve high thermal cycling loads, which may lead to tool failure. The failure of ceramic materials initiates from defects in the volume or on the surface of components and samples. Surface defects, often introduced during finishing, are considerably more harmful than defects in the volume. It was possible to heal surface defects by applying a specifically developed heat treatment – provided the defects are not too large. In water quenching experiments on samples and components, it could be proven that the strength increase induced by the heat treatment also results in an improved thermal shock resistance. Due to high processing costs and in some cases due to the components' geometry, enhancing the surface quality of components is not possible. The proposed heat treatment is an alternative to increase strength, thermal shock resistance (and thus the reliability and service life of silicon nitride components) which requires little effort.

Details of surface finish affect significantly the surface quality of ceramic specimens (e.g. surface roughness, residual stress). To analyse the influence of surface quality on strength five samples of silicon nitride specimens were machined applying different surface finishing procedures. Then they were strength tested using the biaxial Ball-on-three Balls (B3B) test. The test results were compared to those of conventional bending tests. A fractographic analysis was performed on selected specimens to identify the fracture origins. It could be shown that a better surface quality leads to an increased strength but increases the scatter of strength data too.

Kurzfassung Der 4-Kugelversuch ist ein neuer Festigkeitstest zur Ermittlung der Zugfestigkeit spröder Werkstoffe unter bi-axialer Beanspruchung. Eine Scheibe wird auf drei Kugeln gelagert und auf der gegenüberliegenden Seite mit einer vierten Kugel zentrisch bis zum Bruch beansprucht. Wegen der gut definierten und stabilen Lasteinleitung eignet sich dieser Test auch zur Prüfung von Proben mit leicht gekrümmten Oberflächen. Die Prüfung gesinterter Proben ohne aufwendige und kostenintensive Hartbearbeitung ist somit möglich. Beim Kontakt von Kugeln und Platten entstehen in den Platten Zugspannungen, die in spröden Werkstoffen zur Entstehung von Rissen führen können (Hertz Risse). In dieser Arbeit wird der Frage nachgegangen, ob bei der Lasteinleitung solche Schäden entstehen und ob Kontaktrisse zu einer Verringerung der Probenfestigkeit führen. Die hier vorgestellten Arbeiten an Proben mit künstlich eingebrachten Rissen und an Proben ohne Vorschäden zeigen, dass Risse in der Probe im Bereich der Krafteinleitung (Kontakt zwischen Kugeln und Probe) keinen Einfluss auf die Festigkeit haben.