No abstract

No abstract

Knowledge of the fracture characteristics of individual microstructural features such as grains and grain boundaries is essential for a full understanding of the macroscopic behaviour of ceramics. We have investigated the fracture of single crystal, bicrystal and polycrystalline Al2O3 on the micrometre scale by testing microcantilevers containing notches and artificial flaws introduced at features of interest (e.g. grain boundaries) using focused ion beams (FIB). The specimens were loaded to fracture using a nanoindenter and tests were performed under air, water and silicone oil. The results were influenced significantly by ion implantation at the notch tip, moisture assisted slow crack growth and finite notch tip radius. Methods of avoiding these effects or correcting for them are proposed. It is shown that using these methods, the fracture toughness (Kc) and threshold stress intensity for subcritical crack growth (K0) can be measured in individual grains and grain boundaries of typical microstructures.

Flash sintering is a newly developed technique to quickly densify conductive ceramics in the presence of an electric field. At a critical combination of field and temperature, densification takes place in a few seconds. This paper investigates the electrical resistivity during flash sintering of 3YSZ by the four-terminal method in combination with numerical modelling of electrical heating and temperature distributions. Densification rates were similar to those observed previously in specimens rapidly heated by methods not involving direct electrical heating. Temperature gradients caused by heat loss from the surface and non-uniform electrical heating led to differential sintering, specimen distortion and non-uniform grain size. The four point measurements revealed significant contact resistance at the power supply electrodes and allowed accurate measurement of the resistivity under the high current conditions of flash sintering. The specimen resistivity was lower for a given temperature under high current conditions and this was attributed to electronic conduction. Concurrent sintering led to a progressive reduction of resistivity during heating causing an increase in the apparent activation energy for electrical conduction. When the effect of sintering on resistivity was accounted for, the onset conditions for the "flash event" were accurately predicted on the basis of thermal runaway.

No abstract

No abstract

No abstract

No abstract