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For failure assessment of natural cracks caused by surface treatment, the crack resistance for very small crack extensions is necessary. In ceramics with strongly rising R curves, the initial part of a few micrometer crack propagation is of special interest. For this purpose, the use of very small sharp starter notches introduced by a focused ion beam is recommended. As an example, the initial R curve for gas-pressure-sintered silicon nitride at high temperature will be estimated. Whereas for V-notched bending bars a fracture toughness of about 6-6.3 MPa·m 1/2 was determined, the toughness for small strength-relevant cracks was found to be K Ic ≅5.1 MPa·m 1/2 .

Aqueous solutions of boehmite and hydrofluoric acid (HF) were used to prepare homogeneous mixtures of alumina and aluminum fluoride. Calcination at temperatures as low as 1000°C resulted in the formation of well-defined hexagonal-shaped α-alumina platelets. Containment of the aluminum fluoride by covering the calcination crucible promoted crystal growth presumably by a reaction of continuous evaporation–condensation of aluminum fluoride. Hexagonal-shaped platelet α-alumina was observed with average diameters ranging from 7 to 33 μm. Large platelets with a narrow size distribution and average diameter of over 25 μm were prepared by controlling the initial concentration of HF and the calcination time, temperature, and atmosphere.

An improvement for the application of computer-assisted stereophotogrammetry to ceramics is presented. To compensate for the lack of contrast in the fracture facets in scanning electron microscopy fractographs, a special decoration technique has been developed. The structure of the fracture facets is enhanced via heat treatment of a sputtered gold layer. This procedure enables the measurement of the three-dimensional location of more than 10 000 points on the fracture surface by means of automatic stereophotogrammetry.