Estimation of the High-Temperature R Curve for Ceramics from Strength Measurements Including Specimens with Focused Ion Beam Notches

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·m1/2 was determined, the toughness for small strength‐relevant cracks was found to be KIc≅5.1 MPa·m1/2.