The cracks observed after testing on notched specimens (FIGURE ) show a strong deviation from the axis of the notch. There is no obvious relationship between the overall path of the cracks and the initiation toughness (FIGURE ). Indeed, as an example, the AP sample has a lower KJ 0 than the other compositions and however shows a crack deviation similar to the other samples. The structure of the grain boundaries varies between the different samples depending on the presence or on the absence of zirconia particles, glass phase, and alumina nanoparticles. As cracks propagate along the interfaces, one would expect a difference in crack propagation behaviour between the samples. Since the shape of the cracks does not vary much and since the KJ 0 is only representative of the beginning of propagation, the study of the R-curves, i.e. the evolution of toughness with the crack depth (FIGURE ), is of interest. A size effect in crack resistance curves has been clearly noted . In this paper, R-curves are mainly used for comparison purposes, on similar samples tested in the same loading conditions. Thus, this work does not intend to discuss the shape of R-curves or the toughness values after crack propagation (discussions on this aspect can be found in the literature ). R-curves are plotted only at the beginning of crack propagation (0.3 mm, one quarter of the ligament length) in order to compare the different compositions.