However, these slope coefficients actually appear to follow the trend of the toughness at crack initiation KJ 0 (approximately 4 times KJ 0 , using the units of FIGURE ). Intuitively one expects the slope of the curve KJ 0 = f(Δa) to be related to the crack propagation resistance property of the specimen. In our case, it is the interface strength that drives fracture initiation and potentially also the crack propagation, hence the proportionality between these two parameters. It would be beneficial to study this observation in more detail, in particular to determine what is part attributable to the different reinforcement mechanisms potentially present (zirconia phase change, friction and sliding of the platelets...).