Fig. (4) ”Schematic representing how the t-m transformation of ZrO₂ increases fracture toughness. When a part containing metastable t-ZrO₂ is subject to a remote macroscopic tensile stress, the stress intensification due to the presence of a crack tip is sufficient to transform some t-zirconia grains to the monoclinic form. Since this transformation entails a volumetric expansion which is constrained by the surrounding materials, the net result is compressive stress acting on the surfaces of the crack, whose propagation is thus hindered” (From Lughi and Sergo: Dent Mater, 2010; 26: 807-820).