**Figure 13.**

*(a) Zirconia has the ability to phase change from a tetragonal phase to a monoclinic phase to stop ensuing cracks, which is referred to as "transformation toughening" and (b) during phase change, there is approximately 3 to 5% volume increase from tetragonal to monoclinic phase [7].*

to solve the problem of the fragility encountered when using alumina, as implant. Indeed, zirconia owes its importance to the stress-induced transformation of the metastable tetragonal crystallites phase into monoclinic phase, when it is localized around a propagating crack. As shown on **Figure 13**, this transformation contributes to increase the fracture toughness and the resistance to crack propagation by expanding the volume (3–6%) and absorbing a part of the energy required for crack propagation, which leads to crack shielding. The negative aspect of this transformation is "aging", which happens when it is localized on the surface of the implant, in presence of acidic liquids. Hence, the surface of the implant presents some signs of "aging", such as the roughness and the appearance of micro-cracks [115].
