Role of metal additives in mechanochemical phase transformation of zirconia

Reasons for a phase transition in zirconia subjected to intensive mechanical treatment in planetary mill have been considered. If steel balls and vials are used for milling the comminution is accompanied by oxidation of wear metal particles and successive mechanochemical reaction with ZrO₂. Aluminum has been shown to behave similarly and, being deliberately added to zirconia powder, to form solid solutions. Foreign metal cations introduced into the lattice stabilize a modification with higher crystal symmetry thus increasing the threshold size above which the monoclinic modification is stable. An increase of surface energy contribution to the Gibbs’ energy of zirconia plays an important role in phase transformation at the initial stages of mechanical treatment, while henceforth thermodynamic stability is more and more determined by the stabilizing effect of the impurity cations. Under the conditions that rule out contamination of ZrO₂ with wear material or other metal additives, dynamic equilibrium sets in between the direct transition to the tetragonal phase and the reverse transition to the monoclinic form.