Toughening mechanisms for a zirconia-lithium aluminosilicate glass-ceramic

The mechanical properties of a lithium aluminosilicate glass-ceramic and the same glass-ceramic containing 5 and 15 wt% zirconia were investigated. The aim of the study was to assess the contributions to toughening from various toughening mechanisms. For the zirconia-containing compositions, zirconia initially precipitated, upon heat treatment of the glass, as tetragonal zirconia (t-ZrO₂), and upon further heat treatment, transformed to monoclinic zirconia (m-ZrO₂). This transformation could also be induced by grinding samples containing t-ZrO₂. By heat treating, the fracture toughness of all compositions increased with increasing matrix grain size until the matrix grain size exceeded ∼ 1 μm, whereupon both the fracture toughness and strength decreased sharply. The matrix phases, lithium metasilicate and β-eucryptite, have either high thermal expansion mismatch or high thermal expansion anisotropy resulting in large thermal stresses. The initial toughness increases observed in each composition were attributed to the formation of a microcrack zone around the propagating crack. At larger grain sizes, thermal stresses caused spontaneous cracking and loss of strength. Zirconia additions also contributed to the fracture toughness improvement; however, the predominant toughening mechanism was not by transformation but due to crack deflection by the stress fields around the transformed, i.e. m-ZrO₂, particles.