ceramics. Unlike alumina, zirconia is transformed from one crystalline state to another during firing. At firing temperature, zirconia is tetragonal, but at room temperature it is monoclinic, with a unit cell of monoclinic occupying about 4.4% more volume than when tetragonal. Unchecked, this transformation leads to crumbling of the material on cooling. In the late 1980s, ceramic engineers learned to stabilize the tetragonal form at room temperature by adding small amounts (approximately 3 mass% to 8 mass%) of calcium and later yttrium or cerium. However, the tetragonal form is really only “metastable,” meaning that trapped energy still exists within the material to drive it back to the monoclinic state. The highly localized stress ahead of a propagating crack is sufficient to trigger grains of the ceramic to transform in the vicinity of the crack tip. In this case, the 4.4% volume increase becomes beneficial, essentially altering material conditions around the crack tip and shielding it from the outside world. In scientific terms, transformation incrementally increases the local fracture toughness as the transformation zone develops around the growing crack.
Monoclinic-tetragonal-monoclinic transformation of zirconia.
Crack growth in alumina and zirconia.
Zirconia substructure issues
With a fracture toughness higher than that of alumina ceramics, transformation-toughened zirconia represents an exciting potential substructure material. However, possible problems with zirconia ceramics may involve long-term instability in the presence of water, porcelain compatibility issues (thermal issues during firing and cooling), and some limitations in case selection due to their opacity. Two major concerns with zirconia are porcelain chipping and low-temperature degradation. There are early reports of significant percentages (ie, 25% to 50%) of single-unit and multi-unit prostheses having porcelain chipping and cracking. And the propensity for partially stabilized zirconia to autocatalytically transform at surface grain boundaries due to an interaction with water may create major structural issues in the long run.
Porcelain chipping
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