the zirconia system
Pure zirconia (ZrO2) is a beautiful, troublesome material, and its phase behaviour is the reason. On heating it marches through three crystal forms: monoclinic from room temperature up to about 1170 degrees C, then tetragonal up to about 2370 degrees C, then cubic up to its melting point near 2700 degrees C. The trouble is the tetragonal-to-monoclinic change on cooling: it comes with a sudden volume expansion of roughly 4 to 5 percent, which cracks a pure zirconia part to pieces as it cools from firing. Pure ZrO2 is, by itself, almost useless as a structural ceramic.
The fix is a phase-diagram trick: add a stabilizer oxide that forms a solid solution and holds the high-temperature cubic or tetragonal form down to room temperature. Yttria (Y2O3), magnesia (MgO), calcia (CaO), and ceria (CeO2) all work by dissolving into the zirconia lattice, and the ZrO2-Y2O3 diagram is the one to know. Add enough stabilizer and you get fully stabilized cubic zirconia (a single cubic phase, an excellent oxygen-ion conductor); add a little and you get partially stabilized zirconia or fine tetragonal zirconia polycrystal (TZP), where metastable tetragonal grains are held ready to transform. That retained tetragonal phase is the engine of transformation toughening: a crack's stress field triggers the tetragonal-to-monoclinic change locally, and the accompanying expansion clamps the crack shut, giving zirconia its remarkable toughness — an airbag for a crack.
The same physics that makes zirconia tough also makes it fussy, and honesty demands the caveat. In warm, wet service the metastable tetragonal grains can spontaneously transform over time even without a crack — low-temperature degradation, or aging — roughening the surface and weakening the part. This bit famously affected some early zirconia hip implants. So the zirconia system is a two-edged lesson: reading the stabilizer content and temperature off the phase diagram tells you not only how to lock in a tough microstructure, but also where that same metastability can turn against you. Yttria-stabilized zirconia also anchors oxygen sensors and solid-oxide fuel cells, where its cubic form is a fast ionic conductor.
Add about 3 mol% Y2O3 to ZrO2 and sinter to fine grains: you get tetragonal zirconia polycrystal (3Y-TZP), with the tetragonal phase held metastable at room temperature. A crack tip triggers it to pop to monoclinic, and the ~4 percent expansion squeezes the crack closed — that is transformation toughening, read straight off where 3 mol% yttria sits on the ZrO2-Y2O3 diagram.
ZrO2 polymorphs (monoclinic-tetragonal-cubic) plus a stabilizer solid solution: the basis of transformation toughening.
The retained tetragonal phase that toughens zirconia is metastable, so it is also a liability: in warm, damp service it can spontaneously transform and weaken the part over time (low-temperature degradation, or aging). Toughness and durability trade off against each other here.