transformation toughening
Transformation toughening is best pictured as an airbag for a crack. As a crack tries to open in zirconia (ZrO2), a phase change is triggered in the material just ahead of and around it; that change makes the material swell, and the swelling squeezes the crack shut from the sides, forcing you to push much harder to make it advance. It is the mechanism that makes toughened zirconia the toughest of the oxide ceramics, tough enough for knife blades that keep an edge, dental crowns, and the ball of a hip joint.
The trick rests on zirconia's polymorphism. Pure ZrO2 would sit in its monoclinic (m) form at room temperature, but by adding a stabiliser such as yttria (Y2O3) or ceria, keeping the grains fine, and letting the surrounding matrix squeeze them, engineers trap the higher-temperature tetragonal (t) form in a metastable state at room temperature, poised to transform. When a crack arrives, the intense tensile stress at its tip releases that constraint and triggers a martensitic t to m transformation. The catch is that the monoclinic form occupies about 4 percent more volume, plus a shear, so the transformed particles in the crack's wake push outward against the crack faces, putting the tip region into compression and shielding it. More applied stress is now needed to keep the crack moving, and the toughness climbs from around 3 to as much as 8 to 15 MPa sqrt(m). Because the shielding wake builds up as the crack extends, it also produces a strongly rising R-curve. The main forms are partially stabilised zirconia (PSZ), tetragonal zirconia polycrystal (TZP), and zirconia-toughened alumina (ZTA).
The same metastability that toughens zirconia is also its Achilles heel, and this is an important honesty. In warm, humid service the surface tetragonal grains can transform to monoclinic spontaneously, with no crack needed, a slow degradation called low-temperature ageing or LTD. The transforming grains roughen and microcrack the surface, quietly weakening the part over months or years. This is exactly what caused a notorious batch of zirconia hip-joint heads to fail in the body, and it is why medical-grade zirconia is now carefully stabilised and why the mechanism, powerful as it is, must be matched to a service temperature and environment it can survive.
A ceramic kitchen knife of yttria-stabilised tetragonal zirconia (Y-TZP) holds an edge because a chip or crack trying to start meets grains that transform, swell about 4 percent, and clamp the crack shut. Its toughness near 10 MPa sqrt(m) is more than double plain alumina's, which is why it resists chipping in daily use.
A stress-triggered volume expansion clamps the crack shut, but the same metastable phase can age and weaken in warm, wet service.
The metastability that gives zirconia its toughness is also a liability: in warm, wet conditions the surface can spontaneously transform (low-temperature ageing), microcracking and weakening over time. This aging limits zirconia's use in the body and in hot, humid environments.