Zirconia Yttria blends of approximately 3% are called tetragonal zirconia polycrystal or TZP, and have the finest grain size. These grades of Zirconia exhibit the highest toughness at room temperature, because they are nearly 100% tetragonal, but this degrades severely between 200 and 500°C as these irreversible crystal transformations also cause dimensional change.
Zirconia is best known among ceramics as being both hard and fracture-tough at room temperature. Additionally, its fine (sub-micron) grain size enables excellent surface finishes and the ability to hold a sharp edge.
Although it retains many properties including corrosion resistance at extremely high temperatures, zirconia does exhibit structural changes that may limit its use to perhaps only 500°C. It also becomes electrically conductive as this temperature is approached.
Zirconia is commonly blended with either MgO, CaO, or Yttria as a stabilizer in order to facilitate transformation toughening. This induces a partial cubic crystal structure instead of fully tetragonal during initial firing, which remains metastable during cooling. Upon impact, the tetragonal precipitates undergo a stress induced phase transformation near an advancing crack tip. This action expands the structure as it absorbs a great deal of energy, and is the cause of the high toughness of this material. Reforming also occurs dramatically with elevated temperature and this negatively affects strength along with 3-7% dimensional expansion. The amount of tetragonal can be controlled by additions of the blends above to balance toughness against loss of strength.
Cubic Zirconia: This single crystal optically clear form of ZrO2 has a relatively low fracture toughness and strength, but very high thermal shock resistance.
Zirconia PSZ: Cream colored blends with approximately 10% MgO, called partially stabilized zirconia (PSZ), are high in toughness, and retain this property to elevated temperatures. They are somewhat lower in cost but also have larger grain structure.