A ceramic that fights cracks
Ceramics are known for brittleness, but zirconia, or zirconium dioxide, is a notable exception among oxides. Under the right conditions its toughness is several times greater than that of alumina, which has made it a favorite for knives, dental crowns, bearing components and tough wear parts. The reason lies in a crystal phase change that the material can use to its advantage.
Three crystal forms
Pure zirconia can exist in three crystal structures depending on temperature. At very high temperature it is cubic. On cooling, it changes to a tetragonal form and, on further cooling, to a monoclinic form, which is the stable one at room temperature.
The tetragonal-to-monoclinic change involves a volume increase of a few percent and a shear distortion. In a bulk piece, that sudden expansion on cooling can crack the part apart. This is why pure zirconia cannot simply be fired and cooled as an engineering material.
Stabilizing the structure
The fix is to add a stabilizing oxide, most commonly yttria, and sometimes magnesia, calcia or ceria. These dissolve in the zirconia and help keep the high-temperature forms stable, or at least reluctant to transform, at room temperature. Depending on the amount and processing, the product is described as:
- Fully stabilized zirconia, mostly cubic. It is useful for oxygen sensors and fuel cell electrolytes because oxygen ions can move through it, and for high-temperature insulation, but it is less tough.
- Partially stabilized zirconia, a mixture of cubic grains with tetragonal precipitates inside.
- Tetragonal zirconia polycrystal, a fine-grained material made mostly of metastable tetragonal grains held in place by stabilizer and by their surroundings.
The partial and tetragonal grades are the ones prized for toughness. Their grains are in a state of being “ready” to transform, but constrained.
How transformation toughening works
A crack in a brittle material concentrates stress at its tip. In a transformation-toughened zirconia, that high stress at the tip releases the constraint on nearby tetragonal grains, and they change to the monoclinic form. Because the monoclinic form takes up more volume, the transformed grains expand and push against the surrounding material. This compressive squeeze pushes the crack faces together and works against the stress that is trying to open the crack. The transformation itself also consumes energy.
The result is that cracks need more load to advance, so fracture toughness rises. The ceramic effectively uses the threat of fracture to trigger its own defense. The concept is described in more depth in the discussion of fracture toughness versus hardness, since a tougher material is not necessarily a harder one.
Zirconia can also be added to other ceramics. Zirconia-toughened alumina contains small zirconia particles in an alumina matrix, giving a combination of alumina’s hardness and some added toughness. This is used in some orthopedic and cutting applications.
Where it is used
- Dental restorations. Crowns and bridges, valued for strength and tooth-like color.
- Cutting tools and knives. Blades keep a sharp edge, although they may chip if dropped or twisted.
- Wear parts and nozzles. Applications where impact tolerance is more valuable than the maximum hardness.
- Bearings and bushings. Often in corrosive conditions, where its expansion behavior is closer to steel than other ceramics.
- Oxygen sensors and fuel cells. These use the ionic conduction of the stabilized forms.
- Thermal barrier coatings. Yttria-stabilized zirconia forms the insulating top layer in these coatings because of its low thermal conductivity.
Limits of the mechanism
Temperature. Toughening depends on the tetragonal phase being metastable. As temperature rises, the driving force for the transformation drops, and the benefit weakens. Zirconia therefore performs best from room temperature to moderate heat, and it is not the best option for the hottest structural uses.
Low-temperature degradation. In warm, humid or aqueous conditions, tetragonal grains at the surface can transform slowly on their own, which roughens the surface and may lead to microcracking over long periods. Grades and grain size are designed to minimize this, but it has been a point of concern in applications such as implants. Verification with accelerated aging tests is typical.
Hardness and stiffness. Zirconia is generally less hard than alumina or silicon carbide, so for pure abrasion resistance, those may last longer.
Thermal conductivity. Zirconia is a poor heat conductor, which is an advantage for insulation but a disadvantage where heat must be removed, and it makes parts more prone to temperature gradients.
Cost and processing. Powders and stabilizers add cost, and the sintering process must control grain size carefully, because grains that are too large transform spontaneously and ones that are too small will not transform at all.
Diagnosing the phases
Because the fraction of tetragonal and monoclinic material determines properties, makers check it with x-ray diffraction and sometimes Raman spectroscopy. A worn or ground surface may show a higher monoclinic fraction, evidence that transformation has been working to resist damage.