Theoretical description of zirconia ceramics aging kinetics
Andrey O. Zhigachev
Alexey V. Umrikhin
Vyacheslav V. Rodaev
Received: 23 January 2018 / Revised: 21 May 2018 / Accepted: 30 May 2018
Australian Ceramic Society 2018
An analytical approach for fraction calculation of monoclinic phase on zirconia surface under aging conditions was developed.
The approach is based on a model of nucleation and growth of areas which underwent tetragonal to monoclinic transformation.
Nucleation in this model takes place only at the material surface with nucleation rate proportional to the area of non-transformed
zone. The nuclei are conical in shape; their initial dimensions and growth rate are constant. The resulting equations describe
factual and observed monoclinic phase content on the surface of zirconia ceramics. The equations correlate well with experi-
mental data on tetragonal zirconia ceramics aging kinetics.
Zirconia ceramics are structural materials renowned for their
outstanding combination of hardness, fracture toughness, flex-
ural strength, and corrosion resistance [1, 2]. Fracture tough-
ness and bending strength associated with zirconia ceramics
resistance to crack propagation can indeed be considered the
most remarkable among zirconia ceramics properties.
Resistance of zirconia ceramics to crack propagation is due
to stress-induced phase transformation from tetragonal (t-
) to monoclinic (m-ZrO
The stress-induced transformation at the crack tip is associated
with~5%increaseinvolume, causing compressive stress-
es at the tip, hindering further propagation of the crack.
Transformation toughening mechanism allows typical tetrag-
onal zirconia ceramics (TZP (tetragonal zirconia polycrystal))
stabilized with yttria to have fracture toughness up to 8–
along with hardness as high as 12–13 GPa [6, 7].
Transformation toughening is the reason of attractive prop-
erties of zirconia ceramics; on the other hand, this mechanism
causes certain limitations of zirconia ceramics applications.
Indeed, for TZP ceramics, the steadily decreasing efficiency
of tetragonal to monoclinic (t-m) transformation with increase
in temperature is observed [8, 9]. On the other hand, uncon-
trolled t-m transformation in humid environment, especially in
the temperature range 100–400 °C, takes place—phenomenon
known as LTD (low-temperature degradation). LTD is partic-
ularly important for zirconia ceramics used in dental restor-
ative practice, where the material is constantly subjected to
moist environment .
LTD consists in spontaneous t-m transformation, facilitated
by the presence of water molecules. Different possible mecha-
nisms of LTD, taking into account chemical sorption of water
molecules on the surface of zirconia ceramics, causing stress
accumulation and eventually t-m transformation , diffusion
of hydroxyl ions into the zirconia lattice, and stabilizer leaching
by chemical reaction [11, 12], were reported previously.
Microcracks formed in the transformed area facilitate water
molecule penetration deep into the material, allowing transfor-
mation to spread from the surface to the bulk of the material.
The transformation caused by water molecules instead of
crack tip stress field has negative impact on the mechanical
properties of zirconia ceramics. Firstly, volumetric expansion
accompanying t-m transformation induces local compressive
stress near the transformation zone , causing microcracking
and decrease in strength [14–16]; secondly, the transformation
leads to reduction in the tetragonal phase content. It means that
amount of material capable of phase transformation decreases
and so does the toughening effect.
Since decrease in fracture toughness correlates with the
amount of t-ZrO
which has undergone the transformation,
* Andrey O. Zhigachev
Tambov State University, Internatsionalnaya str. 33, Tambov, Russia
Journal of the Australian Ceramic Society