Dentaltown March 2018 - 66
Fig. 1: Maxillary second bicuspid zirconia crown cemented in vivo.
Dental zirconium dioxide is formed into
a polycrystalline ceramic called yttriumstabilized tetragonal zirconia polycrystals,
or simply "zirconia." Materials containing
only tetragonal phase are strongest, while
cubic-containing zirconia is significantly
weaker but more translucent (cubic zirconia). The yttrium oxide, or yttria, content
largely defines the mechanical and physical
properties:
* Zirconia containing 3 mole-percent
yttria is strongest (100 percent
tetragonal phase) but most opaque.
* Zirconia containing 5 mole-percent
yttria produces a more translucent
material, with approximately
50 percent cubic phase.
Yttrium increases the zirconia grain
size and lowers the coefficient of thermal
expansion.
Ceramic flexural strength
vs. fracture toughness
Flexural strength specimens are polished
or milled samples (2 by 4 by 22 millimeters)
and the strength is affected by the surface roughness of the specimen.4 Fracture
toughness describes the ability of a material
to resist crack propagation or how a crack
spreads from a notched specimen. Cracks
originate from flaws in materials-created
by finishing, machining or porosity-and
act as an initiation point for crack growth.
For example, a car windshield hit with a
rock chips the windshield, and the crack
spreads from the chipped area across the
windshield.
The energy required to grow the crack
is measured by fracture toughness. Low
toughness can lead to chipping and bulk
fracture in materials. Fracture toughness
measurements for feldspathic porcelain range
from 1 to 1.3, translucent zirconia, 2-4, and
opaque zirconia ranges from 7-9 MPa*m1/2.
Chipping occurs more frequently in the
less-tough veneering materials compared
with the zirconia frame. 5Y-TZP containing
more than 50 percent cubic phase has little
or no low temperature degradation and lower
fracture toughness, and because the material
has little residual stress, water corrosion is
limited. High-cubic-containing zirconia
is weaker but its strength does not degrade
further from low-temperature degradation.
3Y-TZP: Opaque Zirconia
4Y-TZP: Some Translucency
5Y-TZP: Most Translucent
High mechanical properties
High mechanical properties
Lower mechanical properties
White opaque
Some translucency
High translucency
Low temp degradation
Decreased low temperature degradation
Little or no low temperature degradation
Mainly tetragonal phase
Tetragonal and cubic phases
>Cubic less tetragonal phases
Table 1: Defining zirconia used in dentistry by the yttrium content and its effect on physical and mechanical properties.
66
MARCH 2018 // dentaltown.com
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