This document discusses polycrystalline dental ceramics. It describes the properties and fabrication techniques of aluminous oxide and zirconia oxide, the two main polycrystalline ceramics used. Aluminous oxide is indicated for crowns but cannot be etched, while zirconia has higher strength and toughness due to a phase transformation mechanism. The document outlines the crystal structures and stabilization of zirconia, as well as methods to strengthen dental ceramics including introducing compressive stresses, interrupting crack propagation, and minimizing tensile stresses through design.

1. Polycrystalline Dental
Ceramics
By
Mohamed Mahmoud Abdul-Monem
Dental Biomaterials Department
Faculty of Dentistry
Alexandria University
Egypt

2. What are polycrystalline cermaics?
• Solid-sintered, monophase ceramics are materials
that are formed by directly sintering crystals
together without any intervening matrix to from
a dense, air-free, glass-free, polycrystalline
structure.
• There are several different processing techniques
that allow the fabrication of either solid-sintered
aluminous-oxide or zirconia-oxide frameworks.

3. Aluminous oxide
• The first fully dense polycrystalline material
for dental applications was Procera® AllCeram
alumina (Nobel Biocare) with a strength of
approximately 600 MPa.
• The alumina powder is pressed and milled on
a die and sintered at about 1600°C, leading to
a dense coping but with approximately 20%
shrinkage

4. • Alumina core ceramic is indicated for anterior
and posterior crowns.
• Alumina cannot be acid etched to produce
micromechanical retention silica-coated
alumina particles thus sandblasting the
surface with is required to ensure sufficient
resin bonding.

6. Zirconia
• Zirconia has unique physical characteristics
that make it twice as strong and tough as
alumina-based ceramics.
• Zirconia occurs as a natural mineral called
baddeleyite. This mineral contains 80–90%
zirconium oxide. The major impurities are
usually TiO2, SiO2 and Fe2O3.

7. • This oxide exists in three different crystal structures:
monoclinic at room temperature, tetragonal at
~1200°C and cubic at 2370°C.
• Zirconium oxide is transformed from one crystalline
state to another during firing.
• At the firing temperature, zirconia is tetragonal and at
room temperature, it is monoclinic, with a unit cell of
monoclinic occupying about 4.4% more volume than
when tetragonal.

8. Zirconia phase transformation

9. • ZrO2 adopts a monoclinic crystal structure at room
temperature and transitions to tetragonal and cubic at
higher temperatures.
• The volume expansion caused by the cubic to tetragonal
to monoclinic transformation induces large stresses, and
these stresses cause ZrO2 to crack upon cooling from high
temperatures.
• When the zirconia is blended with some other oxides, the
tetragonal and/or cubic phases are stabilized.
• Effective stabilizers include magnesium oxide (MgO),
yttrium oxide (Y2O3, yttria), calcium oxide (CaO), and
cerium(III) oxide (Ce2O3).

10. • Zirconia is often more useful in its phase 'stabilized' state. Upon
heating, zirconia undergoes disruptive phase changes.
• By adding small percentages of yttria, these phase changes are
eliminated, and the resulting material has superior thermal,
mechanical, and electrical properties.
• In some cases, the tetragonal phase can be metastable. If sufficient
quantities of the metastable tetragonal phase is present, then an
applied stress, magnified by the stress concentration at a crack tip, can
cause the tetragonal phase to convert to monoclinic, with the
associated volume expansion.
• This phase transformation can then put the crack into compression,
retarding its growth, and enhancing the fracture toughness. This
mechanism is known as transformation toughening, and significantly
extends the reliability and lifetime of products made with stabilized
zirconia.

11. Zirconia transformation toughening

14. • Zirconia may be in the form of blocks that are
milled to create the frameworks (CAD/CAM).
• Mostly, they are fabricated from a porous block,
milled oversized by about 25%, and sintered to
full density in a 4 - 6 hours cycle.
• Alternatively, fully dense blocks are milled.
However, this approach requires approximately 2
hours of milling time per unit whereas milling of
the porous block necessitates only 30 to 45
minutes for a three-unit bridge.

15. Properties of zirconia
• Low thermal conductivity (20% that of alumina)
• Chemically inert ant corrosion resistant
• Flexural strength 900 Mpa
• Fracture toughness 8-10 MPa · m1/2
• High fracture resistance
• Wear of opposing dentition(Monolithic Zirconia)
• Difficulty in adjusting occlusion

16. Fracture toughness of zirconia
• Fracture toughness of zirconia tends to
increase with increasing grain size (0.9µm-1.4
µm).
• There is a decrease in strength caused by very
large grain size(1.8 µm) caused by premature
phase transformation leading to microcracking

17. Hydrothermal degradation of zirconia
• Hydrothermal degradation of zirconia occurs
between 200-400 ˚C .
• Longer exposure times at oral temperature
may also degrade zirconia leading to increased
surface roughness,fragmanted grains and
microcracks .

18. Zirconia toughened alumina ZTA
• 70-90% alumina
• 10-20% zironia
• Toughened by a stress-induced transformation
mechanism of zirconia leading to compressive
stresses within alumina.
• The strength of alumina is doubled and
toughness is increase 2-4 times .

20. Methods of strengthening Dental
ceramics
Strengthening
Methods
Strengthening brittle
materials
Introduction of
residual
compressive
stresses
Ion strenghtening
Thermal
compatibility
Interruption of
crack propagation
Incorporation of
crystalline phase
Heat treatment
ceramming
Methods of
designing to
Minimize tensile
stress through
design
Avoid stress raisers

21. 1.Strengthening of brittle materials
a.Introduction of residual compressive stresses
Ion strengthening
• Replacing smaller ions
by relatively larger ones
• As a result crack growth
from surface flaws is
more difficult
Thermal compatibilty
• In PFM ,metals and
porcelain are designed
with a slight mismatch
in COTE (metal slightly
higher)
• Metal contracts more
on cooling
• This leaves porcelain in
residual compression

22. 1.Strengthening of brittle materials
b.Interruption of crack propagation
Incorporation of crystalline
phase
• Tough crystaline material
as alumina or leucite is
added to galss in a
particulate form,the glass
is toughened and
strengthened (Dispersion
tougheneing)
Heat treatment ceramming
• A glass cerammic material
is fabricated in vitreous or
noncrystalline state and
then converted to a
crystalline state by heat
treatment to induce
partial devitrifaction
• Crystalline
particles,needles or plates
formed during ceramming
serve to interrupt cracks

23. 2.Design of dental restorations to :
a.Minimize tensile stress
• In case of PFM ,metal
copings act as the
foundation.
• The strong yet more
ductile metal prevents
the interior of porcelain
portion of the crown from
being subjected to tensile
stresses.
b.Avoid stress raisers
• Sharp line angles in
preparation should be
removed.
• Sharp line angles in
coping surface should be
avoided
• Sudden changes in
porcelain thickness
should be avoided.

24. Thank you