Ceramics strengthening techniques

Ceramics strengthening
techniques

By Radwa El-Dessouky

Griffith flaws

They are minute submicroscopic surface
defects (scratches and cracks) present on the
glass surface and act as stress concentration
centers when subjected to tensile stresses

•Possible causes of ceramic
surface micro-cracks:
•CTE Mismatches between veneer and
core porcelains.
•Heat generation during grinding and
adjustment .
•the destructive, repetitive masticatory
force that occurs in the oral cavity.

Compressive forces
In case of tensile forces, the
forces tend to open crack
sites, resulting in crack
propagation
Compressive
forces, however, tend to
approximate the edges of
surface cracks
Tensile forces

•These imperfections may result
from:
Incomplete fusion of particles during
sintering due to improper Firing time or
temperature

If one crystal is out of line or twisted as
compared to its neighbor, the bonds between
them may be stretched or distorted causing
weakness of ceramic structure.

Ions of the same charge (positive or negative)
may cause electrostatic repulsion leading to
stresses in this region and finally cracks may
occur.

The sizes and shapes of porcelain
particles can be an important factor;

Thermal stresses which may develop
during cooling can create internal
flaws, causing the fused porcelain
particles to separate at their interface
porosity .

•Fatigue refers to the
degradation of strength over time.

• Clinically, ceramic crowns must
function in the presence of
moisture, externally from saliva and
internally from a cementing agent
.
• Two types of loading
conditions can lead to fatigue:
Cyclic (repetitive) loading & static
loading. In the oral
environment, there is a combination
of both conditions.

•The possible mechanism of
ceramic’s fatigue:
I. a chemical reaction between water
molecules and glass surfaces .
II. The absorbed moisture lowers the
energy required for crack
propagation .The pre-existing flaws
grow to critical dimensions.

III. Since stress concentration increases
with length, crack propagation
continues until the load is removed or
fracture occurs.

Reduction of
tensile stresses

•Metal

•Tooth

•Advanced
ceramic core

Dispersion
strengthening

Bonding
to

PFM
System

Fusing the porcelain to
an oxide coated metal
provides rigid support
against propagation of
ceramic surface cracks
when
exposing
to
tensile stresses

•The currently used metal-ceramic systems:
i. Nobel metal alloy systems: high gold, low gold and gold
free
Ii`Base metal alloy systems: Ni-Cr, Ti

Waxing-up the framework

Spruing

Supported framework
fabrication for
high-gold and gold-reduced
alloys

Framework fabrication for
palladium-based
and base metal alloys

Checking the ceramic space with
silicon key

sandblasting

After oxidation firing cycle

Application of opaquer layer (2 layers) with a
brush and then, firing

After ceramic veneering, firing and finishing

Success of Metal-ceramic systems depends
on a strong bond between the metal and
fused ceramic which can be;
•Mechanical Interlocking: the roughened
alloy surface produced by sandblasting
provides irregularities into which porcelain
can flow.
•Van der Waal forces or "wetting bonds”
Depends on surface tension of porcelain in
the liquid state (its contact angle and
wettability)

A contact angle greater than 90 degrees
indicates a lack of wetting
and, consequently, lack of adhesion .

• External surface compression :
thermal expansion coefficient [CTE] of
the veneered porcelain must be slightly less
than
that
of the metal
alloy. During
cooling, the porcelain is held in a state of
compression as shrinkage of the metal
occurs.
Chemical bonding:
through the oxide layer at porcelain-metal
interface a chemical bonding between the
porcelain to metal occurs.

•For precious metal ceramic bond:
Bonded tin foil: platinum foils are electroplated
with a layer of tin oxide to which aluminous
ceramic is attached

•For Base metal ceramic bond:
Base metals can form oxides on their surface
through an oxidative firing

1.Inadequate esthetic, this can be due to:
1. loss of translucency found in natural teeth. In
addition, the underlying metal color often
penetrates the porcelain making it appear
grayer than the surrounding teeth .

2. Opaque porcelains are used
to mask the metal coping;
however,
they
are
highly
reflective causing a less than
natural appearance . In an
attempt
to
avoid
this
reflection,
metal-ceramic
restorations
are
often over
contoured.

2. Galvanism .
3.allergic reactions
in the gingiva.

An all-ceramic
restoration in which
crown is bonded to the
underlying dentin and
Dentin bonded
any available enamel
ceramics
using a composite
resin–based luting
material
This method reinforces the ceramic structure
with no need to internal strengthening
mechanisms.

The bond is mediated through;
1.Mechanical interlocking; through
sandblasting and etching [a hills and valleys
microscopic appearance]. When the resin flows
over the etched surface, it flows into surface
imperfections, and when the resin hardens, the
imperfections act as undercuts firmly bonding the
resin in place.

2.Chemical bond; through application of silane
coupling agent which bonds through the Si
molecule to the silica in the porcelain and to the
acrylic bonding agents in restoration.

Technique

The internal surface of the
sandblasted crown is etched
with hydrofluoric acid

After etching ,The crown is
opaque white.

Silane is applied to the crown

A gingival retraction cord is
placed.

The adjacent teeth are
protected with Teflon tape.

The tooth is etched with 37%
phosphoric acid.

resin cement is applied

gingival retraction cord is
removed

Final result

LUMINEERS
A veneering
system which
can be fabricated
so thin that tooth
reduction is not
usually
necessary.

Its structure and manufacturing resemble that
of EMPRESS I.

•How do Lumineers differ from traditional
porcelain veneers?
•They are thinner. The typical Lumineers will
measure on the order of 0.2 to 0.3 mm thick while
the traditional veneer’s thickness ranged from 0.4
to 0.8mm.
•This decreased thickness means that a dentist can
bond an ultra-thin Lumineer directly to an
unprepared tooth, without creating an end result
that is grossly over contoured.
•Lumineers ® can be placed using a "no drilling /
no shots [anesthesia]" protocol.

•Disadvantages:

•Poor esthetic as the little amount of opaquer
is unable to mask the tooth shade.
• over contoured or look too toothy: if nodrilling technique is applied.

•Indications:
•The patient demands a no-drilling placement
process: ex; people with dental phobias.
•The patient demands a totally reversible
procedure: using a no-drilling technique offers the
possibility that they can be removed if the patient
is unhappy with the appearance they have
created
.
•(In-between visits to avoid the problems
associated with appearance, roughness, or thermal
sensitivity.)

high strength
ceramics

•Pressable
ceramics
(Empress I & IPS e.max)
•Infiltrated ceramics (InCeram
Alumina,
InCeram spinnel & InCeram Zirconia) (pure a
•Machinable
ceramics
luminous core &pure
zirconia core)

When a tough, crystalline material such as alumina
(Al2O3) is added to a glass, the glass is toughened
and strengthened, because the crack cannot pass
through the alumina particles as easily as it can
pass through the glass matrix.

•The magnitude of increased strength
depends on:
 the

crystal type; toughness and their
geometrical shape.
 crystal size; small crystals are better.

 the inter-particle spacing; close approximation
is better
relative CTE to the glass matrix as a close
match between the CTE of crystalline material
and the surrounding glass matrix increases

quartz

Aluminous
reinforcement

Lithium disilicate

leucite

Mica

Residual
compressive
stresses

Ion
exchange

Thermal
tempering

glazing

I. Thermal tempering:
• rapidly cooling the glass
surface while the center is
hot and in the molten state
produces a skin of rigid glass
surrounding the molten core.
•As the molten core solidifies
it tends to shrink. The pull of
the solidifying molten core,
as it shrinks, creates residual
compressive stresses within
the outer surface,

Limitations;
Simple shapes are required such that
uniform stresses distributions can
occur, Dental restorations, however, are
characterized by complex shapes, sharp
angles and varying thickness.

2. Glazing:
•By coating the core
ceramic with a thin layer
of a veneering ceramic
having a slightly lower
(CTE).
•This mismatch allows the core material to
contract slightly more upon cooling; leaving the
veneering ceramic in residual compression

3) Ion Exchange or chemical tempering:
•This process involves the exchange of larger
K+ ions for the smaller Na + ions (a common
constituent of a variety of glasses).
•By placing the glass in a bath of molten
potassium nitrate, K+ ions in the bath exchange
places with some of Na + of the glass particles.
The K+ is larger than the Na + . crowding of the
K+ ions in place previously occupied by the
smaller Na + ion creates residual compressive
stresses in the surfaces of the glass .

Limitation
the depth of the compression
zone is less than 100 μm, so
that this effect would be
easily worn out after long–
term exposure to certain
inorganic acids.

•Proper patient selection [ex;
bruxism or deep bite are
contraindicated]

•Using strong
core materials with
appropriate
thickness; since
these stresses are
distributed on the
inner surface (core
material is in
tension).

Occlusal force
Compressive
stresses

Tensile stresses

cracking

•Adequate
amount of
occlusal
reduction as
too little interocclusal space
during tooth
preparation can
be a potential
cause of
fracture under
occlusal loading

•The marginal designs generally
accepted during ceramic crowns
preparation are;
•deep chamfer
• flat shoulder
• shoulder with rounded internal
angles.

•Acute angled preparations
[beveled or featheredge]finish lines are to be
avoided

•Non-uniform finish line causes the
porcelain at the cervical region to vary in
thickness with a potential for premature
fracture during fabrication procedures, in the
process of seating or after cementation.

•All transitions and line angles are to be
rounded to avoid stress concentrations.

•Accurate registration of
occlusion, avoiding the
premature
contacts
which may act as stress
bearing zones on the
ceramics.

•Adequate cement gap
(internal relief) to avoid
tensile stresses exerted
from excess luting
cement on ceramic
crown

•Adhesive cementation is preferred
because conventional cements are strong in
compression and weak in tension.

In case of a FPD,
The connectors
are the weakest
point and the
most stress
bearing area

•To reinforce the connector area:
1. Increasing the
connectors height
to at least 4mm.
However, in the
posterior region
subjected to higher
loads, the
2. The minimal recommended
connector height
connector cross section
may be limited by
area is 12–16 mm2
the short clinical
although this may interfere
molar crowns.
with biological and esthetic
considerations.

Continuous change in
dimensions

Sudden change

Increasing the Radius of curvature at the
connectors area in the gingival embrasure to
0.45 mm increases the fracture resistance as it
allows the crack to propagate smoothly from
the gingival embrasure toward the pontic
smoothly .

Recommendations
•Good condensation technique
•Programmed firing schedule
•High pressure compaction
•Vacuum firing.
.
•Glazing.
• Gradual cooling is important to avoid
stresses development and cracking

Zirconia (ZrO2) ceramic is a good example for
this mechanism. The material is polymorph
occurring in three forms:
monoclinic (M) at room temperature
tetragonal (T) ≥ 1170C and cubic(C) ≥ 2370°C.

Pure zirconia at room temperature

Pure zirconia at 1170 C

Transformation from the monoclinic to the
tetragonal phase is associated with a 5%
volume decrease.
Reversely, during cooling, the transformation
from the tetragonal to the monoclinic phase is
associated with a 3% volume expansion.

The inhibition of these transformations
can be achieved by adding stabilizing
oxides (CaO, MgO, Y2O3), which allow
the existence of tetragonal-phase
particles at room temperature.

When stress develops in the tetragonal
structure and a crack in the area begins to
propagate,
the tetragonal grains transform to
monoclinic grains.
The associated volume expansion
results in compressive stresses at the
edge of the crack and extra energy is
required for the crack to propagate further.

Ceramics strengthening techniques

Ceramics strengthening techniques

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