Ceramics strengthening techniques

By Radwa El-Dessouky

I - Ceramic’s weak points

1.FRACTURE MECHANICS RELATED TO CERAMIC SURFACE

2.TEXTURE DEFECTS RELATED TOPORCELAIN BULK

3.FATIGUE

1.Fracture mechanics of ceramic surface

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.

ceramics are weak in tension and strong on compression…why?!

Compressive forces

Tensile 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

2. Texture Defects related To porcelain bulk

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.

.

•These imperfections may result from:

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;

Small particles

Large particles

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

porosity .

3.Fatigue

• 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.

2. HOW TO OVERCOME THIS POTENTIAL PROBLEM ?

Bonding to

• Metal

• Tooth

• Advanced ceramic core

Disp

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Resid

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ressive stresses

Red

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sile stresses

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BONDING TO:Ductile Metal

TOOTH STRUCTURE

UnbreakableCeramic

core

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 freeIi`Base metal alloy systems: Ni-Cr, Ti

Waxing-up the framework

Spruing

TECHNIQUE

Framework fabrication for

palladium-basedand base metal alloys

Supported framework fabrication forhigh-gold and gold-reduced 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

•DISADVANTAGES TO METAL-CERAMIC RESTORATIONS

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.

Dentin bonded ceramics

An all-ceramic restoration in which crown is bonded to the underlying dentin and any available enamel 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

The tooth is etched with 37%phosphoric acid.

A gingival retraction cord isplaced.

The adjacent teeth are protected with Teflon tape.

resin cement is applied gingival retraction cord is removed

Final result

LUMINEERSA 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.

•Poor esthetic as the little amount of opaquer is unable to mask the tooth shade.• over contoured or look too toothy: if no-drilling technique is applied.

•Disadvantages:

• 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 (In-Ceram Alumina, In-Ceram spinnel & In-Ceram Zirconia) (pure a•Machinable ceramics luminous core &pure zirconia core)

Dispersion

strengthening

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 strength.

quartz Aluminous reinforcement leucite

MicaLithium disilicate

Residual compressive

stresses

Thermal tempering

glazing

Ion exchange

• 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,

I. Thermal tempering:

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:

•This mismatch allows the core material to contract slightly more upon cooling; leaving the veneering ceramic in residual compression

•By coating the core ceramic with a thin layer of a veneering ceramic having a slightly lower (CTE).

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 .

Limitationthe 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.

Minimizing tensile stressesThrough Optimal design

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

contraindicated]

Occlusal force

Compressive stresses

Tensile stresses

cracking

•Using strong core materials

with appropriate thickness; since these stresses

are distributed on the inner surface (core material is

in tension).

• Adequate amount of occlusal

reduction as too little inter-

occlusal 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 feather-edge]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 connector height may be limited by the short clinical molar crowns. 

2. The minimal recommended connector cross section area is 12–16 mm2 although this may interfere 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 .

Avoiding porosity

during fabrication

proceduresTo increase strength

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

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Recommendations

TRANSFORMATIONTOUGHNESS

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.

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