Dental ceramics part1

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• Keramikos = Burn stuff

i.e. All desirable properties are achieved by Firing

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• General properties:

• Affected by

Ionic bonding Covalent bonding

Both types

atomic bonding

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1. High melting temperature.

2. Thermal & electrical insulators.

3. Transparent unless porous Translucent.

4. Mechanical properties: a) Brittle

b) Low fracture toughness.

c) Hard.

5. Chemically inert due to: a) Formed by oxidation Maximally reacted No free electrons.

b) Strongly bonded No leaching out of constituents.

6. Chemically indestructible Dissolves in strong acids as HF

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• Important terms for silica based ceramics:

Gt (Fictive temperature) Rigid & hard porcelain < Gt< Viscous & soft

Ceraming

Vetrification (Glass forming) Devetrification (Crystallization)

Glass ceramics

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Uses: Denture teeth.

Jacket crowns.

PFM.

Inlays.

Advantages Disadvantages

1. Satisfactory aesthetic.

2. Glazed surface compatible with soft tissue.

3. Resist sudden thermal changes.

4. Adequate strength.

1. Different optical properties than En. & Dn.

2. Additional tooth preparation.

3. Wear of opposing natural teeth.

4. Brittle.

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Composition: (generally vitreous ceramic based on silica network and feldspar)

1.Feldspars:

(75-85%)

Anhydrated aluminum silicate Potassium aluminum silicate (orthoclase)

(K2O. Al2O3.6SiO2) + Sodium aluminum silicate (albite) (Na2O. Al2O3.6SiO2)

1290ºC Glassy phase that gives the form of the restoration.

Potasium feldspar + metal oxides Firing Leucite (Crystalline mineral with α)

2. Silica:

(12-22%)

Quartz Crystalline dispersion in glassy phase Strengthening agent.

3. Kaolin:

(3-5%)

Hydrated aluminum silicate (Al2O.2SiO2 2H2O) With water Sticky Workable mass

Molding of unfired porcelain.

On heating Framework as feldspars Shrinkage and opacity.

4. Others:

a) Glass modifier: Metal ions: Na+,K+ or Ca+ Associated with O atoms Bond strength

Softening temp.

Effects Viscosity of glass.

Softening temp.

Thermal expansion.

↑↑ Conc. Chemical durability of glass Resistance to attack by water,

alkalis and acids.

Crystallization of glass during firing.

b) Fluxes: In low fusion glasses Fusion temp.

c) Pigments:

(< 1%)

As metal oxides Required color.

d) Sugar&Starch: Help manipulation of powder. 7

Manufacturing:

1. Pyrochemical reaction:

All components Blending Melting at temp.

2. Quenching:

The red hot material Quenching Porcelain powder (Frits)

supplied to dental lab.

The frits are composed of

Vitreous (glassy) phase (Feldspars & Kaolin)

Crystalline phase (Silica & Metal oxides)

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a) General classification of

porcelain restorations:

All ceramic

restorations

PFM - Feldspar

1. Core reinforced ceramics: -Aluminous porcelain.

-Hi – Ceram.

-In – Ceram.

-In – Ceram spinell.

-Magnesia core porcelain.

2. Feldspar: -Low – leucite.

-High – leucite (Optec).

3. Cast glass ceramic: -Mica (Dicor).

-Leucite Injection molded glass (Empress).

4. Milled ceramic restorations -CAD/CAM (Cerec).

-Copy milling (Celay).

Classifications of dental porcelain

b) Acc. to fusion temp.

-High fusing 1300ºC denture teeth

-Medium fusing 1100 - 1300ºC

-Low fusing 850 - 1100ºC Cr & Br

-Ultra-low fusing < 850ºC

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Classifications of dental porcelain

I. According to their application:

Core porcelain it must have adequate mechanical

Properties.

Dentin porcelain it gives the shape and color.

Enamel porcelain it is quite translucent

II. According to the method of firing:

At atmospheric pressure.

At reduced pressure (under vacuum).

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III. Accoording to fusion temp.

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High fusing 1300ºC Denture teeth

Medium fusing 1100 - 1300ºC

Low fusing 850 - 1100ºC Cr & Br

Ultra-low fusing < 850ºC

Methods of fabricating ceramic restorations:

- Condensation and sintering (Conventional technique).

- Pressure molding and sintering.

- Casting and ceraming.

- slip- - casting.

- Sintering and glass infiltration.

- Milling by computer control.

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Porcelain manipulation (Technique):

Conventional condensation and sintering for PJC:

Procedures:

Dentist

(In the clinic)

Technician

(In the lab)

1. Tooth preparation

2. Impression

3. Model or Die

4. Platinum foil matrix (0.025 mm)

5. Application of porcelain layers:

-Opaque shade.

-Dentin shade.

-Enamel shade.

Stages of porcelain application:

-Compaction. •Vibration.

•Spatula pressure.

•Sprinkling dry powder

-Firing in electric oven (muffle) Loss of water & Sintering

-Glazing Shiny impervious surface.

Done by Self-Glaze

Applied Glaze

-Cooling Slow cooling Avoid Crazing 13

Tooth preparation Impression Die preparation

Platinum foil matrix

Opaque layer application Porcelain application

& Compaction Firing

Glazing Cooling

Correction and

adjustment

Fired restoration

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Properties:

1. Firing shrinkage: Linear shrinkage = Low fusing 14% , High Fusing 11.5%.

Volumetric Shrinkage = Low fusing 32 -37% , High Fusing 28-34%.

Cause Loss of water Voids Sintering Voids Shrinkage.

2. Porosity:

(1.3 – 4.8%)

Effect Strength & Opacity.

To reduce to 1.3% 1. Firing under vacuum

2. Use powder with different particle size

3. Chemical prop.: Chemically stable Dissolved by strong acids (HF).

4. Mechanical

prop.:

Brittle

% Elong.

Shear St.

0.1%

110 MPa

Tens. St. Diam. Com.

Trans. St.

Comp. St.

34 MPa

62-90 MPa

172 MPa

E.

KHN

69 GPa

460 kg/mm2

5. Specific gravity: 2.2 – 2.3

6. Esthetic & color: Excellent but not perfect

Affected by Nature of incident light.

Porcelain translucency, reflectance, fluorescence.

Lanthamide earth Fluorescence.

7. Biological effect: Very inert No adverse effect to surrounding tissues.

But It is a very bulky restoration Needs excessive tooth reduction.

May produce stresses on the gingiva.

It is very hard Attrition of opposing natural dentition.

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Denture porcelain teeth: Made from high and medium fusing temperature.

Anterior teeth Retained in denture by Pins.

Posterior teeth Retained in denture by Diatoric Channels.

Advantages Disadvantages

1. Aesthetic > Acrylic.

2. More resistance to wear.

3. Allow denture rebasing.

1. Brittle.

2. Clicking sound with opposing.

3. Needs high interarch space

They can not be ground.

4. Very rigid and not resilient

Transmit stresses to bone

Excessive bone resorption.

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I. PROPERTY Acrylic T. Porcelain T.

Mechanical properties Not brittle not resistant to

abrasion

Brittle resistance to a abrasion much

lower than denture base

aesthetics Reasonable More aesthetics

Thermal expansion A teeth similar to denture

base

Have a large difference between the

acrylic base and it So, crazing occur

Density 1.2 gm/C3 2.4 gm/cm3

Retention to denture

base

Chemical bonding Mechanical bonding to dentate base

under cut or pins

Adjustment Can be ground and replaced Cannot ground cannot replaced

Inter ridge space Require smaller space Require larger space

Performance in service • Highly resilient

• Less force transmit to

underling mucosa

• No clicking on contact

with giving natural feeling

• Least resilient

• Low resilience transmits moor

force to underling mucosa.

• Clicking sound occurs on contact.

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Ceramics strength values are much lower than the

predicted values based on their primary intratomic bond.

Why do ceramics fail at

stresses far below their

theoretical strengths?

Scratches and surface defects

http://onlyhdwallpapers.com/wallpaper/ice_winter_snow_cold_frozen_rivers_river_desktop_1600x1200_hd-wallpaper-897222.jpg

Stress concentration and crack propagation

Stress concentration at the tip

Exceeding of theoretic strength

Bond breakage

Crack propagation

Failure

F

=F A’

F

A >

F

STRESS

RAISERS

http://gautampuri.com/research.html

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