Ceramics Lectr Apr2011

8/6/2019 Ceramics Lectr Apr2011

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H. Ralph Rawls Biomaterials

UTHSCSA Dental School


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UTHSCSA Biomaterials 2

Uses of Ceramics in

Dentistry

PFM crowns, bridges & fixedpartial dentures

All-ceramic crowns, inlays,

onlays and veneers

Ceramic denture teeth


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Other Ceramic Materials

- Grinding Wheels


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Ceramic Highly reacted Inert compounds Oxides such as SiO2, MgO,

Al2O3, Gypsum

Glass Ceramic Amorphous

Non-crystalline Single phase


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CERAMICS


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PORCELAIN


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Porcelain - Raw Ingredients

Feldspar the main raw ingredient K2O

Al2O3 6SiO 2

K2OPotash

Al2O3 Alumina

SiO2Silica

(potash feldspar )

Na 2O

Al2O3 6SiO 2

(soda feldspar or albite )

Na 2O Soda


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AmorphousCrystalline


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COMPOSITION

Various ions canreplace Si : Mg, Al, Ca Na, B, KFor example,replacing with: Na reduces meltrange Al increasesmelting range

2-DimensionalSilicate Structure

Glass

Si

Quartz


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Dental CeramicsProcessing Methods

SINTERING OF PORCELAIN

Richard Van Noort: Introduction to Dental Materials, 2nd edition Dental Ceramics http://www.fleshandbones.com/readingroom/

The thermochemical reactionsbetween the porcelain powder components are virtually completedduring the original manufacturing

process

The purpose of firing is to

sinter - FUSE TOGETHER -the particles of powder toform the prosthesis


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High- (1200-1400C) & Medium- (1050 1200C) Low-fusing (800 1050C)

porcelains

Opaque, dentine & enamelporcelains Compaction Firing sintering

Glazing

Production of porcelain jacket crowns


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PFM - Fabrication


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Castable glass

ceramic


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Indications Crowns Inlays Onlays Veneers

Leucite-reinforced ceram ic Based on a glass containing

latent nucleating agent

Pressing heated ceramic material

Permits precise

reproduction of full-contour waxed restorations

IPS Empress :

Hot PressingLeucite-reinforced glass ceramic

Empress


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Mechanical Performance

Metals usually undergoplastic deformation

Ceramics exhibit brittle fracture

Brittle behaviour

Necked region


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Surface Defects Theoretical strength of a glass isapproximately 100 times greater thanits measured strength. This is due to surface defects

voids scratches microcracks

Known as Griffith's flaws.


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Glass

Load

Griffith flaws

When a tensile load is applied . Flaws act as stress concentrators This produces cracks

Propagationof cracks

Cracks propagate and cause fracture failure


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Fracture of ceramics

and glasses Maxstrain that glass can withstand is 0.1%

Glass is extremely sensitive to thepresence of surface microcracks

Tiny flawsin the interior of thecrown act as initiating sites forcatastrophic failure

Two solutions provide support using astronger substrate Produce ceramics that arestronger and tougher


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Methods of strengtheningporcelains


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Stronger substrates tosupport the porcelain

Reinforced ceramiccore systems support is provided

by another ceramicmaterial

Metal-ceramics theceramic is supportedby a strong and toughmetal


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Crack propagation in an non-reinforced glass

Crack propagation

Crack propagation in a reinforced glass


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Strengthening Mechanisms

Glazing/overglazing

Application to metal Dispersion strengthening

Ion exchange Transformation toughening

Glazing/overglazing

Dispersion strengthening

Ion exchange Transformation toughening


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Glazing/Overglazing Glazed porcelain is significantly

stronger than unglazed porcelain. Glazing minimizes the number and

size of surface flaws. This limits crack propagation within

the porcelain.


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FiringAtmosphere

SurfaceCondition

Modulus of Rupture (psi)

Porcelain Air

Vacuum

Ground

Glazed

Ground Glazed

11,000

20,465

11,547 19,187

Material

Surface Glaze and Vacuum

Removal of Defects

Glazing almost doubles strength


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Application of

Porcelain to Metal


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Na+

Na+

Na+

Na+ Na+

Na+

Na+ Na+ Na+

K+

K+ K+

K+

Na+ Na+

Na+

Na+

Na+Na+

Na+

Na+Na+ Na+K+ K+ K+ K+

Compression

Na+

Small ions are replaced by large ions

Introduces compressive stress into the surface Closes surface defects and increases strength

Ion Exchange


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Type of ceramic Flexuralstrength(MPa)

Feldspathic porcelain Alumina coreGlass-ceramic DICOR Non-shrink alumina ceramic Cerestore Leucite-reinforced ceramic Optec

Aluminous core porcelain Hi-Ceram Heat-pressed reinforced ceramic Empress Glass-infiltrated alumina ceramic In-Ceram High alumina, 98% purityGold alloy (yield strength)

65-8592-12490-124

105-114105-120

140-141160-180400-446420-520350-600

Brand name


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CAD/CAM Ceramics that can be milled(machined) to form inlays,onlays , and veneers.

The CEREC [ chair-side ]system

CAD/CAM C omputer Assisted Design /C omputer Assisted Machining

Can produce restorations in oneoffice visit.

Quick turn-around from a lab (Craig, Robert G.. Restorative Dental Materials, 11th Edition .

Elsevier, 2002. 18.5.3).


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Computer-ControlledMilling Machine


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All-Porcelain Application

Pure Alumina Core Procera AllCeram 99.5% sintered alumina Digitized scan of die sent to Sweden Coping returned usually w/in 24 hrs

Veneered with feldspathic Flexural Strengtharound 700 MPa


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All-Porcelain ApplicationGlass-Infiltrated Ceramic Core

In-Ceram Spinel / Alumina / Zirconia

Not Slip-castdry-pressed into blocks

Machined / CAD-CAM Cerec or Celay Flexural Strength

even >700 MPa!


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Other Advantages of

CAD/CAM Systems Bonding ceramic restorations withresin cement helps to compensate for

the problems of poor marginal fit. Fracture of ceramic restorations can

be prevented if the preparation hasadequate thickness to resist occlusalforces.


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Disa dvantages of

CAD/CAM Systems Marginal accuracy can be poor , with values of 100to 150 m

Longevity less than PFM crowns . Durability excellent for inlays; however, full posterior

crowns tend to break within 4 years.

Color match limited Ceramic blocks difficult tocolor match

Technically challenging : Assistants oftenintimidated by CEREC technology High cost CEREC System is ~ $100,000.

This cost is transferred to the patient making the cost of aCEREC inlay $500 - $1,500.


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Translucency Transmission of light via an indirect path through a material Pores or crystalline regions reflect and refract a light beam

Causing it to be scattered in manydirections as it passes through



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Opacity Zero light transmission.A large amount of scattering can

cause opacity . Color (hue ): Blue and violet arescattered more than yellow and red.

Therefore scattering results in a weakchange in color shade (hue).



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ABSORPTION Removal of a portion of light (at certain wavelengths) passing

through a material by electron interaction Principal cause of color Portion not absorbed is perceived ascolor

reflected

scatteredor transmitted



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Color Space Has three dimensions: HUE Dominant wavelength (blue, etc.)

VALUE Grey scale ( black )(high value low value)

CHROMA - Color saturation, or color intensity

Pastels have low chroma:

is low in chroma

but

is high in chroma. Figure 2.A color wheel showing complementary colors.HUE

Component of Color Space


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