Dental Ceramics I

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Metal

Back 1

metallic bonds

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Polymer

Back 2

covalent, ionic, van der Waals

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Ceramic

Back 3

metallic, covalent

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All Ceramic

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Denture Teeth
Anterior/posterior bridges
Crowns
Veneers
Inlays/Onlays

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Metal-Ceramics

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Crowns
Anterior/posterior bridges

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Composition of a Conventional Dental Ceramic

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Silica (quartz)
Feldspar (Al and Silica ion SiO4) - forms glass during firing
Glass modifier - breaks down silica network to &darr viscosity and softening temp and &uarr thermal explansion coefficient and can &darr chemical durability

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Microstructure

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Feldspathic - Glass matrix
Filler particles - quartz

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Glass Modifier

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Glass Network can be altered to have &darr fusion temp
K2O, Na2O, CaO, MgO, PbO and rare-earth oxides disrupt SiO4 or BO3 network

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Fabrication of a Feldspar Porcelain

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Condensation
Firing Procedure
Glazing and Shading

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Condensation

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packing of particles to a desired form prior to firing, including vibration, spatulation, brush technique

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Firing

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fusing powder together to form a solid (sintering)
Preheating to drive off water and sintering in vacuum
Formation of leucite may occur

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Shrinkage caused by Sintering

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no chemical rxn needed
partial melting at the area of contact, densifies the object by elimination of pores

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Air-fired vs. Vacuum-fired

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Vacuum needed when firing porcelain

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Glazing and Shading

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Add-on stain and glaze to provide lifelike appearance

Built into the ceramic rather than merely applied to the surface, also called internal staining

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Glazing eliminates flaws from surface

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Glazing can be add-on or self-glaze
Self-glaze is better (smoother and less plaque retentive)

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Mechanical Properties of Ceramics

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covalent, ionic bond
non-ductile material
no plastic deformation

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Brittle vx. Ductile

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Movement of dislocation consumes energy of crack propagation

Ceramics are brittle - No plastic deformation, but breaking of covalent bonds

Metals are ductile - have dislocations and show plastic deformation

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Lack of dislocation movement

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Dental ceramics should fail near their theoretical strenght under tension, but they always fail at a much lower strength

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Lack of plastic deformation

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results in ceramics having lower tensile strengths than their compressive strength values

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Brittle

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covalent and ionic bonds do not allow dislocation and slip

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Low tensile and shear strength

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inherent porosites and surface defects developed during cooling

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Effect of water

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Alkaline ions may be replaced by hydrogen ions, which attracts water.
Water molecules can modify the glass network

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Strengthening Brittle Materials

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Ceramic materials most likely will fracture under tensile stresses. If we can introduce some degree of residual compressive stress surface on the surface, we should expect to see &uarr resistance to tensile stress. In other words, it &uarr the tensile strength of the material

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How do we introduce residual compressive stresses on the surfaces of brittle materials? (for the purpose of strengthening)

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1. Ion Exchange
2. Thermal tempering
3. Thermal expansion coefficient mismatch

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Ion Exchange

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replaces some of the ions near the surface with larger ions

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Tempering

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Hot glass (no stress and not rigid)
Surface cools down and contracts. Center that remains hot and flowable adjusts to the dimension of the surface. The vertival dimension will increase.

Center cools and contracts more thane the surface. Surface is then under compression by the center and the center is under tension by the surface

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Thermal Expansion and Contraction Mismatch

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If A has a higher thermal coefficient, on heating it, it will become bigger than B

If you bond A and B together chemically at room temp, and then heat the strip, they will expand and contract together

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If materials A and B are bonded together at HIGH temp and then cooled to room temp, what will happen?

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The one that has to stretch (the smaller one) is the one under tension

The one that is bigger will be compressed

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Ideal Arrangement

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If materials A and B are chemically bonded together at high temp, and the difference in thermal contraction coefficient is within a limt, the 2 components will remain bonded with some dimensional adjustment between the 2 components

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expansion coefficient

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the higher the expansion coefficient, the more the contraction

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Prevent Crack during Cooling

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Use a slower cooling rate - it &darr thermal contraction differential between the surface and the core

Use a material with a lower thermal contraction coefficient - it results in the same as above

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Interruption of Crack Propagation

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Dispersion Strengthening
Transformation Toughening

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Dispersion of a Crystalline Phase

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Similar to filler particle reinforced composite resins
Aluminum oxide in a glassy ceramic
Dicor - a glass ceramic, requires a heat txt that causes micron-sized mica crystals to grow in the glass

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Crystal content

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Ceramics containing primarily glass phase can be strengthened by increasing crystal content of leucite, lithia disilicate, alumina, magnesia-alumina prinell, zirconia

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Thermal expansion coefficient of the crystal phase

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should be higher than that of the matrix phase to achieve strengthening

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Transformation Toughening

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Oxides such as MgO, Y2O2 and CaO can slow down or eliminate crystal structure changes, preserving the tetragonal phase at room temp

Stabilizing Zirconia

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ZrO2 TZP

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Tetragonal Zirconia Polycrystal
composed of only the metastable tetragonal phase
when you apply stress to this phase at room temp, it will induce monoclonic