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Ceramics
- Compound involving metallic and nonmetallic materials
- Ionic or covalent bonding
- Crystalline or amorphous
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Esthetic restorations
- Translucent
- Low corrosion and wear
- High modulus
- Veneers, inlays/onlays, crowns, bridges
- Brittle, susceptible to fracture
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Porcelains
- Mixtures of three law ingredients
- -Feldspar (anhydrous aluminosilicates)
- -Quartz (silica)
- -Kaolin (clays, hydrated aluminosilicates)
- Dental porcelains are different from earthenware, stoneware, and domestic porcelain
- Dental have NO KAOLIN
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Kaolin
- Hydrated aluminosilicates (Al2O· 2SiO2· 2H2O)
- Acts as a binder, increasing molding ability
- Opaque omitted for dental porcelains
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Quartz
- Acts as a strengthening agent
- Fine crystalline dispersion throughout the glassy phase of the feldspar
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Feldspars
- Mixtures of potassium aluminosilicate (K2O· Al2O3· 6SiO2) and sodium aluminosilicate (Na2O· Al2O3· 6SiO2)
- Potash (K2O) increases the viscosity of the molten glass
- Soda (Na2O) lowers the fusion temperature
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dental porcelain
- mostly composed of silica & alumina ;
- Powder preparation
- 1. Melting of mixed components and additional metal oxides
- 2. Rapid cooling (quenching) of molten mass (large internal stress, extensive cracking)
- 3. Grounding into a fine powder (frit)
- 4. Fusing individual particles for shape formation(sintering)
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Sintering
- Heating closely packed ceramic particles to a temperature that causes the borders of particles to melt and fuse together
- T(glass transition) < T(sintering) < T(melting)
- Shrinkage during sintering
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Sintering Shrinkage Control
- Particle distribution
- Tightly packed particles to minimize the shrinkage on firing
- ~25 um
- Multimodal particle size(particles of different sizes) distribution to increase the packing density
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Limitations of silica glass for aesthetic ceramics
- Too high sintering temperature
- 3D network of covalent bonds b/w silica tetrahedral
- Too high melting temperature -> melt metal substrates
- Too low thermal contraction coefficient
- Thermal expansion mismatch -> interfacial breakdown
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Addition of glass modifiers
- Lower sintering temperature
- Increase thermal contraction coefficient
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porcelain fused metal system
- (PFM)- porcelain is the outer layer, and metal the inner
- porcelain has too high of a sintering temperature, which will melt the metal
- also, there is differential thermal contraction of metal & porcelain (metal higher than porcelain)-
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interfacial breakdown
thermal expansion mismatch->metal shrinks more than ceramic during cooling
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Glass Modifiers
- Alkali metal ions (Na, K, Ca)
- Interrupt oxygen silicon bonds
- break 3D silica network -> linear chains of silica tetrahedra
- Easy movement of silica chains
- Viscosity↓, softening temperature↓, thermal expansion ↑
- Too high modifier concentrations reduce chemical durability
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Boric oxide
Glass modifier
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Glass Additive Pigmenting oxides
- Simulate natural teeth
- Iron or nickel oxides: brown
- Copper oxides: green
- Titanium oxides: yellowish brown
- Manganese oxide: lavender
- Cobalt oxide: blue
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Glass Additives for Opacity
Cerium oxide, zirconium oxide, titanium oxide, tin oxide
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Glass Additives for Fluorescence
Lanthanide earth
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Glass additive Binder
- Easy manipulation of the powders
- Starch, sugar
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Porcelain jacket crown (PJC)
- Recreation of all of the aesthetic features of a tooth
- Opaque shade-Mask color of the underlying structure (amalgam, metal)
- Dentin shade
- Enamel shade
- Procedure
- -Condensation
- -Firing
- -Glazing and shading ceramics
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Porcelain Jack Crown Procedure
- -Condensation
- -Firing
- -Glazing and shading ceramics
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Condensation
- to avoid volume shrinkage, we want high packing density
- Application of a slurry to the die
- -Powder is mixed with water and binder (slurry)
- -Thin platinum foil coating of the die to separate the crown from the die
- Compaction
- -High density of particles by removing water to minimize the firing shrinkage
- --Spatulation
- --Brush application
- --Whipping or vibrating
- Particle size and shape
- -spherical particle size is best, vs irregular size, to attain highest packing density and less shrinkage
- -Handling characteristics
- -The degree of shrinkage
- Binder
- -Hold the particles together
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Firing
- Sinter the particles of powder together to form the prosthesis
- Fusion of particles as the powder particles are brought into close contact
- High shrinkage of porcelain on firing (~20%)
- Firing for too long will lose form due to pyroplastic flow (flow of the molten glass)
- Cooling
- Slow cooling rate is essential to avoid cracking or crazing
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Air firing
- Air void forming from residual air
- Altered translucency in the areas of pores (light scattering)
- Exposure of air voids after grinding of the surface layer
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Dental Porcelain Firing Temperature Classification
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Vacuum firing
- Air is withdrawn during firing
- Fewer voids, denser and stronger crown
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Glazing
- Exposed air void at the surface
- -Ingress of bacteria and oral fluids
- -Potential sites for the build-up of plaque
- Surface is glazed to produce a smooth, shiny and impervious outer layer
- The glaze is effective in reducing crack propagation within the outer surface
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Glaze ceramic
- Specially formulated ceramic powders are applied to a crown surface after construction
- A short period at a relatively low temperature is sufficient to fuse the glaze
- 50 um thick glazes have adequate durability
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Dental Porcelain Properties
- Excellent aesthetics
- -Chemically stable
- -Do not deteriorate with time
- Thermal conductivity and coefficient of thermal expansion
- -Similar to enamel and dentin
- -Good marginal seal
- Mechanical properties
- -High compressive strength (350~550 MPa)
- -Low tensile strength (20~60 MPa) and toughness, brittle
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surface micro-cracks
- Cracks or defects in the surface of the crown cause the crown to fracture catastrophically
- Brittle materials (glass) are sensitive to the tiny surface flaws, especially under tension (heavy occlusion), unlike ductile metals
- Early dental porcelain was restricted to very low stress-bearing -> anterior applications
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Aesthetic ceramics weakness
- -Lack of strength
- -Micro-cracks: major source of weakness
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PFM Advantages
- Bonding the ceramic to a metal substrate
- -High fracture toughness of metal
- -Metal presents a barrier to the propagation of cracks
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PFM Failure Modes
- Interfacial breakdown of the metal- ceramic bond
- Mismatch between the coefficients of expansion of the ceramic and the metal
- Great mismatch -> stress building up during cooling -> crazing or cracking of the ceramics
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PFM Mechanical Retention
- Interlocking-Ceramic flows into the microscopic spaces in the surface of the metal
- Surface roughness ↑ Interlocking ↑ Alumina-air abrasive, grinding
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Ensuring Porcelain Metal Bond
- Intimate contact
- Good bonding relies on an intimate contact b/w the ceramic and the metal coping
- Remove any remaining contaminants and trapped air on the metal coping before the application of the ceramic
- Degassing cycle of the metal coping in the furnace
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PFM Chemical Bond
- Bond between the ceramic and the oxide coating on the metal
- Migration of the metal oxides into the ceramic through sintering
- Ceramic above Tg can flow and fuse with the oxides on the metal surface
- Adding of oxide-forming elements (Sn, In, Fe, or Zn) to the gold alloy
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Thermally Induced Stresses
- Mismatch in coefficient of thermal expansion
- - α(Ceramics) < α(Metals)
- - Excessive differential shrinkage on cooling
- Difference in the coefficient of expansion between the metal and the ceramic will produce stresses depending upon the type of mismatch
- ideal situation is something between the thermal expansions being equal and the thermal expansion of metal being slightly more than that of porcelain;
- porcelain has high compressive strength, but not tensile strength
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Compressive Stress
- α(Ceramics) < α(Metals)
- metal shrinks more upon cooling and there will be compressive stress on the porcelain
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Tensile Stress
- α(Ceramics) > α(Metals)
- thermal expansion of porcelain is higher than metal
- porcelain shrinks more upon cooling and there will be tensile strength on the porcelain
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Compression Fit
- a slight compressive force in ceramic layer
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Ideal Metal-Ceramic Properties
- coefficient of expansion of the ceramic is only slightly lower than that of the metal
- αp < αm : reduces the potential for the ceramic to crack
- High tensile strength of the alloys: no danger of metal failing
- Too great mismatch may cause crazing or fracture of the ceramic, or debonding from the metal surface
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Early dental porcelains (PJC)
Lack of strength and toughness
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Metal-ceramics (PFM)
Aesthetic ceramic is supported by a strong and tough metal
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Reinforced ceramic core systems
- Aesthetic ceramic is supported by another ceramic materials
- High strength and toughness
- Lack of aesthetics
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Core ceramics
- High strength and toughness core replaces the alloy substructure to support and strengthen the veneering layers
- Core prevents fractures from crack propagation
- Opaque; core ceramic cannot be used for the entire restoration
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Newer all-ceramic crowns
- Aluminum-reinforced porcelain jacket crown
- Glass-infiltrated core ceramics
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Resin-bonded ceramics
- Supported by the tooth structure itself
- Bonding the aesthetic ceramic directly to the enamel and dentine
- Strength depends on the quality of the bond
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Alumina
- Stronger particles than the glass
- Effective at preventing crack propagation crack stoppers -> stop motion of dislocation
- Flexural strength: 60 MPa (feldspathic porcelain) vs. 120~150 MPa (aluminous core porcelain)
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Feldspathic Glass + alumina (40~50%)
- Opaque shade
- Use with the weaker dentine and enamel shades of the feldspathic porcelains
- Anterior teeth restoration
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Resin Bonding Ceramics
- Bonding the ceramic to enamel and dentine
- Porcelain veneers invented by Dr. Charles Pincus of Beverley Hills
- Acid-etch technique: phosphoric acid etching
- Dentine-bonding agents: resin
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Ceramic Veneers
- Replace only the facial and incisal portions of anterior teeth
- Minimal preparation of the tooth enamel: 0.5 mm tooth reduction
- Thin veneer is fragile in the unbonded state
- Durable veneer restorations when bonded to the tooth
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The ceramic should have a coefficient of thermal expansion slightly (less, greater) than that of the alloy in most cases
LESS
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ceramic coefficient of thermal expansion
~14 ppm/C
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Deisireable Loading through slight compression or tension?
COMPRESSION
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Sintering
During firing, the particles of ceramic melt together at their edges
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Types of ceramics
- Porcelain-jacket crowns
- Metal-ceramics
- Reinforced ceramic core systems
- Resin-bonded ceramics
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Dental Ceramics and Fusing Types
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