Materials Ceramics

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emm64
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120200
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Materials Ceramics
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2011-12-01 23:23:00
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Materials M2 Ceramics
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Materials M2 Ceramics
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  1. Ceramics
    • Compound involving metallic and nonmetallic materials
    • Ionic or covalent bonding
    • Crystalline or amorphous
  2. Esthetic restorations
    • Translucent
    • Low corrosion and wear
    • High modulus
    • Veneers, inlays/onlays, crowns, bridges
    • Brittle, susceptible to fracture
  3. 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
  4. Kaolin
    • Hydrated aluminosilicates (Al2O· 2SiO2· 2H2O)
    • Acts as a binder, increasing molding ability
    • Opaque  omitted for dental porcelains
  5. Quartz
    • Acts as a strengthening agent
    • Fine crystalline dispersion throughout the glassy phase of the feldspar
  6. 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
  7. 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)
  8. 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
  9. 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
  10. 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
  11. Addition of glass modifiers
    • Lower sintering temperature
    • Increase thermal contraction coefficient
  12. 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)-
  13. interfacial breakdown
    thermal expansion mismatch->metal shrinks more than ceramic during cooling
  14. 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
  15. Boric oxide
    Glass modifier
  16. 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
  17. Glass Additives for Opacity
    Cerium oxide, zirconium oxide, titanium oxide, tin oxide
  18. Glass Additives for Fluorescence
    Lanthanide earth
  19. Glass additive Binder
    • Easy manipulation of the powders
    • Starch, sugar
  20. 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
  21. Porcelain Jack Crown Procedure
    • -Condensation
    • -Firing
    • -Glazing and shading ceramics
  22. 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
  23. 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
  24. 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
  25. Dental Porcelain Firing Temperature Classification
  26. Vacuum firing
    • Air is withdrawn during firing
    • Fewer voids, denser and stronger crown
  27. 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
  28. 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
  29. 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
  30. 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
  31. Aesthetic ceramics weakness
    • -Lack of strength
    • -Micro-cracks: major source of weakness
  32. PFM Advantages
    • Bonding the ceramic to a metal substrate
    • -High fracture toughness of metal
    • -Metal presents a barrier to the propagation of cracks
  33. 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
  34. PFM Mechanical Retention
    • Interlocking-Ceramic flows into the microscopic spaces in the surface of the metal
    • Surface roughness ↑  Interlocking ↑ Alumina-air abrasive, grinding
  35. 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
  36. 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
  37. 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
  38. Compressive Stress
    • α(Ceramics) < α(Metals)
    • metal shrinks more upon cooling and there will be compressive stress on the porcelain
  39. 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
  40. Compression Fit
    • a slight compressive force in ceramic layer
  41. 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
  42. Early dental porcelains (PJC)
    Lack of strength and toughness
  43. Metal-ceramics (PFM)
    Aesthetic ceramic is supported by a strong and tough metal
  44. Reinforced ceramic core systems
    • Aesthetic ceramic is supported by another ceramic materials
    • High strength and toughness
    • Lack of aesthetics
  45. 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
  46. Newer all-ceramic crowns
    • Aluminum-reinforced porcelain jacket crown
    • Glass-infiltrated core ceramics
  47. 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
  48. 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)
  49. Feldspathic Glass + alumina (40~50%)
    • Opaque shade
    • Use with the weaker dentine and enamel shades of the feldspathic porcelains
    • Anterior teeth restoration
  50. 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
  51. 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
  52. The ceramic should have a coefficient of thermal expansion slightly (less, greater) than that of the alloy in most cases
    LESS
  53. ceramic coefficient of thermal expansion
    ~14 ppm/C
  54. Deisireable Loading through slight compression or tension?
    COMPRESSION
  55. Sintering
    During firing, the particles of ceramic melt together at their edges
  56. Types of ceramics
    • Porcelain-jacket crowns
    • Metal-ceramics
    • Reinforced ceramic core systems
    • Resin-bonded ceramics
  57. Porcelain Metal Bonding
  58. Dental Ceramics and Fusing Types

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