Materials.txt

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emm64
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110377
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Materials.txt
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2011-10-20 15:28:07
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Dental Materials
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Lectures 1-4 Materials
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  1. Metals
    • ionize + in soln
    • 3/4 of periodic table
    • good heat and electrical cond
    • hard and high tensile strength
    • ductile malleble
    • dense, shiny,lustrous
    • high melting/boiling points
  2. Precious metals
    • (high economic value)
    • Au, Pd, Pt, Ir, Rh, Os, Ru, Ag
  3. Noble metals
    • (resistance to corrosion):
    • Au, Pd, Pt, Ir, Rh, Os, Ru
  4. Base Metals
    (strength): Ni, Cr, Co, Mn, Fe……
  5. Metallic Bonding
    Electrostatic attraction between the cations and delocalized electrons
  6. Metal consists of cations held
    • together by negatively-charged
    • electron “glue”
    • Positive metal ions occupy a fixed
    • position in a lattice
  7. Dental Amalgam
    • Ag 67-74%
    • Sn 25-28%
    • Ag-Sn alloy
    • (Ag3Sn)
    • 26%~30% Tin content in the alloy ->desired mechanical properties and handling
  8. Crystal terms
    • Structures Different geometries, Crystal systems
    • Coordination Number - Number of closest atomic neighbors
    • Atomic Packing Factor ― Volume (number) of atoms in unit cell
  9. APF formula
    • APF = (# atoms * Vol atom)/ vol unit cell
    • SC = .52
    • BCC = .68
    • FCC = .74
    • HCP = .74 (hexagonal close packed) (Titanium Alpha)
  10. Crystal systems
    • sides equal = cubic or rhombohedral
    • angles all 90 = cubic, tetragonal, orthorhombic
    • orthorhombic can be SC, BCC, FCC, or Side Centered
    • 14 Bravais Lattices
    • a = b = c: cubic, rhombohedral
    • a = b ≠ c: tetragonal, hexagonal
    • a ≠ b ≠ c: orthorhombic, monoclinic, triclinic
  11. Ductile
    Dislocations move easily (metals)
  12. Brittle
    Dislocations do not move easily (ceramics)
  13. Plastic Deformation
    • motion of dislocation
    • Only a small force is needed to deform the metal by the dislocation moving through the metal one plane at a time.
  14. Ion-pair vacancy
    • Point defect
    • (Schottky defect, pairs of ions of opposite charges)
  15. Interstitialcy
    • (an extra atom if APF is low )
  16. Displaced Ion
    • Point defect
    • (Frenkel defect, extra self-interstitial atom)
  17. Edge dislocation
    • is an extra half plane of atoms “inserted” into the crystal lattice
  18. Screw dislocation
    • forms when one part of crystal lattice is shifted (through shear) relative to the other crystal part
  19. Grain boundaries
    • The zone of crystalline mismatch between adjacent grains
    • Irregular grains (crystal fragments) form as crystals grow together
    • Atoms along the region of mismatch between one grain and the next will dissolve more readily -> lines
    • Each grain appears different color-> different orientation, different light reflectioin
  20. Nucleation of crystals
    When molten metal cools to the solid state, crystals forms around tiny nuclei (clusters of atoms)
  21. Crystal growth
    As the temperature drops, crystals grow until the crystal boundaries meet each other
  22. Grain importance
    • Grain boundaries block the movement of dislocations
    • Small grains (larger grain boundary) improve the elongation and tensile strength of cast gold alloys
  23. Grain Size Controls
    • Cooling rate of solidifying alloy
    • Quenching the hot invested casting in cold water
    • Presence of grain refining elements
    • Adding ruthenium, iridium and rhenium to the alloy
  24. Dental Composites vs. Enamel and Dentin
    • Organic Resin Matrix = polymer vs Collagen, protein, water
    • Inorganic Filler = ceramic vs. Hydroxyapatite
  25. Hydroxyapatite
    Inorganic filler found in enamel and dentin
  26. Crowns
    • All-Porcelain or Porcelain fused to Semi Precious Metal, or Gold
    • Alumina (Al2O3), Silica (SiO2), Zirconia (ZrO2), Titania (TiO2)
  27. Ceramics
    • • Compounds between metallic and nonmetallicelements
    • Oxides, nitrides, silicates
    • • Bonding
    • Ionic
    • Covalent
    • • Structure
    • Crystalline
    • Amorphous (Glass)
  28. Ionic Bonding
    • metal + and non-metal -
    • electrostatic forces of attraction between oppositely-charged ions
  29. Ionic Properties
    • High Hardness = stick to lattice, not easily displaced
    • High Compressive strength
    • High brittleness
    • High melting/bp
    • low electrical conductivity = no free electrons
    • Low plastic deformation and fracture toughness
    • (stress causes ions of like charge to repel each other)
  30. Covalent Bonding
    • • Sharing of a pair of valence electrons by two atoms
    • • Low electrical conductivity
    • ― Electrons are held tightly within covalent bonds, and do not move
  31. Bioinertness
    • Ceramics
    • Results in biocompatibility
    • Minimize inflammatory responses and toxic effects
    • Naturally occurring titanium dioxide (TiO2) layers on the implants result in excellent biocompatibility
  32. Ceramics
    • SiO2
    • Four allotropic forms - polymorphism
    • Glass (Non-Crystalline)
    • Cristobalite (Crystalline)
    • Tridymite (Crystalline)
    • Quartz (Crystalline)
  33. Ceramic Dental applications
    • ― Glass ionomer cements
    • • Alumino-silica glasses
    • ― Ceramic restoration
    • • Mixtures of potassium aluminosilicate and sodium aluminosilicate
  34. Ceramic Structure
    • Crystalline: Long range order
    • Non-crystalline: Amorphous
  35. Crystal Melting
    • Sharp phase change from solid to liquid at a definite melting point
    • At the melting point of the crystal, there is a discrete (i.e. at a specific temperature) discontinuity in the specific volume
    • Specific volume ↔ Density
    • ―Ordered crystalline structure → high packing density
    • ―Disordered liquid → low packing density
  36. Amorphous
    • No long-range periodicity in atom location
    • ―Glass and some types of plastic
    • No sharp phase change from solid to liquid at a definite melting point, but rather soften gradually when they are heated
    • No sudden increase in the volume ―Gradual increase in the volume, with the rate of increase becoming more rapid above the glass transition temperature
  37. Crystalline solid formation
    • Slow cooling of molten silica
    • Rearrangement of molecules (increase in packing fraction)
    • Configurational contraction (sharp reduction in the specific volume)
  38. Glass formation
    • Rapid cooling of molten silica (vitrification)
    • Not enough time for a rearrangement of molecules and growth of crystal nuclei
  39. Vitrification
    Rapid cooling of molten silica
  40. Devitrification
    • Recrystallization of a glass
    • A small amount of crystallization of a glass
    • Reorganization of molecules at an elevated temperature for a long time (annealing)
    • • Translucent appearance (light scattering)
    • • Glass ceramics
  41. Annealing
    Reorganization of molecules at an elevated temperature for a long time
  42. Polyethylene
    • Basis of dental polymers
    • Carbon double bonding (vinyl group)
  43. Acrylic Resins
    • Derivatives of ethylene
    • Contain a vinyl (-C=C-) group
    • Simplest molecule for additional polymerization
  44. Methacrylic acid
    • Central building block of most dental resins
  45. Acrylic acid
    • Acrylic Resin with Carboxyl group (COOH) used in Dental cements
  46. Polymer Bonding
    • ― Covalent bonding (monomer-monomer)
    • ― Secondary bonding (chain-chain)
    • • Hydrogen bonding
    • • Van der Waals bonding
  47. Molecular Weight
    • • Number of repeating unit (mer) x Molecular weight of mer
    • • Random chain propagation
    • ― Many different chain lengths
    • ― Only an average Mw can be defined
  48. Average molecular weight (M)
    SUM (Mean weight of range * number) / total number
  49. Degree of Polymerization
    • Average # of repeating units (mer)
    • =(AVg molecular weight / unit molecular weight)
    • n = M/m
    • M = average molecular weight
    • m = mer molecular weight
  50. High degree of polymerization
    • Fewer polymer chains
    • Longer polymer chains
    • More rigid, less soluble
  51. Low degree of polymerization
    • More polymer chains
    • Shorter polymer chains
    • Less stiff, more soluble
  52. Molecular weight effects
    • Mw Softening/melting temperature
    • < 100 g/mol  liquids or gases
    • ~ 1,000 g/mol  waxy solids
    • > 10,000 g/mol  solid polymers
  53. Polymer Chemical Structure
    • Linear, Branched
  54. Polymer Spatial Structure
    • Linear, Branched, Crosslinked, Network
  55. Thermoplastic polymers
    • Reversible transformation (recycle)
    • Softens upon heating
    • Harden upon cooling
    • Flexible linear polymers w/ some branches
    • Most dental resin
  56. Thermoset polymer
    • (Cured vs Thermoplastic)
    • Irreversible crosslinks
    • Hardens when heated (cannot be softened by reheating)
    • Crosslinked or networked
    • Harder, stronger, more brittle than thermoplastics
  57. Porcelain Fused to Metal Advantages
    • Greater strength than an aesthetic all-ceramic crown
    • Accurate fit over the tooth (gold is a very workable metal)
  58. Porcelain Fused to Metal Disadvantages
    • • Failure modes
    • ― Mismatch between thermal expansion of the ceramic (outer) and the metal (inner)
  59. Thermal conductivity
    • A measure of heat transferred
    • The rate of heat flow per unit temperature gradient
    • Unit: cal/sec/°C/cm
  60. Thermal Insulating Base
    Cements: good thermal insulating bases for the pulp under the metal restoration
  61. Thermal Diffusivity
    • • In the oral environment, temperature are not constant during the ingestion of foods and liquids
    • Time rate of temperature change at one point due to a heat source at another point
    • Diffusivity is Conductivity divided by (specific heat times density)
  62. Thermal conductivity (K)
    The rate of heat flow per unit temperature gradient
  63. Specific heat (Cp)
    The heat energy required to raise the temperature of a unit volume by one degree
  64. Thermal Expansion
    • • Absorbed heat energy increases vibration of the atoms or molecules -> material expansion
    • • Thermal expansion of the restorative material does not match that of the tooth structure
    • ― Differential expansion/contraction -> leakage of oral fluids between the restoration and the tooth
  65. Percolation
    • movement and filtering of fluids through porous materials
    • Decrease with time with dental amalgam
    • Space being filled with corrosion products
  66. Coefficient of thermal expansion
    • Change in length for a 1°C change in temperature
    • Unit: ppm(x10^-6)/°C
  67. Optical Perception
    • The color of an object is a human perception
    • ―Light source
    • ―Object
    • -Background
    • -Observer
  68. metamerism
    • The color of an object appears different under different light sources (Different light spectra)
    • The wavelength and intensity spectrum of the light depends on the source of the light
  69. Fluorescence
    • ― Tooth enamel absorbs light at near UV range (300~400nm), and then release it as light of a longer wavelength (400-450nm)
    • ― Brightness and vital appearance of a human tooth
    • ― Crowns, bridges or composite restoration sometimes look dark next to the fluorescing natural tooth
  70. Translucency
    • ― Different absorption and scattering properties of different restorative materials  different opacity
    • Dentin is more opaque than Enamel
    • Enamel is more transparant than Dentin
  71. VITA Shade Guide
    • • Most widely used color matching system
    • Easy and accurate color communication with laboratory technician for a crown or a veneer to be matched to the patient’s teeth
    • • Specify color characteristics
    • ― Hue: A (reddish-brown), B (reddish-yellow), C (grey), D (reddish-grey)
    • ― Chroma: Intensity of the main color (e.g. A1-A4)
    • ― Value: Grey-scale, lighter to darker
  72. Stress
    • • Force per unit area within a structure subjected to an external force or pressure
    • ―Applied area decrease then stress ↑ (increases)
    • ―Unit: N/m^2= Pa
  73. Stress Types
    • Axial Elongation (tensile)
    • Axial Shrinkage (compression)
    • Shear -> Shear
    • Twisting Moment -> Torsion
    • Bending moment -> Bending
  74. Elastic modulus (E)
    • Slope of the stress-strain curve in the initial straight-line portion
    • (Elastic deformation)
    • Higher=Stiffer
    • Lower=Ductile
  75. Strain
    • Change in length per unit per initial length
    • ε = δ / L

  76. Yield Strength (YS)
    • Stress at which material strain changes from elastic deformation to plastic deformation, causing it to
    • deform permanently
    • where Stress/Strain goes from linear to curved
  77. Ultimate Strength (UTS)
    • • Stress at which fracture occurs
    • ― Tensile strength: fracture from tensile stress
    • ― Compressive strength: fracture from compression
    • Tensile (TS) and compressive strength (CS) of a material are significantly different
    • Brittle materials (e.g. enamel, amalgam, composite): CS >>> TS
  78. Toughness
    • Total amount of energy that a material can absorb before it fracture Total area under the stress-strain curve
  79. Resilience
    • Amount of energy that a material can absorb without undergoing any permanent deformation
    • Area under the linear elastic portion of the curve
  80. Tarnish
    • Surface phenomenon that can result in a discolored restoration
    • Reaction of Ag on the surfaces with sulfur in the saliva from air pollution or food compounds
    • Not very esthetic, but not harmful and does not cause longterm problems
  81. Corrosion
    • Chemical reaction that penetrate into the body of the amalgam
    • Severe and catastrophic disintegration of the metal body
    • Extremely localized corrosion attach may cause rapid mechanical failure
  82. Galvanic Corrosion
    • • Combinations of dissimilar metals in direct physical contact
    • Noble metal (Cu) withdraws electrons from base metal (Al)
    • Base metal (Al) becomes positively charged and positive ions (Al3+) are released -> corrosion!
    • Different metals/alloys assume different corrosion potentials
  83. Galvanic Shock in Dentistry
    • • Dissimilar restoration
    • Dental amalgam ↔ Gold inlay
    • Silver fork (Tin) ↔ Gold inlay
    • Aluminum foil ↔ Gold inlay (electrical shock)
    • • Electrolyte
    • Saliva, tissue fluids
    • Anode: Amalgam
    • Cathode: Gold
    • Electrolyte: Saliva
  84. Dental Alloys
    • ― Mixture of two or more elements, at least one of which has to be a metal
    • ― Binary alloy: two elements; tertiary alloy: three elements
    • ― Alloys can improve the properties of pure metals
    • • Types of alloys
    • ― Solid solution
    • ― Inter-metallic compound
  85. Solid solutions
    • Both metals are completely soluble in one another.
    • One type of crystal is formed.
    • Under a microscope, looks like a pure metal.
    • Usually stronger and harder, but not as elastic
  86. Substitutional solid solution
    • Solute atom substitutes directly for the solvent atom at the normal lattice site of the crystal
    • The atoms have the same crystal structure (e.g. FCC)
    • The atomic size are within 15% of each other
    • The atoms have a similar valency (e.g. Li+ cannot replace Mg2+)
    • ―e.g. 95%Au ― 5%Cu
  87. Interstitial solid solution
    • Solute atom takes up the space in between the solvent atoms
    • olute atom must be much smaller than the solvent atom (r < 60%)
    • e.g. Carbon in iron
    • Hydrogen, nitrogen, boron
  88. Pinning Points
    • Solid Solution Hardening
    • The stresses that these defects create in the crystal lattice are “pinning points” that restrict the motion of dislocations and so strengthen the material
  89. Inter-metallic Compounds
    • • Two or more metals combine, forming a specific composition or stoichimetric ratio
    • ―e.g. Ag ― Sn phase (Ag3Sn) in dental amalgam
    • • Complex crystal structure
    • Limited plastic deformation
    • Hard, brittle
  90. Phase diagram
    • ― A graph of phase stability area at any given temperature, for any given composition of the alloy
    • ― Binary (two metals), ternary (three metals)
    • ― Phase diagram describing > 3 metals are not used because they are too complex
    • In between the liquidus line and the solidus line the alloys are a mixture of solid and liquid, like porridge or
    • wallpaper paste
  91. Latent Heat
    The heat that is used up in the transition from a solid to a liquid
  92. Melting range for Alloy
    • Melting point (Tm) ―Pure copper (1083 °C) < Tm < pure nickel (1453 °C)
    • The temperatures at which solidification starts (liquidus) and solidification ends (solidus) are separate points
  93. liquidus line
    • joins the solidification start points on the phase diagram,
    • tells us that above the line the alloys are liquid
    • In between the liquidus line and the solidus line the alloys are a mixture of solid and liquid, like porridge or
    • wallpaper paste
  94. solidus line
    • which joins the solidification end points on the phase diagram,
    • tells us that below the line the alloys are solid
    • In between the liquidus line and the solidus line the alloys are a mixture of solid and liquid, like porridge or
    • wallpaper paste
  95. Partial solid solution
    • ― Atoms are only partially soluble in one another
    • ― Grain looks like layers of both metals alternating
    • ― Eutectic alloy
    • ― e.g. Ag-Cu
  96. Eutectic Alloys
    • • Components of materials are not sufficiently soluble to form a complete solid solutions
    • ― Atoms are only partially soluble in one another
    • ― Grain looks like layers of both metals alternating
    • Different atomic size: Ag (2.888A), Cu (2.556A), Ni (2.492A), Pd (2.750A)
    • • Liquidus and solidus meet at a mid-range composition
  97. Eutectic Temperature
    • Melting point (vs. melting range)
    • ―From liquid to two solid phases w/o going through a liquid-solid mixture state
    • ―Lower than either of the pure components
    • ―Solder materials with low melting temp.
  98. Ternary Phase Diagram
    • • A phase diagram for a alloy with three components
    • ―Three dimensional representation
    • ―Two dimensional representation (iso-thermal)
  99. Solidification of a metal
    • • Aggregates of atoms regularly arranged in a crystalline structure
    • ―Normally when a material begins to solidify, multiple crystals begin to grow in the liquid and a polycrystalline(more than one crystal) solid forms
    • Nucleation of crystals
    • Crystal growth
    • Irregular grains form as crystals grow together
    • Grain boundaries (as seen in a microscope)
  100. Nucleation
    • ― The moment a crystal begins to grow
    • - Many “nuclei of crystallization” scattered in the molten metal
    • ―Presence of impurity -> nucleation points
  101. Grain Size of Dental Alloys
    • Influence Strength, Workability, Corrosion susceptibility
    • A fine grain is usually desirable in a dental alloy
    • Smaller grain -> more grain boundaries -> higher resistance to deformation
    • Rapid cooling of dental gold alloys
    • Addition of grain refiners in the gold alloys
    • e.g 0.005% iridium in gold alloys
    • Nucleation site ↑ (inc) -> # of grains ↑ (inc)(125 times more grains/unit volume) -> size of individual grain ↓ (decrease)
  102. Soldering
    • Soldering creates a junction between two different metals.
    • The solder must have a LOWER melting point than those of metals to be soldered
  103. As the grain size decreases, What happens to strenght, workability and corrosion succeptibility?
    • strength increases
    • workability decreases
    • corrosion susceptibility increasesfas
  104. Ceramic Properties
    • Hardness (High)
    • Compressive Strength (High)
    • Tensile Strenght (Low)
    • Toughness (Low)
    • Melting Point (High)
    • Electrical Conductivity (Low)
    • Chemical Reactivity (Low)
    • Solubility (Low)
  105. Quartz Vs Glass
  106. Crystal Structures Table
  107. Crystal Structure Summary
  108. Limiting Motion of Dislocation
    • 1. Small Grain Size
    • 2. Strain: Use Solid solution Cu into Au to create Eutectic Solution
    • 3. Intermetallic compound: i.e. tin into silver. Intermetallic compounds are stronger than solid solutions because it’s very organized structure

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