Materials.txt

• 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

• (high economic value)
• Au, Pd, Pt, Ir, Rh, Os, Ru, Ag

• (resistance to corrosion):
• Au, Pd, Pt, Ir, Rh, Os, Ru

(strength): Ni, Cr, Co, Mn, Fe……

Electrostatic attraction between the cations and delocalized electrons

• together by negatively-charged
• electron “glue”
• Positive metal ions occupy a fixed
• position in a lattice

• Ag 67-74%
• Sn 25-28%
• Ag-Sn alloy
• (Ag3Sn)
• 26%~30% Tin content in the alloy ->desired mechanical properties and handling

• Structures Different geometries, Crystal systems
• Coordination Number - Number of closest atomic neighbors
• Atomic Packing Factor ― Volume (number) of atoms in unit cell

• APF = (# atoms * Vol atom)/ vol unit cell
• SC = .52
• BCC = .68
• FCC = .74
• HCP = .74 (hexagonal close packed) (Titanium Alpha)

• 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
• 

Dislocations move easily (metals)

Dislocations do not move easily (ceramics)

• 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.

• Point defect
• (Schottky defect, pairs of ions of opposite charges)

• (an extra atom if APF is low )
• 

• Point defect
• (Frenkel defect, extra self-interstitial atom)
• 

• is an extra half plane of atoms “inserted” into the crystal lattice
• 

• forms when one part of crystal lattice is shifted (through shear) relative to the other crystal part
• 

• 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

When molten metal cools to the solid state, crystals forms around tiny nuclei (clusters of atoms)

As the temperature drops, crystals grow until the crystal boundaries meet each other

• Grain boundaries block the movement of dislocations
• Small grains (larger grain boundary) improve the elongation and tensile strength of cast gold alloys

• 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

• Organic Resin Matrix = polymer vs Collagen, protein, water
• Inorganic Filler = ceramic vs. Hydroxyapatite

Inorganic filler found in enamel and dentin

• All-Porcelain or Porcelain fused to Semi Precious Metal, or Gold
• Alumina (Al2O3), Silica (SiO2), Zirconia (ZrO2), Titania (TiO2)

• • Compounds between metallic and nonmetallicelements
• Oxides, nitrides, silicates
• • Bonding
• Ionic
• Covalent
• • Structure
• Crystalline
• Amorphous (Glass)

• metal + and non-metal -
• electrostatic forces of attraction between oppositely-charged ions

• 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)

• • Sharing of a pair of valence electrons by two atoms
• • Low electrical conductivity
• ― Electrons are held tightly within covalent bonds, and do not move

• Ceramics
• Results in biocompatibility
• Minimize inflammatory responses and toxic effects
• Naturally occurring titanium dioxide (TiO2) layers on the implants result in excellent biocompatibility

• SiO2
• Four allotropic forms - polymorphism
• Glass (Non-Crystalline)
• Cristobalite (Crystalline)
• Tridymite (Crystalline)
• Quartz (Crystalline)

• ― Glass ionomer cements
• • Alumino-silica glasses
• ― Ceramic restoration
• • Mixtures of potassium aluminosilicate and sodium aluminosilicate

• Crystalline: Long range order
• Non-crystalline: Amorphous
• 

• 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
• 

• 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
• 

• Slow cooling of molten silica
• Rearrangement of molecules (increase in packing fraction)
• Configurational contraction (sharp reduction in the specific volume)

• Rapid cooling of molten silica (vitrification)
• Not enough time for a rearrangement of molecules and growth of crystal nuclei

Rapid cooling of molten silica

• 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

Reorganization of molecules at an elevated temperature for a long time

• Basis of dental polymers
• Carbon double bonding (vinyl group)

• Derivatives of ethylene
• Contain a vinyl (-C=C-) group
• Simplest molecule for additional polymerization

• Central building block of most dental resins
• 

• Acrylic Resin with Carboxyl group (COOH) used in Dental cements
• 

• ― Covalent bonding (monomer-monomer)
• ― Secondary bonding (chain-chain)
• • Hydrogen bonding
• • Van der Waals bonding
• 

• • Number of repeating unit (mer) x Molecular weight of mer
• • Random chain propagation
• ― Many different chain lengths
• ― Only an average Mw can be defined

SUM (Mean weight of range * number) / total number

• Average # of repeating units (mer)
• =(AVg molecular weight / unit molecular weight)
• n = M/m
• M = average molecular weight
• m = mer molecular weight

• Fewer polymer chains
• Longer polymer chains
• More rigid, less soluble

• More polymer chains
• Shorter polymer chains
• Less stiff, more soluble

• Mw Softening/melting temperature
• < 100 g/mol  liquids or gases
• ~ 1,000 g/mol  waxy solids
• > 10,000 g/mol  solid polymers

• Linear, Branched
• 

• Linear, Branched, Crosslinked, Network
• 

• Reversible transformation (recycle)
• Softens upon heating
• Harden upon cooling
• Flexible linear polymers w/ some branches
• Most dental resin

• (Cured vs Thermoplastic)
• Irreversible crosslinks
• Hardens when heated (cannot be softened by reheating)
• Crosslinked or networked
• Harder, stronger, more brittle than thermoplastics

• Greater strength than an aesthetic all-ceramic crown
• Accurate fit over the tooth (gold is a very workable metal)

• • Failure modes
• ― Mismatch between thermal expansion of the ceramic (outer) and the metal (inner)

• A measure of heat transferred
• The rate of heat flow per unit temperature gradient
• Unit: cal/sec/°C/cm

Cements: good thermal insulating bases for the pulp under the metal restoration

• • 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)
• 
• 

The rate of heat flow per unit temperature gradient

The heat energy required to raise the temperature of a unit volume by one degree

• • 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
• 
• 

• movement and filtering of fluids through porous materials
• Decrease with time with dental amalgam
• Space being filled with corrosion products

• Change in length for a 1°C change in temperature
• Unit: ppm(x10^-6)/°C
• 

• The color of an object is a human perception
• ―Light source
• ―Object
• -Background
• -Observer

• 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

• ― 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

• ― Different absorption and scattering properties of different restorative materials  different opacity
• Dentin is more opaque than Enamel
• Enamel is more transparant than Dentin

• • 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

• • 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
• 

• Axial Elongation (tensile)
• Axial Shrinkage (compression)
• Shear -> Shear
• Twisting Moment -> Torsion
• Bending moment -> Bending
• 

• Slope of the stress-strain curve in the initial straight-line portion
• (Elastic deformation)
• Higher=Stiffer
• Lower=Ductile
• 
• 

• Change in length per unit per initial length
• ε = δ / L

• 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
• 
• 

• • 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
• 
• 

• Total amount of energy that a material can absorb before it fracture Total area under the stress-strain curve
• 

• Amount of energy that a material can absorb without undergoing any permanent deformation
• Area under the linear elastic portion of the curve
• 

• 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

• 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

• • 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
• 
• 

• • 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
• 

• ― 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

• 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

• 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
• 

• 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
• 

• 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

• • 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

• ― 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
• 

The heat that is used up in the transition from a solid to a liquid

• 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

• 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

• 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

• ― Atoms are only partially soluble in one another
• ― Grain looks like layers of both metals alternating
• ― Eutectic alloy
• ― e.g. Ag-Cu

• • 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
• 

• 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.
• 

• • A phase diagram for a alloy with three components
• ―Three dimensional representation
• ―Two dimensional representation (iso-thermal)
• 

• • 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)
• 

• ― The moment a crystal begins to grow
• - Many “nuclei of crystallization” scattered in the molten metal
• ―Presence of impurity -> nucleation points

• 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)

• Soldering creates a junction between two different metals.
• The solder must have a LOWER melting point than those of metals to be soldered

• strength increases
• workability decreases
• corrosion susceptibility increasesfas

• Hardness (High)
• Compressive Strength (High)
• Tensile Strenght (Low)
• Toughness (Low)
• Melting Point (High)
• Electrical Conductivity (Low)
• Chemical Reactivity (Low)
• Solubility (Low)

• 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