Materials Compiled

Structure of matter

Types of Atomic and Molecular Bonds

• Primary atomic bonds

  • Ionic bonds - Ceramics

    • B/w + and - ions

    • Electron transfer

    • Materials are hard, brittle, electrically, and thermally insulating

  • Covalent bonds - Polymers and ceramics

    • Sharing electrons → Directional

    • Materials are hard, brittle, electrically and thermally insulating

  • Metallic bonds - Metals

    • Sea of donated valence electrons

    • Materials and thermal and electrically conductive, and can readily undergo plastic deformation

• Secondary atomic and molecular bonds - weaker than primary bonds

  • Polar forces (ex/ H-bonds) - Polymers

    • Covalently bonded atoms have dipole

  • Van der Waals forces

Properties from Bonding

• Ceramics (ionic and covalent)

  • Large bond energy

  • Large Tm

  • Large E (elastic modulus)

  • Small CTE (coefficient of thermal expension)

• Metals (metallic)

  • Variable bond energy

  • Variable Tm

  • Variable E

  • Moderate CTE

• Polymers (covalent and secondary)

  • Directional properties with secondary bonding dominates

  • Small Tm

  • Small E

  • Large CTE

Structural arrangement of atoms in solids

Crystal Structure

• Regular arrangement of atoms and molecules in space for minimal internal energy

• Organized in lattice forms (FCC, BCC, HCP)

• Polycrystalline structures: aggregates of many single crystals organized in different orientations → Grain boundaries

Noncrystalline Structure

• Amorphous materials

• Random arrangement of atoms (polymers and some ceramics - porcelain)

Tg vs Tm

• Tg = Glass transition temperature (noncrystalline materials)

  • At Tg, materal is “softer” but not fully a liquid

• Tm = Crystalline melting/fusion temperature

Surface energy and wetting

• Surface energy is higher than bulk → B/c there are “dangling'“ bonds

• A material with a lower surface energy can wet a surface with a higher energy

  • Elevate surface energy via polishing to make it easier to wet

Wetting in Dentistry

• Key for adhesion

• Key to take accurate dental impressions

Physical properties of dental materials

Thermal properties

• Thermal conductivity: How easily heat transfers through a material

  • Metals > Non-metals

    • Large metallic restorations need insulating cement to protect pulp

      • How insulating the base material is (Lining efficiency) = Thickness / sqrt(thermal diffusivity)

• Thermal diffusivity: Rate at which temperature of body changes as heat passes through

  • Depends on thermal conductivity, density, and specific heat

    • Inversely proportional to density

  • Specific heat: quantity of heat to raise temperature of a unit mass of a material by 1 degree C

• Thermal expansion: Change in length per unit of the original length when the temperature is raised by 1 degree C

  • Must know original length

  • \alpha=\frac{\Delta L}{L\cdot\Delta T}

  • CTE of dental materials

• Clinical consequences: Expansion and contraction of material, leakage around restorations, shrinkage of wax patterns, differential shrinkage of PFM crowns

• Biological consequences: Microleakage along tooth/restoration interface, Corrosion products from amalgam reduce microleakage

Rheological properties

• Rheology: Study of flow of materials

• Viscosity: Resistance to flow of a material under an applied stress

  • Resistance is due to internal friction

  • Shear stress = F/A

  • Shear strain rate = V/d

  • Viscosity = Shear Stress / Shear Strain

Rheological Behavior of Fluids

• Newtonian: Viscosity is constant with respect to stress

• Pseudoplastic (shear thinning): Viscosity decreases after initial stress

• Dilatant (shear thickening): Viscosity increases with stress

• Plastic: Some initial stress needed, then linear (like newtonian)

Optical properties

• In dentistry, interaction of light with restorative material must mimic the interaction with teeth

Light and Color in Dentistry

• Visible light wavelength: 400-700 nm

• Wavelength of max. visual perception: 550 nm (green)

Spectral Distribution of Light

• Different light sources have different wavelength distributions

Color measurement system

• Munsell system

  • Hue (wavelength)

    • Circumference

  • Chroma (intensity)

    • Radius

  • Value (light/dark)

    • Bottom → Top

• CTE

  • L*, a*, b* systems

    • L = value

    • a* = red/green

    • b* = blue/yellow

Color Perception

• Metamerism: Change in color matching of two objects under different light sources (have different wavelength distribution)

• Translucency/Opacity: Amount of incident light transmitted by an object that scatters part of the light

• Gloss: Proportion of specualr reflection to diffused reflection of light

  • Gloss = specular / diffused

• Fluorescence: 300-400 nm absorbed → 400-450 emitted (blueish)

  • Near enamel edges - hard to minic

Color Rendering Index

• The degree to which a light source can render the color of an object compared to a reference source

• CRI: Measurement of how well a light source displaces color compared to a natural light source (ex/ sunlight)

  • 0 - 100

Shade Matching

• Light sources (at least 2)

• Background color (neural gray)

Tarnish and Corrosion

• Tarnish: surface discoloration of a metal

  • Presence of saliva, bacteria, chemicals

  • Formation of thin films of oxides, sulfides, and chlorides

• Corrsion: reaction of metallic material with environ,ent

  • Progressive and destructive

  • Degredation of material and possible by-products

Other properties

Electrical - Galvanism

• Can be caused by… opposing metal restorations or be in 1 tooth

Mechanical Properties of Dental Materials

Stress and strain: What are they and why are they used instead of load and deformation

Stress and Strain

• Stress: force per unit area (independent on sample size)

  • Types of stress: Tensile, shear, compressive

• Strain: material deformation in response to stress (change in length per initial length)

  • Dimensionless

  • Types of strain: Tensile, shear, compressive

Stress-strain curve

• PL = Proportional limit

  • Value of stress at which the stress/strain curve deviates from the initial linear relation

• Elastic limit: Stress corresponding to the 1st measurable permanent deformation

  • Somewhere between the PL and YS

• Yield strength: strength corresponding to a designated amount of permanent strain 0.2%

• Ultimate tensile strength: Maximum stress without fracture

Elastic and plastic behaviors: When loads are small, how much deformation occurs?

• Elastic strain: Strain which disappears completely when the applied force is removed

  • Ex/ Pulling on a rubber band

• Permanent (plastic strain): Strain which remained permanently after the applied force is removed

  • Ex/ Bending a metal rod

Elastic modulus and hardness: What are they and how are they determined

Elastic Modulus/Modulus of elasticity/Young's modulus

• E = stress/strain (within the elastic range or the linear portion of the curve)

  • Determined by interatomic and intermolecular forces (the stronger the force, the more rigid the material)

• Material dependent

• Elastic modulus: resistance to elastic deformation

  • Important to consider elastic modulus of implants relative to bone.

    • If elastic modulus of implant much higher than bone, can lead to bone loss

  • Tooth: Enamel = 70-90 GPa; Dentin = 15-20 GPa

Stiffness and Modulus

• Stiffness = force/deflection

• Modulus = stress/strain

• Higher stiffness = Higher modulus

  • Steeper slope in elastic region

Yield strength and tensile strength: What are they and how are they determined

Ultimate Strength

• UTS: The stress corresponding to the maximum value of applied stress a material can withstand without rupturing

  • Peak of stress/strain curve

Fracture Strength

• Fracture point: The value of stress at which a material ruptures into 2 or more portions

Yield Strength

• Yield strength: strength corresponding to a designated amount of permanent strain 0.2%

Toughness, ductility, and resilience: What are they and how do we measure them?

Ductility and Elongation

• Ductility: The ability of a material to undergo permanent tensile deformation without fracturing

  • Elastic + plastic deformation

  • Plastic tensile strain at failure

• The more ductile the material, the more strain it can withstand

Malleability

• The ability of a material to undergo permanent compressive deformation without fracture

Brittleness

• Material behavior characterized by fracture with little or no prior permanent deformation

Modulus of Resilience

• Resilience: The amount of recoverable energy stored in a material during elastic deformation (material does’t suffer damage)

  • Area under linear portion of stress/strain curve

  • The larger the area, the more resilient

Toughness

• Toughness: Energy to break a unit volume of material

  • Area under the stress/strain curve

    • Include elastic and plastic regions

Hardness

• Resistance of a material to plastic or permanent deformation

• In metals, hardness = YS * 3

• Determined by the “scratch test”

  • The harder material can scratch the softer material

Cyclic mechanical properties

• Failure of materials due to cyclic loading and unloading

• Fails occur at stresses lower than UTS

• Growth of small cracks which become larger upon cycling until failure

Fatigue limit

• Stress at which material can withstand an unlimited number of cycles

• Influenced by how polished the surface is, temperature, and wetness

  • The more polished, the greater the fatigue resistance

  • Increase temperature decreases fatigue resistance

  • Wetness decreases fatigue resistance

Mechanical properties of enamel and dentin

Enamel compared to dentin

• Increase: PL, YS, Tensile strength, Compressive strength, UTS

• Decrease: Fracture toughness (AUC smaller than dentin)

Composites

What is the basic composition of dental composites?

Dental Composites

• A combination of materials in which each component retains its identity

• Composed of…

  • Matrix (high molecular weight monomers, low molecular weight monomers, polymerization control additioves)

  • Fillers (particles, fibers, whiskers)

  • Interfacial coupling agents (organic silanes)

What are the roles of the matrix phases, filler particles, coupling agents, and polymerization control additives?

Matrix (Continuous Phase)

• High molecular weight monomers: Increase strength and viscosity, decrease filler loading

  • Ex/ Bis-GMA, UDMA, Sirolane

• Low molecular weight monomers: Decrease viscosity, increase filler loading and polymerization kinetics

  • Ex/ TEGMA

Polymerization Control Additives

• Activators/initiator

  • Light activation = Camphorquinone

  • Chemical activation = Organic peroxide

• Inhibitor

  • BHT and Hydroquinone   

Fillers (inorganic - Dispersed Phase)

• Dental composites primarily have noncrystalline (glassy) silicates

  • Ex/ borosilicate glass, quartz, zirconia

  • Can also add radio-opacity

• Generally harder and stronger than the matrix

• By adding filler to composites….

  • Decrease CTE

  • Increase hardness

  • Decrease shrinkage (fillers physical dimensions won’t change)

  • Decrease water absorption

Interfacial Coupling Agents - Organic Silanes

• Pros…

  • Silanes are the most effective coupling agent and they work best with filler materials have a lot of Si-O bonds (ex/ silica)

  • Reduces viscosity and glass hydrophilicity → Chemical bonding from matrix to filler

  • Raise strength and wear resistance for composites

• Cons

  • Silanation reaction may be reverse in water

  • Problems such as: multi-layers, bonding (increase vulnerability to break between interfaces)

Other Components

• Inorganic pigments for shade development

What are the clinical indications of various composites

Polymerization Shrinkage

• Increased filler → Decreased shrinkage

• High stress due to high elastic modulus → Stiffer

• How to resolve

  • Increase amount of filler

  • Add composite in 2mm increments

  • Use low shrinkage monomers

Working and Setting Time

• Light cure: “On demand”

  • Reaction continues for 24hrs

• Chemically cured: 3-5 mins

Depth of Cure

• 2 mm composite at a time

Mechanical Properties

• Abrasion and wear

  • Toothbrush abrade and wear weak polymers first → rough surface

    • If inter-particle spacing becomes small, wear is greatly reduced (ex/ use of nanohybrids → highly polished surface)

• Occlusal wear resistance

  • Hybrid composites wear rate similar to dental amalgam

Thermal Properties

Water Sorption

Solubility

Color and Color Stability

Biocompatibility

Translucency

Composites for special application

• Multipurpose composites

• Microfilled composites

• Packable composites

• Flowable composites

• Laboratory composites

• Core composites

• Provisional composites

Metals and Alloys

What are pure metals?

Noble metals

• Elements with good metallic surfaces that retain their luster in clean dry air

  • Indicate the inertness of the element

• Resist oxidation, tarnish, and corrosion during heating, casting, and soldering

  • Platinum group (6 metals) - include platinum, palladium, iridium, ruthenium

  • Gold

• The term precious metals indicates how expensive a metal is based on supply and demand

Gold content of a dental alloy

• Karat/Carat (K): Parts of pure gold per 24

• Fineness: Parts of pure gold per 1000 (used for gold solders)

• Pennyweight

Solidification of pure metals

• Have fixed Tm/Tf

  • Supercooling may occur before crystallization

Formation of grain structures

Stages

1. Atoms aggregate to form an embryo

2. Nuclei formation: embryo increases in size

3. Branch/dendrite formation

4. Grains become recognizable

5. Grains are formed

6. A metal has formed grain structures

Crystal Growth

• Homogenous nucleation → Non-uniform grain

  • From nucleus without external agents

• Heterogeneous nucleation → Uniform grain

  • Discreet particles are used to form nucleus

Grain Boundaries

• Junction between grains or crystals

• Have higher energy (due to unsatisfied bonds)

  • Increases strength of material → Cracks don’t propagate as well

Grain morphology and size

• Equiaxed grains: Equal in size in all directions

• Average size of grains in microstructure

• Small grain size → Better physical and mechanical properties

Controlling grain sizes

• Rapid cooling

• Mold design

• Vibration during solidification

• High thermal differential between mold wall and alloy

• Use of nucleating agents (grain refiner) - best way to do it

What are dental alloys?

Alloy

• Mixture of 2 or more metals that are mutually soluble in the molten condition; or a mixture of a metal and a non-metal

• Improves physical and mechanical properties

Categorizing Alloys

• ADA specification #5: Gold-based alloys

  • Can have any composition so long they pass the tests for toxicity, tarnish, yield strength, and percent elongation

    • I → IV (soft → extra-hard)

• ADA classification

  • High noble: >40 wt% Au and >60 wt% of the noble metal elements

  • Noble: > 25 wt% of the noble metal elements (Au, Pd, Pt)

  • (Predominantly) Base metal (PB): < 25 wt% pf the noble metal

• Principle element: Listed in declining order of composition (highest to lowest)

  • Except certain elements that affect physical properties or represent potential biocompatibility concerns

• Descriptive classification

  • Normal-fusing alloys (good for all-metal restorations)

    • Ex/ Silver-palladium

  • High-fusing alloys (mostly for PFM)

    • Ex/ Palladium-silver

Why are pure metals not useful for most dental applications

• Alloys have better physical and mechanical properties than pure metals

Understand phase diagrams for dental alloys

Definitions/Characteristics

• Phase: A physically distinct, homogenous, and mechanically separable portion of a system

• Alloys solidify over a range of temperatures

  • Exist as solid and liquid

  • No one Tm

Solid solutions

• Substitutional: Have similar properties therefore can replace to form alloy

  • Atomic size - variation within 15%

  • Valence - behaves the same with other elements

  • Chemical affinity

  • Lattice type (FCC, BCC, HCP)

• Interstitial: Have different sizes

  • Can distort lattice and make dislocation movement difficult

    • Increase strength, hardness, and proportional limit

    • Decrease ductility and resistance to corrosion

• Liquidus curve: Above line, all liquid (solidification begins once you hit the curve)

• Solidus curve: Below the line, all solid

Know the role of each element in dental alloy

Gold (Au)

• Soft, (most) malleable and ductile

• Relatively low strength

• Tarnish resistant in air and water at any temp

• Insoluble in sulfuric, nitric, or hydrochloric acids

• Soluble in a combination of nitric and sulfuric acids (aqua-regia)

• Physical properties:

  • E: low (weak bonds)

  • Tm: low (weak bonds)

  • CTE: large (weak bonds)

Platinum (Pt)

• Tough, malleable, and ductile

• Very high cost (usually replaced by Pd)

• High corrosion resistance

• Higher melting temp than porcelain

• Physical properties:

  • E: higher than gold → Higher Tm and lower CTE

Palladium (Pd)

• Not used in pure state

• Replaced Pt in dental casting alloys

• Cheaper than Pt

• Helps prevent corrosion of silver in the oral environment

• Physical propertiesL

  • E: higher than gold → Higher Tm and lower CTE

Silver (Ag) - NOT A NOBLE METAL

• Malleable and ductile

• Best known conductor of heat and electricity

• Harder than gold

• Unaltered in clean dry air

  • Severe tarnishing in the oral environment → Pits and porosities

Recognize the importance of some properties of the alloys

• 

• Coring: inhomogenous alloy composition because of non-equilibrium cooling rates

  • Happens during solidification (one element in higher concentration in the center) → can lead to corrosion

• Homogenizing with heat treatment can undo the coring

Eutectic system

• Liquid freezes and forms 2 different phases (alpha and beta) - point is known as the eutectic temperaure

• Alloys in eutectic systems are…

  • Brittle, high hardness and strength, poor tarnish and corrosion resistance

Peritectic Alloys

• Takes place between a previously precipitated phase and the liquid to produce a new solid

• Liquid + Beta solid solution → alpha solid solution (susceptible to coring)

• Alloys in pertectic systems are…

  • Susceptible to coring

  • Brittle

  • Boor corrosion resistance

Intermetallic compounds

• An alloy with definite (fixed) proportions of 2 or more metals (elements in clearly defined atomic ratios)

  • Clearly defined stoichometry

Properties of Metals and Alloys

• Cast metals and alloys = Not deformed

• Wrought metals and alloys = Plastically deformed

  • Have changes in microstructure and physical properties

  • Ex/ direct filling gold, orthodontic wires, titanium dental implantss

Is deformation good or bad?

Mechanism of Deformation

• Deformation occurs when bonds between atoms are ruptured

  • Stress needed = E/15

  • Large stresses required to produce slip in a perfect lattice

• E/15 » YS (through experimentation) because all metals have impurities and defects

What is the reason for deformation?

What types of defects arise in solids?

Types of Imperfections

• Point defects

  • Vacancy atoms: Distorts plane → Collapse

  • Interstitial atoms

    • Self-interstitial: Distorts plane → Extra atoms bulges lattice

    • Interstitial atoms: Typically smaller

  • Substitutional atoms

• Line defects

  • Dislocation: “extra” half plane of atoms → Edge dislocation

    • Atomic arrangement next to a dislocation line is strained

  • Dislocation slips to proceed at stresses lower than predicted yield stress

    • Only need to break one bond at a time (propagates)

    • Movement of dislocation → slip → plastic deformation

    • Stops once reaches the edge of the crystal

• Area defects

  • Grain boundaries: Impedes dislocation motion

    • Need more energy to overcome boundary

• Volume defects

  • Cracks, pores, inclusions

How do defects affect material properties

Wrought

• T/S: high

• Ductility: low

• Corrosion resistance: low

Cast

• T/S: low

• Ductility: high

• Corrosion resistance: high

Are defects undesirable?

• Not necessarily, can strengthen material

What are the methods for strengthening metals and alloys?

Strengthening Strategies

• Decrease grain size: Add grain refiners

  • In metals… increase number of nucleation sites or pin grain boundaries by fine particles

• Solid solution strengthening

  • Substitutional solid solution: Adds local stresses → Increase strength (stress spots making it more difficult to dislocate)

  • Interstitial solid solution: Interstitial strengthening can lock planes from shearing and pit the surface in compression

• Precipitate strengthening

  • Hard precipitates are difficult to shear

    • Ex/ alpha titanium precip. into beta titanium → Locks up beta titanium while maintaining ductility

  • The closer the precipitates are, the more difficult it is to shear

• Cold work: Physically plastic deforming (but becomes more brittle)

  • Dislocation becomes more difficult (trapped by the dislocation that happens during cold-working)

  • Increase YS and tensile strength

  • Decrease ductility (fractures more easily)

Annealing

• Annealing can reduce dislocation density and increase grain size

• Reverse effects of cold work

• Heat metal to half its fusion temperature

• Stages: recovery, recrystallization, grain growth

Application: Implants

• Modern titanium implants are stronger than those from 30 years ago

  • Use CP4 Ti → strengthened via cold working (480 → 760)

  • Can return to 480 after annealing

Introduction to Dental Polymers

Uses in Dentistry

• Impression materials

• Denture base materials: PMMA/acrylic

• Composite resin restorative materials: Bos-GMA, UDMA, Sirolane

What is a monomer?

• Resins: monomer - nonmetallic materials synthesized from organic compounds that can be molded when soft and hardened for use

Plasticizers

• Added to monomers to increase solubility and decrease brittleness of a polymer

  • Facilitates slipping of polymer chains along each other

• Effects:

  • Reduce strength

  • Decrease hardness

  • Lower Tg

What is a polymer?

• Long chain molecules consisting of many repeating units

Classification Based on Origin

• Natural: Proteins and nucleic acids, polyisoprenes, polysaccharides

• Synthetic: PMMA, Nylon, Teflon

Classification based on thermal behavior

• Thermoplastic resins: Soften under heat and pressure and harden when cooled

  • No chemical reaction

  • Generally soluble in organic solvents

  • Ex/ PMMA, waxes

• Thermoset resins: Harden by a chemical reaction and generally insoluble in organic solvents

  • Ex. Alginates, epoxy, Bis-BMA

Cross-linking

• Formation of bridges between chains of polymers to form a 3D network

• Effects:

  • Increase Tg

  • Increase strength

  • Decrease solubility

  • Decrease water-sorption

Physical Properties of Polymers

• Degree of polymerization (DP): Average number of repeating monomeric units in a polymer molecule

  • Large polymer = Large DP = Increase strength

• Degree of conversion: Fraction of DB converted to single bonds after polymerization

  • Increase degree of conversion = Increase strength

Strength Properties

• Strong influence of temperature on strength

  • As temperature increases…

    • Physical properties decrease

    • Strength decrease

    • Ductility increase

• Other influences: Composition, molecular weight, structure, residual monomer (conversion rate)

Biological Properties

• Taste, smell, toxicity, soft tissue irritation influenced by…

  • Water uptake (will structurally change dimensions)

  • Solubility

  • Residual monomer (no less than 25%)

  • Bond to tooth structure

What is the difference between condensation polymerization and addition polymerization?

• Polymerization: A series of chemical reactions by which a macromolecule is formed from a single monomer

Condensation Polymerization (step-growth)

• Repeating units are joined by functional groups

• Reaction is slow (hard to make large molecules)

• Results in a by-product and is exothermic

• Ex/ Mercaptan + lead dioxide → polysulfide rubber + lead oxide + water

  • Lead dioxide = catalyst (survives entire reaction)

Addition Polymerization

• General steps: initiator + monomer → activated monomer → Propagation → Termination

• Successive reactions between monomer molecules to form a polymer without formation of volatile by-products

• Features: the presence of an unsaturated group, readily form giant molecules, chain growth can continue indefinitely, exothermic reaction

• Ex/ polymerization of MMA → PMMA

  • Initiated by benzoyl peroxide

Activators and Inhibitors

• Activators of addition polymerization: Heat, tertiary amine, light (UV, visible), Microwave energy

• Inhibitors of polymerization: Impurities, hydroquinone (esp in composites), oxygen

Structure of Polymers

• Linear

• Branched

• Copolymerization

  • Polymerization of 2 or more monomers to form a polymor

Dental Ceramics

Define ceramic in general terms and give examples of ceramics and their applications in dentistry

• Ceramics: Solid material composed of inorganic nonmetallic compounds (ex/ pottery, clay, cements, glass)

  • Key characteristic - Brittle (stress concentrations at surface imperfections lead to crack initiation, propagation, and failure)

• Dental ceramics

  • Gypsum products

  • Cement powders (ZnO, MgO)

  • Orthodontic bracket

  • Fillings,

  • Veneers

  • Crows and fixed pros.

  • Implants and abutments

Differentiate the composition and structure of crystalline and non-crystalline ceramics

Crystalline vs Non-Crystalline Ceramics

• Crystalline (Ex/ Quartz, cristobalite, tridymite)

• Non-Crystalline (Ex/ Amorphous SiO2 glass, fused silica)

Classes of Dental Ceramics

• Silicate ceramics

  • Porcelain: Mimics optical properties of enamel and dentin

    • Feldspathic ceramics, leucite ceramics, fluor-apatite ceramics

  • Glass ceramics: Add crystallites to strengthen and toughen material

    • Leucite, lithia

  • Glass-Infiltrate (no longer used)

• Oxide ceramics

  • Polycrystalline

    • Aluminum oxide (no longer used)

    • Zirconia

Non-Crystalline: Dental Porcelains

• Feldspathic ceramics: Best mimics optical properties of enamel and dentin

  • Predominantly glassy material

  • Mainly feldspar, minimal clay and quartz components

• Other porcelains: feldspathic porcelain, feldspar, aluminosilicate glass

  • Potash (with K2O) and soda (with Na2O) feldspar

  • Always use oxides in porcelains

  • By adding K2O and Na2O → Lower Tm and Increase CTE (glass modifier)

• Characteristics…

  • Amorphous - non-crystalline

  • Highly translucent

  • Easy to make tooth shades

  • BRITTLE

• Structure and properties of feldspathic ceramics

  • Low strength: Low resistance to crack initiation

  • Low toughness: Low resistance to crack propagation

Glass Ceramics

• Leucite glass-ceramic

  • Add leucite to add strength and toughness

  • Dispersion strengthening

• Lithia glass-ceramics

  • Lithium disilicate (has a higher crystalline content - “log” shaped)

  • By compressing, can force crystals to be perpendicular to applied stress

  • Stronger and tougher than leucite glass-ceramics but still relatively low

  • Best used for anterior bridge and single tooth restorations

Oxide Ceramics - Zirconia (Polycrystalline ceramic - glass free)

• Much higher strength (harder to initiate crack)

• Very strong, but highly opaque

Describe methods of strengthening ceramics for dental application

Leucite Reinforced Feldspar

• Has a similar refractive index to porcelain

• Increases strength

• Faster acid etch rate → Increase mechanical interlocking with cement

Role of Components

• Feldspar

  • SiO2 (glass network)

  • Oxides of potassium, sodium and calcium (glass network modifier)

• Alumina

  • Increase strength and viscosity (glass network intermediate)

• Leucite crystal formation

• Metal oxides (opacity) and pigments

Fracture of Porcelain Crowns

• Feldspathic ceramic crowns

  • Reinforcement with crystals can make it harder

Strengthening Porcelain Restorations

• Strong core materials: zirconia, glass ceramics, and metals

  • Ex/ Porcelain veneered glass-ceramic, PFM

• Strengthen via having a strong framework

List the advantages and disadvantages of ceramic material

Advantages

• Natural appearance

• High resistance to wear and distortion

• Excellent biocompatibility (chemically inert)

• Low or no corrosion

• Considerably less expensive

Disadvantages

• Brittle

  • Tensile stresses can cause crack propagation and fracture

  • High compressive strength (10x the tensile strength)

  • No dislocation motion (slip) - Ionic bonding → too much electrostatic repulsion

• Hard, difficult to polish

• Wear opposing teeth (harder than enamel)

• Produce clicking sound on contact (especially zirconia)

• Difficult to bond to denture base material