Adhesive: the liquid/thin material added to produce adhesion between two surfaces. Adherend: the solid material the adhesive is applied to. Adhesive junction: the transition layer between them.

• 1
  Chemical Bond (True Adhesion) — Ex: Glass Ionomer Cement (GIC) & Zinc Polycarboxylate (ZPC) bond to the tooth via their COOH (carboxyl) group reacting with calcium in the tooth structure.
• 2
  Mechanical Bond — retention through micro/macro surface features rather than a true chemical reaction.

• A
  Related to the Adherend: surface energy, surface irregularities, cleanliness.
• B
  Related to the Adhesive: surface tension, viscosity, film thickness, wetting.
• C
  Related to Both: thermal stresses (CTE mismatch), setting contraction, type of bond formed.

Surface tension is the attractive force between atoms/molecules at the surface of a liquid. It's affected by: 1) nature of the liquid, 2) temperature (↑ temp → ↓ surface tension), 3) chemical impurities/surfactants (↓ surface tension).

Wettability = the ability of a liquid to spread over a solid surface. Viscosity = resistance of a liquid to flow — low viscosity = better flow = better adhesion.

Mixed failure: a combination of both adhesive and cohesive failure.

Physical properties of dental materials include: 1) Mass-relating properties, 2) Thermal properties, 3) Optical properties (covered separately in Lecture 4).

Density = mass per unit volume of a material (Mass / Volume), in gm/cm³.

A. Melting/Freezing Temperature  ·  B. Glass Transition Temperature (Tg)  ·  C. Thermal Conductivity  ·  D. Specific Heat  ·  E. Thermal Diffusivity  ·  F. Coefficient of Thermal Expansion (CTE)

The temperature at which a crystalline material melts or freezes.

Characteristic of non-crystalline structures such as polymers & waxes — they gradually soften as their temperature is raised (a melting range, not a sharp point).

Tg = the temperature at which the material begins to soften on heating (or solidify on cooling) — equivalently, the temperature at which a sharp increase in the Coefficient of Thermal Expansion occurs.

The quantity of heat (calories or joules) per second passing through a body 1 cm thick with a 1 cm² cross-section, when the temperature difference is 1°C.

• 1
  High thermal conductivity of a deep metallic restoration may cause thermal pulp shock — avoided by keeping adequate dentin thickness under the restoration, or, if not possible, placing an insulating cement base under it.
• 2
  High thermal conductivity of a metallic denture base is an advantage — it activates blood circulation, maintaining soft tissue vitality, unlike an acrylic denture base (a thermal insulator).

The quantity of heat needed to raise the temperature of one gram of the material by 1°C. Materials with low specific heat heat up easily.

A measure of transient heat flow: Thermal diffusivity = Thermal conductivity ÷ (Specific heat × Density). Materials with high thermal diffusivity have high thermal conductivity and low specific heat.

The change in length per unit length of a material for a 1°C change in temperature.

• 1
  Filling materials should have a CTE similar to that of the tooth structure to avoid breaking the marginal seal (microleakage).
• 2
  Matching CTE between a denture base and its artificial teeth is essential to avoid their separation during thermal changes.

• 1
  Hue — the dominant wavelength of light (ex: red, blue, or green).
• 2
  Value — the lightness or darkness of a color (a lighter shade has a higher value). In dentistry, it is the most important color parameter, as it represents the vitality of a tooth — a non-vital tooth has a low value and appears gray.
• 3
  Chroma — the degree of saturation (intensity) of a particular hue.

• 1
  Surface finish — an extremely rough surface appears lighter and less chromatic than a smooth surface of the same material, because the rough surface reflects more white light, which mixes with the light reflected from the material and dilutes its color. This is associated with unpolished or worn restorations.
• 2
  Pigmentation — esthetic effects are produced by incorporating colored pigments (metal oxides) into dental materials such as composite resin and denture acrylics.
• 3
  Metamerism — a phenomenon where colors match under one light source but differ under another. Clinical significance: color matching should be done under 2 light sources, one of which is sunlight or white light.
• 4
  Fluorescence (Phosphorescence) — the ability of a material to absorb light of a certain wavelength and re-emit light of a different (usually longer) wavelength.

Natural teeth can emit visible fluorescence — a blue-white color (400–450 nm) — when excited by UV or near-UV radiation (300–400 nm).

• 1
  Fluorescence contributes to the vital appearance of teeth.
• 2
  Restorations lacking a fluorescing agent appear as "missing teeth" when viewed under a black light (UV).
• 3
  Dental porcelains are formulated with fluorescing agents (e.g. Cerium oxide) for a natural appearance, while acrylic resin does not contain one.

• 1
  Opacity — prevents the passage of light entirely; objects cannot be seen through it.
• 2
  Translucency — permits the passage of some light and scatters/reflects the rest; objects cannot be seen clearly through it. Ex: enamel, dental ceramics.
• 3
  Transparency — allows the passage of light without distortion; objects can be seen clearly through it. Ex: glass.

Opalescent materials, such as dental enamel, reflect more of the shorter (blue) wavelengths, while longer (yellow-red) wavelengths are more transmitted. To produce a highly esthetic restoration, materials with an opalescent property should be used (e.g. porcelain veneers).

The most common method for color matching is the shade guide — containing a number of tabs with different Hue, Value & Chroma.

• 1
  Shade guides are arranged either by Hue (VITA Classic) or by Chroma (VITA Pan 3D Master).
• 2
  Disadvantage: color perception may vary from one individual to another.

Stress (σ) is the internal reaction to an external force = Force / Area (unit: Pascal, N/m²). Strain (ε) is the change in length per unit length (ΔL/L) — dimensionless, no unit.

• PL
  Proportional / Elastic Limit — max stress with no permanent deformation; end of the linear (straight-line) portion of the curve.
• YS
  Yield Stress — stress at which the material begins to behave plastically.
• US
  Ultimate Strength — maximum stress the material can withstand before fracture.
• FS
  Fracture Strength — the actual stress at which fracture occurs.
• E
  Elastic Modulus = Stress/Strain = slope of the linear portion. Measures stiffness/rigidity — a high E means a stiff wire that delivers a high, rapid orthodontic force.

• 1
  Flexibility = maximum elastic strain at the proportional limit. High flexibility = large elastic deformation at low stress — important for impression materials so they can flex past undercuts without permanent distortion.
• 2
  Ductility = ability to undergo plastic deformation under tension without fracture (can be drawn into a wire).
• 3
  Malleability = ability to undergo plastic deformation under compression without fracture (can be hammered into thin sheets).
• 4
  Brittleness = inability to show plastic deformation — fracture occurs at or near the proportional limit (the opposite of ductility).

• 1
  Compressive Strength — max stress under compressive force before failure; measured by compressing a cylinder between two flat plates.
• 2
  Diametral Tensile Strength (Brazilian test) — an indirect tensile test used only for brittle materials; a disc is compressed diametrically so the external compression creates internal tension.
• 3
  Shear Strength — studies the strength of an interface, e.g. the porcelain-metal bond.
• 4
  Flexural / Transverse Strength (Modulus of Rupture) — three-point bending test: a rod is supported at both ends while load is applied at the center.
• 5
  Impact Strength — energy required to fracture under a sudden impact force; measured by the Charpy test (horizontal support) or Izod test (vertical support). High impact strength is needed for dentures, which may be dropped suddenly.
• 6
  Tear Strength — resistance to tearing forces; rate-dependent (rapid loading increases tear strength). Important for elastic impression materials and maxillofacial prosthetics — thin sections need high tear strength.
• 7
  Burnishability — the ability to shape cast restoration margins under pressure; depends on ductility and malleability.

Definition

• The resistance of materials' surfaces to be scratched, indented or penetrated by hard objects.

Importance of Hardness

• Indicative of the ease of finishing and polishing.
• Low surface hardness → surface scratches easily.

Hardness Tests — Principle

• Depends on penetration of an indenter into the surface under a standardized load.
• Hardness is determined from the resultant indentation (area or depth).
• ⚠️ Dimensions of indentation vary inversely with hardness → harder material = smaller indentation.

Same load applied to A, B, C → the harder the material, the shallower the indentation.

Indenter Materials & Shapes

Methods for Measuring Surface Hardness

Definition

• The resistance to motion of one material body over another.
• Occurs because the surfaces of two objects touch and their molecules stick together.

Clinical Importance of Friction

Definition

• The mechanical loss of material surface as a result of contact between two or more materials.
• Wear is always recognized at the surface of softer materials.

Examples of Wear

Factors Affecting Wear

1. Abrasive Wear

• A harder material cutting or plowing into a softer material.

2. Corrosive Wear

• Results from interaction of the environment (atmosphere, moisture, bacteria, acids, chemicals) with sliding surfaces.
• Wear is accelerated by corrosion of the sliding surfaces.

3. Adhesive Wear

• One material slides over another with surface interaction and welding (adhesion) at localized contact areas.
• Microregions are pulled from one object and transferred to the other.

4. Surface Fatigue Wear

• Particles (asperities) with small contact area → high localized stresses → surface & subsurface cracks.
• These particles break off under cyclic loading & sliding.

• 1
  In the tray: high viscosity — material stays in tray and does not drip.
• 2
  Through a syringe nozzle (high pressure applied): viscosity decreases — material flows easily around teeth.

A phenomenon where a material exhibits a combination of both elastic and viscous properties.

• 1
  Snap (rapid) removal of impression materials — shortens load application time → reduces permanent deformation → more accurate impression.
• 2
  Allow sufficient time before model pouring — lets the viscoelastic impression material recover its accurate dimensions.

Causes of failure in the oral cavity:

• 1
  Presence of different stresses
• 2
  Frequent changes in pH
• 3
  Warm and humid atmosphere
• 4
  Temperature changes from hot to cold and vice versa
• 5
  Presence of microorganisms

5 Failure modes: Fracture · Tarnish & Corrosion · Crazing · Wear · Fatigue

Causes of tarnish:

• 1
  Thin surface films: oxides, sulfides, or chlorides
• 2
  Pigment-producing bacteria
• 3
  Stain-producing drugs (e.g. iron)
• 4
  Food debris, plaque & calculus on the restoration surface

Components of an electrochemical cell:

• A
  Anode — lower electrode potential; oxidation occurs; releases free electrons (+ions form); loses mass
• C
  Cathode — higher electrode potential; reduction occurs; consumes electrons (+ions gained); gains mass
• E
  Electrolyte — supplies +ions needed at the cathode
• I
  Electric current — the conduction path carrying electrons from anode to cathode

Effects of corrosion:

• 1
  Weakens the restoration & spoils esthetics
• 2
  Roughens the surface → bacterial/plaque retention → bad odor
• 3
  Metallic taste sensation; possible toxicity
• 4
  Local pain & swelling with no evidence of infection
• 5
  Metal ions may migrate to other organs → inflammation

Protection from corrosion:

• 1
  Passivation — a stable surface oxide layer that prevents further oxidation. Best in Al, Cr, Ti.
• 2
  Dental alloys should contain ≥ 70–75% noble metals
• 3
  Maintain good oral hygiene
• 4
  Highly polish alloys used intra-orally (smooth, lustrous surface overcomes concentration-cell corrosion)
• 5
  Avoid dissimilar metallic restorations; if unavoidable, paint the cathode surface with varnish coating

Causes of crazing:

• 1
  Repeated drying/wetting → alternating contraction & expansion
• 2
  CTE mismatch between acrylic denture base and porcelain teeth
• 3
  Solvents (alcohol, acetone, chloroform, monomer) — e.g. monomer contact during a denture repair

Properties of Metals

• At room temperature, all metals are crystalline solids — except Mercury & Gallium (liquid at room temperature).
• Good thermal and electrical conductivity — due to free electrons.
• Opaque and lustrous (shiny).
• High hardness and melting temperature — due to strong primary inter-atomic bond (metallic bond).
• Good ductility and malleability — due to crystalline imperfections (dislocations) which allow slippage of crystals over each other.
• White in color — except Copper (reddish) and Gold (yellow).

Shaping of Metals

Why Alloys?

• Pure metals are too soft and ductile for dental applications.
• Mechanical properties are improved by using mixtures of metals = Alloys.
• An alloy = a mixture containing 2 or more metals.

Classification 1 — According to Number of Alloying Elements

Classification 2 — According to Degree of Solubility

Types of Solid Solution

Properties of Solid Solution Alloys

• Have melting ranges rather than a melting point.
• Homogeneous → resist tarnish and corrosion.
• High ductility, but low strength & hardness compared to other alloy types.

Properties of Eutectic Alloy

• Have a fixed melting point.
• Heterogeneous → low tarnish and corrosion resistance.
• Brittle, but high strength & hardness.

Key Definitions

• Polymer — Long chain molecules consisting of many repeating small units (monomers). Poly = many, Mer = monomer unit. e.g. Poly Methyl Methacrylate (PMMA)
• Monomer — The smallest repeating unit in the polymer chain.
• Homopolymer — Polymer formed of one type of monomer.
• Copolymer — Polymer formed of 2 or more types of monomers.

Classification 1 — According to Spatial Configuration

Classification 2 — According to Thermal Behavior

Polymerization Process

• Process of forming a polymer from monomers.
• Polymerization is never entirely complete → remaining unconverted monomer = Residual Monomers.
• Residual monomers cause irritation of soft tissues and hypersensitivity.

Types of Polymerization Reactions

Definition & Composition

• Inorganic materials processed by firing at high temperature.
• Have ionic or covalent bonding and tightly packed structures.
• Composed of a glassy matrix (amorphous) + crystalline phase (e.g. Al₂O₃, ZrO₂).

Why Are Ceramics Brittle?

• Unlike metals and polymers, ceramics are brittle and difficult to shear plastically due to the ionic nature of bonding and minimum number of slip systems.
• Cracks within ceramics act as stress raisers — the stress at the crack tip can be many times higher than the surrounding stress → stress concentration → weakens the material.

Advantages vs. Disadvantages

Definition & Advantage

• Composites = combination of two or more classes of materials.
• Advantage: fabricate a new material with desirable properties not achievable with one material alone.
• Dental example: Composite Restoration.