Characteristics of Materials

Chapter 2Dental MaterialsDAE/DHE 203

Characteristics of Materials:

Four Classes of Materials

Structure of Materials

Physical Characteristics

Mechanical Characteristics

Biologic Characteristics

Classes of Materials:

METALS:High thermal & electrical conductivityHigh ductility (bend without breakage)

High opacity (do not transmit light)

High luster (reflect light; appear shiny)

Crystalline arrangement of atoms (solid)

Strong metallic bond (high melting point - except Mercury)

Classes of Materials:

METALS:Strong, rigid, and stable materialsCan be “cast” or formed into various shapesWhat restorations can you name that are constructed with metal?

Classes of Materials:CERAMICS:

Compound of metal with non-metal High melting pointsLow thermal & electrical conductivityCrystalline or Non-crystallineInert – biologically compatibleUsed as fillers to reinforce compositesEsthetic - porcelains

Classes of Materials:

POLYMERS:Man-made, long-chain, organic molecules (carbon atoms linked together)

Low thermal & electrical conductivityLow strength and stability Dental acrylics – dentures, sealants,

temps Impression materials Adhesives – dental cements Resin base for dental composites

Classes of Materials:

COMPOSITES:Mixtures of two or more of the other classes (metals, ceramics, polymers)

Example: Dental Composite filling materialResin matrix (polymer) + glass filler (ceramic)

Structure of Materials:

BONDS: forces holding atoms togetherPrimary Bonds – solids Covalent: sharing electrons Ionic: interaction of + and – charges Metallic: share electrons of outer shell

Secondary Bonds – liquids Less stable; weaker attractions Bond liquid to liquid, or liquid to solid

Structure of Materials:

ATOMIC ARRANGEMENTS: (when a material is in a solid state)Amorphous – irregular pattern of atoms; “frozen liquids” i.e. glass, polymers

Crystalline – a regular pattern of columns and rows, stacked upon each other; “cubic” or other crystal patterns i.e. metals

Physical Characteristics:

Electrical & Thermal Conduction:No need for materials to be conductive Metals ARE good conductors Galvanism – a “shock” created by 2 unlike

metals in contact + saliva Heat Capacity – metals have a low capacity

Protect teeth from stimulation; insulate!

Physical Characteristics:

Thermal Expansion: temperature change causing a material to expand or contract can create change in dimension

Coefficient of Thermal Expansion: the amount of dimensional change as

a material expands/contracts Ideally, choose a restoration that

expands & contracts same as tooth.

Coefficient of Thermal Expansion

Material Coefficient of

Thermal Expansion Thermal ConductivityTooth 8 - 11 1 - 2

Porcelain 6 - 15 2 - 3Dental Cement 10 - 12 1 - 3

Gold 14 - 16 710Amalgam 22 - 28 55

Composite 20 - 50 1 - 3Wax 250 - 400 1

Thermal Expansion:

Thermal Conductivity:

Ferracane; 2001; Materials in Dentistry; Table 2-1, p. 21

Physical Characteristics:

Solubility: Ability to dissolve Cause allergic rxn? Least soluble =

porcelains & ceramics

Most soluble = polymers & acrylics

Sorption: Uptake of fluids or

substances Highest sorption

= polymers Swelling of

material Distort or

disintegrate

Ideally, dental materials are resistant to both.

Physical Characteristics:ADHESION:

The force of attraction between molecules or atoms of two different

surfaces brought into contact.COHESION:

The force of attraction between molecules or atoms within a material.

Adhesive: that which is being attached (“glue”)

Adherend: the surface to which it will be attached

Physical Characteristics:

Factors influencing ADHESION, (adhesion): Wettability – the ability of the surface to

become wet; ( wettability) Surface Energy – the available energy

at the surface; ( surface energy) Surface Tension – amount of attraction

the molecules have for one another; ( surface tension)

Viscosity – resistance to flow; ( viscosity)

Wettability: Contact Angle

adherendadherend

Good Wetting:

Low contact angle;

Less than 90°

Poor Wetting:

High contact angle;

Over 90°

Adhesion:

Why is adhesion an important factor in the success of restorative dentistry?

Physical Characteristics:COLOR & ESTHETICS:

Created by light’s interaction with materialHue – dominant colorValue – lightness of a colorChroma – intensity of a colorTranslucency - teeth permit light to transmit through themHigh demand today for materials to match natural tooth

Physical Characteristics:CORROSION:The deterioration of a metal by a chemical or

electrochemical reaction; irreversibleTarnish: surface deterioration; discoloration

Methods of Corrosion:“Battery” created: 2 metals + saliva = release of metallic ions & destruction of metalCrevice corrosion: saliva & plaque acids seep into filling interface

Physical Characteristics:

What happens when the interface or gap between an amalgam and tooth corrodes?

How can this type of corrosion be prevented?

Mechanical Characteristics:

Types of FORCE: Tensile: pulling Compressive: crushing, squeezing Shear: sliding

STRESS: a material’s response to force;generated within it to counter the force

STRAIN: change in dimension as a result of the applied force; deformation

Mechanical Characteristics:

Modulus of Elasticity – proportion of stress is equal to strain (a material can resume its

shape when the stress is removed) Elastic Limit – maximum stress level tolerated by a material without deformation (a material will be deformed if any more stress is put upon it) AKA “proportional limit”; “yield point”

Ultimate Strength – the highest stress tolerated before failure (any more stress will result in breakage of the material)

Stress-Strain Curve:

Strain

Stress

Elastic Limit Proportional Limit

Modulus of Elasticity

Ultimate Strength

Failure

Mechanical Properties:Hardness – ability to resist indentation

Toughness – ability to resist fracture

Fatigue – repeated stress over time creating small cracks/weakness; may lead to failure

Creep – gradual, permanent change in dimension under constant loadWear – direct contact of 2 objects deteriorates surface Attrition – wearing of occlusal/incisal surfaces Abrasion – “toothbrush abrasion”

Biologic Characteristics:

The most prevalent concern: Sealing the interface between restoration & tooth! Prevent leakage of bacteria, saliva, by-products Prevent seepage of fluids into & out of the tooth

Dentinal tubules carry fluids, sealed by enamel Hydrodynamic Theory – tooth pain is a result of

the fluid flow around odontoblastic processes, stimulating nerve fibers

Temperature change can expand/contract these fluids