ch. 3: physical and mechanical properties of dental materials

what is density?

mass of a material per given volume

what factors affect density?

type of atoms, packing of atoms, and voids in the material

why do most metals have a high density?

their atoms are tightly packed and have high atomic numbers

what happens to atomic bonds when a material melts or boils?

thermal energy breaks the bonds

why do some materials have a melting range instead of a specific melting point?

mixtures often melt over a range of temperatures

what dental material has a melting range?

dental waxes

what is vapor pressure?

a liquid’s tendency to evaporate and become a gas

what happens to vapor pressure as temperature increases?

vapor pressure increases

what is thermal conductivity?

the rate of heat flow through a material

why can a metallic restoration close to the pulp cause sensitivity?

metals conduct temperature changes toward the pulp

what is heat capacity?

the amount of thermal energy a material can hold

what is specific heat capacity?

energy needed to raise one unit of mass by 1°C

what is heat fusion?

energy required to melt a material

what is heat of vaporization?

energy required to boil a material

• important when casting gold restorations

what is the coefficient of thermal expansion?

change in volume in relation to change in temperature

• shrink when cold

• expand when hot

what is percolation?

opening and closing of the gap between a tooth and restoration due to different thermal expansion

what can percolation cause?

microleakage, tooth sensitivity, and recurrent decay

which dental materials are good electrical conductors?

metals

which materials are poor electrical conductors?

polymers and ceramics

what is galvanic shock?

electricity flowing from metal to amalgam and through to the pulp

what can galvanic action cause in amalgam?

oxidation and corrosion

what is viscosity?

the ability of a material to flow

how does temperature affect viscosity?

materials generally flow more easily when warmer

what does high viscosity cause?

high contact angle and poor wetting

what does low viscosity cause?

low contact angle and good wetting

what is hardness?

resistance to scratching and indentation

knoop hardness numbers for enamel, dentin, porcelain, and acrylic denture teeth?

enamel = 350

dentin = 70

porcelain = 400-500

acrylic denture teeth = 20

what is the relationship between hardness and abrasion resistance?

harder materials are more resistant to abrasion

what can happen if a restorative material is too hard?

it can wear away opposing teeth

what can happen if a restorative material is too soft?

the restoration itself can wear away

what is solubility?

the amount of material that dissolves in liquid

what can excessive solubility cause?

loss of restorative material and recurrent decay

what is water sorption?

absorption of water by a material

what can affect the perception of a material’s color?

the individual and the lighting

what is used to standardize dental color selection?

shade guides

what are examples of radiopaque dental materials?

metal restorations, bases, and liners

what are examples of radiolucent materials?

some ceramics, denture acrylic resin, and decay

what is force?

weight or load applied to an object

what is stress?

force divided by the area where it is applied

what is elasticity?

ability to change shape and return to the original shape

what is strain?

change in length divided by original length

what is the relationship between stress and strain?

they are proportional and always occur together

what is modulus of elasticity?

the scientific term for stiffness of a material

• the higher the modulus, the stiffer

what is elastic deformation?

the object returns to its original shape after stress is removed

what is plastic or permanent deformation?

the object remains permanently deformed after stress

what is the elastic limit, proportional limit, or yield point?

the point where plastic deformation begins

what is ultimate strength?

the point where the object breaks or fails

what is compression?

pushing or crushing stress

what is tension?

pulling stress

what is shear?

sliding stress

what is torsion?

twisting stress

what is bending?

compression and tension occurring together

what three mechanical properties must a restoration have to withstand mastication?

hardness, yield strength, and stiffness

which is often more important, yield strength or ultimate strength?

yield strength

why is yield strength important for a restoration?

deformation during biting can cause restoration failure

what is resilience?

ability to absorb energy without becoming deformed

what is toughness?

energy absorbed up to the point of failure

what is fracture toughness?

energy required to fracture a material when a crack is present

which materials have high fracture toughness?

metals

which materials have low fracture toughness?

glass and dental porcelain

what is fatigue?

failure caused by repeated stress over a long period of time

what is creep?

small change in shape under continuous compression

what is stress relaxation?

slow decrease in force over time

what is stress concentration?

increased stress around a defect

what can stress concentration lead to?

increased likelihood of fracture

e.g cutting glass, crack in a windshield