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Dental Materials > Properties of materials > Flashcards

Properties of materials Flashcards

1
Q

What factors have to be considered when choosing a material?

A

Unmixed compounds: storage conditions, shelf life
During mixing: heat generation, setting time, viscosity
Set material: strength, toughness, solubility

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2
Q

Define mechanical properties

A

Indicates how material or component will respond in use i.e. how it will perform in response to applied forces

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3
Q

Why are mechanical properties important?

A

To ensure material can function in mouth e.g. not fracture or deform under biting fores or changes in T

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4
Q

What does ISO standard for mechanical properties indicate?

A

Minimum standard it must achieve in MPa

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5
Q

Name 10 mechanical properties

A 
Stress and strain
Fatigue (repetition)
Hardness (resistance to scratching)
Abrasion resistance (wear)
Tension (stretch)
Compression (squash)
Torsional (twist)
Shear (2 planes moving in different directions)
Bending (combination stress - tension outside, compression inside)
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6
Q

Are ceramics better under tension or compression?

A

Good in compression, not in tension

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7
Q

What are the units of stress and strain?

A

Stress is in MPa

Strain is unitless (distance / distance)

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8
Q

What is stress?

A

Stress is the force per unit cross-sectional area acting on a material
Stress = F/A

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9
Q

What is strain?

A

Strain is the fractional change in dimensions caused by the force.
Strain = (L1-L0)/L0 i.e. change in length / original length

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10
Q

What happens to stress as item area dereases?

A

Stress increases

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11
Q

How do you work out force?

A

mass x acceleration

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12
Q

What is resilience?

A

The amount of energy a material can absorb without undergoing any plastic deformation (Elastic Area)

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13
Q

What is toughness?

A

The amount of energy a material can absorb to the point of fracture

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14
Q

What is ductility?

A

The amount of plastic strain at fracture

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15
Q

On a stress-strain curve, which is on the Y-axis / X-axis?

A

Strain on X axis

Stress on Y axis

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16
Q

What is the elastic limit?

A

End of straight line on graph
At yield stress
After that it’s plastic flow region (permanent deformation)

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17
Q

What is elastic (Young’s) modulus?

A

A measurement of the stiffness of material
Worked out by gradient of graph
Units - Pa or MPa

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18
Q

What is stiffness?

A

Resistance to elastic deformation

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19
Q

What is the fracture strength?

A

Force at fracture

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20
Q

What is the ultimate tensile strength?

A

Max force absorbed

21
Q

Shape of stress-strain curve for ceramic

A

Brittle so no plastic region
Put strain on it until it breaks
Can take higher loads than plastics before breaking

22
Q

Shape of stress-strain curve for elastomer

A

Very small elastic region, very large plastic region

Become more brittle as material gets older

23
Q

How do we measure stress and strain?

A

Using a tensile tester

24
Q

How does fatigue occur?

A

When materials are subjected to fluctuating, cyclic loads over time (too small to cause failure)
Accumulation of stress can cause crack propagation –> failure

25
Q

What are fatigue properties given as?

A 
Fatigue life (no. cycles to failure)
Fatigue limit/ strength (cyclic stress required to cause failure for set no. cycles)
26
Q

What needs to be considered to prevent fatigue?

A

Shape, surface and homogeneity of materials for stress

27
Q

What is hardness?

A

The ability to withstand surface indentation by compressive force

28
Q

Which is the most common indentation technique used for dental materials? What does it use?

A

Vickers

Diamond

29
Q

How is hardness determined?

A

Directly proportional to size of indentation (hardness no. is large for soft materials and small for hard)
Or scratch test

30
Q

What does hardness give a good indication of?

A

Materials resistance to wear (abrasion)

31
Q

Name 3 groups of chemical and physical properties

A

Elasticity, viscosity and viscoelasticity (how it responds to the force)
Durability and degradation (how it responds to acids)
Thermal properties (how it responds to heat)

32
Q

Define elasticity, viscosity and viscoelasticity

A

Elastic: strain and instantaneously return to original state once stress is removed
Viscous: resist flow and strain linearly with time. When load is released, do not immediately return to original state
Viscoelastic: elements of both. Exhibit time-dependent strain

33
Q

What are non-Newtonian fluids?

A

e.g. cornflour and water

becomes solid when you put stress on it, otherwise liquid

34
Q

What is the study of viscosity called?

A

Reometry

35
Q

What is durability?

A

Ability of a material to withstand its environment e.g. if it dissolves, erodes or corrodes

36
Q

What is the extent to which a material dissolves in fluid measured by?

A

Its solubility

37
Q

What does erosion mean in dentistry?

A

Destruction of enamel and dentine by acid attack

Can be compounded by mechanical forces such as brushing

38
Q

What is corrosion?

A

Deterioration of a material
Usually a metal with its environment
Electrochemical process, can be considered as destructive (natural) oxidation

39
Q

What are the types of degradation?

A

Corrosion, erosion and abrasion

40
Q

What can degradation lead to? What is therefore important

A

Leaching of constituents into oral environment.

Any leached constituents must be safe locally and systemically

41
Q

When does heat transfer by conduction take place?

A

When T gradient exists and is directional from hotter (high momlecular energy) to colder (low molecular energy)

42
Q

What is thermal conductivity?

A

Rate of heat flow (Watts per meter) per unit of T gradient under steady state conditions
Units: W m-1 C-1

43
Q

What is thermal diffusivity?

A

Measure of readily a material conducts thermal energy to how readily it stores it

44
Q

What is the equation for thermal diffusivity?

A

thermal diffusivity = thermal conductivity / (density x heat capacity)

45
Q

Is a high or low thermal diffusivity preferred? Why?

A

Low. Less heat conducted to the pulp, higher proportion raises T of material

46
Q

Why does thermal expansion occur?

A

Most materials expand when heated due to increase in amplitude of atomic/ molecular vibrations due to absorption of heat energy

47
Q

How is thermal expansion measured?

A

For solids, often stated as linear coefficient of thermal expansion (fractional length change per degree of T change)

48
Q

What is important about thermal expansion in dentistry?

A

Crucial to minimise thermal expansion mismatches between restorative materials and tooth tissue / between neighbouring materials.
e.g. cores/ substructures and crowns

49
Q

What is important about exothermic reactions in dentistry?

A

Use materials formed by combi of 2+ substances, often exothermic i.e. heat generated and transferred to surrounding tooth tissue.
Care required with large restorations, esp. those in close proximity to pulp cavity

Dental Materials (1 decks)

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