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Clinical Dental Science BDS3 > Dental Material Science > Flashcards

Dental Material Science Flashcards

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1
Q

Porcelain has great aesthetics, but why is it not used on its own for restorations

A

Because on its own it isn’t able to withstand all of the loads that it is subjected to ( e.g. biting) so it isn’t inevitable that it will produce micro cracks and eventually fail

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2
Q

What is compressive strength

A

The stress needed to cause fracture

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3
Q

What is elastic/ Young’s modulus

A

Measure of ridigty

The stress strain ratio - the stress required to cause a change in shape

Measure of objects resistance to be deformed elastically ( not permanetly)

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4
Q

What is hardness

A

The resistance of the surface to indentation or abrasion

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5
Q

What three properties can we ascertain from a stress strain curve and which one can we not?

A

Strength , brittleness/ductility, elastic modulus ( rigitdty) - CAN

Hardness cannot be seen on a stress strain curve

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6
Q

On a stress strain curve what does the initial gradient represent

A

The elastic modulus

Steeper gradient would mean the material is more rigid

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7
Q

What is a brittle material ?

A

A brittle material will change shape a FRACTION a percentage of its length and then break

Small distance between proportional limit and fracture stress on stress strain curve

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8
Q

What is a ductile material?

A

A ductile material will stretch SEVERAL percent of its length and then fracture

Much greater distance on stress train curve between proportional limit and fracture stress on diagram

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9
Q

What is the proportional limit of a material

A

This is the maximum stress that a dental material sustains without any deviation

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10
Q

Name 3 good characteristics of porcelain

A

It is rigid - large stress required to cause a strain
It is hard - surface with stands indentations and abrasion well
It is strong - it has a high compressive strength

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11
Q

What are the downfalls of porcelain

A

Low tensile strength - so has a tendency to form surface defects, this leads to fracture at low stresses

It is a brittle material which means it has only a small strain( stretch ability) before it fractures

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12
Q

How is the metal oxide layer produced between the porcelain and the alloy in porcelain fused to metal alloys

A

The porcelain and alloy are subjected to very high temperatures in a furnace which produces the metal oxide layer.

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13
Q

What is the benefit of the metal oxide layer

A

The bonding of the metal oxide to the porcelain helps eliminate defect and cracks on the porcelains surface

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14
Q

What is the purpose of the alloy in porcelain metal restorations

A

The alloy with its own metal oxide layer provides MECHANICAL SUPPORT

Being more rigid, when subjected to large stresses the alloy will change shape very little and return to its original dimensions.

Acts as a support and limits the strain that the porcelain experiences

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15
Q

How is the porcelain alloy restoration made and what must be considered during this

A

It has to be fired in a furnace - raising temp of both materials by hundreds of degrees and then cooling them without developing thermal stresses which could cause the material or the metal oxide layer to form micro cracks

To avoid this the porcelain and the alloy should have similar thermal expansion coefficients.

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16
Q

What is the thermal expansion coefficient of porcelain

A

14ppm degrees Celsius

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17
Q

Why do we want the alloys thermal expansion to be similar to porcelains but ever so slightly greater?

A

This is so that during the cooling stage the alloy is slightly compressing the porcelain

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18
Q

Why would we not use silver palladium under the porcelain

A

Porcelains key feature is the aesthetics, the silver palladium can cause discolouration so don’t use that

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19
Q

Why do we want the melting temp of the alloy higher than the porcelain

A

Because this could lead to creep

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20
Q

What is creep

A

Gradual increase n STRAIN( permanent ) experienced under prolonged appplicaton of stress below the elastic limit

‘’Subjected to low level stresses over a prolonged period of time”

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21
Q

At what temp is creep most likely to occur in an alloy

A

When the alloy reaches half of its melting point

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22
Q

Why is high gold not used in porcelain alloys

A

Melting range too low and Young’s modulus too low

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23
Q

What is the main issue with nickel chromium alloys

A

During the casting process they exhibit a lot of shrinkage making it challenging to use

It’s bond strength to porcelain is also low

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24
Q

Why is there a biocompatibility issue in nickel chromium alloys

A

Because of the allergic response to nicekl

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25
Q

How does the mechanical mechanism of porcelain and alloy work

A

There is said to be irregularities on the surface of thr alloys metal oxide layer and porcelain which allows them to interlock

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26
Q

What is the stressed skin effect

A

Slight differences in the thermal expansion and contraction of hte porcelian and the alloy.

During the furnace stage - the alloy contracts slightly more on cooling and this generates compressive forces on the porcelain essentially gripping it

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27
Q

How does the chemical mechanism of porcelains alloys work

A

Oxides in the metal oxide coating on the allot migrating with the oxides within the porcelain itself - occurs in the firing stage when high temps are reached.

Described as ELECTRON SHARING

28
Q

Ideally if there a was going to be a failure in the porcelain metal bond where would you want it to occur

A

Within the porcelain

29
Q

By what process are wrought alloys manipulated and shaped?

A

Via cold working

30
Q

Iron is allotropic - what does this mean

A

This means that it undergoes two solid state phase changes with temperature

31
Q

What are the two solid crystalline forms of iron

A

Temp >1400 = BBC ( body centred cubic lattice)

900 <temp < 1400= FCC ( face centered cubic)

Temp< 900 - BCC

32
Q

What is an alloy

A

Two metals that form a common lattice structure

33
Q

If in the alloy the two metals in the common lattice are soluble in one another, what kind of solution would this form?

A

Solid solution

34
Q

If two atoms are markedly different in size what kind of solid solution do. They form

A

Interstitial solid solution

35
Q

What is the gamma phase of iron called and what kind of solid solution is it

A

Austenite and it is an intestinal solid solution with a FCC structure

36
Q

Fast cooling of austenite iron leads to what kind of iron

A

Martensite

  • distorted lattice
  • hard
    -brittle
37
Q

What percentage of chromium must steel coat win in order for it to be stainless

A

> 12% chromium

38
Q

How does stainless steel have its corrosion resistance property

A

Due to the chromium oxide layer

39
Q

Name 2 functions of nickel in SS

A

Improves fracture strength and corrosion resistance

40
Q

What are three dental uses of Austenitic steel?

A
  1. Dental equipment and instruments to be sterilised
  2. Wires e.g. ortho
  3. Sheet forms for denture base
41
Q

Why can stainless steel not be continuously manipulated when trying to form desired shape

A

It work hardens rapidly

42
Q

What is cold working

A

Cold working is the work done on a metal/alloy at LOW temperatures ( e.g bending, rolling)

This causes SLIP - dislocations collect at the grain boundaries, therefore the alloy has fewer defects within the lattice structure, making it stronger and harder.

43
Q

What is the springiness of a material ( specific to wire used in ortho)

A

Ability of a material to undergo large defections ( to form an arc) without permanent deformation ( i.e. returning to its natural shape)

44
Q

Why do we want a high ductility in stainless steel wires

A

So that it can undergo bending without fracture

45
Q

What is soldering of stainlesssteel

A

This can be used to Join SS together

46
Q

What happens in weld decay of stainless steel

A

When welding steel there is a risk that weld decay occurs.

This occurs when the temp i raised to 500-900 degrees and this can push the Cr and C atoms to the grain boundaries, allowing CrC to precipitate there.

This causes the SS to become brittle, limiting the amount of manipulation available and it also makes it more likely to corrode

47
Q

As with many alloys which have undergone various process (e.g. cold working), what kind of process is needed to ensure the configuration of the metal atoms

A

Stress. Relief annealing

48
Q

Name 4 advantages of using stainless steel as a denture base?

A

Fracture resistant
Corrosion resistant
High thermal conductivity
Thin

49
Q

Name some disadvantages of using stainless steel as a denture base

A
  1. Possible dimensional inaccuracy
  2. Elastic recovery of steel can lead to inaccuracy
  3. Difficult to ensure uniform thickness
50
Q

Name some ideal properties for an acrylic resin denture base

A

Dimensionally stable
Unaffected by oral fluids
Low density
High softening temperature
Non toxic
Easy and inexpensive to manufacture

51
Q

What is transverse/flexural strength

A

Flexural strength is the ability of the material to withstand bending forces applied perpendicular to its longitudinal axis.

52
Q

What is free radical addition polymerisation

A

Chemical union of two molecules either the same or different to form a larger molecule without the Elimination of a smaller molecule

Involves molecules with C-C double bonds

53
Q

What are the stages of acrylic polymerisation

A

Activation - of initiator to provide free radicals

Initiation - free radicals break the C-C double bonds in the monomer and transfer the free radical

Propagation - growing polymer chain

Termination - of polymerisation

54
Q

What is the initiator in acrylic polymerisation?

A

Benxzoyl peroxide

55
Q

What is the function of the plasticiser in the heat cured acrylic powder

A

Allows quicker dissolving in monomer liquid

56
Q

In the heat cure acrylic liquid there is methacrylate monomer what is its function

A

Dissolves PMMA particles

57
Q

What is the inhibitor in the heat cure acrylic liquid and what does it do

A

Hydroquinone

Prolongs the shelf life - reacts with any free radicals produced by heat, UV light

58
Q

What is the function of the co-polymers in the heat cured acrylic liquid?

A

Improve mechanical properties

59
Q

What is the difference with thermal expansion when using acrylic teeth in denture base compared to porcelain?

A

The thermal expansion is ok if the acrylic tooth is used however if porcelain is used it tends to be slighly higher.

The porcelain teeth expand less than the acrylic base causing a mismatch

60
Q

What is the softening temperature of an acrylic denture and what must we remind the patient of

A

75 degrees

They will be ok with injested hot fluids, however the denture will not cope with boiling water when washing the denture so must tell the patient this.

61
Q

During its usage how does the head cured PMMA make up for the 0.4% contraction that happens during the heating stage?

A

Through water absorption

62
Q

What activates the benzoyl perioxide in heat cured acrylic?

A

Heat

63
Q

Benzoyl peroxide is the initiator in self cure PMMA ( Aswell as heat cured), what is the activator of the initiator in this case

A

Tertiary amine

64
Q

Why might self cured PMMA dentures be more accurate fitting than heat cured?

A

Because the polymerisation stage requires no heating stage - so there will be little thermal contraction experience

65
Q

Why do self cure PMMA have poorer mechanical properties to heat used PMMA

A

Chemical activation of the polymerisation is less efficient, hence it has a lower molecular weight and so poorer mechanical properties

66
Q

Given the polymerisation of self-crushed acrylic is less efficient. One consequence is there is more unreacted monomer, what are some problems with this;

A

It can act as a plasticiser, softening the denture base
Can make the denture more vulnerable to failure
Also means there is more likelihood of the monomer leaking out and causing irritation

67
Q

What % of açylric resin dentures fracture within the first 3 years

A

10%

Clinical Dental Science BDS3 (7 decks)

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