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Flashcards in Dental Materials II Deck (67):
1

Stress/Strain curve for:
Stiff
Ductile
Strong
Tough

2;30

2

Stress/Strain curve for:
Flexible
Brittle
Weak

2;30

3

How does rapid cooling affect the atomic structure of metals and alloys?

Forms more nuclei per unit volume
Decreases grain size

4

What 2 terms are used for the ability of atomic centers to slide against each other into new positions within the same crystal lattice?

Ductility
Malleability

5

For metals and alloys, what raises yield strength, increases ductility, and raises the ultimate strength?

Finer grain size

6

What is related to plastic deformation and fracture in metals and alloys?

Change grain size

7

A fracture due to a crack opening up on a grain boundary is more likely to occur in...

Large grain metals

8

The mechanism of plastic deformation is called...

Dislocation motion

9

What process causes many plastic deformations by producing dislocations (and increases likelihood of fracture)?

Cold Working (wrought)

10

The phase diagram of metals and alloys reveals...

Melting range

11

What are the 3 possibilities when mixed metals are cooled?

Solid Solution - (soluble)
Eutectic - (not soluble)
Intermetallic compound (rxn)

12

What 2 metals always form a solid solution?
Why?

Au and Cu
Soluble

13

What is special about ordered solutions?

They impart greater hardness

14

When Au and Cu occupy specific positions, it is an...

ordered solution

15

When 2 metals are soluble as liquids, but nearly insoluble as solids, they form...

Eutectic alloys

16

A pure Eutectic has a ______ that is ______ than either of the pure components.

Melting point (not range)
Lower

17

What % by weight must a noble alloy contain to be considered noble?

25%

18

Carat content of pure gold:

24 K

19

How is fineness measured?

out of 1000
75% pure = 750

20

What 2 effects will adding 10% Cu to Au have?

Increase tensile strength
Increase BHN

(what is BHN?)

21

What 2 factors does the creation of an ordered solution depend on?

% metals
Cooling rate

22

T/F
An ordered solution will increase hardness, decrease elongation, and increase yield strength.

True

23

T/F
The stress at any point in a Ceramic is the same as the average stress

True

*if stretched

24

In ceramics, where will stress rise significantly?

around a Stress Raiser

*groove and edge of groove

25

What allows water molecules to penetrate polymer chains?

Weak secondary bonds

26

What defines high noble?
Noble?
Base metal?

60% noble / 40% gold
25% noble
less than 25%

27

What adds color to ceramics?

Metallic Oxides

28

What are the 2 post-manufacture phases of feldspathic dental porcelain?

Vitreous (glass)
Crystalline (mineral/leucite)

29

The Vitreous phase of ceramic porcelain is like...

Glass

30

The Crystalline phase of ceramic porcelain contributes what 2 things?

Strength
Thermal Expansion (high)

31

What process joins porcelain particles together?

Sintering

*Biscuit Bake

32

What often fractures in the Oxide layer of ceramics-metal bonds?

Base metals

*layer too thick

33

Where does the fracture take place if proper bonding occurs in ceramic-metal specimens?

Porcelain

34

In a ceramic-metal system, the Coeffecient of Thermal Expansion must be ____ for the metal.

Higher

*puts ceramic into compression where it is stronger

35

What type of wax will soften when heated?

Thermoplastic

36

T/F
Waxes have a low degree of thermal expansion.

False

37

What type of stress in waxes usually results from non-uniform heating/cooling?
How can this be relieved?

Internal

Release Residual stress by heating

38

4 characteristics of all Waxes:

Melting range
Tendency to flow
Residual Stresses
Largest CTE of all dental matls

39

What is the most accurate way to dispense the mixing powder making up aliginate impression materials?

Weighing

40

What ingredient in Alignate retards hardening?

Sodium phosphate

aka tri-sodium phosphate

41

What can cause the shrinkage and distortion of an aliginate impression due to fluid loss?
(not evaporation)

Syneresis

42

T/F
Syneresis is the same as evaporation

False

43

What can cause expansion of an Aliginate impression?

Imbibition

44

What is Imbibition?

The uptake of water in the gel state

45

What can lead to shrinkage of an Aliginate impression and is often due to level of humidity in the room?

Evaporation

46

What is the basis for all gypsum used in dentistry?

Calcium sulfate hemihydrate

47

The 3 types of material derived from the partial dehydration of gypsum rock (plaster, stone, high strength stone) have what major difference?

Manner of driving off water

48

What 3 materials are derived from Mineral Gypsum (dihydrate)?

Plaster
Dental stone
High strength stone

49

The gypsum setting reaction is what type?

characterized by what 3 things?

Nucleated recrystallization

continuous solution of hemihydrate
diffusion of Ca and sulfate ions to nucleation centers
Precipitation of microscopic gypsum crystals

50

What leads to less porosity and more strength in gypsum products?

Less water

51

What leads to more porosity and less strength in gypsum products?

More water

52

Why do thick mixes in water/powder of gypsum products harden more quickly?

Nucleation centers concentrated

53

T/F
Porosity tends to strengthen gypsum products

False

54

What happens if you decrease the water/powder ratio in a gypsum product?

Denser cast

55

What does increasing the Water/Powder ratio do to gypsum products?

Decrease Strength

56

How is Gypsum used in gold alloy casting?

as a Binder

57

What provides expansion to compensate for gold shrinkage?

Refractory
(a silicone dioxide)

58

What are the 2 functions of Binder in Gypsum Bonded Investments?

Forms solid mass
Provides strength

59

What is the composition of Binder in Gypsum Bonded Investments for gold?
For high temp alloys?

Calcium sulfate hemihydrtate

Phosphate, ethyl silicate

60

What is the low temperature phase for a silica?
High temperature phase?

Alpha

Beta

*change between 2 is responsible for most thermal expansion

61

T/F
A Utectic has a melting range and not a melting point

False

*Has a Melting Point

62

What is the difference between a Thermoplastic and Thermoset material?

Thermoplastic - Softens upon heating

Thermoset - no expansion/contraction on heating
(stronger)

63

What gives polymers its properties?

Molecular weights of make-up

64

What is responsible for the Thermal Expansion of porcelain?

Leucite phase

65

What does a refractory component do?

controls expansion

66

Does the refractory material have strength?
What holds it together?

No
Gypsum binder

67

T/F
Metal should have slightly higher coefficient of thermal expansion than porcelain

True

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