Dental Materials II Flashcards
Stress/Strain curve for: Stiff Ductile Strong Tough
2;30
Stress/Strain curve for:
Flexible
Brittle
Weak
2;30
How does rapid cooling affect the atomic structure of metals and alloys?
Forms more nuclei per unit volume
Decreases grain size
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
For metals and alloys, what raises yield strength, increases ductility, and raises the ultimate strength?
Finer grain size
What is related to plastic deformation and fracture in metals and alloys?
Change grain size
A fracture due to a crack opening up on a grain boundary is more likely to occur in…
Large grain metals
The mechanism of plastic deformation is called…
Dislocation motion
What process causes many plastic deformations by producing dislocations (and increases likelihood of fracture)?
Cold Working (wrought)
The phase diagram of metals and alloys reveals…
Melting range
What are the 3 possibilities when mixed metals are cooled?
Solid Solution - (soluble)
Eutectic - (not soluble)
Intermetallic compound (rxn)
What 2 metals always form a solid solution?
Why?
Au and Cu
Soluble
What is special about ordered solutions?
They impart greater hardness
When Au and Cu occupy specific positions, it is an…
ordered solution
When 2 metals are soluble as liquids, but nearly insoluble as solids, they form…
Eutectic alloys
A pure Eutectic has a ______ that is ______ than either of the pure components.
Melting point (not range) Lower
What % by weight must a noble alloy contain to be considered noble?
25%
Carat content of pure gold:
24 K
How is fineness measured?
out of 1000
75% pure = 750
What 2 effects will adding 10% Cu to Au have?
Increase tensile strength
Increase BHN
(what is BHN?)
What 2 factors does the creation of an ordered solution depend on?
% metals
Cooling rate
T/F
An ordered solution will increase hardness, decrease elongation, and increase yield strength.
True
T/F
The stress at any point in a Ceramic is the same as the average stress
True
*if stretched
In ceramics, where will stress rise significantly?
around a Stress Raiser
*groove and edge of groove
What allows water molecules to penetrate polymer chains?
Weak secondary bonds
What defines high noble?
Noble?
Base metal?
60% noble / 40% gold
25% noble
less than 25%
What adds color to ceramics?
Metallic Oxides
What are the 2 post-manufacture phases of feldspathic dental porcelain?
Vitreous (glass)
Crystalline (mineral/leucite)
The Vitreous phase of ceramic porcelain is like…
Glass
The Crystalline phase of ceramic porcelain contributes what 2 things?
Strength Thermal Expansion (high)
What process joins porcelain particles together?
Sintering
*Biscuit Bake
What often fractures in the Oxide layer of ceramics-metal bonds?
Base metals
*layer too thick
Where does the fracture take place if proper bonding occurs in ceramic-metal specimens?
Porcelain
In a ceramic-metal system, the Coeffecient of Thermal Expansion must be ____ for the metal.
Higher
*puts ceramic into compression where it is stronger
What type of wax will soften when heated?
Thermoplastic
T/F
Waxes have a low degree of thermal expansion.
False
What type of stress in waxes usually results from non-uniform heating/cooling?
How can this be relieved?
Internal
Release Residual stress by heating
4 characteristics of all Waxes:
Melting range
Tendency to flow
Residual Stresses
Largest CTE of all dental matls
What is the most accurate way to dispense the mixing powder making up aliginate impression materials?
Weighing
What ingredient in Alignate retards hardening?
Sodium phosphate
aka tri-sodium phosphate
What can cause the shrinkage and distortion of an aliginate impression due to fluid loss?
(not evaporation)
Syneresis
T/F
Syneresis is the same as evaporation
False
What can cause expansion of an Aliginate impression?
Imbibition
What is Imbibition?
The uptake of water in the gel state
What can lead to shrinkage of an Aliginate impression and is often due to level of humidity in the room?
Evaporation
What is the basis for all gypsum used in dentistry?
Calcium sulfate hemihydrate
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
What 3 materials are derived from Mineral Gypsum (dihydrate)?
Plaster
Dental stone
High strength stone
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
What leads to less porosity and more strength in gypsum products?
Less water
What leads to more porosity and less strength in gypsum products?
More water
Why do thick mixes in water/powder of gypsum products harden more quickly?
Nucleation centers concentrated
T/F
Porosity tends to strengthen gypsum products
False
What happens if you decrease the water/powder ratio in a gypsum product?
Denser cast
What does increasing the Water/Powder ratio do to gypsum products?
Decrease Strength
How is Gypsum used in gold alloy casting?
as a Binder
What provides expansion to compensate for gold shrinkage?
Refractory
a silicone dioxide
What are the 2 functions of Binder in Gypsum Bonded Investments?
Forms solid mass
Provides strength
What is the composition of Binder in Gypsum Bonded Investments for gold?
For high temp alloys?
Calcium sulfate hemihydrtate
Phosphate, ethyl silicate
What is the low temperature phase for a silica?
High temperature phase?
Alpha
Beta
*change between 2 is responsible for most thermal expansion
T/F
A Utectic has a melting range and not a melting point
False
*Has a Melting Point
What is the difference between a Thermoplastic and Thermoset material?
Thermoplastic - Softens upon heating
Thermoset - no expansion/contraction on heating
stronger
What gives polymers its properties?
Molecular weights of make-up
What is responsible for the Thermal Expansion of porcelain?
Leucite phase
What does a refractory component do?
controls expansion
Does the refractory material have strength?
What holds it together?
No
Gypsum binder
T/F
Metal should have slightly higher coefficient of thermal expansion than porcelain
True
