Alloys For Porcelain Restorations Flashcards
What is good / bad about using porcelain alone as a restorative material?
It is relatively rigid
It is relatively hard
It has a high compressive strength
Low tensile strength
Tendency to form surface defects leading to fracture at low stress
Brittle - around 0.1% maximum strain before fracture
How does the alloy support the porcelain when bonded?
Alloy limits the strain porcelain can undergo. A stress applied may change porcelain by 1% but an alloy by only 0.1%. Because they are bonded, the porcelain will only change by 0.1% as for the porcelain to move, the alloy must too move.
What properties does the alloy have?
It is very hard, rigid, strong and crucially much more ductile
What are the required properties for an alloy that will be bonded to porcelain?
1- form a good bond to porcelain - good wetting - forms bond with metal oxides on surface
2- must have similar thermal expansion coefficient - to avoid stressed during fusion
3- must avoid discolouration of the porcelain - Ag and Cu can cause green discolouration
4- mechanical properties: good bond strength, high hardness and high elastic modulus
5- must have a higher melting temperature than porcelain to avoid creep
What alloys can be considered?
High and low gold alloys
AgPd (Silver Palladium)
NiCr (Nickel Chromium)
CoCr (Cobalt Chromium)
What is the constituents of a high gold alloy, what is good and bad about it?
80% gold
14% platinum or palladium
1% silver
Indium, tin - forms oxides for bonding
Matches thermal expansion
Very easily cast
Biocompatible
Bonds to porcelain
Poor creep
Low elastic modulus
What are the constituents, Pros and Cons of low gold alloys
50% Au
30% Pd
10% Ag
10% indium, Sn
Suitable melting temperature for low creep
Easily cast
Good elastic modulus
Biocompatible
No real downsides
What are the constituents, Pros and Cons of AgPd?
60% Pd
30% Ag
10% In, Sn
High melting point so good creep resistance
Rigid - high elastic modulus
Bonds to porcelain
Biocompatible
Difficulty casting
What are the constituents, Pros and Cons of NiCr
70-80% Ni
10-25% Cr (oxide bond)
Very high melting point so creep resistance
Bonds to porcelain
Very difficult to cast
Allergy risk so less biocompatible
Pros / Cons of CoCr alloys?
Very high melting point - low creep?
Very rigid and hard
Biocompatible?
Difficulty casting due to shrinkage
Low bond strength
What three bonding forces are there between the alloy and the porcelain?
Mechanical
Stressed skin
Chemical
How do mechanical bonds form between porcelain and alloy?
Created by interlocking of surface irregularity between the two surfaces
(Probably least important)
How do chemical bonds form between porcelain and alloy?
Electron sharing in oxides
During firing in furnace, porcelain flows and oxides in the metal-oxide coating migrate
How do stressed skin bonds form between porcelain and alloy?
Slight differences in the thermal contraction coefficients, allot contracts slightly more.
Leads to compressive forces between materials, aiding bonding.
What are the failure modes between the porcelain and the alloy?
Fracture in the metal oxide layer
Oxide layer delaminating from the alloy
Porcelain detaching from the oxide later
Fracture within porcelain (ideal out of all of them)
