Alloys For Cast Metal Restorations Flashcards
What is added to the porcelain in a crown?
Metal alloy (alloys withstand large forces readily).
What is the other name for the elastic modulus?
Rigidity.
What is a materials compressive strength?
The amount of stress to cause fracture.
What is the elastic modulus of a material?
Stress required to cause a change in shape.
What is the definition of the hardness of a material?
Resistance of surface to indentation or abrasion.
What is the ductility of a material?
The dimensional change experienced before fracture.
What 3 properties of a material can be determined by a stress-strain curve?
Strength
Ductility
Elastic Modulus.
There is a good example of a stress-strain curve on these slides.
.
What feature on a stress-strain diagram lets you know that one material is more brittle than the other?
When the proportional limit and the fracture stress are closer together means that the material is brittle (less ductile).
What are the characteristics of porcelain?
It is rigid, hard, strong but it has a low tensile strength.
What happens as result of porcelain having a low tensile strength?
This makes porcelain have a tendency to form surface defects which leads to fractures at low stresses. Defects are in the crystals – may take a little time to grow into a substantial defect which triggers fracture line.
What substance is bonded to porcelain to help eliminate defects/cracks on the porcelain surface?
Metal oxide.
Why are alloys added to porcelain?
To support and limit he strain that porcelain experiences. It does this because the applied stress has to cause a change in the dimensions of the porcelain AND the alloy, and the alloy is more rigid than porcelain.
Let’s say an applied stress would cause a 1% change in porcelain but only 0.1% in the alloy.
Then once the alloy is bonded and is supporting the porcelain, the applied stress is only capable of moving the alloy 0.1% - for the porcelain to move, so must the alloy.
*With the TWO materials bonded together, the stress applied causes a small strain to be experienced, small enough for the porcelain to withstand it and remain intact!!!
What are different types of porcelain fused metal alloys?
High gold Low gold Silver palladium Nickel chromium Cobalt chromium.
What is creep?
Gradual increase in strain (permanent) experienced under prolonged application of stress (
What are the required properties of porcelain fused metal alloys?
- forms a good bond to porcelain (good wetting)
- the thermal expansion coefficients must be similar
- avoid discolouration of the porcelain
- has to have good bond strength, hardness and a high elastic modulus
- the melting/recrystallisation temperature of the alloy must be higher than fusion temp of porcelain otherwise creep may occur.
Why must the thermal expansion coefficient of porcelain must be similar to the metal oxide?
To avoid setting up stresses during fusing of porcelain on alloy. The alloys melting temp should be no more than 0.5ppm/degrees centigrade so the alloy is slightly compressing porcelain on the cooling stage.
Why is copper not used in high gold alloys?
Can produce a green discolouration.
Why do we want a high elastic modulus?
Elastic modulus- more rigid the alloy- lower amount of stain the porcelain will be subjected to.
What are examples of metals in a high gold alloys?
Pt-pd- 14%- helps match up thermal expansion of porcelain. Increases melting point- potential for alloy to exhibit creep diminishes
Ag 1%
Au 80%
Indium and tin form oxides for bonding.
What are the disadvantages of using high gold alloys with porcelain?
Melting range may be too low
Youngs modulus is also too low (not sufficiently rigid).
Why are low gold alloys better to use than high gold?
Increased melting temperature and slightly better mechanical properties.
What materials are in a low gold alloy?
Au 50%
Pd 30%
Ag 10%
Indium+tin 10%.
What is the composition of silver-palladium alloys?
Pd 60%
Ag 30%
In+Sn 10%.
