Alloys for Cast Metal Restorations Flashcards
PFM crown
PORCELAIN, a ceramic material with excellent aesthetics
Being BONDED to and SUPORTED by a metal or - more accurately - an ALLOY sub-structure
cast metal comping onto which is fired a ceramic veneer
Porcelain
good aesthetics
but microcracks tend to form at the fitting surface, making it prone to mechanical failure
- thus not able to withstand large biting forces
alloys
good mechanical properties
withstand large stresses readily
compressive strength
stress to cause fracture
no longer fit for purpose
elastic modulus (Young’s modulus)
rigidity
stress/strain ratio
i.e. stress required to cause change in shape
brittleness/ductility
dimensional change experienced before fracture
PORCELAIN and ALLOYS differ markedly
hardness
resistance of surface to indentation or abrasion
what mechanical property cannot be ascertained from stress-strain curve
hardness
brittleness/ductility, elastic modulus and compressive strength can
stress-strain diagram elastic modulus
initial gradient
steeper = more rigid
stress-strain diagram fracture point (compressive strength)
end point of curve
stress-strain diagram tensile strength
vertical axis (height)
difference between proportional limit strength and fracture strength on stress-stain diagram
- amount of strain material withstands before fractures
brittle materials
change shape a fraction of a per cent of its length, and then break
Ceramics fall into this category
stretches ~ 0.5% before fracturing
ductile material
stretch several per cent of its length, then fracture.
Alloys tend to be ductile
stretches ~ 3.5% before fracturing
porcelain property axes
Reasonably HARD and STRONG
And QUITE RIGID
HOWEVER, in the other axis, we see that it’s BRITTLE, not DUCTILE
alloys property axes
ALLOYS are much STONGER, HARDER and more RIGID.
crucially more DUCTILE.
So ALLOYS can withstand greater degrees of permanent STRAIN when subjected to very large stresses – when say BITING.
porcelain characteristics
RIGID – large stress required to cause strain
HARD – surface withstands abrasion/indentation well
STRONG - high compressive strength
BUT low tensile strength
tendency to form surface defects
- leads to FRACTURE at low stress
Defects are in the crystals – may take a little time to grow into a substantial defect which triggers fracture lines
BRITTLE / low fracture toughness
- (maximum strain ~0.1% before fracturing) – very small
structure of porcelain metal restorations
simple
- metal oxide layer bonding to each material.
- Bonding of metal oxide to porcelain helps eliminate defects/cracks on porcelain surface.
cast to the desired shape beforehand - a process undertaken by a technician.
Alloy acts as a support & limits the strain that porcelain experiences.
production of porcelain alloy
heating to very high temperatures (many hundreds of degrees Celsius – in a furnace), then cooling them without developing any thermal stresses (could cause either material or the metal oxide layer to develop defects or micro-cracks)
produces an oxide layer on the alloy which will in turn bond to the ceramic.
this metal oxide layer will help to prevent defects or micro-cracks forming on the porcelain surface, which we know is the way it fails.
use of alloy in porcelain-alloy
with its own oxide layer – provides MECHANICAL support to porcelain.
Being more rigid, when subjected to a large stress, the ALLOY will change shape very little, and return to its original dimensions.
When that stress is applied to PORCELAIN on its own it will change shape so much it fractures
When the same stress is applied to the PORCELAIN when fused to the alloy, the strain experienced overall is less than the level which causes porcelain to fracture.
The alloy limits the STRAIN the porcelain is subjected to, helping it from reaching the level for BRITTLE failure
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.
describe how porcelain-alloy is stronger and less prone to fracture
When that stress is applied to PORCELAIN on its own it will change shape so much it fractures
When the same stress is applied to the PORCELAIN when fused to the alloy, the strain experienced overall is less than the level which causes porcelain to fracture.
The alloy limits the STRAIN the porcelain is subjected to, helping it from reaching the level for BRITTLE failure
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.
role of alloy in PFM
acts as a support & limits the strain porcelain experiences
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 must the porcelain and alloy be similar in
thermal expansion coefficients
why do porcelain and alloy need similar thermal expansion coefficients
To avoid developing defects or micro-cracks on cooling the furnaced porcelain-alloy
- both the PORCELAIN and the ALLOY should have similar thermal expansion coefficients.
they will expand at the same rate when being heated and contract at the same rate when being cooled.
- avoids thermal stresses within either material or at their contact surfaces and it ensures a good bond with the metal-oxide layer sandwiched between them.
