alloys for cast metal restorations Flashcards
pfm crown
has porcelain surface and metal alloy substructure
porcelain fused to metal alloys
Porcelain crowns are aesthetically pleasing, but microcracks tend to form at the fitting surface, making it prone to mechanical failure.
We know alloys have good , reliable mechanical properties
So is there a way of combining the benefits of both materials to make a better crown,
by bonding porcelain to an alloy substrate
mechanical properties
compressive strength = stress to cause fracture
elastic modulus = stress required to cause change in shape
brittleness / ductility = dimensional changed experienced before fracture
hardness = resistant of surface to indentation or abrasion
stress-strain curve
STRESS-STRAIN CURVE is a SOURCE of information on RIGIDITY, DUCTILITY AND STRENGTH A MATERIAL
BUT not for surface hardness
porcelain is
Reasonably HARD and STRONG
And QUITE RIGID
HOWEVER, in the other axis, we see that it’s BRITTLE, not DUCTILE
high compressive strength
low tensile strength
low fracture toughness
it only withstands 0.1% permanent strain before fracturing.
That’s a really small amount.
alloys are
are much STONGER, HARDER and more RIGID.
And crucially more DUCTILE.
So ALLOYS can withstand greater degrees of permanent STRAIN when subjected to very large stresses – when say BITING.
between alloy and porcelain there is metal oxide
Bonding of metal oxide to porcelain helps eliminate defects/cracks on porcelain surface.
Alloy acts as a support & limits the strain that 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
A range of alloys have been developed for bonding to porcelain.
This list shows several possible options:
* High gold
* Low Gold
* Silver palladium
* Nickel chromium
* Cobalt chromium
required properties for alloy to bond to porcelain
Form good bond to porcelain. i.e. good wetting.
Thermal expansion coefficient MUST BE SIMILAR* to that of porcelain : (Typ. 14ppm/ oC ) TO AVOID setting up stresses during fusing of porcelain on to alloy.
Avoid discolouration = Ag can produce green discolouration
They must bond well to the metal oxide surface layer generated during the firing stage.
The second characteristic is surface HARDNESS – all the alloys listed are adequate.
The third characteristic is the ELASTIC MODULUS
The more rigid the alloy, the lower the amount of strain that the porcelain will be subjected to.
Melting temperature might be harder than fusion temp = otherwise creep will occur
what is creep
gradual increase in strain under prolonged application of stress
CREEP is more likely to occur whenever the alloy reaches half its melting temperature.
needs high temp
high gold alloys might be too low
low gold alloys are slightly better
silver has high melting point, but care needed in casting
nickel-chromium = high melting point and high casting shrinkage
cobalt chromium
It has a high ELASTIC MODULUS (220GPa)
It’s a hard material
and has a high tensile strength
HOWEVER due to its significant shrinkage during casting, it’s challenging to use
CoCr most used in GDH labs, some Gold, Palladium BUT NOT NiCr
nature of porcelain-metal bond
So the bonding mechanism may be
* Mechanical or Chemical in nature
OR due to what’s called the stressed skin effect
The mechanical mechanism is said to be due to irregularities on the surface of the alloy’s metal oxide layer and porcelain which allows them to interlock.
The stressed skin effect = during the production process, athe alloy contracts slightly more on cooling, and this generates compressive forces on the porcelain – in essence gripping it.
The CHEMICAL mechanism is explained by oxides in the metal oxide coating on the alloy migrating with oxides within the porcelain itself = electron sharing
modes of failure in porcelain
The oxide layer itself fracturing
The oxide layer delaminating from the alloy
Porcelain detaching from the oxide layer
IDEALLY any failure would occur within the porcelain
summary
Considered the various TYPES of alloy AND their characteristics –
We know the weakness of porcelain, and all ceramics is their BRITTLENESS
That an alloy sub-structure can help minimise this.
And we’ve briefly looked at the bonding mechanism between the metal oxide layer on the alloy surface and porcelain.
All materials can fail when subjected to large stresses over long periods.
Ideally the failure mode for porcelain fused alloy restoration is within porcelain itself – it should be the weakest link.
