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.
alloys used to bond to porcelain (5)
High gold
Low Gold
Silver palladium
Nickel chromium
Cobalt chromium (Note that this is a different type of Cobalt chromium alloy from that used as a partial denture framework.)
5 required properties of alloys in porcelain-alloys
form good bond to porcelain
thermal expansion coefficient similar to porcelain
avoid discolouration of porcelain
mechanical
- bond strength
- hardness
- elastic modulus (high)
melting/recrystallisation temperature
importance of alloy bond to porcelain
Form good bond to porcelain. i.e. good wetting, good surface contact
Porcelain forms bond with metallic oxides on surface.
- bond is through a metal-oxide layer on the alloy surface (developed during the firing production stage).
(Ni-Cr alloys more difficult to achieve good bonding)
importance of thermal coefficient of alloy and porcelain
MUST BE SIMILAR* to that of porcelain : (Typ. 14ppm/ oC )
ideally difference of 0.5ppm/ degrees C in alloy’s favour
so that during the cooling stage the alloy is slightly compressing the porcelain.
TO AVOID setting up stresses during fusing of porcelain on to alloy.
(all alloys O.K)
importance of alloys not discolouring porcelain
Key advantage of porcelain – aesthetics (tooth-like appearance)
If the underlying alloy sub-structure disturbs this, then that is clearly undesirable – in fact it would be unfit for purpose.
Ag in AgPd can produce green discolouration (silver palladium alloy)
Thus no COPPER in the HIGH GOLD alloy.
(In contrast, You may recall from your BDS2 alloy lectures, that the alloys used for partial denture frameworks had significant quantities of copper)
mechanical properties of alloy importance
Bond Strength : THREE alloys adequate (not Ni-Cr, nickel chromium)
Hardness: all alloys adequate (though early Ni-Cr alloys too hard)
Elastic modulus: want high value (i.e. rigid) to support porcelain and prevent fracture (Ni-Cr best)
- The more rigid the alloy, the lower the amount of strain that the porcelain will be subjected to.
importance of alloy melting/recrystallisation temperature
MUST be higher than fusion temperature of porcelain - otherwise CREEP may occur
creep
gradual increase in STRAIN (permanent) experienced under prolonged application of STRESS (< EL)
e.g. amalgam
causing permanent deformation whenever it’s subjected to low-level stresses over a prolonged period of time.
occurs when material temperature is more than about half its MPt
during the porcelain firing stage, there could be a permanent change in the shape of the alloy, UNLESS it has an appropriately HIGH melting temperature.)
when does creep in porcelain-alloys occur
occurs when material temperature is more than about half its MPt
during the porcelain firing stage, there could be a permanent change in the shape of the alloy, UNLESS it has an appropriately HIGH melting temperature.)
high gold porcelain fused metal alloy
Au 80%
Pt/Pd 14%
- match thermal expansion
increase melting point – minimise creep
Ag 1%
Indium, Tin
- PIVOTAL enable oxides to form, thus allowing bonding
NO Cu - otherwise green hue imparted to porcelain
Shortcomings
- Melting point is too low
- Not sufficiently rigid (Young’s modulus)
low gold porcelain fused metal alloy
Au 50%
Pd 30% double high gold
Ag 10%
Indium, Tin 10%.
- increased melting temperature
- slightly better mechanical properties
silver palladium porcelain fused metal alloy
Pd 60%
Ag 30%
In,Sn 10 %
- high melting point
- care needed in casting – challenge for technicians
nickel chromium porcelain fused metal alloy
Ni 70 - 80%
Cr 10 - 25% (oxide bond)
- high melting point desirable
- high Young’s Modulus/ Rigid wanted
- high casting shrinkage - challenging to use
- quite low bond strength - drawback
cobalt chromium alloys in porcelain fused metal alloy
high melting point (1300 degrees C – 1400 degrees C)
casting shrinkage (2.3%) – significant, challenging to use
low-ish bond strength (50MPa)
high Young’s Modulus/Elastic Modulus (220 GPa) GOOD
high tensile strength (850MPa)
high hardness (360 – 430 VHN)
2 shortcomings of high gold porcelain fused metal alloy
- Melting point is too low
- Not sufficiently rigid (Young’s modulus)
2 short comings of nickel chromium porcelain fused metal alloy
- high casting shrinkage - challenging to use
- quite low bond strength - drawback
shortcoming silver palladium porcelain fused metal alloy
- care needed in casting – challenge for technicians
advantage of silver palladium porcelain fused metal alloy
high melting point
3 advantages of nickel chromium porcelain fused metal alloy
- high melting point desirable
- high Young’s Modulus/ Rigid wanted
NOT BIOCOMPATIBLE
2 drawbacks of cobalt chromium alloys in porcelain fused metal alloy
casting shrinkage (2.3%) – significant, challenging to use
low-ish bond strength (50MPa)
3 advantages of cobalt chromium alloys in porcelain fused metal alloy
high melting point (1300 degrees C – 1400 degrees C)
high Young’s Modulus/Elastic Modulus (220 GPa) GOOD
high tensile strength (850MPa)
high hardness (360 – 430 VHN)
most common alloy used in porcelain alloys in GDH labs
CoCr
some Gold, Palladium BUT NOT NiCr – biocompatibility issues
key issue with nickle chromium alloys
biocompatibility concerns
- due to allergic responses attributed to nickel
alloys which have difficult casting process (3)
AgPd, NiCr and CoCr
most satisfactory alloy
LOW GOLD proves to be satisfactory in each criteria
- not hard to cast
types of mechanisms for bond between oxide layer and the alloy and porcelain
Mechanical
Stressed Skin
Chemical
van der Waal’s forces
- now disregarded
mechanism for bond between metal oxide layer and porcelain and alloy surface
Mechanical or Chemical in nature
OR due to what’s called the stressed skin effect
OR a combination of these.
mechanical bond
surface irregularities – allows interlocking
probably least important
stressed skin effect bond
slight differences in thermal contraction coefficients
- lead to compressive forces which aid bonding
During the production process, after the furnace stage, the alloy contracts slightly more on cooling, and this generates compressive forces on the porcelain – in essence gripping it.
chemical bonding
may be ELECTRON SHARING in oxides
during firing when high temperatures reached, porcelain flows and oxides in the metal-oxide coating migrate
- oxides in the metal oxide coating on the alloy migrating with oxides within the porcelain itself
4 failure modes of porcelain- metal bond
- oxide layer itself fracturing
- oxide layer delaminating from the alloy
- porcelain detaching from the oxide layer
- any failure occurs within the 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.
reason for using porcelain fused alloys
alloy sub-structure can help minimise porcelain brittleness
