Alloys For Cast Metal Restorations Flashcards
(47 cards)
1
Q
What are other names for alloys for cast metal restorations? (2)
A
- Crown and bridge alloys
- Porcelain-fused-metal alloys
2
Q
What is the main advantage of porcelain?
A
- Aesthetics
3
Q
What is the main problem with porcelain?
A
- Microcracks tend to form at the fitting surface, making it prone to mechanical failure
- So on its own porcelain isn’t able to withstand all the loads, the large biting forces it will be subjected to
4
Q
What is the main advantage of alloys?
A
- Good mechanical properties so withstand stresses readily
5
Q
What is compressive strength?
A
- The stress required to fracture a material - making it no longer fit for purpose
- Stress to cause fracture
6
Q
What is the elastic modulus?
A
- It is a measure of the rigidity (the stress strain ratio)
- That is for a given stress what change in shape - amount of strain - would the material undergo
7
Q
What is brittleness/ductility?
A
- Dimensional change experienced before fracture
- Whether a material is brittle or ductile is related to how much dimensional change it undergoes before it fractures
- This is where porcelain and alloys differ markedly
8
Q
What is hardness?
A
- Resistance of surface to indentation or abrasion
9
Q
Which one out of the 4 of these properties cannot be ascertained from a materials stress-strain curve:
- Strength (compressive/tensile)
- Brittleness/ductility
- Elastic modulus (rigidity)
- Hardness?
A
- Hardness
10
Q
What does the initial gradient of a material on a stress-strain curve indicate?
A
- The materials elastic modulus
11
Q
Look at stress strain curve on lecture slides
A
- Good explanation on slides
12
Q
What is a brittle material like?
A
- A BRITTLE material will change shape with a fraction of a percent of its length and then break (ceramics fall into this category)
13
Q
What is a ductile material like?
A
- A DUCTILE material will stretch several % of its length, then fracture (alloys tend to be ductile)
14
Q
What mechanical properties do we ideally want porcelain to have? (4)
A
- Hard
- Strong
- Rigid
- Ductile (it is not ductile)
15
Q
Is porcelain ductile or brittle?
A
- Brittle - low fracture toughness
- (max strain around 0.1% before fracturing)
16
Q
Is porcelain hard or soft?
A
- Reasonably hard
- Surface withstands abrasion/indentation well
17
Q
Is porcelain strong or weak?
A
- Reasonably strong
- High compressive strength BUT low tensile strength (tendency yo form surface defects which leads to fracture at low stresses)
18
Q
Is porcelain rigid or flexible?
A
- Rigid
- Large stress required to cause strain
19
Q
What mechanical properties do alloys have in relation to porcelain?
A
- Contrast this with porcelain-fused ALLOYS.
- ALLOYS 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.
20
Q
Are alloys hard or soft?
A
- Hard
21
Q
Are alloys strong or weak?
A
- Strong
22
Q
Are alloys rigid or flexible?
A
- Rigid
23
Q
Are alloys brittle or ductile?
A
- Ductile
24
Q
What does bonding of metal oxide to porcelain help eliminate?
A
- Helps eliminate defects/cracks on porcelain surface
- Alloy acts as a support & limits the strain that porcelain experiences
25
Explain strain with porcelain, alloy and them both together?
- The 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.
26
How can we ensure to avoid thermal stresses when forming a porcelain-fused-alloy restoration?
- When forming a porcelain-fused-alloy restoration, it has to be fired in a furnace.
- This means raising the temperature of both materials by hundreds of degrees, and then cooling them without developing any thermal stresses that could cause either material or the metal oxide layer to develop defects or micro-cracks.
- To avoid this outcome, both the PORCELAIN and the ALLOY should have similar thermal expansion coefficients.
- That way they will expand at the same rate when being heated and contract at the same rate when being cooled.
- This 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.
27
A range of alloys have been developed for bonding to porcelain. Give examples of these? (5)
- High gold alloy
- Low gold alloy
- Silver palladium (AgPd)
- Nickel Chromium (NiCr)
- Cobalt chromium (note that this is a different type of cobalt chromium alloy from that used as a partial denture framework)
28
Alloys must form a good bond to porcelain. How do they do this?
- i.e. good wetting
| - Porcelain forms bond with metallic oxides on surface (NiCr more difficult to achieve good bonding)
29
The thermal expansion coefficient of the alloy must be similar to porcelain. What is it ideally and why do we need this?
- Must be similar to porcelain (14ppm/degree Celsius - ideally difference of 0.5ppm/degree Celsius in alloys favour (the alloys value should be just a little greater, so that during the cooling stage the alloy is slightly compressing the porcelain)
- This is to avoid setting up stresses during fusing of porcelain on to the alloy
30
When bonding an alloy to porcelain we want to avoid discolouration of porcelain. What may cause discolouration? (2)
- Ag in AgPd can produce green discolouration
| - Copper not used in high cold alloy for this reason
31
Do all of the allots have adequate bond strength?
- THREE alloys adequate (not Ni-Cr)
32
Is the hardness of all the alloys adequate?
- All alloys adequate (though early Ni-Cr alloys too hard)
33
What do we want the elastic modulus of the alloys to be like?
- Want a high value (i.e. rigid) to support porcelain and prevent fracture (Ni-Cr best)
34
What must the melting, recrystallisation temperature of the alloy be like compared to porcelain?
- MUST be higher than fusion temperature of porcelain -otherwise creep may occur
35
What is creep?
- Gradual increase in strain (permanent) experienced under prolonged application of stress (
36
Give the composition of high gold alloys and explain why this is advantageous?
- HIGH GOLD
- Which consists of 80% Gold
- 14% Platinum and/or Palladium
- and this helps match up the thermal expansion of the alloy with porcelain
- while also increasing its MELTING point temperature (which helps to minimise the potential for the alloy exhibiting CREEP
- There’s 1% silver and a small amount of INDIUM and/or tin
- Indium and tin play a PIVOTAL role because they enable a metal oxide layer to form which enables the bonding to porcelain
- There is no Cu - otherwise green hue imparted to porcelain
37
What are the shortcomings with high gold alloy?
- Melting range may be too low
| - Young's modulus too low (not sufficiently rigid)
38
Give the composition of low gold alloys and explain why this is advantageous?
- Moving on to LOW GOLD alloy.
- It consists of 50% gold
- 30% palladium (double that in HIGH GOLD)
- 10% silver
- And 10% indium and tin
- LOW GOLD, besides having a higher melting temperature,
- Also has better mechanical properties than HIGH GOLD.
39
Give the composition of silver-palladium alloys and explain why this is advantageous?
- Let’s consider: silver palladium
- Which consists of:- 30% silver
- 60% Palladium
- And 10% indium and tin
- This gives it a high melting temperature
- BUT casting this alloy is a challenge for technicians
40
Give the composition of nickel-chromium alloys and explain why this is advantageous or disadvantageous?
- Nickel chromium alloy consists of
- 70 to 80% Nickel
- And 10 to 25% chromium
- It has a high melting point – which is desirable
- And it’s a RIGID material – that’s advantageous (high young's modulus)
- HOWEVER during the casting process it exhibits a lot of shrinkage – making it challenging to use.
- IN ADDITION it’s bond strength to porcelain is quite low.
- These make two significant drawbacks.
41
Give the characteristics of cobalt-chromium alloys? (6)
- High melting point (1300-1400 degrees Celsius)
- Casting shrinkage (2.3%)
- Low-ish bond strength (50MPa)
- High Young's modulus (220 GPa)
- High tensile strength (850MPa)
- High hardness (360-430 VHN)
42
Look at the properties table in the slides
V useful
43
What are the possible natures of the bond between the oxide layer and the alloy and porcelain? (4)
- Mechanical
- Stressed skin
- Chemical
- Van der Wall's forces (now disregarded)
44
What is the mechanical nature of the bond between the oxide layer and the alloy and porcelain?
- 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.
- (this is probably least important)
45
What is the 'stressed skin effect' nature of the bond between the oxide layer and the alloy and porcelain?
- The stressed skin effect depends on the slight differences in the thermal expansion/ and contraction of the porcelain and the alloy.
- 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 which aids bonding
46
What is the chemical nature of the bond between the oxide layer and the alloy and porcelain?
- The CHEMICAL mechanism is explained by oxides in the metal oxide coating on the alloy migrating with oxides within the porcelain itself.
- This occurs during the firing stage, where high temperatures are reached.
- This mechanism has been described as ELECTRON SHARING
47
What are the possible modes of failure in the porcelain fused restoration?
- The possible MODES of FAILURE in a porcelain fused restoration are:
- The oxide layer itself fracturing
- Then we have the oxide layer delaminating from the alloy
- Thirdly we have porcelain detaching from the oxide layer
- IDEALLY any failure would occur within the porcelain
