noble metal alloys Flashcards
What are noble metals?
- are those with good and retained metallic luster in dry air
- their resistance to tarnish and corrosion during heating, casting, soldering or use in the mouth is very good
- gold and platinum group (platinum, palladium, iridium, rhodium, osmium and ruthenium)
What are precious metals?
noble metals together with silver
What is casing?
process of pouring a molten alloy into a mold to form restorations
What are the functional requirements of dental casting alloys?
- fatigue strength and fatigue limit should be high to resist cyclic loading
- yield strength should be high to resist permanent deformation
- tarnish and corrosion resistance should be high
- stiffness (elastic modulus) should be high to resist elastic deformation of long span bridges and denture framework
- resilience should be high to absorb mechanical energy without permanent deformation
- compatibility to porcelain
- sag resistance
What is sag resistance?
resistance of the alloy to flow or melt during porcelain firing or soldering (important property)
What should the dental casting alloy be compatible with porcelain in?
- coefficient of thermal expansion
- modulus of elasticity
- the oxides formed should help bonding and all of its components must not discolor the porcelain
What are the working requirements of dental casting alloys?
- ease of casting i.e. easy melting and flow to fill the mold easily
- ease of soldering i.e. easy wetting by the solder alloy
- ease of burnishing i.e. easy adaptation of its margins to tooth
What are the properties of gold?
- soft, malleable, ductile metal with high nobility that resist tarnish and corrosion
- rich yellow color with strong metallic luster
- has a low strength and hardness
- melting temperature is 1063 degrees and the specific gravity is 19
What are the properties of platinum?
- noble metal which resists tarnish and corrosion
- blue white in color and tends to lighten the color of yellow gold
- increases the hardness and strength of gold
- ductile
- melting temperature is 1755 degrees and specific gravity of 21 which increase the weight of gold alloys
What are the properties of palladium?
- cheaper than platinum and it is often used as replacement for platinum
- white metal
- specific gravity is 11
- ductile and malleable noble metal - - the melting point is 1555 degrees
Why do gold alloys used with porcelain should contain higher amounts of platinum & palladium?
to increase its sag resistance, so it does not flow during porcelain firing
What effect does 5% palladium have?
whitening the color of gold alloy and it decreases the greening of silver and the reddening
What are the properties of silver?
- ductile and malleable metal
- stronger and harder than gold
- white in color and so it is important in developing the white color of gold alloys particularly when combined with platinum and palladium
- its melting temperature is 960 degrees and the specific gravity is 10.4
- also improves the mechanical properties of gold alloys
- pure silver occludes oxygen in the molten state which is evolved during solidification as a result, porous and rough casting may develop
What is th eproperty of iridium?
effective in refining the grain size of gold alloys
What are some base metals added to gold?
- copper
- zinc
- tin
- indium
- iron
What arethe properties of copper?
- ductile and malleable metal with characteristic red color
- increases strength and hardness of gold alloys and is an important factor in heat treatment
- its melting temperature is 1083 degrees
- specific gravity is 8
What are the properties of zinc?
- blue white in color with melting temperature of 419 degrees
- it is present in gold alloys to act as a scavenger (deoxidizing agent) during melting and casting of the alloys
- improves the fluidity and castability of the alloy
What re the properties of tin?
lustrous white metal with melting temperature of 232 degrees
- it produce oxides to help bonding with porcelain
What are the properties of indium?
- white metal with melting temperature of 156 degrees
- the same as tin it produces white oxides in alloys for porcelain which help bonding with porcelain
What is the property of iron?
added with porcelain alloys to produce precipitation hardening (Pt3Fe)
What is the classification of gold alloys according to gold content?
- carat (k)
- fineness (F)
What is carat?
- the number of parts of pure gold in 24 parts of alloy
- for example, 24 K is pure gold, 21 K means 21 parts pure gold and 3 parts other metals, and 18 K means 18 parts pure gold and 6 parts other metals
What is fineness?
- the number of parts of pure gold in 1000 parts of the alloys
- for example, 1000 F is pure gold, 750 F means 750 parts pure gold and 250 parts other metals
Is carat used in dentistry?
no
Is fineness used in dentistry?
yes, in describing gold solder alloys
What is the classification of gold alloys according ADA, hardness and uses?
- type 1 (soft)
- type 2 (medium)
- type 3 (hard)
- type 4 (extra hard)
What types have highest and lowest gold composition?
- type 1 (highest gold composition)
- type 2
- type 3
- type 4 (lowest gold composition)
What is the use and properties of type 1 (soft)?
- use: inlays subjected to very slight stress
- heat treatment: do not respond to hardening heat treatment
- ductility: 30% (highest)
- hardness: 70 (lowest)
- Y.S. in MPA: 100 (lowest)
- M.P.: 1000 degrees
What is the use and properties of type 2 (medium)?
- use: inlays subjected to moderate stress
- heat treatment: do not respond to hardening heat treatment
- ductility: 20%
- hardness: 85
- Y.S. in MPA: 150
- M.P.: 950 degrees
What is the use and properties of type 3 (hard)?
- use: onlays subjected to high stress and as crowns and bridges
- heat treatment: respond to hardening heat treatment
- ductility: 20% (soft) 10% (hard)
- hardness: 100 (soft) 115 (hard)
- Y.S. in MPA: 200 (soft) 300 (hard)
- M.P.: 930 degrees (soft) 1000 degrees (hard)
What is the use and properties of type 4 (extra hard)?
- use: onlays subjected to very high stress and as denture frameworks
- heat treatment: respond to hardening heat treatment
- ductility: 20% (soft) 5% (hard)
- hardness: 130 (soft) 225 (hard)
- Y.S. in MPA: 300 (soft) 500 (hard)
- M.P.: 900 degrees (soft) 950 degrees (hard)
What is the modulus of elasticity in noble metal alloys types?
- ranges between 75,000 and 150,000 MPa, depending on the composition of the alloy
- not altered by heat treatment
What is the classification of gold alloys according to alloy description?
- white gold away
- economic gold alloy
What are white gold alloys?
- gold alloys which are white in color because palladium % is more than 5%, also silver and platinum are present
What are the advantages of white gold alloys?
- more aesthetic
- less expensive
What are economic gold alloys?
- gold alloys that contain gold in the range of 42% - 55% which is below than that required by the ADA specification (65% Au in type IV gold alloys)
- these alloys are comparable to type III and IV gold alloys in properties and uses
What is found in economic gold alloys?
- 1% palladium is needed for every 3% silver to stop the high tarnish effect of silver
- if the silver–copper ratio were not carefully balanced, the alloy would also tarnish even with the correct palladium and silver content
- this is due to the precipitation of a silver-rich phase in the microstructure
What is heat treatment of gold alloys?
- conventional cast gold alloys that contain sufficient copper (above 12%) may be easily heat treated to produce a softer or a harder alloy
- they are softened by heating gold alloys at 700 degrees for 15 minutes and then cooling rapidly to room temperature in water
- a hardening heat treatment takes place when gold alloys are heated at 700 degrees for 15 minutes and then cooled slowly
- hardening heat treatments produces an increase in strength but also a reduction in ductility
- in practice, dental gold castings are heat treated by either quenching or slow cooling the casting while it is still in the mold
- to produce a softer casting, the mold is quenched in water when the metal appears dull red
- however, a harder and stronger but less ductile casting is produced by allowing the casting to cool to room temperature in the mold
What is the result of softening heat treatment?
- decrease strength
- decrease hardness
- increase ductility
What is the result of hardening heat treatment?
- increase strength
- increase hardness
- decrease ductility
What is the explanation of softening heat treatment?
- heating causes atomic movement
- quenching stop them in a discolored state
What is the hardening heat treatment?
- heating causes atomic movement
- slow cooling gives atoms the chance to arrange themselves ideally in an ordered state
What is recasting of gold alloys?
- gold alloys can be recast for 2 or 3 time without a change in composition
- the only elements likely to get lost are the more volatile base metals such as zinc, indium and iron
- this loss can be treated by the addition of at least equal amounts of the new alloy to the scrap (old) when recast
- the scrap should be cleaned and foreign materials removed before recasting
