Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Yr 3 Restorative - biomaterials and CSL > Casting alloys > Flashcards

Casting alloys Flashcards

1
Q

What is an alloy?

A

An alloy is a mixture of two or more metallic elements (binary, ternary or quaternary)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is the benefit of an alloy over pure metals?

A

usually has better/ improved properties

e.g.

    • cost (i.e. cheaper than pure gold but keeping similar properties)
    • biocompatibility (resistance to corrosion and tarnish)
    • mechanic properties (strength, stiffness, hardness)
    • ease of casting/fabrication (melting point, shrinkage & density)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Why are very light (not very dense) materials not good for casting?

A

Force is not sufficient to get all the detail

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Which alloys are used commonly in dentistry?

A
  • Amalgam
  • Steel
  • Nickel-titanium
  • Gold
  • Cobalt chrome
  • Titanium
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

When are amalam alloys used?

A

dental fillings

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is amalgam an alloy of?

A

Mercury, tin, silver and copper

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What are steel and nickel-titanium alloys used to make?

A

Instruments and endodontic/orthodontic wires (shape memory)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What are steel alloys made of?

A

Iron and carbon

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What are gold and cobalt chrome alloys used for?

A

Crowns, inlays, onlays and denture bases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are titanium alloys used for?

A

Implants and fixed/removable partial dentures

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What different fabrication processes of alloys are there?

A
  • Forming
  • Amalgamation
  • Casting
  • Powder metallurgy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is forming?

A

Plastic deformation, including hot or cold working (e.g. forging, rolling, extrusion and drawing)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is amalgamation?

A

The blending of mercury with other metals

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is casting?

A

Pouring molten metal into a mould

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is powder metallurgy?

A

Sintering, injection moulding (for metal and polymers = mix in polymerising chamber under pressure = injected into mould) & 3D printing

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is the current problem with 3D printing?

A

Coarse particles used = thick layers and needs polishing etc. afterwards

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is the typical method for dental alloy casting?

A

Lost wax mould casting

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What are the typical steps in lost wax mould casting?

A
    • preparation of the dention
    • preparation of an impression
    • pouring of a model
    • waxing of the desired shape
    • investing the wax pattern
    • burn-out and heating the mould
    • melting and casting the alloy
    • finishing and polishing
    • heat treatment
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is the history of alloys?

A

1907: lost wax technique

1932-1948: standardisation of dental casting alloys

1950s -1960s: development of PFM alloys (adding palladium and platinum to gold lowers the coefficient of thermal expansion sufficiently to ensure the physical compatibility between porcelain and the metal substructure)

1970s: gold placed on the free market = increased prices = search for low gold and base metal alloys

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What are the general (dental) requirements of a casting alloy?

A

Physical properties:

  • reasonably low melting point (flow)
  • moderately high density (castibility)
  • low coefficient of thermal expansion

Chemical properties:

  • chemical corrosion (tarnish) resistance
  • electrochemical corrosion resistance

Mechanical properties:

  • high modulus (stiffness)
  • moderately high yeild strength (resistance to plastic deformation)
  • hardenable by heat treatment (retention of polish)

Biological properties:

  • biocompatible: no toxic soluble phases
  • non-reactive in the oral environment
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What are the two classes of dental casting metal alloys?

A
  • Noble (precious metal) alloys
  • Base metal casting alloys
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Which alloys are classed as noble (precious metal) alloys?

A

Gold alloys

Silver-palladium

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Which alloys are classed as base metal alloys?

A

Cobalt-chromium

Nickel-chromium

Titanium

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What is a noble metal?

A

Elements with good metallic surfaces that retain luster in clean dry air

= resists oxidation, tarnish and corrosion during heating, casting and soldering

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

Which metals count as noble metals?

A

Gold

Platinum group (6 metals) =

  • platinum,
  • iridium.
  • osmium,
  • palladium,
  • rhodium,
  • ruthenium

-> n.b. sometimes these are mixed together to make a very good alloy

Silver (? less good tarnish resistance than the others but expensive)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

Why should we avoid the old term of precious and semiprecious metals?

A

Indicates how expensive a metal is based on supply and demand = should be avoided because they are imprecise terms

27
Q

What are the fundamental properties of Gold (Au)?

A
  • Soft, (most) malleable and ductile
  • Tarnish resistant in air and water at any temp
  • Attacked by only a few of the most powerful oxidizing agents
  • Insoluble in sulfuric, nitric or hydrochloric acids
  • Soluble in a mixture of nitric acid and hydrochloric acid (aqua regia)
  • Relatively low strength/hardness
  • Small amounts of impurities (i.e. lead, mercury. base metals have a pronounced and usually detrimental effect on its properties)
  • * Fusion temp = 1063 degrees C
  • * Density 19.3g/cm cubed
  • * Thermal coefficient of expansion = 14.2 x10^-6/ degree C
  • * Youngs modulues (MOE) = 80 GPa (very close to dentine)
28
Q

What are the fundamental properties of Platinum (Pt)?

A
  • Tough, malleable and ductile
  • High corrosion resistance
  • Very high cost (usually replaced by Palladium in most modern alloys)
  • Higher melting temp than porcelain
  • * Fusion temp = 1755 degrees C (>Au)
  • * Density = 21.37 g/cm cubed (>Au)
  • * Thermal coefficient of expansion = 8.9 X 10^-6 per degree C (Au)
29
Q

What are the fundamental properties of Palladium (Pd)?

A
  • Not used in the pure state in dentistry (properties aren’t good enough)
  • Has replace platinum in dental casting alloys (cheaper)
  • Helps prevent corrosion of silver in the oral environment
  • Absorbs hydrogen gas when heated improperly (properties worsen)
  • * Fusion temp = 1555 degrees C (>Au)
  • * Modulus of elasticity = 112 GPa (>Au)
30
Q

What are the fundamental properties of Silver (Ag)?

A
  • Malleable and ductile
  • Harder than gold
  • Best known conductor of heat and electricity (bad= damages pulp = sensitivity)
  • unaltered in clean dry air BUT combines with Sulfur, Chlorine and phosphorus = severe tarnish in the oral environment
  • occludes large quantities of oxygen in molten state (= defects -> evolves during solidification = pits and porosities and affects properties)
  • * Fusion temp = 960.5 degrees C (Au) (>Pt) (>Pd)
  • * Modulus of elasticity = 120 GPa (>Au) (Pd)
31
Q

What are the two minor alloying elements?

A

Iridium (Ir) and Ruthenium (Ru)

32
Q

What so the two minor alloying elements do?

A

Grain refining (stops crystals growing so much by producing as many boundaries as possible)

-> addition of as little as 50ppm (0.005%) of Ir and Ru = 100 X increase in the no. of grains per unit volume

  • = increased alloys tensile strength and elongation by >30%,
  • increases tarnish resistance and
  • slightly increases yield strength -> does not appreciably affect hardness
33
Q

Tell me about gold casting alloys:

A
  • used as a dental material for last 2500 years
  • pure form too soft to maintain shape = strength and hardness improved by alloying
  • n.b. pure gold can be used in fillings = cohesive gold; alloys of gold used for cast restorations
34
Q

What are the typical compositions of gold alloys?

A
35
Q

What is a Karat, Carat (K)?

A

Parts of pure gold per 24

e.g. 12k = 50%, 18k, 24k

36
Q

What is fineness?

A

Parts of pure gold per 1,000

e.g. a 650 fine alloy has a gold content of 65% n.b. this is primarily used for gold solders

37
Q

How does gold content/ nobility vary from type I gold alloys to type IV?

A

Gold content/ nobility decreases from type I to type IV

38
Q

What does adding copper do to the properties of a gold alloy?

A
  • Strengthening (order-hardening),
  • reducing melting temperature

BUT increases tarnish, adds reddish tint

39
Q

What does adding silver do to the properties of a gold alloy?

A
  • Slightly strengthening,
  • counteracts the reddish tint of the copper
40
Q

What does adding platinum and palladium do to the properties of a gold alloy?

A
  • Strengthening,
  • increasing melting temperature
41
Q

What does adding Zinc do to the properties of a gold alloy?

A

A scavenger during casting = prevents oxidation and improves castability

42
Q

What is class IV (hardened) gold alloy?

A

Post heat treated

43
Q

What are the comparitive properties of gold alloys?

A
44
Q

When are Type I casting gold alloys used?

A

Inlays that are well supported with no large masticatory

High ductility allows burnishing = improves hardness and fit

45
Q

When are Type II casting gold alloys used?

A

Most widely used alloys for inlays = better mechanical properties than Type I but less ductile

46
Q

When are Type III casting gold alloys used?

A

Areas where less support and opposing stress

e.g. Class II cavities in molars, all inlays, onlays, full-coverage crowns and short-span bridges

47
Q

When are Type IV casting gold alloys used?

A

Extensive use of construction of partial denture components, long-span bridges (stiffer and stronger)

48
Q

What classifies a gold alloy as a low content gold alloy?

A

<65%

(some as low as 10% but typically 45-50%)

49
Q

What is the characteristic colour of high palladium content?

A

Whiteish colour

50
Q

What are the properties of a low content gold alloy?

A

They vary but similar to type III & IV casting alloys except for ductility = significantly lower

51
Q

Which casting technique is used for low content gold alloys?

A

Same as for gold alloys

52
Q

What are low content gold alloys used for?

A

Widespread use on grounds of acceptable properties, good performance,

same equipment requirements and lower cost when compared to coventional gold alloys

53
Q

What are Silver-palladium alloys made from?

A
  • Silver
  • A minimum of 25% Palladium
  • With small quantities of copper, zinc, indium and occasionally gold
54
Q

What are base metal casting alloys?

A

Alloys which contain no gold, silver platinum or palladium

55
Q

What are Nickel chromium alloys used for?

A

Crown and bridge casting

n.b. some concern about biocompatibility of nickel

56
Q

What are cobalt chromium alloys used for?

A

Partial denture framework casting

57
Q

What are titanium and titanium aluminium vanadium alloys used for?

A

Implants

58
Q

What are the properties of Cobalt/Nickel chromium alloys?

A
  • * Melting point = 1300-1500 degrees C
  • * Modulus of elasticity = 250 GPa (>Au) = STIFF
  • * Vickers hardness number 350-400 (>Au = 250-300)
59
Q

What is the composition of cobalt/Nickel chromium alloys?

A
  • Chromium (30% = difficult to cast and brittle)
  • Cobalt and Nickel (65-78%)
    • = pretty interchangeable
      • Ni alloys have decreased strength, hardness, modulus of elasticity, fusion temps and increased ductility & % elongation compared to cobalt
    • Minor alloying elements = control the majority of the physical properties
        • Carbon (0.1-0.5%) = increases strength, hardness and brittleness -> if increased by 0.2% = too hard and brittle for dental use, if decreased by 0.2% = decreases yield strength and ultimate tensile strength to unacceptable levels
        • Molybdenum (3-6%) = refines grain size = increases strength, hardness and % elongation
        • Aluminium 4-5% = forms a Nickel Aluminium in NiCr alloys = contributes to precipitation hardening = increased tensile and yield strength
        • Beryllium (0.5-2%) = decreases the fusion temp by approx 100 degrees C, increases fluidity during casting, allows for electrolytic etching (with resin bond prosthesis)
      • -Manganese (5%)
      • Silicon (0.5%) = increases fluidity and cast-ability of molten alloy
      • (+ Boron = deoxidizers = essential in Ni containing alloys)
        • Iron and copper = increase hardness

N.b. composition changes slightly dependant on the company that manufactures them

60
Q

What are the properties of titanium and titanium alloys?

A
    • Light weight (Density: 4.5 g/cm cubed n.b. not ideal for casting = why we often use alloy)
    • Strong (>100MPa)
    • Corrosion resistant (dynamically formed titanium oxide on the surface = most important reason for increased use in dentistry
    • Biocompatible - Low cost
61
Q

What is commercially pure titanium (cp Ti) used for?

A
    • Dental implants
    • Crowns
    • Partial and complete dentures
62
Q

How do we improve osteointegration of Titanium implants?

A

Sandblast or etching surface left

Cells = like growing on titanium oxide

63
Q

Which Titanium alloy is more widely used?

A

Ti-6Al-4V

= stronger (225MPa), smaller diameter implants

-> concern with aluminium (not scientifically proven) but never actually contacts tissue directly as oxide layer does

64
Q

What are the problems with casting titanium alloys?

A
  • High melting point (approx 1700 degrees C)
  • Chemical reactivity (reacts with gaseous elements easily especially at high temps (>600 degrees C) = needs a well controlled vacuum in processing (inert gas atmosphere & centrifugal force applied)

= technology required makes casting titanium expensive

Yr 3 Restorative - biomaterials and CSL (16 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions