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base metal alloys Flashcards

1
Q

What are base metal casting alloys?

A
  • alloys which do not contain any noble metals but resist corrosion as gold alloys by the passivity
  • have been introduced in dentistry to substitute gold alloys because of the increased cost of gold
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2
Q

What are the types of base metal alloys?

A
  • cobalt chromium alloy
  • nickel chromium alloy
  • titanium and titanium alloys
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3
Q

What is the use of cobalt chromium alloys?

A

the same as type IV gold alloys, they are used as partial and complete denture base

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4
Q

What are the major elements of cobalt chromium alloys?

A
  • cobalt (30-60%)
  • chromium (15-30%)
  • nickel (0-30%)
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5
Q

What is the function of cobalt in cobalt chromium alloys?

A

it increases the strength, hardness and rigidity (modulus of elasticity)

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6
Q

What is the function of chromium in cobalt chromium alloys?

A
  • it gives passivity which is defined as the formation of a thin, uniform, non porous (continuous) adherent lustrous layer of chromium oxide which prevent the underlying alloy from corrosion
  • less than 15%: the alloy will not resist corrosion
  • more than 30%: the alloy will be too brittle due to the formation of a brittle sigma phase
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7
Q

What is the function of nickel in cobalt chromium alloys?

A
  • The same functions as cobalt but to lesser degree
  • also increases the ductility of the alloy
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8
Q

What are the minor elements of cobal chromium alloys?

A
  • molybdenum 3-6%
  • carbon 0.2-0.4%
  • beryllium 1%
  • silicon and manganese
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9
Q

What is the function of molybdenum in cobalt chromium alloys?

A

acts as grain refiner, therefore it increases the strength and hardness of the alloy

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10
Q

What is the function of carbon in cobalt chromium alloys?

A
  • extremely important element
  • increases the strength and hardness of the alloy
  • small changes in the carbon % can significantly affect the properties of the alloy because it can combine with any of the other metals to form carbides
  • if more than 0.5%, the alloy becomes too hard and too brittle
  • brittleness is the result of the stop of dislocation (slip interference) due to the formation and precipitation of carbides at the grain boundaries.
    If less than 0.5%, the alloy will be with low tensile strength
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11
Q

What is the function of beryllium in cobalt chromium alloys?

A
  • decreases the melting range of the alloy by 100 degrees thus better castability
  • has a carcinogenic effect to technician during alloy melting or grinding and finishing, thus most recent formulations exclude it.
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12
Q

What is the function of silicon and manganese in cobalt chromium alloys?

A

give better fluidity and thus better castability

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13
Q

Which metal in cobalt chromium alloys causes allergy?

A

nickel is a well known metal to produce allergy for some patients; therefore it is advisable to use a nickel free Co-Cr alloy

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14
Q

What is th color of cobalt chromium alloys?

A

silver white i.e. more aesthetic

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15
Q

What is th color of type 4 gold alloys?

A

golden yellow color

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16
Q

What is the density of cobalt chromium alloys?

A
  • 8 gm/cm3 i.e. two times lighter than type 4 gold alloys
  • advantage: more upper denture retention
  • disadvantage: only centrifugal casting machines must be used to produce high pressure to force the light molten alloy into the mold
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17
Q

What is the melting temperature of type 3 gold alloys?

A
  • 900-1000 degrees
  • use air-gas flame
  • only the reducing (blue) zone is used because it is the hottest and free from oxide
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18
Q

What is the casting shrinkage of type 4 gold alloys?

A

during solidification, gold alloys shrink by 1.6%, therefore either gypsum bonded investment or phosphate bonded investment mixed with water can be used

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19
Q

What is the density of type 4 gold alloys?

A
  • 17 gm/cm3 i.e. two times heavier than cobalt chromium alloys
  • either air pressure or centrifugal casting machines can be used
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20
Q

What is the melting temperature of cobalt chromium alloys?

A
  • 1300-1500 degrees, (very high)
  • use either electric induction melting method which is a rapid, easy and safe method or using oxy-acetylene flame
  • in this method , oxygen and acetylene ratio should be carefully adjusted because if there is too much oxygen, oxidation takes plce and if there is too much acetylene, pick up of carbon
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21
Q

What is the casting shrinkage of cobalt chromium alloys?

A
  • during solidification, Co-Cr alloys shrink by 2.3% therefore, either phosphate bonded investment mixed with silica gel or silicate bonded investments can be used to withstand its high melting temperature and compensate its high solidification shrinkage
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22
Q

What is the hardness of cobalt chromium alloys?

A
  • 320 B.H.N i.e. 30% harder than gold alloy type IV.
  • advantage: the highly polished (lustrous) surface is retained for a very long time
  • disadvantage: difficult to polish
  • require the use of special equipments as sand blasting and electrolytic polishing
23
Q

What is sand blasting?

A

mechanical smoothness of the casting by sand to remove the adherent investment

24
Q

What is electrolytic polishing?

A
  • the opposite of electro-plating
  • the restoration is placed in the anode position
  • the rough surface is removed leaving a smooth surface
25
Q

What is the hardness of type 4 gold alloys?

A
  • 220 B.H.N.
  • advantage: easy finishing and polishing
26
Q

What is the ductility of cobalt chromium alloys?

A
  • about 4% i.e. brittle
  • this give rise to a common problem with partial dentures clasps fractures
27
Q

What is the ductility of type 4 gold alloys?

A

20%(soft) or 5%(hard) i.e. gold alloys have greater ductility which is an advantage since it means less brittleness

28
Q

What is the modulus of elasticity of cobalt chromium alloys?

A
  • 30 X 106 P.S.I. i.e. stiffer two times than gold alloy IV
  • this high modulus has the advantage that the denture can be made thinner in cross section whilst maintaining sufficient rigidity (this is of great importance to the comfort of the patient) and load distribution on the denture and underlying tissues without bending
29
Q

What is the modulus of elasticity of type 4 gold alloys?

A
  • Half that of Co-Cr alloy
  • must be used in double thickness to obtain the same rigidity as Co-Cr denture
30
Q

What is the yield strength of cobalt chromium allous?

A
  • 450 MP/m2
  • determines when permanent deformation will occur
    (more than type 4 gold alloys)
31
Q

What is the yield strength of type 4 gold alloys?

A
  • 400 Mp/m2
  • less than cobalt chromium alloys
32
Q

What is the ultimate strength of cobalt chromium alloys?

A
  • 850 Mp/m2 i.e. stronger
  • moe than type 4 gold alloys
33
Q

What is the ultimate strength of gold alloys type 4?

A
  • 800 Mp/m2
  • less than cobaly chromium alloys
34
Q

Can cobalt chromium alloys be recast?

A

cannot be recast because of the fear of the increase of carbon %

35
Q

Can type 4 gold alloys be recast?

A

yes

36
Q

Can cobalt chromium alloys be heat treated?

A

no

37
Q

Can type 4 gold alloys be heat treated?

A

yes

38
Q

What is the microstructure of cobalt chromium alloys?

A

larger grains with carbides dispersed along the grain boundaries

39
Q

What is the microstructure of type 4 gold alloys?

A

smaller grains

40
Q

What are the advantages of cobalt chromium alloys compared to type 4 gold alloys?

A
  • less expensive
  • lighter in weight
  • more aesthetic
  • more rigidity (thinner)
  • more strength (stronger)
41
Q

What are the advantages of type 4 gold alloys compared to cobalt chromium alloys?

A
  • more ductile
  • can be recast
  • easier to melt
  • cast and polish
  • not allergic to all patient
42
Q

What are the disadvantages of cobalt chromium alloys?

A
  • “technique sensitivity” i.e. need
  • special melting methods
  • special investments
  • special casting machine
  • special finishing and polishing machines
43
Q

What is the disadvantage of type 4 gold alloys?

A

expensive

44
Q

What are nickel chromium alloys?

A

base metal alloys recently developed with high percentage of elongation (20%) i.e. high ductility with lower hardness than Co-Cr alloys to be suitable for use for crown and bridge work the same as type III gold alloys

45
Q

What is the composition of nickel chromium alloys?

A
  • 70% nickel: to increase ductility of the alloy
  • 20% chromium: to give passivity
  • 10% other ingredient: the same as for Co-Cr alloys
46
Q

What are the properties of nickel chromium alloys?

A
  • nearly the same as Co-Cr alloys except:
  • hardness = 220 B.H.N. so that not abrade the opposing natural tooth
  • ductility = 10-20% so can be used with crowns and bridge
47
Q

What are the uses of wrought base metal alloys?

A
  • wires and bands in orthodontic treatment
  • clasps in partial denture
  • reamers and files in endodontic treatment
48
Q

What are the types of wrout bse metal alloys?

A
  • stainless steel (the commonly used type)
  • cobalt – chromium – nickel
  • nickel chromium
  • nickel - titanium
  • titanium
49
Q

What is the composition of stainless steel wires?

A
  • iron and carbon to form steel which is an interstitial solid solution
  • chromium (18%) to give the wire its stainless property (corrosion resistance or passivity)
  • passivity layer formed on the steel surface is characterized by being thin, strongly adherent (prevent further oxidation), continuous, lustrous, and transparent so that can not be seen even at high magnifications
  • nickel (8%) to give ductility to be drawn into wires
  • other elements as titanium, silicon, and manganese as stabilizing elements to prevent the formation of carbides between the carbon present in the alloys and chromium at the surface and as a result they are described as stabilizing ingredients and the alloy is known as “stabilized stainless-steel”
50
Q

What is the manufacturing and supply of stainless steel wires?

A
  • to form wires, the cast alloy is pulled through dies
  • this cold working produces an internal fibrous or wrought structure and hence ductility is reduced, also internal stresses will be induced in the wires
  • therefore, there are three types of stainless-steel wires (soft, half hard and hard)
  • if much cold working is to be carried out in the clinic or patient mouth the soft type should be selected
  • the most common type of stainless steel alloys used in dentistry is the austenite or 18-8 alloy
51
Q

Why is the most common type of stainless steel alloys used in dentistry the austenite or 18-8 alloy?

A
  • highest corrosion resistance
  • liable to undergo more cold working without fracture
  • easy welding and shaping
52
Q

What is the heat treatment of stainless steel wires?

A
  • with stainless steel wires it is not possible to apply heat treatment to restore the ductility lost during cold working because heating causes:
  • a minimum 500 degrees is necessary to relief internal stresses if the wire is heated to such temperature carbon will react with chromium –> chromium carbide formation that precipitate at grain boundaries and the wire becomes more and more brittle
  • corrosion resistance is reduced because there is less chromium left at the grain surfaces to produce the required passivity (this phenomena is called “weld decay”)
  • full ductility requires heating the wire to above 950 degrees where recrystallization occurs
  • this will destroy the fibrous structure of the wire on which “springiness” depends
  • therefore, heat treatment should be limited to below 500 degrees to relief internal stresses only
53
Q

How to avoid weld decay?

A

using an alloy with lower carbon %.
ii. using a stabilized stainless steel alloy

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