Cobalt dental alloys
Introduction:
- according to content of cobalt , which is higher than the content of
chrome, these alloys name as a chrome-cobalt dental alloys (after
content of individual elements should be term these alloys as a
cobalt-chrome alloys)
- if the content of chrome was higher than cobalt, it was these
alloys very strong and wroughting is very hard
- the casting temperature of chrome is 1860°C, chrome has a high
affinity to the oxygen ( corrosion), it is very strong and fragile
- these properties can decrease value of alloy and so it is better,
when the alloys are cobalt-chrome
- cobalt belong to the most expensive metal of general metal alloys
The development of cobalt dental alloy:
- pure cobalt has not appearance in the nature
- it is depend on arsenic and sulphur
- at first was isolated pure cobalt, as pure metal, at 1742y.
- the pure cobal is not accommodeted for dental using and from it
results its adjusting by chrome, molybdenum and wolfram
The development of cobalt dental alloys:
- the first alloy on Cobalt base with adjusting chrome was prepared
in America at 1899 y.
- it was term as a Stellit
- nowadays it is using as a Haynes Alloy 21 or Stellit 21
- american company Austenal at 1929 y. began cast from Stellit 21
removable dentures and this alloy names as Vitallium
- so began the period of using cobalt dental alloys
Chemical chracteristic:
- the cobalt dental alloy have additional standard ISO 6871-1
- after this standard, cobalt alloys have to contain at least 25% Cr,
4% Mo
- cobalt alloys are from chemical view allotropic and ferromagnetic
- ferromagnetical properties are showing at the temperature 1125°C
- by the narrower teperatures create the reorganisation of atoms
into the atomic lattice
- this change is appllyed by the volume expansion
- the casting temperature of cobalt alloys is 1400°C
- by the cooling create the centres of crystalisation and they make
chemical nonhomogenous structure and from it result the beginig
of chemical corrosion
Physical properties:
- density of cobalt is 8,92 g/cm3, chrome 7,14 g/cm3 a molybdenum
10,2 g/cm3
- chrome in alloys decreases density, molybdenum increases it
- according to content of chrome in alloys, final alloys has density
between 8,2-8,4 g/cm3.
- the color of alloy is not depend on content of chrome and
molybdenum, because chrome makes on the surface of alloy
passive lyaer
- from it result, that reflex and brightness is permanent
Mechanical properties:
- the Co-Cr dental alloys are very strong and hard, because size of
atoms of Co, Cr and Mo is similar (Co 0,25; Cr 0,26; Mo 0,28
mikrometra)
- atomic lattice is very resistant to the external forces
Mechanical properties:
- for the better mechanical resistance is added carbon or nitrogen
- between atoms carbon and cobalt predominate a repulsine forces
- between atoms carbon and chrome , carbon and molybdenum are
adhesive forces
- result of this connection are carbid moleculles, which can
incresed hardness of alloy
- similar result give connection with nitrogen and so can create very
stable nitrides
- whroughting of these materials is very difficult and it is directly
proportional of alloy hardness
- for practice it means, that cuting and abrading constructions of
these alloys is very difficult and is very high consuption of time and
drills
Casting:
- casting of quality reconstruction is depend on:
- melting
- reactivity of free ions
- material convergence
- setting contraction
- reactivity of cobalt alloys is induced very intensive by chrome
- it has strong afinity to the free oxygen and so can induced
creation of oxides
- these oxides are on the surface of alloy and can begin the
electrochemical corrosion
- the presence of oxides can increased of viscosity and decreased
convergence. So can be changed the properties of alloy.
Casting:
- after casting of alloy follow the cooling. It is applyed with volume
contraction
- volume contraction of Cobalt alloys is 2,2%
Soldering:
- soldering is important to the connection of two metal parts
- solder is alloy with casting temperature less than is the casting
temperature of origin alloy. From it result, that the chemical
composition and mechanical properties is different.
- this different is the base for creation of electrochemical potential
- for minimalisation of these diffrent is important than the solder and
origin alloy were similar. It means that the solder has the same or
very similar composition as a origin alloy.
- other form to connection is welding by laser. It is better, because
so can uprise smaller different between soldering part and origin
alloy
Corrosion of Cobalt dental alloys:
- corrosion of cobalt alloys is begin as a creating free cations in
aequeous solution
- protection before corrosion is passive layer, which create by the
chrome
- the cobalt alloys are more resistant before corrosion than nickel
alloys, because the cobalt alloy dont make easy free cations
Corrosion of cobalt alloys:
- for creation of corrosion is important:
1/ pH must be less than 7
2/ very alkaline character pH
- neutral pH or very little different from it , is not condition of
corrosion
- in this pH area is alloy stabile, because on its surface is passive
layer, which separate alloy before aqueous solution
Biocompatibility of cobalt alloys:
- biocompatibility of cobalt alloys is abillity to change:
- properties of soft tissue
- create inflammation
- allergic reaction
- these changes are depend on dissolving of metal in contact with
saliva, tissue and drinks
Biocompatibility cobalt alloys:
- the results of examination biocompatibility show that:
1/ alloys with cobalt are nontoxic for organism
2/ they dont change structure on molecular layer
3/ they are without cytotoxic potential
4/ they dont make hypersensitivity reactio
5/ they dont irritate skin
This is the main different between nickel and cobalt alloys. Nickel
alloys are more agressive on the organism, because they create the
solubile macromolecules and so tissue reactions can not be out.
Conclusion:
- the cobalt – chrome alloys are regarding as a best biocompatible
dental material from general metals, besides titanium
- according to properties titanium, cobalt alloys are beter melting
and soldering
- for the creation of protective passive layer must be in alloy 25%hm
Cr a 4hm% Mo
- names of commercial products: wiron 99, wiron 77, wironit (6g
roller), Vitalium
Porovnanie Ni a Co zlitin:
Výskyt v prírode
viazaný na S, As
viazaný na S, As
Zliatiny
Ni-Cu; Ni-Fe-cr
Co-Cr
Teplota tavenia
1455°C
1860°C
Prvá zliatina
r.1906
r.1899
Charakter zliatiny
nehomogénny
nehomogénny
Teplota tav.zliatin
1250°C-1350°C
1400°C
2,2%
2,2%
8,9 g/cm3
8,92 g/cm3
Spevnenie
Al,Ti
C, N, Ti,Cr,Mo
Spájanie
pájka
pájka, laser
rozpustné
makromolekuly
nerozpustné
zlúčeniny
Rozpúšťanie
rýchlejšie
pomalšie
Pasivačná vrstva
pomalšie
rýchlejšie
Priem. Kontr
Hustota
Biokompatibilita
END.