Lecture 9: Metallic Materials Flashcards
what are the common metallic dental biomaterials?
- amalgam
- casting alloys
- titanium and titanium alloys
- stainless steel
- co-cr alloys (cobalt chromium)
- ni-ti alloys (nickel-titanium)
what is a casting alloy?
high noble, noble and base metals used in labratory and restorations
what are the noble metals and what do they do?
- gold, palladium and platinum
- high resistance for corrosion
what are the base metals and what do they do?
- nickel, copper, silver, cobalt, zinc and titanium
- high tendancy to corrode
what are some characteristics of gold?
- best known
- excellent corrosive resistance
- good malleability
- low melting point
what are some characteristics of palladium?
- excellent corrosive resistance
- medium melting point
- much harder than gold (not practical by itself)
- when mixed with gold: increased hardness, melting temp and whitens the color
what are some characteristics of platinum?
- high melting point
- harder than Pd
- low dental use due to price and mixing
what is nobility of alloys?
sum of weight percentages of the noble metals in the alloy
what are characteristics of high noble alloys?
- expensive
- high density (easy to cast)
- copper and silver added to increase hardness and strength
- addition of Pt or Pd
- excellent corrosion resistance
- ## not high moduli
what are characteristics of noble alloys?
- most compositionally diverse
- moderate densities
- strength is greater or equal to high noble alloys due to Pd content
- lower cost than high noble alloys
- gold based contain about 40% gold
- higher amounts of silver and copper than high noble
- au based contain low metlting so cannot be used for ceramic alloy restorations
- Pd based contains 70% pd and almost no gold
- Pd-Cu or Ag-Pd used for crowns or fixed partial dentures with or without ceramic dentures
what are characteristics of base-metal alloys?
- minor amounts of noble elements
- most complex and contains 6-8 elements
- extremely high yield strength and hardness (difficult to polish)
- low densities (difficult to cast)
- least expensive
- Nickel-cobalt base have high corrosion and questionable bio-compatibility
- crowns, fixed partial dentures, impalnts
what are the mechanical and physical alloy properties?
- melting range
- young’s module and strength
- hardness
- casting shrinkage
- density
- color
explain melting range
alloys don’t melt at a single temperature but have a melting range
what is liquidus?
temp a which all alloy melts
what is solidus?
temp at which all alloy freezes
why is modulus and strength relevant?
for clinical success and prevention of restoration failure
why is high modulus required?
resist occlusal forces
why is high strength needed?
to prevent permanent deformation
* can be increased for some alloys by heat treatment
explain alloy hardness
-indication of how easy it is to indent or to polish
- related and proportional to yield strength
all metals do what?
expand when heated
shrink when cooled
alloys with what two properties are able to produce more accurate casting?
little solidification and cooling shrinkage
definition of metals
any of the several solid material elements that are malleable under heat or pressure and can conduct heat/electricity
definition of alloys
the mix of two or more substances where at least one is a metal
amalgam is an alloy made of?
- silver alloy powder: silver + tin + copper
- liquid mercury
- small contents of zinc and palladium
what are the two types of silver alloy powder?
high copper alloy (13-30%) and low copper alloy (2-4%)
what are the differences between low and high copper alloy?
high copper alloy has
- higher strength, less corrosion, less creep, better longevity,
low copper alloy has
- zn causing long term expansion
what are the three silver alloy powder shapes?
irregular, spherical and combination
what is amalgamation?
- reaction that forms amalgam matrix where copper, silver and tin are dissolved into the mercury
- once reaction is complete little to no mercury remains
the crystallization of new products during amalgamation continues until???
all the liquid mercury is consumed
what are the clinical properties of amalgam?
- mechanical strength
- dimensional change
- creep
- tarnish and corrosion
what are the mechanical properties of amalgam?
brittle material where the Hg concentration controls the strength (maximum after 7 days)
** strength changes with time after restoration placement
what are the two types of dimensional changes that can occur during setting?
- contraction (negative) during alloy dissolution
- expansion (positive) during impingement of reaction product crystals
** most significant property
explain creep % of amalgam?
associated with breakdown at the margins of the restoration (loss of marginal integrity) where thin margins stretch beyond their limit
what is corrosion and what does it lead to?
- progressive destruction of a metal by chemical or electrochemical reaction
- increased porosity, reduced marginal integrity, loss of strength and release of metallic ions
list some of the corrosion products
- tin oxides, tin hydrochlorides, copper oxides
**due to salty environment
** saliva can provide protection
why do you want to limit exposure to mercury?
- its toxic
- slow elimination from the body
what is minamata convention?
reducing the use of amalgam
what are some characteristics of titanium alloy?
- corrosion resistance
- low toxicity (no release of ions)
- excellent mechanical properties
- low thermal conductivity
what are the two most common titanium formulations?
- commercially pure titanium
- titanium-aluminum-vanadium
in CP-ti alloy what determines the grade?
the amount of oxygen
explain the ti-biocompatibility/corrosion protection properties
- oxide layer: titanium spontaneously forms an oxide surface on exposure to air or saline environment
** self healing layer, inhibits low charge transfer, prevents corrosion
explain dental titanium implants
- implant surfaces are coated for bioactivity
- silver copper fluoride (antibacterial)
- improves osseointegration due to porosity
