amalgam
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what is dental amalgam
an alloy formed by the reaction of > mercury (liquid) and > silver, tin, copper and other metals (powder)
what is an alloy
consists of 2 or more metals
what are the 2 ways to classify amalgam
> composition > particle size and shape
classify amalgam in terms of composition
> traditional > copper enriched
what does particle size and shape of amalgam affect
affects handling and final properties
what is the the average composition of amalgam
• 70% Ag • 25% Sn • 3% Cu • 1% Zn (a lot of today’s amalgams don’t have zinc and are zinc free - traditional amalgams have a small %) • 1% Hg
what is included in the powder constituent of amalgam
> silver, tin copper zinc Hg in powder
what is the function of silver, tin
intermetallic compound Ag3Sn gamma phase - reacts with mercury (Hg) to form amalgam
what is the function of copper
increases strength and hardness added to improve material
what is the function of zinc
scavenger during production preferentially oxidises and slag formed / removed some amalgams are zinc free
what is the function of mercury in powder
only a few materials have this pre-amalgated alloys so it will react faster [silver, tin and copper then mix with amalgam grounded down]
what is included in the liquid constituent of amalgam
mercury (Hg)
what is the function of mercury in liquid
reacts with other materials it is triple distilled = very pure
name 2 particle types
> lathe cut > spherical, spheroidal
what are lathe cut particles
> coarse, medium, fine formed by filing ingots creates lots of fragments / shards / shavings
what are spherical, spheroidal particles
> range of particle sizes formed by spraying molten metal into inert atmosphere easier to control form globules on surface underneath
what is the setting reaction of amalgam
Ag3Sn + Hg = Ag3Sn + Ag2Hg3 + Sn7Hg9 gamma = gamma + gamma 1 + gamma 2 powder + liquid = unreacted particles + amalgam matrix
what is present in the set structure of amalgam
> gamma gamma 1 gamma 2
what is gamma
intermetallic silver tin (Ag3Sn) gives good strength and corrosion resistnace
what is gamma 1
Ag2Hg3 gives good corrosion resistance its like a soup which encapsulates the rest
what is gamma 2
Sn7Hg9 it is weaker and has poor corrosion resistance spherical particles
what do voids do
decrease strength increase corrosion
what is the tensile strength of the different amalgam components
> gamma = 170 gamma 1 = 30 gamma 2 = 20 = amalgam = 60
what is the setting dimensional changes in traditional amalgam
initial contraction - solution of alloy particles in mercury then expansion - gamma 1 crystallisation