Metal

advantage  x6

• dense
• high melting and boiling point (big energy to overcome the force between positive ions and the delocalised electrons)
• heat conductor (delocalized electrons transmit energy from one vibrating positive ion to another)
• electricity conductor
• malleable and ductile (distortion doesn't disrut the metallic bonding)
• lustrous (free electrons reflect light; non-metals are transparent)

Unit cells

4 types

primitive cubic

body centered cubic (BCC)

face centered cubic (FCC)

hexagonal closed pack (HCP)

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-- atoms do not line up in regualr fashion

-- grain boundaries

-- hinder rolling of atoms over one another

-- hardness

Metal corrosion

definition

reaction

chemical reaction of a metal degrade into soluble compounds (e.g. oxidies, hydroxides)

Fe + O2 + H2O = Fe(OH)2

stainless steel

compostion and why  x3

properties  x4

12% chromium

• Cr passivation gives corrosion resistance
• oxidized Cr form a sealing film on surface to provide other oxidants from reaching the metal
• Cr weakens the material so Ni is added

1. high elastic modulus (rigid)
2. stress corrosion cracking, pitting and crevice corrosion (temporary use)
3. intergranular corrosion at grain boundaries
4. wear fragments in adjacent giant cells

gold and platinum

advantage and disadvantage  x3

• no self-protecting oxide layer on surface
• used as electrodes due to corrosion resistance, durability and stability
• expensive

cobalt-based alloy

application  x3

advantage  x5

disadvantage x2

detal, knee, hip

1. long service life
2. wear reistant
3. fatigue strength
4. ultimate tensile strength
5. corrosion resistance

denser

stiffer

titanium alloy

advantage

disadvantage

• high strength to weight ratio
• modulus of elasticity lower than stainless steel and cobalt alloys, but higher strength per density
• biocompatible
• corrosion resistance due to TiO2 passivation coating

• modulus of elasticity does not match bone causing stress shielding
• seize when in sliding contact with metals
• surface wear

Ni-Ti shape memory and superelasticity

process  x2

application  x2

• activated by temperature t return to programmed shape and function
• malleable until transition temperature, at which it returns to a programmed shape

braces, stent

precious metals: resistant to corrosion

stainless steel: economical

Co-vased alloys: wear resistance

Ti-based alloys: biocompatibility

Corrosion!

• lowest free energy in O2 and H2O is an oxide
• ionized metal atoms go into solution, or combine with O2 to form a compound that flakes off or dissolves
• at the anode (+) electrode (more electronegative metal), metal oxidizes (lose electrons; at the cathode, reduction occurs

dissimilar metals implanted together should be avoided

add an oxide film (like TiO and CrO)

biocompatibility