Platinum–iridium alloy

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international prototype of the kilogram (IPK) is an artifact standard of platinum–iridium alloy that was defined as having a mass of exactly one kilogram
.

Platinum–iridium alloys are alloys of the platinum group precious metals platinum and iridium.

Typical alloy proportions are 90:10 or 70:30 (Pt:Ir). These have the chemical stability of platinum, but increased hardness. The

Vickers hardness of pure platinum is 56 HV while platinum with 50% of iridium can reach over 500 HV.[1][2] This improved hardness has also been considered as beneficial for use in platinum jewellery
, particularly watch cases.

Owing to their high cost, these alloys are rarely used. They have been used for

synthetic fibres.[3]

Their well-known use is in

international prototype of the metre, although both have been superseded during the 2019 redefinition of the SI base units
.

The other extremely widespread use for Pt/Ir alloy is fabrication of metal microelectrodes for electrical stimulation of nervous tissue[4] and electrophysiological recordings.[5][6][7] Pt/Ir alloy has an optimal combination of mechanical and electrochemical properties for this application. Pure iridium is very difficult to pull into small diameter wires; at the same time, platinum has a low Young's modulus which makes pure platinum wires bend too easily during insertion into nervous tissue. Additionally, platinum–iridium alloys containing oxides of both metals can be electro-deposited onto the surface of microelectrodes.[8]

References

  1. doi:10.1038/186211a0
    . Retrieved 2008-10-13.
  2. doi:10.1595/147106705X24409
    .
  3. S2CID 135705244
    .
  4. S2CID 19297044
    .
  5. PMID 18429704
    .
  6. S2CID 7973265
    .
  7. S2CID 23762187
    .
  8. ^ "Platinum Group Coatings".
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