Platinum–iridium alloys are alloys of the

platinum group The platinum-group metals (PGMs) are six noble, precious metallic elements clustered together in the periodic table. These elements are all transition metals in the d-block (groups 8, 9, and 10, periods 5 and 6). The six platinum-group ...

precious metals

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

and

iridium Iridium is a chemical element; it has the symbol Ir and atomic number 77. This very hard, brittle, silvery-white transition metal of the platinum group, is considered the second-densest naturally occurring metal (after osmium) with a density ...

. 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. This improved hardness has also been considered as beneficial for use in platinum

jewellery Jewellery (or jewelry in American English) consists of decorative items worn for personal adornment such as brooches, ring (jewellery), rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be attached to the body or the ...

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

spinneret A spinneret is a silk-spinning organ of a spider or the larva of an insect. Some adult insects also have spinnerets, such as those borne on the forelegs of Embioptera. Spinnerets are usually on the underside of a spider's opisthosoma, and ar ...

s in the manufacture of

synthetic fibre Synthetic fibers or synthetic fibres (in British English; see spelling differences) are fibers made by humans through chemical synthesis, as opposed to natural fibers that are directly derived from living organisms, such as plants like cotton or ...

s. Their well-known use is in

metrology Metrology is the scientific study of measurement. It establishes a common understanding of Unit of measurement, units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to stan ...

, where they were used to make the international prototypes used by international standards bodies for mass standards such as the

International Prototype of the Kilogram The International Prototype of the Kilogram (referred to by metrology, metrologists as the IPK or Le Grand K; sometimes called the ''wiktionary:ur-#Prefix, ur-kilogram'', or ''urkilogram'', particularly by German-language authors writing in Engli ...

and the

International Prototype Metre During the French Revolution, the traditional units of measure were to be replaced by consistent measures based on natural phenomena. As a base unit of length, scientists had favoured the seconds pendulum (a pendulum with a half-period of o ...

, although both have been superseded during the

2019 revision of the SI In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artefacts such as the standard kilogram. Effective 20 May 2019, the 144th ...

. The other extremely widespread use for Pt/Ir alloy is fabrication of metal microelectrodes for electrical stimulation of nervous tissue and electrophysiological recordings. 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

yield strength In materials science and engineering, the yield point is the point on a stress–strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and w ...

, 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.

References

{{DEFAULTSORT:Platinum-iridium alloy Iridium, Platinum alloys Iridium Precious metal alloys