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

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...

and

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

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

Vickers hardness The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials. The Vickers test is often easier to use than other 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 ( UK) or jewelry ( U.S.) consists of decorative items worn for personal adornment, such as brooches, rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be attached to the body or the clothes. From a w ...

, 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 are ...

s in the manufacture of synthetic fibres. Their well-known use is in

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

, where they are 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 of the metre, although both have been superseded during the

2019 redefinition of the SI base units 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 artifacts 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

Young's modulus Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied ...

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