Platinum Iridium Alloy: A Key Material in Medical, Aerospace, and Semiconductor Industries

Platinum iridium is described in most short introductions as platinum with a small addition of iridium, and that phrase removes the only number a buyer needs. The iridium content is the specification. It sets the hardness, it sets the corrosion resistance, it sets how the alloy can be worked, and it sets the price, and the range that is actually sold runs from a few per cent to thirty per cent with very different behaviour at each step.

The same property that makes the alloy useful in three industries is the reason the number matters. Iridium is one of the few metals that raises the hardness of platinum without destroying its chemical stability, so a small addition converts a soft and easily deformed metal into a spring temper material that holds an edge and resists wear. Add more and the hardness rises further while the ability to cold work falls, until the alloy has to be hot worked and the yield of finished wire becomes the dominant cost.

This page sets out the composition steps that are actually traded, what each one buys in a medical electrode, in an ignition component, and in a test probe, and the high temperature limitation that the alloy shares with pure iridium and that no datasheet headline mentions.

The table gives the properties that change with iridium content, using published values for the common compositions. Where a property is not published for a composition, the cell says so, because an interpolated number in a materials table is worse than a blank.

Two things are worth noting in that table. The density rises with iridium content and does so monotonically, from 21.45 for pure platinum to 21.69 at 30 % iridium, because iridium is the denser of the two metals at 22.56 g/cm3. And the hardness step from pure platinum to PtIr 10 is much larger than the step from PtIr 10 to PtIr 20, which is why the 10 % alloy carries most of the medical and instrument work: it captures most of the hardness benefit while remaining a material that can be drawn, rolled, and formed with normal tooling.

The composition range that is routinely stocked is narrower than the range that is technically possible. DLX supplies platinum iridium in the 5, 10, and 15 % iridium grades as standard, with higher contents produced to order, and the practical advice for a new design is to begin at 10 % and to move up only when a wear or hardness measurement shows that 10 % is not enough.

A stimulating electrode has to do three things at once. It has to pass charge into tissue without corroding, it has to survive millions of cycles without changing shape, and it has to be manufacturable into a small and often complex form. Platinum iridium at 10 % is the composition that balances those three, and the reasons are electrochemical and mechanical and not marketing.

The electrochemical reason is the width of the window in which the metal is passive. Platinum and its alloys are used because they do not dissolve at the potentials a stimulation pulse reaches, and the iridium addition improves the resistance to corrosion under repeated cycling without introducing a phase that would corrode selectively. The mechanical reason is that the 10 % alloy is hard enough to resist deformation during crimping and insertion, and ductile enough to be drawn into wire or formed into a ring without cracking.

Three product forms follow from those requirements. A pacemaker or neuromodulation electrode is usually a wire, a ring, or a tip machined from bar. A stent marker band is a thin ring. A surgical instrument insert is a small machined part. All three are ordered by composition, by form, and by a surface condition, and the surface condition matters because a scratch on an implanted electrode is a site for charge concentration.

The aerospace application that is usually described as a rocket engine component is more precisely an ignition component. Platinum iridium appears in spark plug electrodes and igniter tips, where the alloy resists arc erosion and keeps a sharp edge under repeated discharge, and it appears in small sensor and contact parts. It is not the material of a combustion chamber or a nozzle, which are built from nickel alloys, copper alloys, or carbon composites depending on the cooling strategy.

That distinction matters to a buyer because the two enquiries go to different suppliers. A request for platinum iridium ignition electrodes is a small precious metal part with a wear requirement. A request for a thrust chamber is a large structural component with a thermal requirement, and platinum iridium is not a candidate at any price.

The same logic applies to the semiconductor list. The alloy appears in test probes and in some crucible and evaporation sources, where its hardness and its chemical inertness are used at a small scale. It does not appear as a structural material in a wafer fab, and a page that lists semiconductor manufacturing without naming the probe or the crucible has described an industry and not an application.

Iridium oxidises in air at high temperature to a volatile oxide, and the oxide leaves the surface and does not stay on it as a protective scale. The consequence for a platinum iridium part is a slow loss of iridium from the surface layer at temperatures above about 1,000 °C in an oxidising atmosphere, which changes both the composition and the dimensions of a thin section.

This is the reason a platinum iridium electrode is used in an ignition application and not as a furnace element, and the reason the alloy is specified for vacuum, inert atmosphere, or reducing conditions when it runs hot. In air at temperature the metal does not fail by a dramatic mechanism. It simply gets thinner, and the iridium that gave it the hardness is the part that leaves.

Two practical consequences follow. A thin wire or a fine probe tip in air above 1,000 °C should be treated as a consumable with a finite life and not as a permanent component. And a part that has been in that service should be checked by weight and by dimension and not by appearance, because the loss is uniform enough to be invisible until the section is gone.

Platinum iridium work hardens quickly, and the working window narrows as the iridium content rises. The practical sequence for wire is to start from a cast or sintered billet, to hot work it into a bar, and then to draw it with an anneal between passes. The number of passes between anneals falls as the iridium content rises, and at the top of the range the material is difficult to draw at all.

The commercial consequence is that the price of a platinum iridium part is driven by the yield of the working route as much as by the metal itself. A specification that asks for a diameter and a tolerance which are difficult to draw will carry a cost that has nothing to do with the iridium content, and a buyer comparing two quotations for the same nominal alloy may be comparing two different yields.

Six items decide a platinum iridium quotation, and the first one is the item that most enquiries leave vague.

DLX supplies platinum iridium in Changzhou, Jiangsu, as wire, tube, ring, strip, and bar, in the 5, 10, and 15 % iridium grades as standard and in higher contents to order. The forms include fine wire for electrodes and probes, tube for marker bands, ring for instrument parts, and spring wire for precision mechanisms.

Material is supplied against the composition named on the order, with an EN 10204 3.1 certificate on request and the heat number traceable to the test results. The question that decides a quotation is the iridium content together with the form, because those two items set the working route and the yield before any price is discussed.

It usually means 10 %, and sometimes 5 or 15 %. The phrase is used because the iridium addition is small compared with the platinum content, but the difference between 5 % and 15 % is the difference between an easily drawn wire and a material that needs a careful schedule. A specification should state the percentage and not the adjective.

Because it captures most of the hardness benefit of iridium while remaining a material that can be drawn and formed with normal tooling. It resists corrosion under repeated charge cycling, it holds its shape during crimping and insertion, and it can be made into the wire, ring, and machined forms that an implant needs. Higher iridium contents are harder and harder to work, which raises the cost of a part that is usually small.

It is used in ignition components such as spark plug electrodes and igniter tips, and not as a structural material for a chamber or a nozzle. Those larger parts are built from nickel alloys, copper alloys, or carbon composites depending on the cooling strategy. A request for platinum iridium engine hardware is usually a request for an electrode.

Well enough for an electrode and not well enough to be chosen for conductivity. Platinum has a resistivity roughly six times that of copper, and the iridium addition does not improve it. The alloy is selected for chemical stability, hardness, and wear resistance, and the conductivity is a property that happens to be adequate and is not the reason for the choice.

Iridium forms a volatile oxide that leaves the surface instead of building a protective scale, so the alloy slowly loses iridium from the surface layer. In a thin section that shows up as a reduction in dimension and a change in composition. The alloy is used hot in vacuum, in inert gas, or in reducing conditions, and a part that runs hot in air should be treated as a consumable.

For two reasons that compound. Iridium is much more expensive than platinum, so the metal cost rises with the percentage. And the working window narrows as the iridium rises, so the number of drawing passes between anneals falls and the yield of finished wire drops. The second effect is often larger than the first for a fine wire.

Platinum iridium is a platinum alloy whose specification is its iridium content. At 10 % it is hard enough to resist wear and ductile enough to be drawn into the wire and formed into the rings that medical and instrument work needs. At 20 % and above it resists arc erosion and wear further, at the cost of a working window that narrows until hot working becomes the only route.

Two limits belong in the specification and not in the brochure. The alloy loses iridium as a volatile oxide in air above about 1,000 °C, so a hot part needs vacuum, inert gas, or a consumable life. And its conductivity is adequate and not exceptional, so a design that selects it for electrical performance has selected it for the wrong reason.

Published property data for platinum iridium alloys, giving a density of 21.56 g/cm3 for the 10 % iridium grade, 21.61 for 20 %, and 21.69 for 30 %, and a melting point of about 1,800 °C for the 10 % grade.

Published supplier data for platinum 90 iridium 10, giving a Brinell hardness of 130 to 190, a tensile strength of 380 to 620 MPa, a density of 21.56 g/cm3, and a melting point of about 1,800 °C.

Published reference data for the platinum iridium system, giving a Vickers hardness of about 56 for pure platinum, the common 90:10 and 70:30 compositions, and the increase in hardness that the iridium addition produces.

Published material data for iridium, giving a density of 22.56 g/cm3 and a melting point of about 2,446 °C, as the basis for the density trend across the alloy series.

Published oxidation studies of iridium and iridium containing alloys, reporting weight loss from oxidation in air at 1,000 °C and identifying the volatile oxide as the mechanism by which iridium leaves the surface.

Published data on the vapour pressure of platinum, iridium, and rhodium over the 1,000 to 1,750 °C range, as the basis for the high temperature loss statement.

ISO 5832 and ASTM F75 family documents for implant materials, as the general framework within which a medical electrode material is qualified, noting that a specific platinum iridium grade is qualified by the device manufacturer against the applicable standard.

EN 10204, inspection documents, for the definition of the 3.1 certificate as a document carrying actual test results and endorsed by the manufacturer independent inspection representative.

The compositions and property values above are published values for the grades named, and they are not acceptance limits for a delivered lot. Hardness and tensile properties depend on the amount of cold work and on the annealing treatment, so a value quoted for one temper does not describe another. Density is a property of the composition and is used here to show the trend across the series. The oxidation behaviour described applies to air at temperature, and the rate depends on temperature, gas flow, and the surface to volume ratio of the part, so a hot application should be qualified by test and not by a single temperature limit. Platinum iridium grades for medical use are qualified by the device manufacturer against the applicable standard, and the alloy supply does not by itself constitute that qualification. Product specifications are revised, and the edition named on the purchase order governs. Nothing in this page is a substitute for an engineering release or for the qualification testing agreed between buyer and supplier.