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IntroductionThe metal Ir has a unique combination of prop-

erties, including a high melting temperature,strength at high temperature, oxidation resistanceand corrosion resistance, that are useful in a rangeof applications, particularly at elevated tempera-ture. However Ir is also one of the more difficultmaterials to process into finished products, inmany cases due to the same properties that recom-mend its use. This was highlighted by the review ‘AHistory of Iridium’ published twenty years ago inthis Journal with the subtitle ‘Overcoming theDifficulties of Melting and Fabrication’ (1). Thepurpose of the present review is to summariseadvances in the processing of Ir during the pasttwo decades. In some cases processing methodshave been refined and improved, while in othersentirely new processing methods have been devel-oped to serve new applications of Ir. The scope ofthis review is limited to the processing of pure Irand Ir alloy materials in which Ir constitutes themajority of the composition. The processing meth-ods that are reviewed include purification, melting,powder processing, forming, joining and coating.

PurificationThe purity of Ir has been shown to have impor-

tant effects on the mechanical properties of boththe nominally pure metal and its alloys (2, 3). Ir isseparated from platinum group metal (pgm)concentrates and purified either by conventionalchemical refining methods or by a solvent

extraction process (4). In the conventional meth-ods Ir oxide is dissolved in aqua regia (a mixture ofconcentrated nitric and hydrochloric acids) andprecipitated with ammonium chloride. After aseries of dissolutions and precipitations the salt isheated in a hydrogen atmosphere to produce Irsponge. In the solvent extraction method, a seriesof organic liquids are used to concentrate variouspgms from aqueous solutions. The Ir can then beprecipitated and heated in hydrogen to produce Irsponge. Solvent extraction methods may offer costand environmental benefits over conventionalchemical precipitation methods.

The purity of the sponge is sensitive to both thedetails of the processing and the starting materials.A method of chemical purification of Ir com-pounds following dissolution is reported toachieve removal of platinum, rhodium, rutheniumand palladium to levels below 5 parts per millionby weight (ppm) (5). A method of scrap purifica-tion, as well as fabrication, uses electrodepositionof Ir from molten salt solution (6). Purification iseffective in removing many base metals, but less sofor some others including pgms. A pyrometallurgi-cal method for purification of Ir scrap employsinduction melting in a crucible of magnesiumoxide with an air atmosphere, to remove bothvolatile impurities through vaporisation and oxide-forming impurities as a vapour or as a slag (7). This is a relatively low-cost method, althoughsome impurities, including iron, are not easilyremoved by it.

Processing of Iridium and Iridium AlloysMETHODS FROM PURIFICATION TO FABRICATION

By E. K. OhrinerOak Ridge National Laboratory, Materials Science and Technology Division, PO Box 2008, Oak Ridge, TN 37831, U.S.A.;

E-mail: ohrinerek@ornl.gov

Iridium and its alloys have been considered to be difficult to fabricate due to their high meltingtemperatures, limited ductility, sensitivity to impurity content and particular chemical properties.The variety of processing methods used for iridium and its alloys are reviewed, includingpurification, melting, forming, joining and powder metallurgy techniques. Also included arecoating and forming by the methods of electroplating, chemical and physical vapour depositionand melt particle deposition.

DOI: 10.1595/147106708X333827

Platinum Metals Rev., 2008, 52, (3), 186–197

Electron beam melting has been used forpurification and is effective for removal of mostimpurities with the exception of refractory metalelements (8, 9). Recent work treated removal of alarge number of impurity elements by electronbeam melting (10). It showed that titanium, vana-dium and zirconium were not removed byelectron beam melting, due to negative deviationsfrom ideal solution behaviour (10). In addition,several other elements were only partiallyremoved even under optimal conditions of meltstirring. The impurity ratio, defined as the ratio byweight of the final impurity content to the initialimpurity content, was measured for electron beammelting of Ir buttons under conditions producingIr vaporisation of 5.6% by weight. Low ratioswere observed for iron, aluminium and chromiumof 0.004, 0.014 and 0.06, respectively.Intermediate ratios were obtained for the impuri-ties platinum, silicon and carbon of 0.11, 0.2 and0.3, respectively. The results were shown to beconsistent with ideal mixing and vaporisation. Theobserved purification behaviour for the impurityelements Fe, Al, Cr and Si was explained by theirlow activity coefficients.

MeltingMelting of Ir is performed by induction melt-

ing, electron beam melting and arc meltingmethods. Induction melting, frequently used forinitial melting (11), is performed in air with zirco-nia or magnesia crucibles. Due to excessivevolatilisation of the crucible material at the Ir melt-ing temperature (5), the crucibles are not suitablefor use in vacuum. Ceramic inclusions are avoidedby the use of electron beam, plasma or arc meltingwith water-cooled copper crucibles. Plasma melt-ing of Ir is performed at a moderate pressure ofabout 50 Pa and provides for some purification byvaporisation of impurities, although to a lesserextent than with electron beam melting. In somecases, the final melting operation prior to defor-mation processing (12) is electron beam melting(discussed above under purification). Button arcmelting is used for relatively small quantities of Irand Ir alloys, particularly those with alloyingadditions subject to vaporisation during melting.

Vacuum arc remelting (VAR) is applicable to larg-er-size melts, and provides ingots with little or nointernal porosity (13). The VAR ingots have a rel-atively large grain size with some degree ofdirectional solidification. Melting of 27 mm dia-meter electrodes to produce 63 mm diameteringots of an Ir alloy is performed with a directcurrent (DC) of 3000 A at 31 V in a vacuum ofabout 13 mPa with a melt rate of about 3 kg min–1.Control of porosity is important even in the caseof ingots to be forged or rolled, since porosity inthe melted ingot may never be completely sealedeven with extensive hot deformation, such as byhot extrusion (14).

Powder MetallurgyThe powder metallurgy methods of pressing

and sintering can be used to prepare billets forsubsequent hot working to produce a fully denseproduct (15), but progress in producing a fullydense near-net shape by these means has been lim-ited. In a study of powder metallurgy methods forpgms and their alloys, including pure Ir and Ir-Ptalloys, neither pure Ir nor Ir-30 wt.% Pt wereamenable to pressing and sintering, nor to hot iso-static pressing (HIP) (16). The problems with Irinclude its high melting temperature, which pre-cludes standard gas atomisation methods, andcontamination from can materials during HIP. AnIr-50 wt.% Pt alloy was processed to powder by aplasma rotating electrode process and brought tonear theoretical density by HIP (16). Ir powder orsponge typically consists of agglomerates of Ircrystals, with crystal sizes of the order of 1 μm andagglomerates in the size range 10 μm to 150 μm.Sintering of Ir powder with subsequent hot press-ing has been used for the fabrication of Ircrucibles (17). An Ir composite with 15% byweight of yttrium oxide, selected for the necessarycombination of high melting temperature andelectrical conductivity together with a low electronwork function, was developed as an electrodematerial for plasma cutting (18). The material wasprepared by pressing and sintering in hydrogen at2273 K for 30 minutes to obtain a density of 94%.Ir-base alloys containing up to 15% alloy additionsof niobium, titanium, zirconium or hafnium have

Platinum Metals Rev., 2008, 52, (3) 187

been consolidated on a laboratory scale from pre-alloyed powders using a pulse electric currentsintering method (19). Homogeneity wasimproved over earlier work with elemental pow-ders, and densities close to 98% were achieved.The use of elemental powders was also foundunsuitable for preparing quaternary alloys of Irbecause the desired microstructures could not beobtained in melted alloys (20).

Powder metallurgy methods have been used toproduce porous Ir for use as filters or in otherapplications. Conventional pressing and sinteringmethods have been used to produce a porous Irmetal filter which is bonded to Ir alloy compo-nents (21). The bonding and final sintering at 2173K were performed in a vacuum furnace under anapplied load of 22 N over an area of 0.5 cm2 usinggraphite tooling. A density of 47% of theoreticalwas achieved. Compression at room temperaturesubsequently increased the density to 67% in orderto obtain the desired flow rates for the filter. Slurrycasting has been investigated as a method for pro-ducing porous Ir components with controlleddensity and pore size (22). After removal of thewax binder the samples were presintered in argonfor one hour at 1273 K to achieve a density of33%. Sintering in vacuum for one hour at 1473 Kand 1573 K resulted in densities of 35% and 46%respectively.

Deformation ProcessingIr and its alloys can be processed using standard

metalworking methods including forging, extru-sion, rolling and drawing, but only with somedifficulty (23). In general deformation is per-formed at elevated temperature in order to avoidcrack formation and propagation (15). The defor-mation behaviour depends on impurity content,impurity distribution, microstructure and texture.Annealing at temperatures of 2273 K and higherwas shown to result in homogenisation of somepotentially deleterious impurities and to decreasethe tendency for grain boundary fracture (24). Incontrast, small additions of elements such as thori-um and cerium have been shown to segregate tograin boundaries and to improve ductility (2, 25).The very coarse grain structure of Ir and Ir alloys

in cast ingot form makes the material particularlysensitive to cracking at relatively small tensilestrains. Preheat temperatures for the initial defor-mation of cast Ir by extrusion, forging or rollingare 1500 K or higher (26). Initial working temper-atures for Ir ingots can be as high as 2075 K (27).Hot extrusion minimises tensile stresses during theinitial deformation processing. Canning in molyb-denum for extrusion minimises cooling of the Irduring the extrusion process and permits preheattemperatures in the range of 1600 to 1700 Kwith an extrusion ratio of 6.4:1 (28). (The extrusionratio is defined as the ratio of the container borearea to the total cross-sectional area of extrusion.)

Hot rolling of Ir and its alloys following initialingot breakdown is generally performed with pre-heat temperatures in the range 1100 to 1500 K. Inorder to minimise chill during rolling, covers ofMo have been used (29). In-process recrystallisa-tion is also used to minimise cracking duringrolling. Ir alloy sheet of about 0.5 mm thickness orless can be rolled to foil at room temperature withcold work levels of more than 80%. A schematicflow diagram of the production of the DOP-26alloy sheet material from Ir powder through multi-ple melting and deformation processing steps isshown in Figure 1. (DOP-26 contains (by weight)3000 ppm tungsten, 60 ppm thorium and 50 ppmaluminium.)

Ir wire is normally produced from a melted Irstock with deformation performed at elevated tem-perature. An enriched 191Ir isotope was forgedsquare at about 1775 K and then rod rolled begin-ning at 1675 K preheat temperature (12). The finalcircular cross section of about 0.6 mm was achievedby swaging. Another method uses repeated hotextrusion of a melted ingot followed by warmdrawing, and is applicable for initial ingotsas small as 30 g (30). Trials of upset formingof annealed Ir wire determined that limited colddeformation of up to 25 to 30% could beachieved, but forming of complex geometries wasunsuccessful (27).

Forming of sheet or plate of Ir and its alloys isgenerally performed at elevated temperature.Warm drawing, that is deformation below therecrystallisation temperature, has been successfully

Platinum Metals Rev., 2008, 52, (3) 188

performed on Ir and its alloys. Hemispherical cupsof an Ir alloy were hydroformed using tooling pre-heated to 775 K (31). Prior to forming, the blankswere encapsulated in evacuated stainless steel cov-ers that were preheated to 1175 K. Similarencapsulation in stainless steel was used for deepdrawing with preheated steel tooling (32). Theencapsulation and subsequent removal of the cap-sule materials does add processing steps andpotentially decreases dimensional control. Crack-free cups may be formed from similar Ir DOP-26alloy without encapsulation, using preheated tool-ing, isothermal forming temperatures of 825 K to875 K and a draw ratio of about 2 (33). Crackingoccurred at a draw temperature of 775 K. An ear-lier work on deep drawing of Ir cups reportedwrinkling of the drawn cups (34), and incorrectlyconcluded that drawing below the recrystallisationtemperature was not possible. Increasing the hold-down pressure minimises wrinkling, and improvedlubrication minimises loads on the cup wall. By

way of example, blanks of 51 mm diameter and0.65 mm thickness, in stress-relieved conditions,were drawn with a hold-down force of 65 kNusing flexible graphite sheet as a lubricant (33).The advantages and disadvantages of various deepdrawing methods for DOP-26 Ir alloy have beenevaluated (35). No substantial differences werefound between the inspection yields of formedcups with and without encapsulation. Additionalprocessing steps associated with encapsulation andlater removal of the encapsulation materialincrease processing costs, but the potential for sur-face contamination of the Ir alloy is minimised.Also, as reported elsewhere (36), the tendency forfracture near the cup radius might possibly beminimised by maintaining the punch at a lowertemperature to increase the strength of the mater-ial in that region. Hot drawing of Ir has also beenreported, using a preheat temperature up to1625 K with tooling heated to about 650 K. Theblanks were of 2 mm thickness and the draw ratio

Platinum Metals Rev., 2008, 52, (3) 189

DOP-26 Iridium Alloy Processing

Iridium powder Compact Electron beam button

Drop-cast segmentElectrodeArc-melted ingot

Extruded bar Sheet Blank

Fig. 1 Schematic diagram of the processingof iridium powder to a DOP-26 iridium alloyforming blank. Note: photographs are not tothe same scale

was 1.1 (27). In general, Ir sheet material has beenformed by deep drawing, spinning or pressing attemperatures in the range 1175 to 1675 K (26).The minimum working temperature increaseswith increased material thickness and increasedapplied tensile strain levels.

JoiningIr is weldable by a number of methods, as are

some Ir alloys. The applications for welding Ir andits alloys include the fabrication of spark plug elec-trodes, nuclear fuel containers and crucibles forcrystal growth. Arc welding 0.63 mm thickIr metal was performed in a helium atmospherewith a 3.2 mm diameter thoriated tungsten elec-trode, an arc gap of 1 mm, a weld velocity of5 mm s–1 and a current of 41 A DC with straightpolarity (37). The weld metal along the centrelineexhibited an unfavourable microstructure, withsingle grains extending through the thickness ofthe weld. An increase in weld velocity, with corre-sponding increases in welding current, reducedthe size of grains along the centreline. Oscillationof the arc across the weld centreline by magneticdeflection, equivalent to a square wave at a fre-quency of 6.25 Hz, produced a microstructurewith smaller and more equiaxed grains along theweld centreline.

Some modifications to these methods wererequired for welding of the DOP-26 Ir alloy con-taining 60 ppm Th (38). The alloy was found to bemore susceptible to hot cracking during welding, aphenomenon associated with grain boundary sep-aration during the solidification process. A weldcurrent of 83 A was used and hot cracking wasminimised by using long initial and final tapers,and a short arc length. The use of arc deflectionwith a four-pole oscillator tended to furtherreduce hot cracking and gave a smaller averagegrain size in the weld (39). The grain orientationsand grain sizes in the weld were associated withchanges in the shape of the weld pool fromteardrop to elliptical (40). This improvement ingrain structure with four-pole oscillation resultedin increased tensile elongation from 4% to 14%, asmeasured in tests at 923 K.

The effect of weld width on the DOP-26 alloy

was studied under similar conditions at a weldspeed of 12.5 mm s–1, with current adjusted to giveweld bead widths of 3.7 mm or 2.5 mm (41). Thenarrow welds exhibited a finer grain size, and atensile impact elongation more than double thatfor the wider bead as measured at 5000 s–1 andtemperatures of 1253 and 1373 K. Scanning elec-tron microscopy showed equiaxed particles ofIr-Th intermetallic along grain boundary fracturesurfaces within the narrow weld, whereas thewider weld gave aligned intermetallic precipitatessimilar to a eutectic structure. The cracking atgrain boundaries during solidification was attrib-uted to segregation of Th and local melting pointdepression, resulting in strain concentration atweld grain boundaries.

Arc welding of the Ir alloys has been auto-mated. Autogenous, full joint penetration gastungsten arc (GTA) girth welds were made to jointwo DOP-26 Ir alloy shells over a plutoniumoxide fuel pellet using computer-based control(38). This process was later updated to use a PC-based commercial controller that continuouslymonitors and controls welding current, rotationalspeed, and the composition and flow of the torchgas (42). The use of precision tooling and controlsfor location, rotation and joint loading allowedtack welding to be eliminated and improved yieldsof defect-free welded components (43).

Electron beam welding provides some advan-tages in the welding of Ir alloys that are difficult tojoin by arc welding. An alloy of Ir with 200 ppmTh was successfully welded without cracking byelectron beam, but was not weldable by arc weld-ing (44). At low speed, successful electron beamwelds could only be made over a narrow range ofbeam focus conditions, whereas at high speedswelds could be made over a wide range of focusconditions. This behaviour at high welding speedswas associated with fusion zone grain structureand positive segregation of Th at the fusionboundary under conditions leading to cracking.High heat input led to more rapid solidification,finer grain size and lower levels of segregation(45). The micrograph in Figure 2 shows the align-ment of grain boundaries in the weld metal alongthe centreline of a weld obtained at the relatively

Platinum Metals Rev., 2008, 52, (3) 190

low travel speed of 2.5 mm s–1. At higher speedsthe grain boundaries become less uniformly ori-ented. The shape of the weld pool during electronbeam welding was also found to influence the ori-entation of grains in the weld pool (46), asdiscussed above for arc welding of Ir. Electronbeam welding of Ir alloy components has beenpracticed, although control of heat input and tool-ing is essential to obtaining acceptable welds (47).Figure 3 shows an example of an electron beamweld in a DOP-26 Ir alloy cup assembly.

Laser welding has shown benefits in welding anIr alloy that is subject to hot cracking (48). Analloy containing 200 ppm Th was welded with acontinuous-wave high-power carbon dioxide lasersystem without hot cracking. This was explained ina manner similar to that for electron beam weld-ing, a highly concentrated heat source and arelatively fine fusion zone microstructure. Laserwelding has been used to join Ir wire segments tonickel or nickel alloy for use in spark plug applica-tions (49). Electric resistance welding, a potentiallylower-cost alternative for this joining process, wasalso evaluated using a programmable high-fre-quency power supply. Direct bonding of Ir to the

Ni was not achieved but metallurgical bonding wasachieved with the use of an intermediate layer. Theintermediate layers were chosen to assist in alloy-ing between the dissimilar joint materials and toprovide an orderly transition in the thermal expan-sion coefficient of the material across the joint.

Ir alloys have been characterised with respectto weldability, defined as the capacity to producecrack-free welds. Since there are multiple mecha-nisms by which cracks may initiate and propagate,there are a variety of methods for measuring weld-ability of Ir and its alloys.

The simplest method of determining whetherautogenous welds can be made without cracks insheet material under specific conditions wasemployed to show that Ir containing up to 100ppm Th produced sound welds by arc welding,whereas alloys containing 200 ppm or greater didnot produce crack-free welds (44). The more sen-sitive modified circular patch test employs a diskof material which is clamped to a test fixture andarc welded to produce two concentric autogenouswelds in sequence (50). This test introduces ther-mal stresses from mechanical constraint as well asincreased residual stresses from the multiplewelds. The modified circular patch test was used

Platinum Metals Rev., 2008, 52, (3) 191

Fig. 2 Optical micrographs of an electron beam weld iniridium alloy sheet, showing regular grain alignmentalong the weld centreline associated with a relatively lowweld traverse speed (Reproduced with permission from(45))

Fig. 3 DOP-26 iridium alloy cups with electron beamwelded frit vent component. The light coloured area, seenas four semi-circular regions, is a porous iridium metalfilter or frit (Photograph courtesy of G. B. Ulrich, OakRidge National Laboratory)

Welding direction

300 μm

(a)

(b)

200 μm

5 mm

to characterise various individual lots of the DOP-26 alloy with respect to hot cracking. Another testemployed the detection of underbead cracks inarc-welded DOP-26 Ir alloy capsules (51). Here, aclosure weld was made by arc welding the circum-ference of two mating Ir alloy cups, followed byadditional circumferential welds and shorter arcwelds in the same locations as the previous welds.The welded cups were examined for cracks byboth non-destructive and destructive methods.This permitted the selection of improved weldparameters, in particular current ramp rates, as wellas characterisation of various lots of Ir alloy sheetmaterials.

In the Sigmajig weldability test for hot cracking,an initial tensile stress is applied to the test sample,and threshold cracking stress is determined; this isthe maximum applied stress at which a crack-freeweld can be made (52). This test has been usedboth to characterise the weldability of Ir alloymaterials and to characterise the effects of varyingsome welding parameters (53). A study of theeffect of Th concentration in Ir-0.3 wt.% W alloyshowed that the threshold cracking stressdecreased from 170 MPa at 37 ppm Th to 85 MPaat 94 ppm Th. Neither oxygen impurities up to2000 ppm by volume nor water vapour up to 1000ppm in the argon atmosphere of the glove boxaffected the threshold cracking stress. However,significant increases in weld width were observedwith increased gas impurity levels, an effect attrib-uted to changes in the surface tension of the liquid.The test was also used to evaluate the weldabilityof alloys with Ce or both Ce and Th at levels (inatom fraction) up to 100 ppm (54). An alloy with50 ppm Ce and alloys with 40 ppm Ce and 10 ppmTh or 30 ppm Ce and 20 ppm Th all showedthreshold cracking stresses of 170 MPa or greater.A number of other alloys with boron and Y addi-tions exhibited low threshold cracking stresses.

The performance of welded Ir crucibles hasalso been characterised. Crucibles fabricated bywelding of electron beam melted Ir have shownlonger service life than those made by powder met-allurgy (55). Grain growth during service attemperatures of 1800 to 2400 K resulted in largegrains, up to 10 mm in size. Crucible failures were

attributed to segregation of impurities at theseboundaries. One unusual application is the autoge-nous welding of an entire wrought Ir crucible, witha goal of increasing crucible life (56). The grains inthe welded structure, although large by normalstandards, can impede further grain growth anddelay crucible failure.

Nondestructive examination of Ir welds andbase metal has been performed using both dyepenetrant and ultrasonic methods. Fluorescent dyemethods have been used for deep drawn cups (57).Ultrasonic inspection methods initially used in the1980s (38, 58) have been further developed to pro-vide better sensitivity and diagnostic capability (59).

Deposition ProcessesDeposition processes for Ir include electrolytic

deposition, chemical and physical vapour deposi-tion and melt deposition. Each of these methodshas advantages and limitations depending on therequirements of the application. The plating of Irfrom aqueous solutions has been the subject of arecent review (60). Plating of Ir from Ir chloridesolutions with sulfamic acid produces deposits upto 25 μm thick, although the deposits exhibitcracks. Plating of Ir from solution in hydrobromicacid produces crack-free deposits of up to 1 μmthick using a plating rate of about 1 μm per hour.Improved plating efficiencies and decreased crack-ing of the coating were reported for sodiumhexabromoiridate(III) baths with additions ofoxalic acid. While typical thicknesses of Ir platingof 1 μm or less can minimise corrosion and servefor many electronic applications, thicker coatingsare necessary for use at elevated temperature.

Electrodeposition from molten salts has beenused to produce coatings of Ir up to 0.4 mm thick,and net shape components up to 3 mm thick.Initial work on electrodeposition demonstratedthat Ir plating could be performed in a bath offused sodium cyanide or a mixture of sodiumcyanide and potassium cyanide under inert atmos-phere at rates up to 10 μm h–1 (61). Coatings up to0.125 mm thick were produced in a single coatingcycle (62) and coatings up to 0.4 mm wereproduced in multiple cycles with intermediate sur-face removal (63). Later work demonstrated that

Platinum Metals Rev., 2008, 52, (3) 192

Platinum Metals Rev., 2008, 52, (3) 193

deposition of Ir coating from fused chlorides wasalso possible, with thicknesses up to 350 μmdeposited at temperatures of 800 to 920 K (64).Coatings have been made using a ternary eutecticmolten salt bath of NaCl-KCl-CsCl containing 2 to7 wt.% Ir (65). This bath composition has alsobeen used to electroform crucibles by depositionof thick deposits on to a graphite mandrel that islater removed (66). Deposition rates up to 100 μmh–1 were reported. The grain size was 10 to 15 μmfor direct current deposits with a thickness of200 μm. The grain size was about 70 μm usingintermittent current reversal. Electrodepositionhas been used for a variety of other components,including crucibles, tubes, nozzles and jewellery(6, 67). Some examples are shown in Figure 4.

The chemical vapour deposition (CVD) pro-cessing of Ir has progressed significantly in recentyears, particularly for very high-temperature appli-cations. Ir was first applied by CVD to rheniumrocket thruster chambers for high-temperatureoxidation protection in 1986, and a 490 N cham-ber was flight-qualified in 1997 (68). Ir coatings aredeposited by CVD to a thickness of about 50 μmonto a Mo or graphite mandrel of the appropriateshape, and Re, to a thickness of about 2 mm, issubsequently deposited over the Ir by CVD andmachined (69), as shown schematically in Figure 5.Typical deposition rates are about 10 μm h–1 for Irand 40 μm h–1 for Re at a temperature stated to beabout 1475 K (70). Further performance improve-ments and weight savings are reported using acarbon/carbon composite outer shell and CVD Re

inner liner with a CVD Ir coating (71).The range of methods reported for CVD of Ir

was recently reviewed (72). Ir hexafluoride can beused to produce non-porous coatings at rates of upto 10 μm h–1, but the compound requires highdeposition temperatures and is corrosive. Organiccomplexes of Ir offer the potential for lower depo-sition temperatures. Ir acetylacetonate has beenused to produce Ir coatings up to 50 μm thick atrates up to 25 μm h–1, although these deposits con-tain up to 20% carbon by weight. Controlledadditions of oxygen can produce essentiallycarbon-free coatings, but deposition rates areabout 0.2 μm h–1. A variety of carbonyl, allyl and cyclooctadienyl complexes of Ir have beenevaluated for CVD of Ir coatings. Temperatures

Fig. 4 Iridium products produced by electrodepositionfrom a molten salt bath (Reproduced with permissionfrom (6))

Coat mandrelwith iridium

Overcoat withrhenium

Removemandrel

Completediridium/rhenium

combustion chamber

Rhenium(structure)

Mandrel Iridium(oxidationprotection) Fig. 5 Schematic diagram of the

production of an iridium-coated rheniumnozzle by chemical vapour deposition(Reproduced with permission from (69))

for decomposition are in the range of 400 to 760 Kand oxygen or hydrogen is generally added to con-trol the carbon content of the coating. Amethylcyclopentadienyl complex of Ir,Ir(COD)(MeCp) (COD = 1,5-cyclooctadiene) hasbeen used to produce coatings of 1 to 2 μm thick-ness, of good purity, at temperatures in the range573 to 673 K, but with low deposition rates of0.25 μm h–1 or less (73).

Physical vapour deposition methods for Irinclude both thermal evaporation and sputtering.Electron beam vapour deposition (EBVD) hasbeen used to produce thin coatings on mirrors forinfrared telescopes (74). Ir alloys have beendeposited by EBVD for use as diffusion barrierson coated Ni-base superalloys (75). Pulsed laservaporisation of Ir has also been studied (76).Studies of coatings of Ir for high-temperature oxi-dation protection of carbon materials showed thatcontinuous coatings were produced via radio-fre-quency magnetron sputtering, but not with directcurrent sputtering methods (77).

A number of melt deposition processes havebeen investigated for Ir component production.The production of near-net shape parts with Ir bydirected light fabrication has shown some promise(78). In this method metal powder is transported ina stream of inert gas and fused to a surface in thefocus of a high-power laser beam, to form fullyfused near-net-shaped components. Initial workon this process indicates that porosity originatingfrom gases during melting and solidification is anissue. Plasma spray, and, in particular, vacuumplasma spray or low-pressure plasma spray of Irhas been proposed as a method for achieving high-density coatings. While there is little published

literature available on plasma spraying of Ir, it isexpected to perform similarly to that of a numberof refractory metals (79). The use of sphericaland/or pre-alloyed powders of Ir may alsooffer advantages, as it does for other refractorymetals (80).

ConclusionsDuring the past twenty years improvements

have been made in the processing of Ir and itsalloys and also in the fundamental understandingof some processing methods. These advances havesupported the use of Ir and its alloys in applica-tions such as rocket combustion chambers, fuelcontainers for nuclear power in space, radiationsources for medical treatments, engine ignitiondevices and crucibles for the growth of electronicand photonic materials. Research on new methodsfor Ir processing, including novel powder metallur-gy and metal deposition techniques, may facilitatefuture applications such as the production of Iralloys as high-temperature structural materials.

AcknowledgementsThe author acknowledges the assistance of

George B. Ulrich and Stan A. David, both of theOak Ridge National Laboratory, in providing someof the illustrations and in reviewing the manu-script. This work was sponsored by the Office ofRadioisotope Power Systems (NE-34) of theUnited States Department of Energy and per-formed at the Oak Ridge National Laboratory,managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725.

Platinum Metals Rev., 2008, 52, (3) 194

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The AuthorDr Evan K. Ohriner is a Distinguished Development Staffmember in the Materials Science and Technology Division ofthe Oak Ridge National Laboratory, U.S.A. His main interestis in processing of refractory metals and alloys. In 2005 DrOhriner was honoured by ASM International as an ASMFellow ‘for the development of iridium alloys and high-temperature and wear-resistant materials used in spaceexploration and energy generation and transmission’.