Intermetallic Compounds of the Platinum Group SELECTED MATERIALS AND THEIR PROPERTIES

By I. R. McGill Group Research Centre, Johnson Matthey & Co Limited

towards advances in coating technology. ivany of the platinum group inter- The demand for high purity single crystals metallic compounds possess unique of mixed refractory oxides is rapidly properties which may occur in most increasing. Consequently, the choice of interesting combinations. This paper suitable non-reactive container materials be- draws attention to a number of these comes more selective. At temperatures and briejly describes some applications around zo00"C iridium and rhodium are for them. It suggests that further study obvious choices, although are more often is required before a full appreciation used under an atmosphere of nitrogen to can be made of their perhaps con- minimise precious loss. In terms of siderable potential for use in hostiIe thermodynamic stability and resistance to environments. environmental attack, a number of the plati- num group intermetallics offer exciting alter- natives to the present range of crucible The search for new high temperature materials. materials has gained increasing impetus over From an extensive survey by Paine et a1 (I) the last ten years. Greater demands are in 1960 covering 798 binary metal systems, being placed on existing materials as the only 95 were shown to either contain or environmental conditions under which they expected to contain intermetallic compounds must operate become increasingly severe. In with high temperature ranges of stability. particular the trend towards optimising Only two systems containing platinum group performance and efficiency in modern gas metals were studied with respect to high turbines requires the development of high temperature oxidation resistance, namely strength materials with structural stability beryllium-platinum and beryllium-palladium. and corrosion resistance at temperatures in As a result of the unexpected volume ex- excess of 10oo"C. With few exceptions the pansion of the intermetallics during isother- desire for higher creep rupture strength mal oxidation tests, experimental work on linked to better corrosion resistance cannot these systems was discontinued. It was be economically fulfilled by the use of any unfortunate that other precious metal inter- single material. Greater emphasis has there- metallics were not considered. The following fore been placed on the development of text gives some insight into the current protective coating systems which are com- and potential uses of intermetallics of the patible with new generation high strength precious metals and is based on theoretical materials and offer suitable corrosion pro- prediction supported by a selection of pub- tection at operating temperatures. Already lished experimental data. the platinum group metals, and in particular The extraodinary stability of intermetallic the platinum aluminides, have contributed compounds formed by combination of the

Platinum Metals Rev., 1977, 21, (3), 85-89 85 platinum group metals with elements from therefore, be based on attaining the right the left of the 3d, 4d and 5d transition series combination of transition elements such that has been satisfactorily explained by the d orbital overlap can be maximised. This Engel-Brewer approach to . condition can be fulfilled by the interaction It would be appropriate, therefore, to sum- of suitable electron-deficient and electron- marise briefly the basis and implications of rich elements. The position of the platinum the Engel-Brewer correlation before con- group metals in the 4d and 5d transition sidering a selection of published data on the series is most appropriate as their electron stability and high temperature properties of configuration and in particular d orbital a number of platinum group intermetallics. structure make them primary source electron donors. The choice of electron acceptor can A Basis For Prediction therefore be made from the transition ele- In general the high melting points of ments of Groups V to 111. intermediate phases in binary alloys of the A simple illustration of the Engel-Brewer transition elements can be explained on the concept can be made by comparing the melt- basis of availability of unpaired s, p and d ing or decomposition temperatures of the bonding electrons. Although s andp electrons compounds AIr3, where A is a transition do contribute to bonding their concentration element taken from Group I11 or IV. This and ratio have more influence on long range is shown in the Table. order and crystal symmetry. The d electrons, in transition metal alloys are, however, directly Intermetallies responsible for the degree of metallic bonding and Their Application and as such their distribution affects bonding Generally high stability compounds of the capacity. Elements to the left of and includ- platinum group metals have received little ing rhenium and technetium in the 4d and 5d attention, reference only being made to them transition series make use of all their valence as prime examples of the Engel-Brewer electrons for bonding as their electron con- approach to metallic bonding. However, their figurations are not limited by the Pauli stability and high melting or decomposition exclusion principle. Optimisation of d elec- temperatures make them attractive materials tron bonding is obtained for elements with for use in demanding environments. To the d5 configuration and is clearly reflected limit the scope of transition metal combin- by the melting points of Group VI elements, ations with the platinum group metals, only molybdenum and tungsten. The effect of those elements from Groups IVB and VB resonance coupling of unpaired d electrons will be considered with occasional reference on atomic bond strength is reflected by a to other combinations worthy of mention. lowering of melting point of the elements A general lack of ductility restricts the from ruthenium to palladium and osmium use of these materials for structural com- to platinum. Similarly, the general effective- ponents, although a number of exceptions ness of d electron bonding increases from the havc been reported in the literature. In first to third transition series. The d electrons, particular a number of isostrvctural phases therefore, ultimately dictate thermodynamic with narrow ranges of compositional stability stability and in this sense are of more con- in the systems of niobium and tantalum with cern to the metallurgist as a means of pre- iridium and rhodium are known to have dicting the degree of transition metal somewhat higher degrees of ductility than interaction and the potential existence of would normally be expected of intermetallic high temperature materials in a number of compounds. selected systems. The constitution of the tantalum-iridium The genesis of such a prediction must, system was first reported by Ferguson et a1 (2)

Platinum Metals Rev., 1977, 21, (3) 86 An Indication of the High Temperature Stability of Same Platinum Group Intermetallics Resulting from Predictions Made by the Engel-Brewer Concept I Transition Series I Group 111 I Group IV Compound Melting Point Compound Melting Point "C "C

3d Tilr, 2115 Vlr, 21 008 4d Zrlr, - Nblr, 2435 5d Hflr, 2470 Talr, 2450

decomposition temperature in 1963 from which confirmation was then improvement in oxidation resistance of the made on a number of terminal solid solutions refractory elements niobium and tantalum at and intermediate compounds. The iridium- temperatures above IOOO~Cby alloying with rich compound u-TaIr, melts congruently a variety of suitable elements has proved to at ~450°C.Three other phases y, cc, and be somewhat ineffective. Single phase ac-Nb az were also reported all of which are stable containing 5 atomic per cent iridium shows up to at least 1860°C.The phase ccl, nominal- only a marginal improvement over pure ly TaIr, decomposes peritectically at 2120°C niobium. and shows a respectable degree of ductility Although further alloying with elements and toughness. The tantalum-rhodium such as tungsten, and nickel might system (3) shows close resemblance to the well be foreseen, the interesting combination tantalum-iridium system and similar prop- of strength, ductility and high decomposition erties were recorded for a1 (TaRh), although temperatures of a number of alloys of niobium no explanation could be given. The com- and tantalum with the platinum group metals pound TaRh decomposes peritectoidally at are such as to warrant further attention. 1860°C. Ritter et a1 (4) reported the con- Of the many and varied coating methods stitution of the niobium-rhodium system in developed for high temperature corrosion 1964 and it was used to form the basis for a protection of nickel and cobalt-based alloys, comparative evaluation of the intermediate pack aluminising remains one of the most phases of the related systems tantalum- economical and convenient processes. The iridium and tantalum-rhodium. Although a mode of degradation of aluminide coatings number of these phases were reported to be is well understood and has prompted the hard but ductile, none had stability above development of superior coatings containing 1430°C. The isostructural phase ap in the precious metal diffusion barriers (5-9). There niobium-iridium system is known to have still remains some controversy as to the similar mechanical properties to the a1 mechanism by which elements such as plat- phase in the tantalum-rhodium and tantalum- inum and rhodium can effectively increase iridium systems. the durability of conventional aluminide A measurable degree of ductility and high structures. The stability and oxidation resist- stability are obviously two criteria which ance of the platinum and rhodium aluminides have some relevant importance to high tem- is, however, an important consideration. perature coating technology. However, the Figure I shows a typical platinum-aluminised

Platinum Metals Rev., 1977, 21, (3) 87 Fig. 1 A typical platinum-alurninised structure is shown in the photomicrograph. The diflusion zone consists basically of a platinum-rich surface layer, an intermediate nickel-rich Zaycr and, at the diffusion ronejsubstrate interface, a layer which has a more characteristic aluminised structure

intermetallic particles at the protective oxide / coating interface. As yet there seems to be no experimental evidence to corroborate this particular theory although work has been conducted on CoCrAlY coatings which shows that the presence of platinum can have a structure while the type of component which substantial effect upon the adherence of could benefit from this coating process is alumina scales (10, 11). shown in Figure 2. Some of the highest melting point com- Precious metals are now being considered as pounds as predicted by the Engel-Brewer beneficial additions to CoCrAlY and NiCrAlY theory are those which combine a platinum clad coatings. Yttrium, hafnium and the rare group metal, in particular platinum or iridium earths have been used for some time in these with zirconium and hafnium. Holcombe (12) protection systems and are known to create has suggested that the compound HfPt, may fine dispersions of inert oxides which improve be used for containing reactive oxides. More scale adherence by pinning mechanisms. The recent work by Ficalora et a1 (13) on both addition of platinum and/or rhodium to the HfPt, and ZrPt, showed no deterioration MCrAlY compositions where M is cobalt, when oxidised in air at rooo"C for 5 hours. nickel or iron, may well contribute to this Fine dispersions of ZrPt, and ZrRh, in mechanism by the formation of discrete rhodium-platinum alloys are thought to

Fig. 2 High temperature applications are one of the more obvious uses for the intermetallic compounds of the platinum group metals

Platinum Metals Rev., 1977, 21, (3) 88 improve significantly high temperature creep Keferences strength (14). Obviously there is scope for I R. M. Paine, A. J. Stonehouse and W. W. an improvement in the high temperature Beaver, U.S. Air Force Tech. Rpt. W.A.D.C. TR 59-29, Part I, Jan. 1960 mechanical properties of other alloy systems 2 W. H. Ferguson, B. C. Giessen and N. J. by platinum-group intermetallic dispersion Grant, Trans. AIME, 1963, 227, 1401 strengthening. In particular it would be 3 B. C. Giessen, H. Ibach and N. J. Grant, Ibid, 1964,230, 113 interesting to speculate whether small addi- 4 D. L. Ritter, B. C. Giessen and N. J. Grant, tions of zirconium or hafnium to the ductile Ibid, 1964, 230, 1250 phase compositions in the niobium-iridium 5 G. Lehnert and H. W. Meinhardt, Electrudcp. Surf. Treatment, 1972i73, I, (I), 71 (rhodium) and tantalum-iridium (rhodium) 6 Idem, Ibid, 1973, 1, (3), 189 systems would: (a) improve oxidation resis- 7 British Patent 1,350,855; 1974 tance by the formation of complex mixed 8 US. Patent 3,961,910; 1976 oxide scales of the type Zr(Hf)O,.Nb,O,, 9 U.S. Patent 3,999,956; 1976 E. J. Felten, Oxid. Met., 1976, 10, (I), 23 having some plasticity and; (b) improve the 10 11 L. Aprigliano and G. Wacker, 3rd U.S. /U.K. overall creep properties of the alloys by Conference on Gas Turbine Materials in a selective dispersion of intermetallic com- Marine Environment, 1976, 2-23 Sept., University of Bath, England pounds of the form Zr(Hf),(PGM),. Cer- I2 C. E. Holcombe, J. Less-Common Metals, tainly Barrett and Corey (15) have suggested 19769 44,331 13 P. J. Ficalora, V. Srikmishman and L. Pecora, that fine dispersions of inert platinum group Naval Ordinance Systems Command Con- metals in oxides which have very high volume tract NOOOI7-72-C-4424, 1974 ratios would form a unique basis for a pro- I4 British Patent 1,238,013; 1971 tection system. I5 C. A. Barret and J. L. Corey, Nat. Aero- nautics and Space Agency, Note D-283, 1960 Apart from exploiting these intermetallics 16 L. R. Testardi, W. A. Royer, D. D. Bacon, as high temperature materials, a number of A. R. Storm and J. H. Wernick, Trans AIME, alternative applications can be suggested 1973343 2195 I7 U.S. Patent 3,912,611; 1975 based on the prediction that low temperature ( < 4~0°C)properties are comparable with those of the compounds Mo,Ru, and W,Ru, Palladium for Electrical Contacts (16, 17). These two compounds produced as The use of electrodeposited palladium thin films have hardness values comparable instead of gold on electrical contacts appears with sapphire and are virtually inert in the economically attractive at the present time. presence of toxic etchants up to temperatures At the Annual Technical Conference of the around 550°C. Ficalora et a1 (13) reported Institute of Metal Finishing held recently similar results, although mainly qualitative, at Windermere, in a session devoted to noble metals, this was one of the topics discussed. on the chemical inertness of HfPt, in various In a paper, “High Speed Plating Solutions etchants. Equally attractive is the possible for Selective Electroplating”, F. I. Nobel application of the platinum-group intermetal- and R. T. Hill of Lea-Ronal dealt with the lics as bearing and electrical contact materials special plating solutions required to produce utilising their high melting point and cor- the desired results when plating both select- ively and continuously. For palladium plating rosion and wear resistant properties and as it has been established that proprietary general wear resistant coatings for high solutions, at present being successfully used quality cutting tools and extrusion or in barrel and rack plating operations to plate wire drawing dies. electrical contacts, can be modified for use It seems evident from the literature that in spot plating machines by increasing the metal content from about 8 to grams per the high temperature intermetallics of the 10 litre to 15 to 25 grams per litre. With the platinum group metals are still somewhat correct conditions 5 microns of sound bright novel and that their potential has yet to be palladium, as good as barrel plating, can be fully appreciated. deposited in 15 seconds.

Platinum Metals Rev., 1977, 21, (3) 89