Platinum Metals Rev., 1982, 26, (3), 98
Dispersion Strengthened Gold-Platinum
A New Material for Glass Handling Equipment
Platinum alloys containing gold exhibit marked non-wetting characteristics with molten glasses. Simple gold-palladium alloys, however, are relatively weak while those that have rhodium added to increase their strength can be difficult to work, and also suffer from embrittlement during service. Now the binary alloys can be dispersion strengthened with zirconia to produce a material with the ductility of the simple binary and the higher hot strengths of rhodium-containing ternary alloys. This dispersion strengthened gold-platinum alloy has enhanced resistance to progressive contamination and offers a longer service life. It is being used successfully for specialised applications in the glass industry, for which it has potential, as well as for the apparatus in which samples for X-ray fluorescence analysis are prepared.
The marked effect that gold has upon the contact angle between a platinum surface and molten glass is well documented. In particular, Selman, Spender and Darling carried out considerable work on this subject using soda glass and ‘E’ glass—a glass commonly used for the production of reinforcement fibre (1,2,3). The remarkable increase in contact angle that the addition of gold produces, particularly at levels of 3 per cent and above, is shown in Figure 1. Also of importance is the elimination of the contact angle “trough” that occurs with both platinum and rhodium-platinum alloys between about 900 and 1250°C, see Figure 2.
This phenomenon is not restricted to conventional glasses but is also seen with other glassy compounds such as the borate fluxes commonly used in the preparation of samples for X-ray fluorescence analysis (XRF) (4). As a result, gold-platinum and gold-rhodium-platinum apparatus is widely used in analytical and glass research laboratories.
The most common binary composition used is 5 per cent gold-platinum. Higher gold contents produce little further increase in contact angle, as can be seen in Figure 3, but can bring considerable problems with inhomogeneity due to segregation resulting from the considerable difference in solidus and liquidus temperatures in the gold-platinum system, as shown in Figure 4. For many applications the simple binary alloy has insufficient hot strength to give an acceptable service life and a ternary alloy of platinum, gold and rhodium provides increased strength and is commonly used in XRF analysis applications (4,5,6). The exact choice of alloy is, however, something of a compromise. An alloy of 5 gold, 10 rhodium and 85 per cent platinum provides good wetting resistance and strength, see Figures 5 and 6, but compared to the binary alloy is difficult to form and has inferior chemical resistance (4). Additionally, the grain boundary gold-rich precipitation noted during Selman’s work is often realised in practice, particularly during the preparation of sample beads for XRF, leading to embrittlement and subsequent cracking. Reducing the gold content to 3 per cent but maintaining the rhodium content significantly reduces the problems of corrosion and embrittlement while retaining a useful strength; a relevant U.S. Patent 3,672,880 identifies platinum alloys with gold and rhodium contents in the ranges 1 to 3 per cent and 12 to 25 per cent, respectively. However, the contact angle of such alloys is reduced as shown in Figure 3, and they are sensitive to any loss of gold from the surface.
All the alloys mentioned so far experience grain growth during use at typical service temperatures. This is particularly relevant to XRF analysis work in two respects, namely the surface finish of sample beads and also the ability to resist the effects of grain boundary contamination of the type described (7).
To attain a high contact angle, the binary alloy of 5 per cent gold-platinum represents the best option, and also has the benefit of being easily worked. Attempting to provide this alloy with good hot strength leads in turn to other problems outlined earlier, and no previously available option overcame the problem of grain growth. It was clear that a need existed for an alloy with the workability and wetting characteristics of 5 per cent gold-platinum but combined with the hot strength of the rhodium-containing ternary alloys; resistance to grain growth and progressive contamination resistance were also desirable. The solution lay in utilising the techniques developed by Johnson Matthey for the production of dispersion strengthened platinum and rhodium-platinum alloys (8,9). The dispersion of 0.08 to 0.1 per cent zirconia in the binary 5 per cent gold-platinum alloy, coupled with suitable thermomechanical treatment, resulted in the development of an alloy possessing the strength of the ternary alloys and the wettability and the ease of fabrication of the binary alloy. In addition, grain growth at service temperatures was dramatically reduced and the contamination resistance noted by Knapton was clearly demonstrated. This new material has been designated ZGS 5 per cent gold-platinum, and its ductility and stress rupture characteristics are given in Figures 7, 8 and 9.
The XRF Analysis Application
ZGS 5 per cent gold-platinum has been thoroughly evaluated in the market for which it was primarily developed, that is for making casting dishes used in the preparation of XRF sample beads. A typical moulding dish is illustrated in Figure 10. Service lives in the order of 4 or 5 times that of the conventional alloys have been reported (6). Additionally, and equally important, the surface finish of beads produced in casting discs made from this material is dramatically improved (see Figure 11) with a consequent improvement in analytical accuracy being reported in many cases. As a result these dishes are particularly useful for the automated methods of analysis currently gaining wide acceptance for industrial purposes.
The superiority of ZGS 5 per cent gold-platinum over conventional alloys in this application is without doubt, and the response from the market confirms this, even though the alloy is more expensive to produce than conventional alloys. As a result the demand for ternary alloys has fallen considerably and it is not difficult to envisage a situation where gold-platinum is used for low temperature applications and ZGS gold-platinum at above about 800°C, rendering the troublesome ternary alloys redundant.
Glass Fibre Production
The production of continuous glass fibre utilising platinum bushings as described by Loewenstein is a complex and highly skilled process (10). The stability of the meniscus illustrated in Figure 12 is greatly affected by the wettability of the alloy and the use of gold-containing alloys is an obvious development, either to improve the current designs or to enable jets to be more closely packed and hence increase bushing efficiency.
Of the previously available materials, the poor creep strength of the simple binary gold-platinum alloy precludes its use as a bushing baseplate material. Ternary alloys, especially 5 gold-10 rhodium-85 per cent platinum, would be difficult if not impossible to fabricate for this purpose by the existing coining and deep drawing method indicated in Figure 13. Although the wettability of 3 per cent gold-rhodium-platinum is better than that of both 10 per cent rhodium-platinum and 20 per cent rhodium-platinum the alloy has proved to be unstable in service due to the deleterious effect of any loss of gold, however small. This is clearly shown in Figure 3. Unfortunately, adding gold-platinum to a rhodium-platinum jet leads to the diffusion of gold into the baseplate during fabrication and use, so denuding the tip of the jet and reducing the contact angle.
The advent of ZGS gold-platinum has enabled “non wetting” baseplates having the strength of rhodium-platinum alloys to be produced by the existing method (11) and these have performed satisfactorily over a 12 month period. Initial trials were made with conventional designs where flooding of the jets was not necessarily a severe problem; nevertheless, the amount of glass contamination of the baseplate was dramatically reduced. Trials are now proceeding with baseplates of more radical design which exploit the improved wetting characteristics of the alloys.
This extension of the ZGS process has resulted in the development of a new material, ZGS 5 per cent gold-platinum, that reconciles the hitherto conflicting needs of XRF analysis equipment, and which has considerably improved the performance of the apparatus and the product, and thus the accuracy of the results. The availability of this unique material has resulted in considerable interest from other markets, notably the glass fibre industry which foresees its use for the development of new high performance bushings for the production of continuous fibre.
- 1 G. L. Selman,, M. R. Spender and A. S. Darling, Platinum Metals Rev., 1965, 9, (3), 92
- 2 G. L. Selman,, M. R. Spender and A. S. Darling, op. cit., 1965, 9, (4), 130
- 3 G. L. Selman,, M. R. Spender and A. S. Darling, op. cit., 1966, 10, (2), 54
- 4 H. A. Foner, Lab. Pract., Aug. 1977, 625
- 5 A. Wittmann,, J. Chmeleff and H. Herrmann, X-Ray Spectrom., 1974, (4), 137
- 6 P. I. Roberts, Proc. Thirty-Third Chemists’ Conf., British Independent Steel Producers Association, Scarborough, England, June 3–5 1980, p. 74
- 7 A. G. Knapton, op. cit., ref. 1, 1979, 23, (1), 2, 11, 12
- 8 G. L. Selman,, J. G. Day and A. A. Bourne, op. cit., ref. 1, 1974, 18, (2), 46
- 9 G. L. Selman and A. A. Bourne, op. cit., ref. 1, 1976, 20, (3), 86
- 10 K.L. Loewenstein, op. cit., ref. 1, 1975, 19, (3), 82
- 11 British Patent Appl. 2,085,028 A