MATERIALS FORUM VOLUME 37 – 2013 Edited by P. Howard, P. Huggett and D. Evans © Institute of Materials Engineering Australasia Ltd COATINGS OF COLOURED INTERMETALLIC COMPOUNDS FOR DECORATIVE AND TECHNOLOGICAL APPLICATIONS S. Supansomboon 1 , A. Dowd 1 , E. van der Lingen 2 , V. J. Keast 3 and M.B. Cortie 1 1 Institute for Nanoscale Technology, University of Technology Sydney PO Box 123, Broadway, NSW 2007, Australia 2 Graduate School of Technology Management, University of Pretoria Lynnwood Road, Hatfield, Pretoria, South Africa 3 School of Mathematical and Physical Sciences, University of Newcastle, Callaghan NSW 2308, Australia ABSTRACT The coloured intermetallic compounds AuAl 2 and PtAl 2 are attractive and unique, with AuAl 2 being deep purple and PtAl 2 golden yellow. These phases are formed in a very narrow range within the Al-Au or Al-Pt phase diagrams and, for this reason, it is challenging to manufacture pure samples of them. Here we show that physical vapour deposition (PVD) can be used to form pure, thin film coatings by co-sputtering of the elements. The chemical compositions, X- ray diffraction patterns, microstructures and reflectance spectra of the nanoscale films are compared to those of bulk samples that had been produced by vacuum arc furnace melting. A range of colours can be obtained if the thickness of the films is varied. Simulated and measured reflectance and transmittance data of coloured nanoscale films are compared and the CIE chromaticity indices (a* and b*) of films of various thicknesses determined. 1. INTRODUCTION The colour of gold-based alloys is generally varied by making alloying additions of other elements. For example, the system Au-Ag-Cu is well-known for the various colours that it can produce, with compositions capable of providing traditional yellow gold as well as white, red or rose gold 1, 2 . However, an alternative method to produce colour in metallic systems is by combining two or more elements to form an intermetallic compound. There are tens of thousands of silver-coloured intermetallic compounds, but only a few coloured ones. Binary intermetallic compounds with the cF12 crystal structure or CaF 2 crystal structure are the best known of these (Figure 1). Their interesting colours are the result of their particular electronic structures, which are quite different from those of their constituent elements. For example AuAl 2 is purple 3, 4 while PtAl 2 is yellow 5 . Apart from these fascinating colours, their properties and technological applications are also interesting. In addition to their applications in jewellery, PtAl 2 can be used as a corrosion or oxidation resistant coating, particularly for high-temperature turbine blades 6 , while AuAl 2 has potential applications as a superconductor, resistor and selective solar absorber 7-9 . It has also been predicted that nanostructures of AuAl 2 and PtAl 2 should manifest a localized surface plasmon resonance 10, 11 . There are several methods to fabricate these coloured intermetallic compounds such as traditional investment casting, powder metallurgy, and vacuum-arc melting. Both AuAl 2 and PtAl 2 can only be formed in a very narrow area in the phase diagram (Figures 2 and 3) 12 . The 33 at.% Au or Pt content corresponds to about 78 to 79 wt.% precious metal. This exceeds the requirements for 18 carat hall-marking in the case of gold but falls well below the traditional lower limit for Pt in Pt jewellery alloys. The precise compositional control possible during physical vapour deposition makes it an attractive alternative method to produce coatings of these coloured intermetallic compounds. Here the co- sputtering technique was applied to produce AuAl 2 and PtAl 2 thin films which were then characterized by a variety of techniques. Characterization was also performed on bulk samples of the intermetallic compounds prepared by vacuum arc melting, and the two types of sample compared. Fig. 1. Coloured intermetallic compound (AuAl 2 ) with CaF 2 crystal structure Au Al