The Hardening of Platinum Alloys for Potential Jewellery Application

60) to be used for fabricating jewellery, so alloying additions are made to increase the hardness. Platinum jewellery alloys usually have platinum contents of 90 wt.% and higher. The most common alloys are hallmarked as 950 platinum (95 wt.%). Unlike carat gold (Au) jewellery alloys, where relatively large additions can be made (to alter the properties of the alloy such as its hardness or colour): for instance 18 ct gold contains 25 wt.% of alloying additions, the 950 platinum alloy hallmarking, only allows alloying additions of up to 5 wt.% (to alter properties, such as increase its hardness). Several platinum jewellery alloys are available, and usage depends on national preference and hallmarking regulations. Typical alloying elements include copper (Cu), palladium (Pd), cobalt (Co), gallium (Ga) and indium (In). Cu is often added and creates a general-purpose alloy which casts well and is easy to work. Adding Co results in a very good casting alloy, while additions of Ga or In produce alloys with good springiness. Other popular alloying additions are iridium (Ir) and ruthenium (Ru). Pd can be added to platinum, but while this alloy has a good surface finish, softness limits its use. Examples of hardening platinum by alloying additions are shown in Figure 1 (1, 2). There has been much work on hardening platinum by alloying and this is demonstrated in several patents. Citizen Watch Co. holds a patent for an alloy of 85–95 wt.% Pt, 1.5–6.5 wt.% Si with the balance being one or more of Pd, Cu, Ir, Rh, Au, Ag, Ni and Co (3). This company also patented a Pt-Fe-Cu-Pd alloy (85–90 wt.% Pt, 2.5–3.5 wt.% Fe, 7.5–12.5 wt.% Cu and 0–4 wt.% Pd) (4). A patent on hard Pt alloys for jewellery application states the hard, high-purity Pt alloy contains 10–100 ppm Ce with a minimum Pt content of 99 wt.% (5). Another patent is concerned with maintaining the high purity of platinum while increasing its hardness by minor additions (0.01 to 1 wt.%) of titanium or a rare earth metal. No age hardening was reported (6). Hard, but still workable, platinum alloys have been reported with good abrasion resistance. These were achieved by modifying a surface layer to induce hardening (7, 8). An intermetallic layer of platinum, containing especially aluminium and chromium, developed on the surface. One patent