Precious-metal-base advanced materials

Abstract

The precious metals are basically known for their applications in jewely, coins, bullions and catalysis. The reality is thas precious metals constitute also the base of several advanced materials used in the industry in hundreds of metric tons. Since forties, platinum alloys have been used as structural materials for equipments in the glass industry. The essential reason for this is the excellent resistance of platinum alloys to oxidation and electrolytical corrosion in molten glasses at temperatures as high as 1200-1500 o C. The major drawback is a weak creep resistance. In the case of all conventional platinum alloys, the 100 h creep resistance at 1100 o C is smaller than 10 MPa while the stress to cause rupture in 1000 h at 1100 o C for superalloys is about 100 MPa. The unique way for significant improvement of platinum base materials creep resistance is a strenghtening by an oxide dispersion (ODS). In the cas of CLAL's patented Plativer materials, 0.05 wt% of Y 2 O 3 is incorporated within the alloy matrix by the flame spraying process. The creep behaviour of such ODS materials shows a 100 times creep rate reduction comparing to conventional platinum alloys. Thanks to the fact that the mean size of oxide particles is included between 2000 and 4000 A, the obtained materials don't exhibit any loss of plasticity in comparison of conventional Pt alloys. Further improvement of platinum base materials is related, in the authors opinion, to the development of precious metals base intermetallics. Anothe interesting applications of precious metals are silver base electrical contacts. They are in fact silver matrix composites containing varying amounts of well-dispersed particles of constituents such as Cd0, Sn0 2 , Ni, WC or C. In the case of such materials, particular properties are required and tested: resistance to arc erosion, resistance to welding and contact resistance. Choice of the second phase depends on the specific service conditions. During last 10 years, an interesting and successful battlle took place in order to replace, without alteration of the properties of conventional materials, toxic Cd0 by inert Sn0 2 . Electrical contacts are manufactured by various powder metallurgy techniques, including coprecipitation from solution, internal oxidation of atomized powders, unitary pressing and hot extrusion. A supplementary difficulty comes from the fact that in order to enable brazing or welding of contacts to supports, the parts had to be manufactured as sandwiches: silver composite/pure silver. Many other technically facinating precious metals base materials exists: brazing alloys for assembling metals, superconductors and ceramics; dental materials including magnetic biocompatible alloys; silver composites for superconductor wire jackets