Troisiéme Conférence Européenne sur les Matériaux et les Procédés Avancés
J. Phys. IV France 03 (1993) C7-509-C7-518
Precious-metal-base advanced materialsT. NOWICKI and C. CARBONNAUX
CLAL, Centre de Recherches, 8 rue Portefoin, 75003 Paris, France
The precious metals are basically known for their applications in jewelry, 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 eletrolytical corrosion in molten glasses at temperatures as high as 1200-1500°C. The major drawback is a weak creep resistance. In the case of all conventional platinum alloys, the 100 h creep resistance at 1100°C is smaller than 10 MPa while the stress to cause rupture in 1000 h at 1100°C for superalloys is about 100 MPa. The unique way for significant improvement of platinum base materials acep resistance is a strengthening by an oxide dispersion (ODs). In the cas of CLAL's patented "Plativer" materials, 0.05 wt% of Y2O3 is incorporated within the alloy matrix by the name 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 exihibit 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 fad silver matrix composites containing varying amounts of well-dispersed particles of constituents such as CdO, SnO2, 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 specfic 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 CdO by inert SnO2. Electrical contacts are manufactured by various powder metallurgy techniques, including coprecipitation from solution, internal oxidation, 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 fascinating precious metals base materials exist : brazing alloys for assembling metals, superconductors and ceramics ; dental materials including magnetic biocompatible alloys ; silver composites for superconductor wire jackets. One can ask the fundamental question if the development of materials science will induce the replacement of precious metals by cheaper materials. The observation of current evolution indicate very clearly the opposed tendency : precious metals cannot be replaced by common metals because of their unique characteristics due to their atomic level properties. The relatively high cost of raw materials authorizes the use of advanced metallurgical techniques, the conversion cost being only a small portion of the total material value.
© EDP Sciences 1993