Platinum Metals Rev., 1994, 38, (3), 108
Platinum Cladding for High Temperature Alloys
Materials which retain their strength at high temperatures and remain ductile are needed for use in hydrogen-fuelled engines for hypersonic vehicles, in heat exchange tubes operated at 1260°C and combustion chamber linings. Molybdenum-based alloys can provide the strength at temperatures up to 1300°C, and adding rhenium improves ductility and lowers the ductile to brittle transition temperature. Molybdenum-47 weight per cent rhenium alloy is suitable, but rapidly undergoes oxidation in these arduous conditions. Therefore if protection from oxidation could be provided by a non-reactive and impermeable barrier its service life might be increased.
Researchers from NASA Langley Research Center in Virginia, U.S.A., have used platinum to clad the molybdenum-rhenium alloy, relying on its high melting point (1790°C) and chemical inertness at high temperature. (R. K. Clark and T. A. Wallace, Scr. Metall. Mater., 1994, 30, (12), 1535–1540). Diskshaped alloy samples were foil diffusion bonded with platinum of thickness 0.0178 cm, by placing each sample in a platinum sandwich, wrapping in graphite foil and applying hot isostatic pressure for 10 hours at 1094°C. Platinum cladded and unprotected disks of molybdenum-rhenium then underwent high temperature dynamic and static testing.
Oxidation effects on the alloy and interactions between alloy and cladding were examined, and while unprotected disks had catastrophic oxidation under dynamic oxidation at 595°C, the platinum cladding gave good protection from oxidation under both static and dynamic conditions for moderate times of 12.5 hours at 1260°C. The cladding also remained fixed to the alloy during the dynamic testing.