High-Temperature Crystal-Growth Cartridge Tubes Made by VPS
- Wednesday, 22 October 2008
Mechanical properties and maximum useful temperature exceed those of tungsten-alloy tubes.
Cartridge tubes for use in a crystal-growth furnace at temperatures as high as 1,600°C have been fabricated by vacuum plasma spraying (VPS). These cartridges consist mainly of an alloy of 60 weight percent molybdenum with 40 weight percent rhenium, made from molybdenum powder coated with rhenium. This alloy was selected because of its high melting temperature (≈2,550°C) and because of its excellent ductility at room temperature. These cartridges are intended to supplant tungsten/nickel-alloy cartridges, which cannot be used at temperatures above ≈1,300°C.
Graphite mandrels were used as substrates for VPS to form the cartridge tubes to the desired size and shape. A mandrel was placed in the VPS chamber, oriented vertically. Before spraying, the plasma gun was used to heat the mandrel to a temperature of about 1,093°C. Then, the Mo/Re alloy precursor powder was deposited by VPS on the mandrel to a thickness between 0.51 and 0.64 mm. The deposition was done in one pass, spraying from the top to the bottom of the mandrel.
Then a tantalum coat was deposited in a similar manner onto the Mo/Re deposit to a thickness between 0.13 and 0.18 mm. The tantalum coat serves as a sealing layer, increasing the protection of the Mo/Re alloy against the formation of such volatile oxides as MoO3.
Next, the pressure in the chamber was reduced to <100 mtorr (less than about 13 Pa) and the cartridge allowed to cool. Once the cartridge had cooled to room temperature, the chamber was opened to the atmosphere and the cartridge was removed from the mandrel.
A cross section of a representative cartridge tube fabricated in this process showed a good bond between the tantalum coat and the main body of Mo/Re alloy. Both the Mo/Re and the Ta were dense. Because this tube was not heat treated, the Mo/Re-alloy layer still contained two phases — one Mo-rich and one Re-rich. Tests of the mechanical properties of tubes like this one in the as-sprayed condition have revealed a vast improvement over similar tungsten-alloy tubes in the as-sprayed condition.
This work was done by Richard Holmes of Marshall Space Flight Center and Scott O’Dell, Timothy McKechnie, and Christopher Power of Plasma Processes, Inc. For further information, contact Sammy Nabors, MSFC Commercialization Assistance Lead, at sammy.a.nabors@ nasa.gov. MFS-31540-1