Refractory metal alloys oxidation-resistant high temperature

Two methods for the evaporation of precursors may be employed - resistance heating and electron beam collision. The first method employs a simple alumina crucible that is heated by a W filament. Temperatures as high as 1,800°C may be reached inside the chamber, which is enough for some metals or metal salts to vaporize. Deposition rates for this method are 1-20 A s . The use of an electron beam to assist in the precursor evaporation results in temperatures on the order of 3,000°C, being more suited for the deposition of refractory metals/alloys and metal oxides such as alumina, titania, and zirconia. Since the temperature of the chamber interior is much higher than the walls, the gas-phase ions/atoms/molecules condense on the sidewalls as well as the substrate this may lead to film contamination as the nonselective coating flakes off the chamber walls. 

It is instructive to note that Stringer (1973) expressed pessimism about the development of oxidation-resistant refractory metal alloys (Nb, Ta, Mo, W) and that Meier (1996) opines that attempts to protect Nb-base compounds by the selective oxidation of Al or Si are fruitless . Although we are not quite that pessimistic, it is clear that successful development of oxidation-resistant Nb-Al intermetallics for high- temperature structural applications will certainly require a truly radical, new approach. 

New prospects for the development of coating materials resistant to high-temperature sulfide corrosion have been created by combined alloying of common metals with molybdenum and aluminum and in particular by the development of novel nanocrystalline aluminum based refractory metal alloys containing silicon which are highly resistant to both sulfidizing and oxidizing environments. 

To identify suitable materials for an acid complex such as Hl, one can begin by surveying materials applicable to the individual acid/chemical. Table 4.5 to table 4.7 list the corrosion properties of various materials in I2, HI acid, and H2SO4.12 is a strong oxidizer, especially in liquid form at high temperature. The corrosion rates of a number of corrosion-resistant materials in I2 at 300 and 450°C are listed in table 4.5. Even though the data show that gold and platinum are stable in an I2 environment, they have been found to dissolve in HI. Refractory metals such as Ta and Nb alloys are probably the best candidates within the l2-rich environment in Section 1. 

---