PROTECTIVE COATINGS FOR TANTALUM-BASE ALLOYS.
Final technical rept. 1 May 63-30 Apr 64,
IIT RESEARCH INST CHICAGO IL
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This program has been concerned with developing protective coating systems for tantalum-base alloys useful in an oxidizing environment at temperatures above 3400 F. The study has been confined to very refractory, thermodynamically stable materials. The most significant discovery, and that which has the most direct practical utilization, is the relative oxidation resistance of Hf-Ta alloys at very high temperatures. In the binary system, alloys in the composition range of 20-35wt- Ta appear to be most oxidation resistant. At higher temperatures under torch testing conditions the maximum tantalum alloy content which provides satisfactory oxidation resistance is 25wt-. At higher tantalum levels a scale which is partially molten forms and is rapidly eroded in the oxidizing gas stream. Ternary alloy additions to the more oxidation resistant binary compositions generally decrease oxidation resistance. Tungsten additions to Hf-25 Ta accelerate the rate of oxidation at 2500 F but have only a minor effect on oxidation rate at 3150 F. Both zirconium and titanium increase the rate of subscale formation. The oxidation behavior of iridium-base alloys containing Zr, Hf, and Y has been investigated at 3400 -3500 F. Although only preliminary oxidation studies were performed on the refractory carbides and nitrides, it appears that the thermal stress sensitivity of these materials will severely limit their usefulness in coating systems. Author