VERY HIGH TEMPERATURE COATINGS FOR TANTALUM ALLOYS

Candidate coating alloys were subjected to plasma-arc oxidation at 3860 F in a simulated air environment and were found to have the following order of increasing surface recession rates: Ir-30 Rh, Ir-30 vol% ThO2-5vol% Ta2O5, Ir-10 vol% HfO2, Ir-10 vol% SiO2, Ir-10 vol% Al2O3, Ir-10 vol% ThO2, Unalloyed irridium, Ir-150s, Ir-10 vol% MgO, and Hf-25TA. Heating Ir-W, Ir-Re, Ir-30Rh/W, and Ir-30Rh/Re diffusion couples for one hour at 3600 or 4000 F revealed that 3 mils of either W or Re would be adequate to keep Ir and Ta from coming into contract for times up to 1 hour at 4000 F. Plating Ir onto W from fused NaCN-KCN baths, into which Ir had been added by either passing an alternating current between Ir electrodes or by direct addition of K2IrCl6, was found to deposition from fused ZnCl2 or KcL-MgCl2 baths and also from aqueous baths. Two methods were studied to plate Ir-Rh alloy onto W. The first method involved an alternate layer deposition of Ir and Rh, followed by an homogenization anneal. The second method corresponded to the codeposition of Ir and Rh by the direct addition of the alloys to the fused NaCn-KCN bath. Pinholes or cracks in the Ir or Ir-Rh alloy coatings caused premature failure during oxidation testing. Unsuccessful attempts were made at using gold as a fugitive solvent in sealing the pinholes or cracks. HfO2 was applied to Ir for emittance enhancement purposes.