The United States Department of the Interior, Bureau of Mines, has studied the effectiveness of dif-fusion barriers in preventing interdiffusion between a molybdenum alloy (TZM), Mo-0.5Ti-0.07Zr-0.01C, and electrodeposited platinum coatings. Studies were conducted at 1200°, 1300°, and 1400 °C for 10, 5, and 2.5 hours, respectively, in a vacuum furnace. Diffusion barriers tested were TiC, TiN, TaC, TiB2, TiB2/TiC, and TiB2/TiN. Coatings of the diffusion barriers were applied by chemical vapor deposition (CVD) with the exception of the TiB2 coatings, which were applied by electrodeposition from a molten metaborate electrolyte. The platinum coatings were applied by elec-trodeposition from a molten cyanide electrolyte. Of the barrier materials tested, TaC shows the most promise because it extended the useful lifetime of platinum coated TZM by a factor of at least 2.

Evaluation of high-Temperature diffusion barriers for the Pt-Mo system

The phase diagram of the Ni-W-O system at 1200 K was established by metallographic and X-ray identification of the phases present after equilibration at controlled oxygen potentials. The oxygen partial pressures over the samples were fixed by metered streams of CO+CO2 gas mixtures. There was only one ternary oxide, nickel tungstate (NiWO4), in the Ni-W-O system at a total pressure of 1 atm, and this compound decomposed to a mixture of Ni+WO2.72 on lowering the oxygen potential. The Gibbs' free energy of formation of NiWO4 was determined from the measurement of the e.m.f. of the solid oxide galvanic cell, Pt, Ni+NiWO4+WO2.72/CaO-ZrO2/Ni+NiO, Pt and thermodynamic properties of tungsten and nickel oxides available in the literature. For the reaction, NiO(s)+WO3(s)rarrNiWO4(s) DeltaG°=–10500–0.708 T (±250) cal mol–1.

http://repository.ias.ac.in/12952/1/365.pdf

Phase relationships in the system Ni-W-O and thermodynamic properties of NiWO,

A new approach to the development of diffusion-barrier coatings which can effectively reduce the interaction of refractory filaments such as tungsten and tantalum with a nickel-alloy matrix is presented. This consists of reacting the oxide film deliberately grown on these metals with a nickel coating prior to fabricating the composite. The nickel-oxide film reactions proceed under vacuum conditions in the temperature range 650 to 800° C with the formation of intermediate phases which produce sufficiently strong bonds for composite strengthening.

Development of Oxide Diffusion-Barrier on Refractory Metals,

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Development of Oxide Diffusion-Barrier on Refractory Metals,