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Platinum Metals Rev., 1993, 37, (4), 219

Iridium Coatings on Structural Carbon Materials

Carbon reacts rapidly with oxygen at temperatures as low as 800 K and therefore carbonaceous materials intended for high temperature structural applications have to be protected against oxidation. Silicon- or boron-based coatings can provide reasonable protection for limited times at temperatures up to about 1700 K, but above this other refractory coatings are required.

Iridium appears to be a favourable material for such coatings for the following reasons: it melts at 2713 K, it has a very low permeability to oxygen, it does not react with carbon below 2553 K and it forms an excellent barrier to carbon diffusion. The preparation of iridium coatings on carbon-based materials, their structure, and the effects of mismatch in the thermal expansions have not been widely reported. Now a group of researchers at Tohoku University, Sendai, Japan, have published the results of their investigations of the surface and growth morphology, and the structural changes of iridium coatings on carbon materials (K. Mumtaz, J. Echigoya, T. Hirai and Y. Shindo, “R.f. Magnetron Sputtered Iridium Coatings on Carbon Structural Materials”, Mater. Sci. Eng., 1993, A167, (1–2), 187–195).

Iridium was deposited by radio frequency magnetron sputtering onto isotropic graphites and carbon-carbon composite substrates held either at room temperature or 1073 K, and the coatings were examined in the as-deposited state and after heat treatment. Regardless of substrate temperature, columnar grained homogeneous coatings were obtained, with those deposited at room temperature having the finest grain size. However, coatings deposited onto substrates held at 1073 Kwere denser and less strained. After heat treatment in nitrogen at 1973 K or 2173 K iridium coatings which had been deposited at 1073 K onto isotropic graphite having a coefficient of thermal expansion of approximately 7.6 × 10-6/K were crack-free. Furthermore there was no outward diffusion of carbon, no dislocations were observed and porosity was either reduced or eliminated from the equiaxial grain structure.

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