New Techniques for Applying Coatings on Graphite

A reliable, easily worked process for depositing coherent, compact and pore-free layers of iridium could, when fully developed, be the means of making use as never before of the outstanding and indeed unique characteristics of iridium. Iridium has a very high melting point2443OC-aIId as a platinum group metal it does not scale or tarnish when heated in air even to incandescence. I t is true that it loses weight through formation of a volatile oxide when it is heated to above about 10oo"C in air, but it is not difficult to contain the loss within reasonable limits in most circumstances by controlling free circulation. Like the other platinum metals it does not when solid react with carbon or graphite, but unlike platinum, palladium or rhodium it is unaffected by molten tin, lead, copper or most base metals and can be heated unchanged in contact with such refractory metals as tungsten, molybdenum, tantalum, and zirconium. In the past, however, although small iridium crucibles are standard equipment, the difficulties of fabricating iridium have often restricted its wider use. Moreover, iridium cannot be electroplated from aqueous solutions, and only limited experimental success has been achieved with the very difficult process of electrodeposition from molten salts. More recently, the high melting point of iridium and its compatibility with graphite at high temperatures have made it of special interest to designers of nuclear reactors and missile shields. It has become apparent that iridium coatings would be of particular value in protecting graphite from oxidation at around 20oocC. For this application, iridium has the unexpected additional advantage that its thermal expansion is a good match with that of graphite at temperatures up to IIOOT, over the range in which the metal is relatively unyielding. It is with this background that the results of experiments recently reported in applying methods of chemical vapour deposition to the formation of iridium coatings on graphite may be viewed. The process of chemical vapour deposition is probably best known by such examples as the Mond process for making nickel shot or powder by the decomposition of nickel carbony1 and by the Van Arkel process for purifying such metals as titanium. In none of these applications, however, has much control been possible of the density and uniformity of the deposit. The last decade has seen a considerable interest in expanding and controlling methods of vapour deposition. In particular, the Battelle Memorial Institute and some large American industrial research laboratories have been active in the field and have claimed success in such divergent fields as the production of tungsten tubes and sheets from tungsten fluoride and of titanium oxide pigments from titanium tetrachloride.