Development of Metal-Ceramics
Development of Metal-Ceramics
Possibilities of the Platinum Metals in Silicide Cermets for High Temperature Service
The possibility that one of the platinum metals might meet all the exacting requirements of a metallic binder for a silicide cermet having strength, stability, and oxidation resistance at high temperatures is put forward by Dr. Alan W. Searcy, associate professor of ceramic engineering in the Division of Mineral Technology of the University of California, as a result of a theoretical survey he has just published on “Predicting the Thermodynamic Stabilities and Oxidation Resistances of Silicide Cermets” (1).
Cermets, which have been the subject of a great deal of intensive—and for the most part disappointing—research during the past ten or fifteen years, are essentially composite bodies made by incorporating particles of some refractory (usually silicide, oxide, boride or nitride) in a continuous, ductile, metallic network. The familiar cemented carbides, made by bonding particles of tungsten and other carbides with cobalt, are the prototypes of all cermets.
The most optimistic seeker after the perfect cermet hardly hopes that he will ever develop a kind of super heat-resisting alloy, capable of being hammered home in a force fit or adjusted to shape after installation; what is needed is a highly refractory body, almost certainly destined to be ground to shape, but resistant to the blows of normal service.
The number of combinations possible is obviously enormous, and in recent years many scores have been made and tested experimentally. Dr. Searcy now suggests that the area of investigation can be very appreciably narrowed down by choosing those combinations which are most likely, from thermodynamic considerations, to be (a ) resistant to oxidation and (b ) chemically stable—the refractory and the metal must not react at high temperatures. He has therefore collected the available thermodynamic data and applied it to the problems of those cermets incorporating, as a refractory, the high-melting silicides.
Among the known silicides, molybdenum disilicide is outstanding in its resistance to oxidation, and this is attributed to the formation on the surface of the particles of a closely-adherent layer of nearly pure silica glass, the MoO3 which is formed at the same time escaping by volatilisation. Very protective silica coatings also form on silicon carbide and silicon nitride by a similar reaction. In general, therefore, Dr. Searcy suggests that investigation of further silicides should be confined to those of high silica content which on oxidising form silica glass together with a volatile metal oxide. He concludes that “the only uninvestigated silicides that appear to meet this requirement are the disilicides of rhenium, ruthenium, rhodium, palladium, osmium, iridium and platinum”. The stability of none of these, however, seems likely to be better than that of molybdenum disilicide.
The bonding metal must similarly be resistant to oxidation and must not react with the silicide. Here again, the platinum metals appear to have attractive properties and are well worth further consideration. At first sight, therefore, molybdenum sulphide bonded with platinum would seem to have promise, but two American investigators, De Vincentis and Russell (2), have recently reported that reaction takes place between these materials at 1425°C. Dr. Searcy and his collaborators are now investigating the behaviour of the remaining platinum metals with molybdenum disilicide, but apparently have little hope of finding a stable combination. There seems greater hope in bonding silicon carbide or silicon nitride with one of the platinum metals, since the latter do not form stable carbides or nitrides, and Dr. Searcy writes that “cermets of platinum metals with silicon carbide or silicon nitride seem to be well worth investigating”.
Dr. Searcy concludes his review with these words: “Unfortunately the materials that seem to have the greatest promise are rare and expensive. In certain areas of critical needs, such as in high-temperature engine parts, the high costs of such materials would not bar them, however, from limited application if they prove to be really satisfactory.”
A. W. Searcy J. Amer. Ceram. Soc, 1957, 40 (12), 431 – 435
H. A. De Vincentis and W. E. Russell Nat. Advisory Comm. Aeronaut. Research Memo., 1954, E54B15