Discussion
Abstract Generated abstract
This discussion examines quantitative and theoretical approaches to metal, oxide, and ceramic bonding, especially diffusion processes and transition layer formation. Participants note calculations using Frenkel’s and Hines’s methods for heavy oxide diffusion into ceramics and metallic powder sintering on ceramics, and relate bonding quality to powder dispersity, grain shape, thermal expansion, oxide type, and possible acid, base interactions. The exchange also considers solid phase reactions and valence bonding explanations, while cautioning that the proposed theory requires broader experimental testing with additional metals, ceramics, and methods such as thermographic analysis.
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Discussion
G. I. Skanavi.—What quantitative calculations have been made?
V. A. Presnov.—Quantitatively, by Frenkel’s method and by Hines’s method, the diffusion of heavy oxides into ceramics and the sintering of metallic powders on ceramics have been calculated.
G. I. Skanavi.—What conclusions does the calculation make it possible to draw?
V. A. Presnov.—For example, the following: the fine dispersity of the metallic powder used, and the nonspherical shape of its grains. Diffusion improves with an increase in the coefficient of linear expansion of the ceramics, etc.
Ivanov.—How does diffusion theory explain the diffusion of a metal into ceramics and the obtaining of a bond?
V. A. Presnov.—If the bond is produced as a result of acid–base interaction, then one can specify the conditions of bonding and, in particular, indicate the type of oxide on the metal that gives the best bond. The kinetics of the process of formation of the transition layer has a diffusion character. Diffusion is possible both of cations and of whole groups.
R. L. Müller.—1. Reactions in solid phases do occur, although not in all the cases assumed by Hedvall and Tammann. The complete denial of solid-phase reactions by Ginstling and Budnikov is erroneous. This has already been stated this year in Leningrad at one discussion in the Technological Institute. 2. From the standpoint of the atomic-valence conception that I am developing, a strong bond is due to the emergence of stable valence bonds between the metal and the ceramic. The oxide \(MoO_3\), having closed valence-electron shells, cannot simultaneously provide a valence bond with the ceramic and a metallic bond with the metal. In this respect lower oxides of metals are more suitable, and moreover metals of the secondary subgroups of the elements of the periodic system. In general, the phenomenon of bonding should resemble the wetting of coal by water, for which the introduction of polar molecules of the fatty series of organic compounds is required.
I. G. Skanavi.—The available experimental facts seem to speak in favor of the proposed assumption concerning acid–base interaction. However, it is still too early to speak of a theory; it is necessary to set up the experiments more broadly, to test the proposed proposition on other metals and ceramics. In addition to those listed, it is necessary to employ other methods of investigation as well, in particular thermographic analysis. Its application is especially convenient in studying processes that occur during the firing of ceramics. In the case of studying the bond, the application of thermographic analysis is more difficult.