reaction glass metal

Received June 20, 1985; accepted August 22, 1985.
Adherence, or bonding, of glass to metal is extremely important in producing dental restorations, glass/metal seals, and protective coatings on metals. For maximum strength of an assembly, strong or chemical bonding is required between the dissimilar phases. Although completely satisfactory bonding tests providing quantitative values have not yet been developed, it is possible to determine whether satisfactory "good or strong" bonding exists at an interface by distorting the metal substrate and observing whether fracture occurs in the ceramic or at the interface. It is possible to realize satisfactory mechanical bonding when liquid glass wets to form an intimate interface with an irregular or "rough" metal surface, and the device is geometrically-shaped so that the glass at the interface is in compression. Maximum adherence or strength of bonding is realized by the additional existence of a chemical bond which is represented by the presence of continuous electronic structures across the interface. An additional advantage of chemical bonding is a greater resistance to penetration by fluids along the interface. Chemical bonding across the interface is achieved by having thermodynamically stable equilibrium in the interfacial zone and having both phases saturated with an oxide of the substrate metal, even though the whole glass/ metal system may not be in stable equi-
Chemical reactions and adherence at glass/metal interfaces: an analysis
Tomsia AP, Pask JA. Chemical reactions and adherence at glass/metal interfaces: an analysis. Dent Mater 1986: 2: 10-16. Abstract - A review of wetting, spreading and reactions of sodium discilicate glass on metals is presented. The selected metals represent a wide range of reactivity. Most of them are either principal constituents of dental alloys or are present as alloying additions. Conditions for the occurrence of the different types of redox reactions at glass/metal interfaces are analyzed. All of the reactions have an effect in some way, both chemically and physically, on the bonding of glass ting, within the normal heating time, especially if the solubility of the oxide in the glass is high. This problem does not exist at the oxide/ glass and metal/metal interfaces as solution reactions in the absence of redox reactions occur more rapidly than the diffusion of the dissolved oxides from the interface into the adjoining phases. The necessary saturation is, thus, maintained in the interfacial zone. The ideal endpoint, in most cases, would correspond to a remaining monomolecular layer of the oxide. Understanding the nature of the glass/metal interface and the conditions under which the redox reactions occur is thus necessary. It can be seen that reactions involving multi-oxide glasses and two or more component alloys would be a great deal more complex. This summary, therefore, will concern itself with a relatively simple glass and with "pure" single element metals to which alloying elements were not intentionally added. Studies on complex alloys are in progress. Furthermore, this summary will not concern itself with the physical problems of making a glass/metal seal, e.g., strains introduced because of differences in thermal expansion characteristics.
librium (1-2). No difficulties exist in realizing this type of equilibrium continuity across metal/metal or ceramic/glass interfaces because of similar types of electronic structures in each couple. There are problems, however, with metal/glass or more generally metal/ non-metal interfaces because of differences in electronic structures. When dealing with oxide ceramics electronic continuity across a glass/metal interface can be realized if equilibrium relative to the low valent oxide of the substrate metal exists in both phases at the interface. In addition, the interfacial zone must be saturated with the metal oxide; the thermodynamic activity of the oxide in the metal and the glass is then basically equal to one. Atomistically, this condition corresponds to the presence of a metal oxide layer, ideally a monomolecular layer, at the interface which is compatible and bonds with both phases. Generally, this condition is achieved most easily by controlled solution of the oxide layer formed on a metal substrate by preoxidation. The problems arise when this condition is to be achieved or retained by a redox (reduction-oxidation) reaction wherein substrate metal atoms are oxidezed and some cations in the glass are reduced(I). The reason is that the redox reaction is generally slower than the oxide diffusion processes in the glass. Consequently, the glass in the interfacial zone is not readily saturated with the substrate oxide, a requirement