Interfacial Reactions in Glass–Ceramic‐to‐Metal Seals

Abstract

During the sealing of glass–ceramics to metals, undesirable interfacial reactions may occur between constituents of the glass and diffusing metal species, and any reaction products formed may have serious consequences on the lifetime behavior of seal components. This paper reviews the factors influencing the lifetime behavior of glass–ceramic‐to‐metal systems, with particular emphasis given to the effect that such interfacial reactions can have on the resultant and longer term properties. Steps that can be taken to enhance desirable interfacial reactions and minimize those that are undesirable are highlighted. In addition, in order to aid in the understanding of complex alloy systems, it is of interest to study both the interfacial reactions and diffusion characteristics of simplified systems without the complicating issue of multiple element diffusion. Thus, a preliminary investigation is highlighted in which a lithium zinc silicate glass nucleated with P2O5 has been sealed to high‐purity Fe, Ni, and Cr metals and the resultant diffusion into the glass of each metal monitored as a function of sealing temperature and time. The data generated have been compared with information from multicomponent alloy systems and an attempt has been made to explain the differences noted.

Recent developments in the preparation, characterization and applications of glass- and glass–ceramic-to-metal seals and coatings

“…This is in marked contrast to seals to the multi-component metal alloys, where reactions occurring in the vicinity of the interface are often far more extensive. The anomalous sealing behaviour noted for LZS seals to Cr when heat-treated in contact with copper, in which CuZn-rich precipitates are formed at the Cr/glass interface, suggests that reactions can occur between constituents of a glass and specific metal species in the presence of additional metal species, and this may explain the differences noted between sealing to pure metals and sealing to complex alloys [8,9]. The precise reason why these apparent differences occur is presently the subject of additional studies.…”

“…A study of the influence of surface cleaning treatments and how these affect seal quality has, for example, been reported by Donald et al [175]. When sealing to the pure unoxidized metals, extensive diffusion of Fe and to a lesser degree Cr into lithium zinc silicate glass has been observed, whilst Ni diffusion is less severe [8,9]. Noteworthy is the fact that there is little evidence for interfacial reactions in the case of pure Fe or Ni and only minor reaction for Cr.…”

“…Firing under air rather than Ar was found to reduce the evolution of oxygen by favouring reaction (8) over reaction (9); however, the seals were still found to be mechanically weak due to the continued presence of the brittle Ti 5 Si 3 phase. Firing at a lower temperature of 900°C did not produce a good seal due to rapid crystallization of the glass powder which inhibited sintering and prevented adequate wetting of the Ti surface.…”

“…In a recent investigation in this area [8,9], seals have been prepared between a LZS glass and high-purity Fe, Ni 46:1975-2000 1995 and Cr metals, and the diffusion of these metals into the glass as a function of temperature and time and their effect on the crystallization behaviour monitored. Line scans and spot analyses were performed by EDS, from which the maximum depth of diffusion of each metallic element was estimated.…”

“…This requires that not only the thermal expansion characteristics of the metal and bulk glass-ceramic are closely matched, but also that interfacial reactions are kept to a minimum and do not lead to the formation of reaction products and precipitated phases with thermal expansion or elastic characteristics that differ markedly from those of the bulk phases, the formation of which may lead to unacceptably high residual stresses within the interfacial reaction zone. In addition, reaction between diffusing metallic species and glass constituents, including the nucleating agents employed for specific glass-ceramic systems, must be avoided [1,3,4,8,9]. One example of an undesirable reaction is that which may occur between Fe or Cr, a constituent of many common metallic alloys, and P 2 O 5 , employed as a nucleating agent in various glass-ceramic systems, leading to the formation of iron and/or chromium phosphide precipitates within the interfacial region [1].…”