Effect of double oxide layer on metal–glass sealing

One of the main requirements of a metal–glass sealing is hermeticity along with mechanical strength of the metal–glass bond. For this purpose, the thermal expansion coefficient of the metal needs to be as close as possible to the glass expansion coefficient to limit stress development in the glass. Second, an oxide layer should be developed on the metal surface that works as an interface, for the transition from glass properties, gradually, to metal properties. For these purposes, a ferritic stainless steel stabilized with titanium has been developed. The thermal coefficient expansion of the metal was measured using a Chevenard differential dilatometer. To evaluate adhesion of the oxide film on the metal, oxide spallation after deformation has been performed. Scanning electron microscopy (SEM) was done to determine the oxide structure and X-ray photoelectron spectroscopy (XPS) was performed to analyze the superficial composition of the oxide. It is shown that chromium affects the thermal expansion coefficient of the metal. A study of the oxide film formed by a thermal treatment under wet hydrogen atmosphere is presented. During this thermal treatment a double oxide layer is built up. The outer layer, made of a spinel type MnCr2O4 with titanium oxide in the extreme surface, dissolves in the glass. The inner layer, made of a chromia scale ensures the mechanical cohesion. Moreover, it is shown that silicon contained in the steel segregates at the metal–oxide interface and decreases metal–oxide adhesion, and consequently metal–glass sealing.