(2) Glass Adhesives.


GLASSES are in many respects ideal adhesives. They wet ceramics, allow a wide working temperature range, dissolve impurities and the extensive collection of silicate phase equilibria in the literature allows reactions to be predicted. Glasses can be applied as fine powders (in slips), by means of sputtering, glazing or as thin solid sheets. Glass joints can also be molded by vacuum or pressure
application [26].
    On the other hand, the usefulness of glasses for high T applications is limited by the relatively low glass transformation point, which for most glass compositions is lower than the temperature of creep onset in crystalline ceramics, and by low fracture toughness, Table II. There are other incentives, however, for considering glasses as adhesives, perhaps the most important being that intergranular phases in polycrystalline ceramics are often amorphous. Therefore an ideal glass dhesive should mimic the composition of the intergranular phase of the adherend. Unfortunately, this composition is rarely known, and its quantitative determination is difficult. Other glasses, not identical with it, have to be used as substitutes.
    Typically, joining is carried out at temperatures well above the T_g point of glass, with or without applied pressure, in circumstances where the glass wets and joins opposing surfaces. This is followed by relaxation annealing. The feasibility of this approach has been proved [7]. Namely, simple butt joints wih rectangular cross sections were prepared at elevated T, using various glasses. The reactions at the cearamic/glass interface were minimized by short heat treatment times. The mechanical behavior of such joints was then evaluated in terms of stress disrtibutions, fracture toughness and strength.
    Thermal expansion mismatch between the adhesive and adherends was one of the variables to be controlled. The Young's moduli of the adherends (alumina) and glass adhesives were 350 and 65 GPa respectively. The coefficient of thermal expansion was 7.2 x 10^-6 ºC^-1 for the ceramic and from 3.5 to 11.1 x 10^-6 ºC^-1 for the seven glass compositions used [7]. The concept of the model is generic, and is used here to illustrate the imoortant influence of residual stresses on the mechanical properties of the joints.
    The distribution of residual stresses calculated by finite element analysis is shown in Fig.8. Stress profiles indicate the presence of stress concentrations at free surfaces and at the adhesive/adherend interfaces. The shear stresses at the interface are the principal means for the stress transfer induced by thermal strain. The sign of the shear stress and of the axial stress s_x, depends on the sign of the difference in the thermal expansion (Ća) and elastic mismatch (ĆE) between the adhesive and adherend: