THERMAL-EXPANSION AND ADHESION OF CERAMIC-TO-METAL SEALINGS - CASE OF PORCELAIN-KOVAR JUNCTIONS

Glass to metal sealings necessitate a pre-oxidation of the metal, producing a thin superficial oxide layer. In the case of porcelain-kovar junctions a glassy interphase is necessary, which reacts and dissolves the oxide layer during the bonding thermal treatment. In the interfacial zone this leads to a good fitting of the thermal expansion coefficients of the phases present: in the alloy the superficial part is impoverished in iron and its thermal expansion coefficient is close to that of the FeO-rich glass which has dissolved the oxide layer. The resulting buffer zone limits the effect of strains appearing during cooling at the end of the bonding thermal treatment. Moreover, the FeO-rich glass penetrates the open porosity of the superficial alloy layer (due to its pre-oxidation) providing a good physical adherence of glass to kovar. At the porcelain-glass interface there is no problem because of the similarity of dilatometric behaviour of the two materials.