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

Glass to metal sealings necessitate a pre-oxidation of the metal, prod ucing a thin superficial oxide layer. In the case of porcelain-kovar j unctions a glassy interphase is necessary, which reacts and dissolves the oxide layer during the bonding thermal treatment. In the interfaci al zone this leads to a good fitting of the thermal expansion coeffici ents of the phases present: in the alloy the superficial part is impov erished in iron and its thermal expansion coefficient is close to that of the FeO-rich glass which has dissolved the oxide layer. The result ing buffer zone limits the effect of strains appearing during cooling at the end of the bonding thermal treatment. Moreover, the FeO-rich gl ass penetrates the open porosity of the superficial alloy layer (due t o its pre-oxidation) providing a good physical adherence of glass to k ovar. At the porcelain-glass interface there is no problem because of the similarity of dilatometric behaviour of the two materials.