SEM and ESCA study of fractured borosilicate glass to Kovar seals

Abstract

A borosilicate glass (Corning No. 7052) was bonded to an iron, nickel, cobalt alloy (Kovar ASTM F15 alloy) by firing at 900°C in flowing nitrogen, air or oxygen. The shear strength of the resulting seal was determined using a universal testing machine (Instron) and could be correlated with the oxygen partial pressure of the firing atmosphere. The oxygen-fired samples showed the strongest bonding, yielding an average failure stress of 6.3±0.4 MPa. The samples fired in air or nitrogen exhibited average failure stresses of4.5±0.3 MPa and 3.6±0.4 MPa. The fracture surfaces were examined using scanning electron microscopy (SEM) to determine fracture morphology and failure mode. Electron spectroscopy for Chemical Analysis (ESCA) was used to characterize the composition of the fracture surfaces. Surface elemental compositions determined using ESCA were consistent with the failure mechanisms suggested by SEM analysis. Additional Kovar samples were degreased, decarburized, preoxidized and subjected to the same firing schedule and atmospheres as the Kovar-glass seals. ESCA analysis of the fired alloy samples showed the surfaces to be depected in nickel and enriched in cobalt relative to the bulk composition. The extent of cobalt enrichment could be correlated with firing atmosphere.