EVALUATION AND ELIMINATION OF DEFECTS IN GLASS TO METAL SEALING USED IN AEROSPACE APPLICATIONS

Abstract

Glass-to-metal seals are used in a variety of applications specifically in the field of medical, military, consumer products, aerospace and aviation etc. These seals are composed of various alloying metals and different combination of glass powders such as a low silica glass powder with stainless steel 304 or a borosilicate glass with Titanium, Molybdenum or Kovar alloy (54%Fe, 29%Ni, 17% Co). In our study, the microstructural basis of the bonding of borosilicate glass with Kovar alloy joints has been investigated where the Kovar alloy was pre-oxidized at 750°C for 10min in air. Glass beads were prepared from two different types of glass powders having almost closer true density and matching thermal coefficient of expansion (CTE). The green compact of glass beads was pre-sintered at 150°C for 60min followed by burn out of paraffin at 450°C for 60min. Pores were observed in the glass beads by SEM investigation which are beneficial in the glass-metal seals. The glass was bonded to the alloy by melting at 925 °C keeping different speeds of the moving belt in the belt furnace in a controlled N2 environment by purging N2 gas at different rates. The effects of various process parameters studied by scanning electron microscopy revealed that a belt speed of 3cm/min and purging N2 gas at 5m3/hr in the furnace resulted in a defected glass-metal seal. Keeping lower belt speed at 2.5cm/min by allowing more time to the glass-to–metal joint to form under the N2 environment at 6m3/hr were the optimum conditions for gaining a hermetic glass-metal seal. The SEM of the different specimens revealed an iron oxide (FeO) interlayer in the joint formed under these conditions. The controlled thickness of FeO layer improves the hermeticity between the glass powder and the Kovar alloy.