Experimental investigations on time, temperature and atmosphere influence on microstructure and mechanical properties of borosilicate glass to Kovar-alloy seals

Abstract

Glass-metal seals have widespread use in chemical industries and electronic engineering. The sealing of Kovar alloy (Fe-Ni-Co) to borosilicate glass was performed by the use of an intermediate oxide layer which matched well with the two materials at both of its sides. The oxide layer was obtained by pre-oxidation of Kovar alloy in a humid nitrogen atmosphere at 850 °C and 20 min and was mainly composed of Fe3O4. The sealing of the glass to the pre-oxidized metal was then carried out considering temperature, time, and atmosphere. The optimum sealing conditions consisted of a nitrogen atmosphere, a temperature of 950 °C, and a time of 28 min. Other requirements were comprised of an adequate wetting of the pre-oxidized alloy surface by the glass and an appropriate dissolution of the surficial metal oxide in the glass, without formation of detrimental crystal phases. These requirements were shown to depend on the values of temperature, time and atmosphere of sealing, being optimized with the values mentioned above. The microstructural evolution was studied by the SEM and EDS analyses. The results showed that at temperatures of 1000 °C and higher, the more is the sealing time, the more is the extent of nucleation and growth of the fayalite phase, excessive growth of which results in embrittlement and mechanical-property degradation. Strength studies showed that the temperature and the time of sealing have significant effects on the seal strength, the maximum of which is met with sealing at 1000 °C for 4 min. The value of shear strength thus obtained was 9.84 MPa.