Thermal cycle stability of glass-to-metal seals with glass preforms produced via powder-metallurgy and casting-machining methods

Abstract

In order to evaluate the influence of preform preparation processes on thermal cycle stability of glass-to-metal seals, this work embraced two different methods to produce the preform for seals. For the conventional powder metallurgy (PM) method, the molten glass was quenched to form frits, then the frits were ball milled to prepare glass powders. These glass powders were pressed into green bodies and heated to prepare preforms. While for the casting-machining (CM) method, the molten glass was cast into a graphite mold and annealed before accurate machining to preforms. In contrast to the PM method, the CM method provided an ultralow-porosity preform structure and a low porosity glass seal region. Field emission scanning electron microscope (FE-SEM) was conducted to investigate the bubbles and cracks in glass region. Furthermore, thermal cycling tests confirmed that these two tremendously different glass regions strongly affected the thermal cycle stability of the seals. To support the understanding of cracking in seals, the damage features of the samples were observed by FE-SEM and the extended finite element method (XFEM) was used to simulate the crack initiation and propagation. The experimental results demonstrated that cracking in the seals made from CM preforms occurred in the glass region near the sealing interface. However, cracks initiated from the bubbles in the seals made from PM preforms, which was verified by the XFEM simulation results. In addition, the CM seals demonstrated little degradation of the leakage rate until 105 thermal cycles, while cracking was found in the PM seals after 70 thermal cycles, indicating a decreased thermal cycle stability and resulting in hermetic failure.