An investigation into microwave bonding mechanisms via a study of silicon carbide and zirconia

It is known that the glassy grain-boundary phase present in low-purity alum inas has two primary functions during direct microwave bonding. Firstly, it increases the dielectric loss of the host ceramic, allowing heating to occ ur; secondly, the bonding mechanism itself has been found to be based on vi scous flow of the glassy grain-boundary phase. However, some evidence has a lso been found for the bonding of individual grains where they come into di rect contact across the join line. To investigate the role of grain-boundar y phases further, the microwave bonding of two different grades of silicon carbide and one grain of zirconia has been studied. A single-mode resonant cavity operating at 2450 MHz was used for both studies. The temperature and axial pressure were varied and the bonding time was kept to a minimum. Ana lysis of the resultant bonds indicated that both reaction-bonded silicon ca rbide and partially stabilized zirconia could be successfully joined using microwave energy with bonding times typically 10min or less. For reaction-b onded silicon carbide ceramics, the silicon grain-boundary phase softened a t the bonding temperature, allowing the butting faces to be "glued" togethe r. Unlike the glassy grain-boundary phase for alumina ceramics, the silicon phase did not allow grain motion but always formed a discrete and continuo us layer at the interface, even under optimum joining conditions. The work with zirconia confirmed that it is possible to join ceramics without the pr esence of a substantial grain-boundary phase. The mechanism is thought to b e either solid-state diffusion and/or grain-boundary sliding. (C) 1998 Kluw er Academic Publishers.