Analysis of the Reaction Layer Formed during Sapphire–Sapphire Brazing Using a Ag–Cu–Ti Filler Metal for Gas-Pressure Sensors

The present research paper introduces a reliable and economical method for direct brazing using an active Ag–Cu–Ti filler metal that can ensure effective brazing. The sapphire–sapphire brazing process has been conducted at temperatures in the range of 830–900 °C for durations in the range of 15–30 min in a high-vacuum (10–4 Pa) furnace. Material and microstructural characterization involved the use of scanning electron microscope-energy-dispersive system (SEM-EDS) and transmission electron microscope (TEM) for investigating the bonding joint morphologies. Two uniform reaction layers were observed to have formed at the sapphire/brazed area interface. On the sapphire side, the first reaction layer, which was uniform and had a thickness in the range of 0.13–0.17 μm, was identified as a TiO phase, whereas the second layer was identified as a Cu3Ti3O phase with a thickness in the range of 1.72–2.43 μm. The uniformity of the Cu3Ti3O phase was found to decrease with increasing brazing temperature and time. Therefore, in this research, a lower brazing temperature and time (830 °C/15 min) were chosen so as to avoid bonding degradation.