Microstructural Characterization of Ag-Cu-Zr Active Braze Alloy at Varying Temperatures

Emily Hopkins1,2, Brittany Muntifering1, Sandra Stangebye1, Cory Gibson1, Caitlin Taylor1, Daniel Berg1, Rajan Tandon1

  1. Sandia National Laboratories
  2. University of New Mexico

It is often necessary to join ceramic and metal parts. One of the most reliable joining methods is ceramic-to-metal brazing. Brazing is the process in which two materials are adjoined by inserting a filler metal, called braze, within the material joint. The structural pieces and braze create a “stack” which is brought to the melting point of the braze and is allowed to cool and solidify, completing the braze process.
Historically, conventional brazing has involved five complex steps in addition to the heating and cooling process. However, interest in simplifying the process led producers to develop and utilize the method of active brazing. An active braze contains a surface-active ingredient that diffuses through the braze and spontaneously reacts with the oxygen atoms in the structural pieces at high temperatures.
This talk will focus on the benefits of active brazing over conventional brazing, its associated challenges, and the research being conducted to overcome these challenges via microstructural characterization.
This study focuses on a Ag-1Cu-2Zr active braze alloy which is used to join 94% alumina to Kovar. These joints are required to be hermetic, have adequate strength, and display little to no runout or underfill.
The temperature at which braze samples were fired in this study varied from 940 to 980˚C, with the goal of determining optimal brazing parameters. The reported solidus temperature for this braze alloy is between 940-950˚C while the liquidus temperature is between 960-970˚C. The experimental temperature range covers the entire solidus-liquidus range, where the lowest and highest peak temperatures studied are respectively below and above the melting temperature of the braze alloy.
The talk will focus on the characterization of the resulting microstructure of the braze joint at each peak temperature, and at different peak temperature hold times. A complex, multi-layered structure was observed at the Kovar interface at all temperatures tested, including those below melting point. In addition, a lacey Zr reaction layer at the alumina interface was observed at all temperatures. The composition and thickness of the reaction layers at each temperature was characterized, with the goal of tying microstructure to braze performance and defect parameters.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.