Brazing ZrB2 ultra-high-temperature ceramics to Zr metals for hypersonic thermal protection systems

Porous and dense zirconium diboride (ZrB₂) materials are promising ultra-high-temperature ceramics (UHTCs) for transpiration cooling in hypersonic applications, providing an effective means to reduce component temperatures and mitigate oxidation effects. While porous ZrB₂ enables coolant flow, dense ZrB₂ offers structural integrity. However, achieving a reliable ceramic-to-metal connection remains a significant challenge due to the inherent differences in their physical, chemical, and mechanical properties. This study investigates the brazing process of both porous and dense ZrB₂ UHTCs to zirconium metal (Zr). Zr was selected due to its thermal expansion properties closely matching those of ZrB₂, reducing thermal stress at the interface and enabling coolant transfer into porous ZrB₂ from the supply reservoir. The brazing temperature of 1650°C was selected based on the Zr–ZrB₂ phase diagram to ensure eutectic melting without compromising the structural integrity of the base materials, and the filler composition was selected to enhance wetting and spreading. The results demonstrate that dense ZrB₂ forms robust and continuous interfaces with Zr metal, while porous ZrB₂ presents greater challenges due to thermal expansion mismatches and paste infiltration. These findings provide valuable insights into ceramic–metal joining for aerospace high-performance thermal protection systems under extreme conditions, which would reinforce the real-world impact.