Blackening Mechanism and Mechanical Properties Variation of Zirconia Ceramic Induced by Active Metal Brazing

Blackening of zirconia (ZrO₂) ceramics often occurs in active metal brazing, however, its discoloration mechanism and induced variation in mechanical properties remain unclear. Herein, ZrO₂ ceramic and Ti are successfully brazed by Ag–Cu–Pd filler in vacuum along with the blackening phenomenon. In the brazed joints, the reaction layers of TiO₂ + Ti₃(Cu, Pd)O are formed adjacent to ZrO₂, indicating that partial oxygen in ZrO₂ is taken by active Ti in the liquid filler that dissolved from the Ti substrate. Furthermore, the height of the blackened ZrO₂ ceramic near the brazing seam is in line with the amount of Ti content added to the filler, which infers that bulk‐sized black ZrO₂ can be efficiently obtained by the interfacial reaction between Ti and ZrO₂. Additionally, the black ZrO₂ is having abundant Zr³⁺ defects and more tetragonal phase content than the original ZrO₂. The formation of Zr³⁺ defects results in the blackening of ZrO₂, while the phase transformation leads to reduced hardness, improved impact toughness, and enhanced wear resistance of black ZrO₂. Thus, this blackening phenomenon of ZrO₂ ceramics provides a new way to tune the mechanical properties of ZrO₂.