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Vacuum breakdown in CuCr contacts severely affects the operational reliability of vacuum circuit breakers. Although previous studies have mainly focused on the macroscopic breakdown characteristics and microstructure regulation of CuCr contacts, the intrinsic vacuum breakdown behavior associated with individual constituent phase (e.g., Cu phase and Cr phase) remains unclear. In this work, an in situ phase-resolved characterization and electrical testing method was developed based on a FIB-SEM dual-beam system, enabling direct investigation of the intrinsic vacuum breakdown behavior of individual Cu phases, Cr phases, and Cu-Cr phase boundaries in CuCr contacts. The Cu-Cr phase boundary was identified as a preferential weak point for vacuum breakdown initiation. First-principles calculations further showed that defect accumulation and preferential impurity segregation at the phase boundary enhance local field emission. This conclusion differs from the commonly accepted view that the Cr phase is the weak phase for breakdown, which was mainly inferred from post-breakdown erosion morphology rather than the phase-scale intrinsic breakdown characterization. In addition, within the Cr particle size range of 20-60 μm, particle refinement improved the dielectric strength of the contact gap by interrupting continuous weak phase-boundary regions and enhancing heat dissipation. These findings clarify the critical role of the Cu-Cr phase boundary in vacuum breakdown initiation and provide a new experimental approach for the optimization of CuCr contact materials.
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