Phase-resolved in-situ insight into vacuum breakdown in CuCr contacts

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 to directly investigate the intrinsic breakdown behavior at the individual constituent phase scale on CuCr contacts. The Cu–Cr phase boundary was identified as a preferential weak point for vacuum breakdown initiation, which is attributed to the accumulation of impurities and defects at the interface that enhances 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 μ m, 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 guidance for the optimization of CuCr contact materials.