Background Alumina (Al₂O₃) ceramics are extensively employed as insulating components in vacuum electronic devices. However, under high voltage, charge accumulation on their surface can easily lead to surface flashover, which severely degrades the insulation performance of the device and affects its operation. Therefore, enhancing the vacuum surface insulation performance of Al₂O₃ ceramics holds significant academic value and practical implications. Surface coating represents a widely adopted strategy for enhancing the insulation performance of Al₂O₃ ceramics. Nevertheless, the specific influence of the glass phase within the coating on the insulating properties remains largely unexplored.

Purpose The present work is dedicated to exploring how the glass phase in coatings affects the vacuum insulation performance of Al₂O₃ ceramics.

Methods A Cr₂O₃-based coating was fabricated on the surface of Al₂O₃ ceramics, and the effects of the glass phase within the coating on phase structure, surface morphology, secondary electron emission coefficient (SEE), surface resistivity, and vacuum insulation performance of the coated ceramics were systematically investigated.

Results The results indicate that Al from the substrate diffuses into the coating under high-temperature firing. The content of Cr₂O₃ phase in the coating exhibits a gradual decrease and eventually disappears with the rise of the glass phase content, causing it to fully react with the ceramic substrate to form Al_2-xCr_xO₃ (0＜x＜2)、Mg(Al_2-yCr_y)O₄ (0＜y＜2), along with small amounts of ZnAl₂O₄ and (Na,Ca)Al(Si,Al)₃O₈. The coating improves the surface grain homogeneity and the density of the ceramic surface, although variations in the glass phase content have a negligible effect on its microstructure. Additionally, the Cr₂O₃ coating reduces both the SEE coefficient and the surface resistivity of the Al₂O₃ ceramic. However, as the glass phase content in the coating increases, both the SEE coefficient and surface resistivity of the coated ceramics exhibit a gradual upward trend. The optimal insulation performance is achieved when the glass phase content reaches 20%. At this point, the vacuum surface hold-off strength attains 119.63 kV/cm.

Conclusions Modulation of the glass phase content in the surface coating enables the tunability of the vacuum surface insulation performance of the Al₂O₃ ceramics, with the performance improvement stemming from the decreased SEE coefficient and the appropriate surface resistivity.