Surface insulation degradation of alumina ceramics induced by metal deposition in vacuum

Abstract With the increasing application of electric propulsion on in deep space exploration, the long-term reliability of ion thruster grid assemblies has become a critical factor influencing overall system lifetime. During operation, charge-exchange ions continuously sputter the grids, causing metal species such as molybdenum to be sputtered and progressively deposited onto adjacent ceramic insulation surfaces. This accumulation is essentially irreversible and cannot be removed spontaneously, leading to a sustained increase in the likelihood of vacuum flashover. In this study, controlled deposition experiments were performed to reveal how the duration of metal deposition alters the surface composition of alumina ceramics and drives the degradation of their vacuum surface insulation. Scanning electron microscopy was employed to observe the surface morphological evolution, x-ray photoelectron spectroscopy was used to examine the changes in surface chemical composition, and key electrical parameters including surface resistivity, trap characteristics, and vacuum flashover voltage were measured. The results show that with increasing deposition time, the surface resistivity of the ceramics decreases by up to six orders of magnitude, the trap density is markedly reduced, and the flashover voltage continues to degrade, with the initial flashover voltage decreasing by as much as 67.9%. The deposited metal forms conductive pathways or metallic island structures on the ceramic surface, enhancing local electric field distortion and strengthening field electron emission, which significantly promotes the early appearance of flashover. Based on these findings, a mechanism-based interpretation of flashover behavior on metal-contaminated ceramic surfaces is proposed on the basis of the secondary electron emission avalanche, providing insight into the physical processes underlying insulation degradation induced by metal deposition. This work offers both experimental evidence and theoretical support for understanding metal-contamination driven insulation failure in ion thruster grid assemblies.