Influence of mechanical grinding and polishing operations of brittle polycrystalline alumina on the pulsed surface flashover performance

Abstract

The degradation of the pulsed surface flashover performance of 99.9% pure polycrystalline alumina in an insulator-bridged vacuum gap, due to the mechanical grinding and finishing of the insulator is reported. The flashover characteristics of three different surface finishes—as-fired (0.8 μm), a ground surface (0.25 μm), and a polished surface (0.05 μm)—were studied using time-coordinated voltage, current, and luminosity diagnostics with a temporal resolution of ∼1.5 ns, for 0.5/15-μs pulsed voltage excitations. The flashover strengths of the ground samples are lower than the as-fired sample by about 50% while the strengths of the polished surfaces are intermediate between the two. These results contradict earlier reports which attribute the lower holdoff of smooth finishes to the enhanced secondary emission yield as compared to the rougher surface finishes. The lower flashover strengths for the ground surface as compared to the as-fired surface are attributed by us to the surface defects consisting of intergranular and transgranular fracture features induced by the mechanical grinding operation. The improved performance of the polished surface over the ground surface is proposed to be due to the partial removal of the damage created during the grinding operations. The suggested increase in the defect density in the surface layers of ground and polished surfaces relative to the as-fired finish is substantiated by characteristic defect-dominated signatures in the luminosity-current profiles. These results are in agreement with an earlier report describing the role of trapped carriers in the flashover process. It is concluded that the insulating properties of the surface as influenced by the microstructural features are strongly dependent on the precise mode of material removal during grinding and polishing operations and hence cannot simply be correlated to the gross surface topography in terms of surface roughness, particularly for hard and brittle ceramic materials.