Influence of surface microstructure on the electric and spectroscopic characteristics of dielectric surface flashover

Spectroscopic investigations were performed on high purity polycrystalline alumina in vacuum to understand the effect of surface microstructure as altered by mechanical surface treatment, on the optical as well as the electrical characteristics of dielectric surface flashover. HV pulses were applied to unpolished samples as well as to samples subjected to mechanical polishing by using SiC (268 to 16 μm) and diamond abrasives (15 to 0.25 μm). The samples finished using diamond paste exhibited lower conditioned voltages and holdoff strength than those prepared with SiC abrasive finish. The spectrum of diamond-finished samples showed early development of molecular bands associated with the dielectric material. This behavior could be due to a larger density of defects developed on the surface during final polishing with diamond paste. There seems to be a correlation between the electrical performance of a sample, the polishing medium used for its surface finish, and the development of some of the spectral lines from flashover. The experimentally-observed phenomena in the studies point to different phases of surface conditioning, in which the dominating factors for the origin of surface flashover seem to depend on the surface state of the insulator