INFLUENCE OF SURFACE MICROSTRUCTURE ON THE ELECTRIC AND SPECTROSCOPICCHARACTERISTICS OF DIELECTRIC SURFACE FLASHOVER

Spectroscopic investigations were performed on high purity polycrystal line alumina in vacuum to understand the effect of surface microstruct ure 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 subje cted to mechanical polishing by using SiC (268 to 16 mum) and diamond abrasives (15 to 0.25 mum). The samples finished using diamond paste e xhibited lower conditioned voltages and holdoff strength than those pr epared with SiC abrasive finish. The spectrum of diamond-finished samp les showed early development of molecular bands associated with the di electric material. This behavior could be due to a larger density of d efects developed on the surface during final polishing with diamond pa ste. There seems to be a correlation between the electrical performanc e of a sample, the polishing medium used for its surface finish, and t he development of some of the spectral lines from flashover. The exper imentally-observed phenomena in our studies point to different phases of surface conditioning, in which the dominating factors for the origi n of surface flashover seem to depend on the surface state of the insu lator.