EFFECT OF RESIDUAL-STRESS ON THE SURFACE FLASHOVER OF ALUMINA CERAMICS

The influence of surface residual stress on the surface flashover beha vior of polycrystalline alumina in vacuum is investigated. The samples for the present investigation were prepared by using SiC and diamond abrasives of particle size ranging from 268 to 0.25 mum. Two types of polishing procedures, namely forward and reverse sequence polishing, a re adopted in the present work. The surface residual stress of the spe cimens is determined by the two-exposure X-ray diffraction method. The flashover strength and the surface stress of the samples, subjected t o forward sequence polishing (i.e., coarse to fine finish), are found to exhibit a random behavior with respect to the surface finish. Inter estingly, a linear dependence between flashover strength and surface s tress with respect to the surface finish is observed for the samples s ubjected to reverse sequence polishing (i.e., fine to coarse finish). The observed dependence between the residual stress and the flashover strength is discussed in terms of the high energy defect centers (e.g. , deep traps) formed within the forbidden gap of the alumina insulator .