Axial discharge degradation of mica

This paper reports the initial results of a large investigation of the discharge degradation of mica. In this instance, two grades of mica, amber phlogopite and ruby muscovite have been investigated in air and nitrogen under a.c., d.c. and impulse stress conditions. All samples, following discharge degradation, were examined by optical and scanning electron microscopy and chemically analysed. Results indicate that above a critical discharge level breakdown may occur through surface discontinuities by a surface charge field effect. Below this critical level, discharge breakdown occurs as a slow erosion process involving the removal of cations from the mica structure, probably by a polarity dependent mechanism.