Effect of aging on the microstructure evolution, thermal and mechanical properties of mica/epoxy composite

The mica/epoxy composite used in generator (rated 22 KV and 560 MVA) stator windings was exposed under air or hydrogen atmosphere at 180/spl deg/C for 1000 hours. The measurements of glass transition temperature and storage modulus were conducted using differential scanning calorimeter (DSC) and dynamic mechanical analyzer (DMA). The defect evolution was investigated by scanning electron microscope (SEM) in control and aged specimens. The SEM microstructural analysis showed that the degradation occurred fast aged specimens in air as the size of cavities increases at the mica/epoxy interface. The aged specimens in hydrogen showed retarded generation and growth of cavities. All specimen showed cavities and cracks at the mica/epoxy interface. Which could cause premature failure. The microstructural analysis indicates that reducing defect formation in operation would greatly extend the lifetime of mica/epoxy composite.