Internal field distributions in multilayer polycarbonate/poly(vinylidene fluoride)-hexafluoropropylene films at onset of breakdown

Multilayer polymer films comprising alternating layers of polycarbonate (PC) and polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) show enhanced dielectric strength relative to single component films of either source polymer. Previous failure analysis on films subjected to breakdown under divergent field conditions revealed that multilayer films produced distinct surface treeing patterns whereas monolithic films did not. The choice of surface layer (PC or PVDF-HFP) contacted by a needle electrode influenced the nature of these treeing patterns. Additionally, damage within the film was largely localized to the interfaces between layers. To help explain these empirical results, we model the divergent field based on the geometry of our experimental setup and calculate the internal electric field distribution using the boundary integral equation method (BIEM). All fundamental charges, including: free, bound, trapped, and space charges are accounted for in the calculations, based on current and voltage data recorded during prior breakdown measurements. The calculations show that when PC is used as the surface layer in contact with the needle anode, there is significant field intensification in the top PC layer, in excess of 2000 V/μm. This is many times higher than the measured dielectric strength of monolithic PC and is at least partially due to charge injection from the needle anode. In contrast, the PVDF-HFP sub-layer in this configuration has very low field. These observations are consistent with breakdown occurring near the surface of the film, resulting in large-range surface treeing. When PVDF-HFP is the top layer, field intensification occurs deeper in the film, which is again consistent with the observed optical and FIB/SEM imaging results where less surface treeing and more internal damage is observed. The calculations suggest that the large contrast in field between adjacent layers generates a nexus for localized breakdown at the layer interfaces, again consistent with large internal voids formed by layer delamination in films subjected to divergent field breakdown.