Mixed-mode traction-separation relations between graphene and copper by blister tests

A blister test with associated analysis was developed to characterize the traction-separation relations associated with interactions between graphene and a copper substrate to which it had been transferred. Graphene grown by chemical vapor deposition was backed by a photoresist film and transferred to a highly polished copper substrate from its seed copper foil. The graphene/photoresist composite film was then pressurized with deionized water through a hole in the substrate. The blister profiles and normal crack opening displacements (NCOD) were measured by two microscopes with synchronized cameras. Different mixed-mode conditions were achieved by varying the thickness of the backing layers. The measured adhesion energy for the graphene/copper and photoresist/copper interfaces showed a strong dependence on the mode-mix. Cohesive zone models associated with traction-separation relations were then developed to study the damage initiation and crack propagation under various mixed-mode conditions. The numerical solutions for the resistance curves, pressure vs. deflection, and NCOD were in good agreement with measurements. The cohesive zone model was extended to a wider range of mode-mixes by making use of the asperity locking model which had provided a mechanism for the observed toughening effect. The interactions between graphene and copper were found to be stronger in all respects than those associated with photoresist and copper. Because the monolayer graphene was sandwiched between photoresist and copper, this result suggests that graphene was not transparent to interactions between photoresist and copper, but opaque. The use of pressure could provide another approach to transfer large-scale graphene.