Nanoindentation of explosive polymer composites to simulate deformation and failure

Crack initiation and propagation is a common concern for molecular composites such as plastic bonded explosives (PBXs) and pharmaceutical tablets. Under compressive stresses, cracks form at contacts between crystals and propagate along crystal-binder interfaces, causing composite failure. To investigate this process, crystal-binder interfaces have been characterised and their mechanical properties tested. Here, samples were created with interfaces representative of those in PBXs and characterised with surface energy measurements and neutron reflectometry (NR). Nanoindentation was performed to simulate the deformation and cracking that occurs at crystal-crystal contacts through the binder. NR revealed that use of a plasticiser disrupts typical crystal-binder intermixing and results in a mechanically weaker interface. During nanoindentation, a plasticised binder was observed by atomic force microscopy to delaminate around indentation impressions, whereas a non-plasticised binder did not. Differences in interfacial adhesion and incompatible strain, dictated by the elastic-plastic film compliances, were used to explain the contrasting delamination behaviours.