It has been long recognized that the BDT in bulk materials may be associated with enhanced plastic energy dissipation. This can be achieved by either changing the state of stress (plane strain to plane stress) or by raising the test temperature (lowering the yield stress). The situation is somewhat different in thin films where the BDT can be achieved by increasing film thickness or perhaps, even in a limited temperature range, by raising the test temperature. To study the latter we use a superlayer technique with a 1 μm tungsten film on top of thin copper films bonded to SiO2/Si wafers. This involves indenting into the superlayer which stores and then releases large amounts of elastic energy into the thin film/substrate interface. Here, preliminary data on 500 nm thick Cu demonstrates more than an order of magnitude increase in fracture energy from about 10 to 200 J/m2 as the test temperature is raised from 20°C to 130°C. As the amount of plastic energy absorption would appear to be limited by film thickness, this relatively large value was unanticipated. This interfacial fracture energy translates to a stress intensity of 5 MPa-m1/2. In context of the highest possible nanocrystalline Cu yield strength, this still represents a plastic zone of nearly 30 μm. This illustrates the quandary associated with explaining such high apparent toughness values as one generally expects plasticity to be truncated by film thickness. Is this associated with:
–some artifact of assessing local stresses during nanoindentation at elevated temperature:
–extending the plastic zone in the direction of crack growth much further than the film thickness;
–a shielding mechanism from an organized dislocation array in a ductile film sandwiched between a brittle substrate and a higher yield strength superlayer;
–some plastic energy dissipation in the superlayer;
–or by enhanced mode II at higher temperatures?
A few of these will be addressed in some detail with a goal of narrowing the field of the most promising candidates.