The role of plasticity in bimaterial fracture with ductile interlayers

Evaluation of the plasticity effects in fracture along ductile/brittle inte rfaces requires appropriate models for plastic dissipation in a ductile com ponent. For thin ductile films, constitutive properties appropriate to the small volumes involved are essential for adequate modeling. Here, yield str ess is of primary importance. With nanoindentation, one can obtain both a l arge strain flow stress as well as the far field yield stress representing the small strain elastic-plastic boundary. Using these to estimate an appro priate plastic strain energy density, the crack tip plastic energy dissipat ion rates associated with the interfacial crack extension can be estimated for a ductile film. With the preceding analysis, plasticity effects on the interfacial toughness have been evaluated for external measures of strain e nergy release rates as obtained from indentation tests using the axisymmetr ic bilayer theory. Comparison involved RF sputtered 200- to 2000-nm-thick C u interlayers between oxidized silicon and sputtered tungsten. Experimental values for the Cu/SiO2 interface increased with Cu film thickness from 1 t o 15 J/m(2). This was in qualitative agreement with the theoretical predict ions for plastic energy dissipation rates. In contrast, first-order estimat es suggest that the observed interfacial toughness increases cannot be attr ibuted to either mode mixity effects or increased intrinsic interfacial fra cture energies. As such, crack tip plasticity is identified as the dominant mechanism for increasing interfacial toughness.