Use of self-assembled monolayers at variable coverage to control interfacebonding in a model study of interfacial fracture: Pure shear loading

The relationships between fundamental interfacial interactions, energy diss ipation mechanisms, and fracture stress or fracture energy in a glassy ther moset/inorganic solid joint are not well understood. This subject is addres sed with a model system involving an epoxy adhesive on a polished silicon w afer containing its native oxide. The proportions of physical and chemical interactions at the interface, and the in-plane distribution, are varied us ing self-assembling monolayers of octadecyltrichlorosilane (ODTS). The epox y interacts strongly with the bare silicon oxide surface, but interacts onl y weakly with the methylated tails of the ODTS monolayer. The fracture stre ss is examined as a function of ODTS coverage in the napkin-ring (nominally pure shear) loading geometry. The relationship between fracture stress and ODTS coverage is catastrophic, with a large change in fracture stress occu rring over a narrow range of ODTS coverage. This transition in fracture str ess does not correspond to a wetting transition of the epoxy. Rather, the t ransition in fracture stress corresponds to the onset of large-scale plasti c deformation within the epoxy. We postulate that the transition in fractur e stress occurs when the local stress that the interface can support become s comparable to the yield stress of the epoxy. The fracture results are ind ependent of whether the ODTS deposition occurs by island growth (T-dep = 10 degreesC) or by homogeneous growth (T-dep = 24 degreesC).