Using self-assembling monolayers to study crack initiation in epoxy/silicon joints

Abstract

The effect of the density and in-plane distribution of interfacial interactions on crack initiation in an epoxy-silicon joint was studied in nominally pure shear loading. Well-defined combinations of strong (specific) and weak (nonspecific) interactions were created using self-assembling monolayers. The in-plane distribution of strong and weak interactions was varied by employing two deposition methods: depositing mixtures of molecules with different terminal groups resulting in a nominally random distribution, and depositing methyl-terminated molecules in domains defined lithographically with the remaining area interacting through strong acid-base interactions. The two distributions lead to very different fracture behavior. For the case of the methyl-terminated domains (50 m on a side) fabricated lithographically, the joint shear strength varies almost linearly with the area fraction of strongly interacting sites. From this we infer that cracks nucleate on or near the interface over nearly the entire range of bonded area fraction and do so at nearly the same value of local stress (load/bonded area). We postulate that the imposed heterogeneity in interfacial interactions results in heterogeneous stress and strain fields within the epoxy in close proximity to the interface. Simply, the bonded areas carry load while the methyl terminated domains carry negligible load. Stress is amplified adjacent to the well-bonded regions (and reduced adjacent to the poorly bonded regions), and this leads to crack initiation by plastic deformation and chain scission within the epoxy near the interface. For the case of mixed monolayers, the dependence is entirely different. At low areal density of strongly interacting sites, the joint shear strength is below the detection limit of our transducer for a significant range of mixed monolayer composition. With increasing density of strongly interacting sites, a sharp increase in joint shear strength occurs at a methyl terminated area fraction of roughly 0.90. We postulate that this coincides with the onset of yielding in the epoxy. For methyl-terminated area fractions less than 0.85, the joint shear strength becomes independent of the interfacial interactions. This indicates that fracture no longer initiates on the interface but away from the interface by a competing mechanism, likely plastic deformation and chain scission within the bulk epoxy. The data demonstrate that the in-plane distribution of interaction sites alone can affect the location of crack nucleation and the far-field stress required.

Effect of Cross-Linker Functionality on the Adhesion of Highly Cross-Linked Polymer Networks: A Molecular Dynamics Study of Epoxies

“…For C < 0.1 we have purely adhesive failure. These three regimes have been observed experimentally by Kent et al

8 Failure stress in tensile (closed squares) and shear (open squares) simulations as a function of the fractional number of interfacial bonds for the case of f = 6.

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“…One would then expect that varying the number of bonds across the interface may change the location of failure from cohesive to interfacial. Experimentally, this has been done by coating the surface with a mixture of self-assembled monolayers (SAMs) which allows continuous variation of the number of bondable sites. , The mixture is a combination of methyl-terminated and bromine- or amine-terminated molecules. The methyl-terminated molecules do not form bonds with the epoxy while the bromine- and amine-terminated ones do form bonds.…”

“…Following the success of the initial work, , we are now ready to study the effects of varying key aspects of the adhesive system in a systematic manner. An important issue is the transition between adhesive and cohesive failure as a function of the interfacial bond density. , The cross-linker functionality f is a key element of the network structure, and understanding its influence is a fundamental issue. Here we present extension of our previous work to include varying f and cohesive failure.…”

“…The bulk network structure has been investigated using neutron scattering. , The bond strength is strongest in the highly cross-linked limit. Recently, Kent et al , have studied failure for a system involving an epoxy adhesive on a silicon wafer. Using a mixture of self-assembled monolayers (SAMs), they systematically vary the number of chemical bonds at the interface.…”

Molecular Tailoring of Interfacial Failure

“…The prospect of using SAMs to modulate bonding in thin film components requires testing of larger contact areas. At the macroscopic scale, the influence of several self-assembled monolayer chemistries on interface adhesion has been investigated using the superlayer test method, fiber pull out method, tape peel test, four-point-bend method, , and other sandwich specimen configurations. − Zhuk et al adapted a superlayer test configuration to measure the strength of an Au–epoxy interface . An increase in fracture energy was measured with incorporation of higher fractions of methyl/epoxy bonds to COOH/epoxy bonds.…”