Paper BO-TuP6
Partitioning Fracture Energy of a Molecularly Tailored Interface: Bond Cleavage and Plasticity

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

Separating the work of adhesion and plastic energy contributions to fracture toughness is essential to understand the mechanisms of interface debonding in thin film stacks and tailoring thin interfaces with desired properties for many applications. Here, we quantitatively separate the two contributions for Cu-silica interfaces modified with a molecular nanolayer by four-point bend testing at controlled environments to reveal the controlling mechanisms of energy dissipation. Recent work has shown that annealing Cu-silica interfaces treated with sub-nm-thick molecular nanolayers (MNLs) of a mercaptan-terminated organosilane MNL can yield manifold increase in toughness due to Cu-S bonding and thermally-activated siloxane (Si-O-Si) bridging. The increased fracture toughness, however, exceeds that of fused silica, indicating the importance of secondary absorbing processes in the Cu layer. Since Si-O-Si bridges are susceptible to hydrolysis, varying the water content provides a facile means for tuning the strength of the Si-O-Si bridges, and hence for isolating the contributions of the work of adhesion and the plastic energy. We measured interface toughness of Cu/MNL/SiO2 structures by four-point-bend testing as a function of water partial pressure pH2O under fixed displacement. In all cases, X-ray photoelectron spectroscopy measurements of the fracture surfaces indicated debonding due to siloxane bridge fissure at the MNL-SiO2 interface. The resultant plots of debond rate, V, vs. debond driving energy, G, show two distinct regimes. At pH2O < 1100 Pa, the plasticity in the Cu layer is the dominant contributor to the fracture toughness, as indicated by a high dG/d(ln pH2O), which captures the relative extents of plastic energy dissipation and work of adhesion. At pH2O > 1100 Pa, dG/d(ln pH2O) decreases by a factor of 3, due to the diminished role of large scale plasticity. Thus, our results indicate that interfacial strengthening has a multiplicative effect on the fracture toughness through a factorial contribution due to plastic energy. This result is supported quantitatively by density functional theory calculations of bond stretching in the MNL and siloxane bond breaking in the presence of water. Further, our calculations indicate water decreases the Si-O-Si bond energy by at least a third of its value in vacuum. Our results are of significance for many applications involving molecularly tailored interfaces exposed to environmental and mechanical stresses.