The Effect of Self-Assembled Monolayers on Interfacial Fracture

This paper describes a series of experiments and analyses that were used to examine crack growth near sapphire/epoxy interfaces. Adhesion of the epoxy to the sapphire was enhanced by coating the sapphire with mixtures of two silane coupling agents that form self-assembled monolayers. A new biaxial loading device was used to conduct a series of mixed-mode fracture experiments. Crack opening interferometry, atomic force microscopy, and angle-resolved X-ray photoelectron spectroscopy allowed cohesive zone sizes, fracture surface topographies, and loci of fracture to be established. The experiments were complemented by finite element analyses that accounted for the rate- and pressure-dependent yielding of the epoxy. The analyses also made use of traction-separation laws to represent the various interphases that were produced by the mixed monolayers. The intrinsic toughness (defined as the area underneath the traction-separation curve) of the bare sapphire interfaces was independent of mode-mix and lower than values from previous experiments with glass/epoxy and quartz/epoxy specimens. The increase in overall toughness with mode-mix was completely accounted for by viscoplastic dissipation in the epoxy outside the cohesive zone. The minimum toughness of the coated sapphire interfaces was about five times higher than the mode-mix independent intrinsic toughness of the uncoated specimens. The increase in overall toughness with mode-mix was almost completely accounted for by increases in the intrinsic toughness as the traction-separation law varied with mode-mix. As a result, viscoplastic dissipation outside the cohesive zone was minimal. Atomic force fractography and X-ray photoelectron spectroscopy indicated that the crack growth mechanisms and the loci of fracture in the coated and uncoated specimens were quite different.