We present a combined chemical (auger electron spectroscopy) and microscopic (optical microscopy, scanning electron microscopy and scanning probe microscopy) study of the work of adhesion and delamination mechanisms at interfaces between a glassy polymer (glycol-modified polyethylene terephthalate) and Al covered with different types of surface hydroxides. A clear correlation between the measured work of adhesion and the chemical nature of the Al surface, specifically the hydroxyl coverage and the iso-electric point is found. The magnitude of the work of adhesion points to important contributions from plastic deformation in the glassy polymer for some cases. Delamination is shown to be accompanied by the formation of microscopic shear bands at such interfaces. The non-monotonous stress-strain behaviour of the glassy polymer that gives rise to the shear bands is also shown to lead to peculiar pinning events at the crack front. Evidence indicates that the occurrence of protrusions in the crack front deriving for example from the presence of stress concentrators and crack initiation sites ahead of the front, combined with mode and rate dependence of the local energy release rate along the front may pin the front at positions adjacent to a protrusion. It is believed such microscopic mode-dependent pinning phenomena may be relevant for the adhesion on patterned interfaces.
Application of organic coatings on aluminium alloys is commonplace for corrosion protection. The adhesion of coatings is of great importance to the final protection properties. It is therefore necessary to understand on a molecular level the mechanisms with which a coating is able to bond. In this paper, we explore the possibilities of combining model molecules for a poly(ethylene terephthalate) (PET) type coating, di-methyl terephthalate (DMT), with differently pre-treated samples of AA1050 and AA5182 alloys. Bonding is studied by means of Fourier-transform infrared (FTIR) spectroscopy. Because the type of bonding gives a direction for adhesion of a coated system, we also test (macroscopically) the adhesion of PET coatings with a novel technique: asymmetrical double cantilever beam (ADCB). In this method, a thin knife is used as a wedge on the interface of the alloy and the polymer. The displacement of the crack front as measured from the knife's contact point with the coating is used as an input parameter to obtain the adhesion energy for various systems. We show that there is a relationship between the character of bonding of DMT molecules and adhesion energies of PET on both alloys after pre-treatments in alkali and acid and boiling in water.
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