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Abstract
The internal stresses and Young's moduli of different-thickness organic layers made of DGEBA epoxy monomer and IPDA hardener were determined. Coatings were deposited on aluminum alloy (5754) after degreasing, chemical etching or anodizing. Using the same stoichiometric ratio (a/e) and the same curing cycle, interphase and bulk properties were determined using both Fouriertransform near-infrared (FTNIR) spectroscopy and differential scanning calorimetry (DSC). Young's modulus and the radius of curvature of coated samples were determined by a three-point flexure test. For thin films (h c < 200- 250 μm), different gradients in Young's modulus, physical and chemical properties, corresponding to the interphase formation, were observed for different surface treatments. Interphase thicknesses of 200 μm were obtained for both degreasing and anodizing, and of 250 μm for chemical etching. For thick coatings (h c > 200- 250 μm), each coating can be divided into two different layers. The first one corresponds to the interphase and the second one to the remaining part of the coating having mechanical, physical, and chemical properties similar to those of the polymer bulk. To understand the real interphase formation and the gradient of mechanical properties observed experimentally, a three-layer model (bulk coating/interphase/substrate) was developed to evaluate the residual stress profiles generated in such three-layered materials. This model was based on the determination of adhesional strains, which can be of either intrinsic or thermal origin, from the experimental curve of the curvature variation versus the coating thickness. Maxima in residual stress intensities were observed at the interphase/substrate interface for all surface treatments. Experimentally, an adhesional (interfacial) failure was observed in all cases. Moreover, an increase in practical adhesion was observed when residual stresses at the interphase/substrate interface decreased, leading to a correlation between 'practical adhesion' and 'residual stress' for different surface treatments.