ABSTRACT
Abstract-Internal stress and Young’s modulus of different thickness organic layers made of DGEBA epoxy prepolymer and IPDA hardener were determined. Coatings were deposited on aluminum alloy (5754) after chemical etching or anodizing. Using the same formulation and the same curing conditions, coating, interphase and bulk properties (reaction extent and interphase thickness) were determined by using FTNIR spectroscopy. Young’s modulus and curvature of the coated samples were determined by a three-point flexure test. For thick coatings (> 2 0 0 -2 5 0 μιη), mechanical and chemical (amine and epoxy conversion) properties are found to be similar to those of bulk. For thin films, different gradients in both Young’s modulus and chemical properties were observed depending on the surface treatment. Interphase thicknesses of 200 μιη and 250 μιη were obtained, respectively, for anodizing and chemical etching. Considering both the real interphase created between the organic coating having the bulk properties and the substrate and the gradient of mechanical properties observed experimentally, a three-layer model was developed to evaluate the residual stress profile generated in such three-layered material. This model was based on the identification of adhesional strains (i.e. strains which can be of either chemical or thermal origin) using data for radius of curvature from layers thicker than interphase thickness of our systems. The maxima in residual stress intensities are obtained at the interphase/substrate interface for both surface treatments. To calculate the interphase residual stress intensities, only a model considering constant interphase properties (e.g. Young’s modulus) can be used. Based on such model, when adhesional failure was observed, the practical adhesion increased when internal stresses at the interface between the metal and the formed interphase decreased.
