Determination of residual stresses in coated metallic substrates

The internal stresses and Young's moduli of different-thickness organic lay ers 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 curin g cycle, interphase and bulk properties were determined using both Fourier- transform near-infrared (FTNIR) spectroscopy and differential scanning calo rimetry (DSC). Young's modulus and the radius of curvature of coated sample s were determined by a three-point flexure test. For thin films (h(c) < 200 -250 mum), different gradients in Young's modulus, physical and chemical pr operties, corresponding to the interphase formation, were observed for diff erent surface treatments. Interphase thicknesses of 200 mum were obtained f or both degreasing and anodizing, and of 250 mum for chemical etching. For thick coatings (h(c) > 200-250 mum), each coating can be divided into two d ifferent 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 observ ed experimentally, a three-layer model (bulk coating/interphase/substrate) was developed to evaluate the residual stress profiles generated in such th ree-layered materials. This model was based on the determination of adhesio nal strains, which can be of either intrinsic or thermal origin, from the e xperimental curve of the curvature variation versus the coatings thickness. Maxima in residual stress intensities were observed at the interphase/subs trate interface for all surface treatments. Experimentally, an adhesional ( interfacial) failure was observed in all cases. Moreover, an increase in pr actical adhesion was observed when residual stresses at the interphase/subs trate interface decreased, leading to a correlation between 'practical adhe sion' and 'residual stress' for different surface treatments.