Internal stresses, Young's modulus and practical adhesion of organic coatings applied onto 5754 aluminium alloy

Residual stress, Young's modulus and practical adhesion of different thickness organic layers made of DGEBA epoxy resin and TPD hardener were determined. Coatings were deposited on aluminum alloy (5754) after degreasing and chemical etching. Both bi-layer (having a perfect interface with a null thickness) and three-layer models (having a real interphase between the organic coating and the substrate) were considered to evaluate residual stress and Young's modulus of the entire coating, the polymer/metal interphase and the remaining part of the coating which have the bulk properties. Coatings, interphase and bulk properties (T-g, reaction extent and interphase thickness) were determined by using differential thermal analysis (DSC) and FTNIR spectroscopy. Young's modulus, curvature radii of coated samples and practical adhesion were determined by three point flexure test. Results show the bi-layer model limitation when a coating/substrate interphase is formed. Interphase thicknesses of 200 and 250 mu m were obtained for respectively degreasing and chemical etching. For the same coating formulation and the same curing conditions, amine conversion, T-g of the entire coatings, residual stresses at interfaces interphase/metal and bulk coating/interphase, Young's modulus and practical adhesion depend on the nature of the substrate treatment and on the coating thickness. For thick coatings mechanical, physical and chemical properties are equal to the bulk one. For thin coatings significant differences compared to the bulk properties were observed. Variations of both residual stress and Young's modulus were observed according to different surface treatments. Residual stress intensities at the interphase/substrate interface are higher than the ones at the coating/substrate interface. For thin coatings on chemically etched substrate, Young's modulus is more important than degreased substrate one. On the contrary, residual stress intensities are lower for chemical etching than the degreasing ones. When adhesional failure was observed, the adherence increases when internal stresses at the interface between the metal and the formed interphase decrease. (C) 1999 Elsevier Science S.A. All rights reserved.