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 thickn ess organic layers made of DGEBA epoxy resin and TPD hardener were determin ed. Coatings were deposited on aluminum alloy (5754) after degreasing and c hemical etching. Both bi-layer (having a perfect interface with a null thic kness) 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 th e remaining part of the coating which have the bulk properties. Coatings, i nterphase and bulk properties (T-g, reaction extent and interphase thicknes s) were determined by using differential thermal analysis (DSC) and FTNIR s pectroscopy. Young's modulus, curvature radii of coated samples and practic al adhesion were determined by three point flexure test. Results show the b i-layer model limitation when a coating/substrate interphase is formed. Int erphase thicknesses of 200 and 250 mu m were obtained for respectively degr easing and chemical etching. For the same coating formulation and the same curing conditions, amine conversion, T-g of the entire coatings, residual s tresses at interfaces interphase/metal and bulk coating/interphase, Young's modulus and practical adhesion depend on the nature of the substrate treat ment and on the coating thickness. For thick coatings mechanical, physical and chemical properties are equal to the bulk one. For thin coatings signif icant differences compared to the bulk properties were observed. Variations of both residual stress and Young's modulus were observed according to dif ferent surface treatments. Residual stress intensities at the interphase/su bstrate interface are higher than the ones at the coating/substrate interfa ce. For thin coatings on chemically etched substrate, Young's modulus is mo re 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 stre sses at the interface between the metal and the formed interphase decrease. (C) 1999 Elsevier Science S.A. All rights reserved.