The role of the residual stresses of the epoxy-aluminum interphase on the interfacial fracture toughness

When an epoxy-diamine system (DGEBA-IPDA) is applied onto aluminum alloy (5754) and cured, an interphase having chemical, physical, and mechanical properties quite different from those of the bulk polymer is created between the substrate and the part of the polymer having bulk properties. To get a better understanding of the role of the interphase on the interfacial fracture toughness either a tri-layer (bulk coating/interphase/substrate) or a bi-layer model (bulk coating/substrate) were used for quantitative determination of the critical strain energy release rate (noted G(c)). Indeed, as the interphase formation results from both dissolution and diffusion phenomena, we were able to control the interphase formation within coated systems by controlling the liquid-solid contact time and then to make tri- or bi-layered systems. The particularity of models used is to consider residual stress profiles developed within the entire system leading to an intrinsic parameter representing the work of adhesion between the polymer and the metallic substrate. The aim of this publication is to clearly establish the role of the interphase mechanical properties, such as Young's modulus and residual stress on the interfacial fracture toughness. Results are presented and discussed for three different aluminum surface treatments (chemical etching, degreasing and anodizing).