How do residual stresses and interphase mechanical properties affect practical adhesion of epoxy diamine/metallic substrate systems?

Abstract

The practical adhesion, characterized by either ultimate parameters (F max or d max) or the critical strain energy release rate (G Ic) using the three-point flexure test (ISO 14679-1997), and the residual stress (σ ) profiles within systems of organic layers made of DGEBA epoxy monomer and IPDA diamine hardener were determined. The prepolymer (DGEBA-IPDA) was deposited as thin and thick coatings onto degreased or chemically etched aluminum alloy (5754). To understand the role of the interphase, 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. 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. In the three-point flexure test used to determine the practical adhesion, the failure may be regarded as a special case of crack propagation. The model considers residual stresses developed within the entire system leading to an intrinsic parameter representing the practical adhesion between the polymer and the metallic substrate. Moreover, to determine the profiles of residual stresses generated in such systems, the Young's modulus gradient of the interphase was also considered. The maxima in residual stress intensities were found at the interphase/substrate interface for a tri-layer system and at the coating/substrate interface for a bilayer system leading for all systems to an adhesional (interfacial) failure as experimentally observed. A comparison between the results obtained from the three-point flexure test and the Tapered Double Cantilever Beam (TDCB) was made. The determination of the critical strain energy release rate shows that residual stresses cannot be neglected. G Ic depends on the substrate surface treatment when the residual stresses were neglected. Moreover, we have determined the role of the interphase formation on the practical adhesion before and after hydrothermal aging. The results obtained emphasize that the epoxy/metal interphase affects significantly the initial practical adhesion. However, organo-metallic complex formation improves considerably the hydrothermal durability, as these complexes act as corrosion inhibitors.