Alain André Roche

Mechanical properties of thin and thick coatings applied to various substrates. Part I. An elastic analysis of residual stresses within coating materials

Internal residual stresses significantly influence the overall mechanical properties of multi-layer systems and consequently affect the coated materials performance. The determination of residual stresses within coatings has been extensively carried out for thin (the coating thickness being less than the substrate thickness) films (Stoney, Roll, etc.) and for thick (the coating thickness being approximately equal to the substrate thickness) films (Timoshenko, Inoue, etc.). This work extends currently existing models to cover cases where the coating thickness approaches that of the sheet substrate. We developed relationships for the determination of the first-order residual stress. The construction of these models was carried out using a one-dimensional analysis based on beam theory (the width-to-thickness ratio of the system being less than 5). As suggested by Timoshenko and later on by Hoffman, we also introduced the bi-axial modulus for isotropic stresses when the thin plate theory (the width-to-thickn...

The role of the polymer/metal interphase and its residual stresses in the critical strain energy release rate (Gc) determined using a three-point flexure test

The critical strain energy release rate (G c), the residual stresses (σ), Youngs modulus (E), and the practical adhesion, characterized by ultimate parameters (Fmax or dmax), of organic layers made of DGEBA epoxy monomer and IPDA diamine hardener were determined. The prepolymer (DGEBA-IPDA) was deposited both as thick coatings and as a mechanical stiffener onto degreased aluminum alloy (5754) or chemically etched titanium alloy (Ti-6Al-4V). During the three-point flexure test used as a practical adhesion test [this test is also called the double cantilever adhesion test (DCAT)], the failure may be regarded as a special case of crack growth. To understand the real gradient properties of the interphase, substrate, and bulk polymer properties, a three-layer model was developed for quantitative determination of the critical strain energy release rate (Gc). The particular characteristic of this model was to consider the residual stresses developed within the entire three-layered system, leading to an intrinsic parameter representing the practical adhesion between a polymer and a metallic substrate. Moreover, to determine the residual stresses generated in such three-layer systems, the gradient of interphase mechanical properties was considered. The maxima of residual stress intensities are found at the interphase/substrate interface, leading to an adhesional (interfacial) failure that is experimentally observed. The determination of the critical strain energy release rate by the three-point flexure test (DCAT) shows that residual stresses cannot be neglected. A comparison between the results obtained from the three-point flexure test (DCAT) and those obtained by the tapered double cantilever beam (TDCB) test is presented.

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

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 Youngs 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.

Residual stresses and practical adhesion: effect of organo-metallic complex formation and crystallization

Epoxy-amine liquid pre-polymers are often applied onto metallic substrates and cured to obtain painted materials or bonded joint structures. The overall performance of such systems depends on the interphase created between the epoxy-amine polymer and the metallic substrate. When epoxy-amine liquid mixtures are applied onto a metallic oxide layer, concomitant amine chemisorption and oxide dissolution occur leading to organo-metallic complex formation. Depending on the amine nature, as soon as the organo-metallic complex concentration is higher than the solubility product (e.g., isophoronediamine (IPDA)), these organo-metallic complexes crystallize as sharp needles. At the same time, the uncrystallized organo-metallic complexes react with the epoxy monomer to form, after curing cycle, a new network. Moreover, the crystal size increases with the solid/liquid contact time leading to an increase of intrinsic residual stresses and Youngs modulus. When aliphatic diethylenetriamine (DETA) was used, no crystallization occurred, but the interphase formation was observed. The aim of this study was to understand and to establish the role of crystallization of organo-metallic complexes formed within the interphase on the practical adhesion performance. As the crystallization of the organo-metallic complex depends on the nature of the amine, two amine hardeners were used (IPDA inducing the formation of crystals and DETA without formation of crystals). For DGEBA-IPDA systems, the ultimate load decreases while residual stresses increase when the liquid/solid contact time increases. When no crystal formation was observed (e.g., DGEBA-DETA system), residual stresses, coating Youngs modulus and ultimate load values all remained nearly constant irrespective of the liquid/solid contact time.

Tapered double cantilever beam test used as a practical adhesion test for metal/adhesive/metal systems

To quantify the practical adhesion between an epoxy-amine adhesive (DGEBA + IPD) and a titanium alloy (Ti6A14V), a tapered double cantilever beam (TDCB) test was used. The critical strain energy release rate (GIc) was evaluated. An appropriate size of the sample allows the fracture to propagate within the adhesive/metal interfacial region. The crosshead speed, the joint thickness, and the adhesive formulation all influence the experimental results. For low crosshead displacement speeds (< 1 mm min-1), the crack speed was controlled by the crosshead displacement. For higher crosshead displacement speeds, the crack propagation was controlled by the interfacial region critical strain energy release rate. By adding a reactive liquid rubber (ETBN 13) to the initial adhesive formulation, an increase in the critical strain energy release rate was observed.

Mechanical properties of thin and thick coatings applied to various substrates. Part II. Young's modulus determination of coating materials*

To determine Youngs modulus of coating materials when they are applied to substrates, theoretical and experimental analyses are performed. Significant residual stresses are generated within thin and thick coatings applied to substrates. As a result of these stresses, the bi-material strip assumes a certain curvature. The curved beam theory was used to establish the equivalent bending stiffness of bi-layer materials as functions of (a) the initial radius of curvature generated by residual stresses, (b) the mechanical radius of curvature during flexure testing, and (c) mechanical (Youngs moduli) and geometrical (widths and thicknesses) characteristics of bi-layered systems. The relevant expression was transformed to a second- or third-order equation in order to calculate Youngs modulus of the coating undergoing residual stresses (using models developed in Part I and by Stoney, Roll, and Inoue).

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 Gc). 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 Youngs 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).