Stress Gradient Determination in Anti-Corrosion Multilayer Coating

To reduce maintenance and to increase the corrosion protection and lifetime of maritime structures while complying with environmental standards, multilayer coatings are applied to protect steel sections. A new generation of hybrid sol-gel and/or HiPIMS Ni-based thin films appear to constitute an efficient pre-treatment before the anti-corrosion paint application. However, increasing the number of coatings and associated interfaces may lead to coating failure due to stresses induced by the different deposition processes. Therefore developing smart models to assess the stress distribution along these multilayers appears of significant importance. The well-known Stoney formula cannot be used for multilayers and owing to the large dimensions of the object to be protected. To assess an easily measurable curvature after deposition, thin steel sheets are used but do not respect any more the Stoney hypotheses. So we set up an analytical thermo-elasto-plastic model to evaluate the stresses induced by depositions in each layer. This model is based on the various thermal expansion coefficients of every coat. After extrapolation along the complete thickness, combining sol-gel and PVD deposition smoothens the stress difference between steel and paint. The shear stresses at interface seems thus to be reduced. The evolution of the stress difference between layers with the imposed deflection can predict the mechanical strength and the interface failure. In order to evaluate the quality of the model, in-situ four-point bending in SEM was performed to study of the adhesion between the various layers. The results deduced from the model are in good agreement with SEM images.