New generation of protective nanocomposite coatings - Chemical, microstructural and functional approaches – CHAMELEON

Surface modification of steel by sacrificial Al or Zn coatings has been used for a long time to increase its corrosion resistance. However, these soft layers suffer from too poor mechanical properties, which is unfortunately a critical property for a multifunctional purpose. The challenge of this project is to deliver hard wear resistant sacrificial films through an environmental-friendly economic process. In the past decade, new perspectives were opened with the wide development of tools protected by nanocomposite coatings. Film has to be structured so as its properties may accommodate external stresses imposed to the coated part throughout its lifetime. In this context of improved materials designed at a nanometer-scale, we propose to study the interest, on the durability of steel, of incorporating by electrodeposition, hard nanoparticles into a multilayered Zn-based anodic coating.
By conventional process (solid particles added in the electrolytic bath) functional characteristics of films indeed increases, but the film growth is very difficult to control owing to the chemical instability of baths, inherent to the presence of solid species prone to sedimentation and/or flocculation… In the present project, we aim to develop an original process involving a single-phase bath. In such an innovative configuration, we profit from the modification of the local chemistry at the growing interface due to the metal deposition (pH variation especially), to induce simultaneously a controlled precipitation of the hardening oxides into the coating.

The first research will be limited to a "model layer" composed of cerium or yttrium oxides introduced into a pure zinc matrix, in order to highlight the role of the second phase on the electrochemical, mechanical and tribological behaviours. In a second step, effect of an alloyed matrix, obtained with a commercial Zn-Ni bath and more interesting for a potential industrial transfer will be then analysed.
A great advantage of the "chameleon's baths" relies on the fact that they are composed of only one liquid phase that contains both types of metallic cations: those giving rise to the metallic matrix on the one hand and those leading to the ceramic nanoparticles on the other hand. Therefore, it gets possible to control the nature and amount of particles introduced in the matrix thickness. Gradient layers can be then easily deposited characterised, for instance, by a hard top-layer (high particle density) to withstand wear and corrosion, and a ductile bulk (low particle density) to promote a high toughness. Besides, applying pulsed current to steel substrates immersed in a Zn-Ni electrolytic bath, it can be deposited bi-nanostructured films composed of oxides nanoparticles embedded in a nanostratified Zn-Ni matrix. Such a complex architecture combining a three-dimensional distribution of oxides with a 2-D structure of the metallic matrix (multilayer) has never been studied, and should present outstanding properties to assess and explain.

The objective of this project is first to optimise the electrodeposition of binanostructured coatings, and then to understand why, and how, presence of nanoparticles may influence the durability of the protected steel. To succeed in this work, skills of three well-known complementary laboratories have been gathered. With such a "consortium" we expect to thoroughly understand the relationships linking the deposition parameters, the microstructure of films and their functional properties.