While mechanical properties of zinc coatings are well-known and have been studied in detail during the last decades [1,2], the fundamental behaviour of coatings of zinc- or aluminium-based alloys is much less understood. Understanding the contribution of the different micro-phases in their complex solidification structure requires new approaches.
Nanoindentation coupled with data post-processing has been proven to be a powerful technique to draw two-dimensional mechanical mappings and to obtain the mean elastic modulus and hardness of constituent microscale phases in heterogenous materials [3,4]. The microstructure of low-alloy zinc-based coatings consists mainly of phases having hexagonal close-packed (HCP) crystal structure, like η-Zn in the form of dendrites or eutectic phases (Zn/Al or Zn/Al/MgZn2) and MgZn2 in the form of eutectic phases (Zn/MgZn2 or Zn/Al/MgZn2). The Al-rich phase in the Zn/Al and Zn/Al/MgZn2 eutectic phases, is a solid solution of Al and Zn having a face-centered cubic (FCC) structure. The elastic behaviour of HCP single crystals is usually highly anisotropic, meaning that the stiffness of the material strongly depends on the direction of loading. In addition, the hardness of Zn crystal has been reported to be also highly anisotropic [5].
In this study, the microstructure of Zn-5Al and Zn-3.7Al-3.0Mg coatings was investigated using scanning electron microscope (SEM) coupled with an electron probe microanalyzer (EPMA). Mechanical property maps were built using a grid nanoindentation technique [3,4] and were quantitatively correlated to the microstructure map. Electron backscatter diffraction (EBSD) was used to determine the crystallographic orientation (Euler angles) of each indented phase in order to determine the angle between the direction of indentation and the hexagonal orientation [0001] of the phase (η-Zn or MgZn2). By combining all these techniques, the effect of the anisotropy of η-Zn and MgZn2 crystals on the mechanical properties of Zn-Al and Zn-Al-Mg coatings was quantified.
1. R. Parisot, S. Forest, A. Pineau, F. Grillon, X. Demonet, J.-M. Mataigne, Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part I. Deformation modes, Metall. Mater. Trans. A. 35 (2004) 797–811. doi:10.1007/s11661-004-0007-x.
2. R. Parisot, S. Forest, A. Pineau, F. Grillon, X. Demonet, J.-M. Mataigne, Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part II. Damage modes, Metall. Mater. Trans. A. 35 (2004) 813–823. doi:10.1007/s11661-004-0008-9.
3. N.X. Randall, M. Vandamme, F.-J. Ulm, Nanoindentation analysis as a two-dimensional tool for mapping the mechanical properties of complex surfaces, J. Mater. Res. 24 (2009) 679–690. doi:10.1557/jmr.2009.0149.
4. D. Mercier, J.-F. Vanhumbeeck, M. Caruso, X. Vanden Eynde, M. Febvre, Microstructural and mechanical characterisation of electroplated nickel matrix composite coatings, Surf. Eng. 35 (2019) 177–188. doi:10.1080/02670844.2018.1433270.
5. Y.T. Pei, G.M. Song, W.G. Sloof, J.T.M. De Hosson, A methodology to determine anisotropy effects in non-cubic coatings, Surf. Coatings Technol. 201 (2007) 6911–6916. doi:10.1016/J.SURFCOAT.2006.11.044.