Thermo-physical properties of plasma electrolytic oxide coatings on aluminium

Plasma electrolytic oxide coatings appear to offer attractive combinations of hardness, wear resistance, corrosion resistance and interfacial adhesion. In order to optimise such characteristics, however, more basic thermo-physical property data are required, together with an understanding of how they are affected by processing conditions and microstructure. In the present study, coatings were produced on 6082 aluminium and characterised using profilometry, scanning electron microscopy, X-ray diffraction and nanoindentation. The in-plane thermal expansivity of detached coatings was measured by dilatometry to be about 8 microstrain K−1. There is thus a rather substantial mismatch between the expansivities of coating and substrate, amounting to about 15 microstrain K−1. The global in-plane Young's modulus was estimated using cantilever bending of sandwich coated substrates and also by measuring the curvature generated in a bi-material beam on cooling to low temperature. It was found to lie in the approximate range of 10-40 GPa. Values of this order, which are low compared with the figure of around 370 GPa expected for fully dense polycrystalline alumina, are thought to be associated with the presence of a network of microcracks and voids. A low value is expected to be beneficial in terms of conferring good strain tolerance, and hence resistance to spallation driven by differential thermal expansion.