Structural, mechanical and corrosion resistance properties of Al2O3-CeO2 nanocomposites in silica matrix on Mg alloys by a sol-gel dip coating technique

The physical, chemical and mechanical properties of magnesium alloys make them attractive materials for automotive and aerospace applications. However, these materials are susceptible to corrosion and wear. This work discusses the potential of using sol-gel based coatings consisting of Al2O3 and CeO2 (15 wt.% of each) in silica matrix. The CeO2 component provides enhanced corrosion protection, while Al2O3 impart scratch resistance properties as well as wear resistance. Coating deposition was performed by the dip coating technique on two magnesium alloy substrates with different surface finishes: AZ91D) (as-casted, sand-blasted, and machined) and AZ31 (rolled and machined). All as-deposited coatings (xerogel coatings) were then subjected to 10 h annealing: a temperature of 180 degrees C was applied to the AZ91D alloy and 140 degrees C to the AZ31 alloy. Morphological and structural properties of the annealed coatings were investigated by scanning electron microscopy, atomic force microscopy and transmission electron microscopy. Coating composition was examined using energy dispersive X-ray analysis. Adhesion of the annealed Al2O3-CeO2 in silica matrix coatings on the substrates, assessed by scratch tests, showed critical loads indicative of coating perforation of up to 32 N. Hardness and elasticity, measured using depth-sensing nanoindentation tests, gave a hardness and elastic modulus of 4.5 GPa and 98 GPa, respectively. Salt spray corrosion tests performed on these coatings showed superior corrosion resistance for AZ91D (as-casted and machined) and AZ31 (machined), while severe corrosion was observed for the AZ31 (rolled) and AZ91D (sand-blasted) magnesium alloy substrates. (c) 2006 Elsevier B.V. All rights reserved.