Characterisation of multilayered sol–gel silica coatings on aluminium–SiC composites

Silica coatings up to 20 μm thick have been obtained through a sol–gel route on aluminium matrix composites reinforced with SiC particles. Single silica layer and multiple ones have been deposited on the substrates and extensive crack-free surfaces have been achieved with both methods. The topography of the layers has been characterised by means of Scanning Electronic Microscopy and Atomic Force Microscopy, and dense flat surface has been observed. However, topographical details consisting of spherical zones tend to appear on the surface of multilayered coatings. The mechanical properties of the different coatings have been measured using the nanoindentation technique independently from the substrate underneath, and hardness and elastic modulus similar and even higher than those of fused silica have been obtained for sol–gel multilayered coatings. The microstructure of the coatings has been studied by Transmission Electronic Microscopy, and the appearance of nanocrystals on the flat surfaces has been considered responsible of the mechanical properties measured.