Alumina–silica coatings were prepared on polyimide substrate by sol–gel method using aluminium isopropoxide and tetraethyl orthosilicate as the precursors. The molar ratios of Si/Al were designed as 1∶3, 1∶1 and 3∶1. The surface morphology and structure of as-deposited coatings were investigated by various complementary analysis techniques. The results indicate that all silica–alumina coatings show a smooth and dense surface and have a thickness of ∼400 nm. After treatment at 230°C, the alumina–silica coatings are a diphase structure of AlOOH and amorphous silica, the same as the corresponding gel powders. Moreover, after calcination at 550°C, the alumina–silica gel powders still remain a diphase structure, but AlOOH transforms into γ-Al2O3. The Si content has no obvious effect on the phase structure. However, increasing the Si/Al ratio in the as deposited coatings causes the binding energies of Si 2p, Al 2p and O 1s peaks to shift to higher energy.
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