Sol-gel coatings of different composition in the ZrO2-SiO2 system have been prepared, starting from several precursors (alkoxides and alkylalkoxides). Common soda lime silicate glass slides were used as substrates for coating deposition. The Vickers microhardness, H-V, of the coated glass substrates have slightly higher values than that of uncoated glass. Young modulus, E, values for coated glass substrates are higher than that of uncoated glass. To compare the mechanical properties of the samples, the ratio H-V/K-IC (where K-IC is the toughness) has been calculated and the concept of fragility discussed. The fragility of coated samples increases as the ZrO2 content diminishes and the SiO2 increases. Both scanning and transmission electron microscopy observations were performed to characterise the textural and microstructural properties of the coatings. (C) 2003 Elsevier Ltd. All rights reserved.