The thick alumina films (0.2-0.5 mum) were prepared on quartz glass and nickel substrates by a dipping method using an alumina sol derived from an aqueous AlCl3 solution. The hardness of this coating was compared to those of similarly coated substrates that had been heat-treated at various temperatures. The hardness of the alumina coating increased with increasing heat-treatment temperature. This sol gave crack-free and well-coated substrates that could be heat-treated to temperatures up to 800degreesC. This may have been due to the formation of an amorphous mixed oxide interface layer between the alumina films and the substrates. At temperatures higher than 900degreesC, problems with the coating started to appear because the oxide layer formed into alpha-alumina accompanying evolution of stress in the interface. (C) 2003 Kluwer Academic Publishers.