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The thick alumina films (0.2-0.5 µm) were prepared on quartz glass and nickel substrates by a dipping method using an alumina sol derived from an aqueous AICI3 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 800°C. 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 900°C, problems with the coating started to appear because the oxide layer formed into a-alumina accompanying evolution of stress in the interface. [ABSTRACT FROM AUTHOR]