Two-layer hybrid sol-gel system's thermal parameters investigated with the optical nondestructive photoacoustic method in the frequency domain

•Hybrid sol-gel coatings have been investigated with the non-destructive technique.•Thermal parameters of these thin coatings were determined depending on their composition.•Optical photoacoustic method was applied for experimental investigations.•Two layer model of the samples was used for interpretation of experimental results. This work presents results of studies of thermal parameters such as thermal diffusivity, thermal conductivity, thermal effusivity and volumetric heat capacity of thin hybrid sol-gel corrosion protective layers deposited on the aluminum substrate. In the studies two groups of samples were investigated. The first group was: TMZ.1, TMZ.2 and TMZ.3 (TMZ- Si/Zr-based hybrid sol-gel coatings), with different amount of zirconium, and the second group was: TMZ.1/Ce, TMZ.2/Ce and TMZ.3/Ce (TMZ/Ce - Ce-doped Si/Zr-based hybrid sol-gel coatings) with the same amount of zirconium and an addition of cerium. This work presents the influence of the content of zirconium and cerium on their thermal parameters. The possibility of using the nondestructive photoacoustic method (PA), with the microphone detection, to characterize this type of layers was analyzed theoretically and experimentally too.