Effects of coating thickness on thermal conductivities of alumina coatings and alumina/aluminum hybrid materials prepared using plasma electrolytic oxidation

The aim of this paper was to investigate the effects of coating thickness on the thermal conductivities of alumina coatings and alumina-coated aluminum substrates as hybrid materials prepared using plasma electrolytic oxidation (PEO). The coating thermal conductivities were measured with the comparative guarded heat flow method (also called cut-bar method). The guarded insulation system adopted in this method can effectively reduce the radial heat exchange and the axial shunting exchange, thus improving the precise of the measurement. The experimental results show that with the coating thickness increasing from 10 μm to 100 μm, the thermal conductivity has a slight rise from 1.9 W/m·K to 2.5 W/m·K. The gradient structure of the coating in grain size was believed to be one of the possible reasons to cause this slight variation. For the alumina-coated substrates (i.e., alumina/aluminum hybrid materials) with a determined substrate thickness, the increase in the coating thickness has more influence on the equivalent thermal conductivity of the hybrid materials than that in the effective thermal conductivity of the coating themselves.