The Thermal Contact Conductance of Hard and Soft Coat Anodized Aluminum

An experimental investigation of the thermal contact conductance of anodized coatings, synthesized at different bath temperatures and in different electrolyte solutions, was performed, and the results were compared with previously published information. Electrolyte solutions of sulfuric acid at bath temperature of 7°C (Type III) and 25°C (Type II) and chromic acid at a bath temperature of 54°C (Type I) were used to grow coating thicknesses ranging from 3.2 to 61 μm (0.11 to 2.4 mil). Experimental thermal contact conductance data were obtained for a junction between anodized aluminum 6101-T6 and uncoated aluminum A356-T61 as a function of apparent contact pressure and anodized coating thickness. Apparent contact pressure ranged from 172 to 2760 kPa (25 to 400 psi) and the mean interface temperature was maintained at 40°C (104°F). The thermal contact conductance for the low-temperature sulfuric acid anodized (Type III) coatings varied from 300 to 13,000 W/m2 K, while the conductance of the room temperature sulfuric anodized (Type II) coatings varied between 100 to 3000 W/m2 K. The chromic acid (Type I) coatings yielded conductance values of 60 to 3000 W/m2 K. In general, the use of elevated temperatures for the anodizing bath will lead to lower surface microhardness and lower thermal contact conductance. The greatest conductance measurements were obtained for coatings grown in low-temperature sulfuric acid.