The thermal conductivity of plasma electrolytic oxide coatings on aluminium and magnesium J.A. Curran, T.W. Clyne * Department of Materials Science and Metallurgy Cambridge University, Pembroke Street, Cambridge CB2 3QZ, UK Available online 25 February 2005 Abstract Plasma electrolytic oxide coatings have been produced on both aluminium and magnesium substrates. Their microstructures have been studied and deductions made about formation conditions. The thermal conductivities of the coatings have been measured using a simple steady state method. The values obtained are relatively low (~1 W m 1 K 1 ). This is explained in terms of the microstructure, which exhibits an extremely fine grain size and a significant proportion of amorphous phase. The porosity levels are low, so the low conductivity is not due to the presence of pores. It is noted that, even with a thickness limit of the order of 100 Am, coatings with such low conductivity may prove useful as thermal barrier layers, particularly since they exhibit excellent adhesion characteristics. D 2005 Elsevier B.V. All rights reserved. Keywords: Plasma electrolytic oxidation; Thermal conductivity; Amorphous; Grain structure; Nanocrystalline; Alumina coatings; Discharge channels; Microarc oxidation 1. Introduction Plasma electrolytic oxide (PEO) coatings can be formed on a variety of alloys, with a wide range of thickness [1]. They are reported to offer attractive combinations of wear resistance [2–6] and interfacial adhesion [7,8]. The high resistance to interfacial spallation is undoubtedly due, at least in part, to strong bonding at the interface, which is formed by partial consumption of the substrate. However, it also seems likely that these coatings exhibit high strain tolerances, as a consequence of relatively low stiffness, which is caused by the presence of micro-cracks and other microstructural defects [9]. This inhibits the generation of large stresses and associated high strain energy release rates. Thus, differential thermal expansion stresses, which have been identified [1,8] as a potential source of problems for high temperature use, may in practice be unlikely to reach high levels in PEO coatings. There has been little systematic study so far of any thermal properties of PEO coatings, although there are reports [10,11] indicating that they have good high temper- ature stability. In the present work, coating microstructures have been examined using X-ray diffraction and scanning electron microscopy. The thermal conductivities of thick PEO coatings were then measured, using a novel steady state method based on double-substrate specimens. 2. Experimental procedures 2.1. Sample preparation Substrates were prepared in the form of cylinders, 30 mm in diameter and 20 mm in length, designed to fit the thermal conductivity rig (see Section 2.4). Aluminium alloy BS Al- 6082 was used for study of coatings on aluminium, while AZ 91 magnesium alloy was used for study of coatings on magnesium. PEO coatings were grown on the flat end surfaces of the cylinders, using the Keronitek process. AC power was applied with a 50 Hz modulation, applying both positive and negative potential pulses to the substrates, immersed in commercially available electrolytes. Power was controlled so as to maintain a constant current density and 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.11.045 T Corresponding author. E-mail address: twc10@cam.ac.uk (T.W. Clyne). Surface & Coatings Technology 199 (2005) 177– 183 www.elsevier.com/locate/surfcoat