Porosity in plasma electrolytic oxide coatings

• Published in: Acta materialia Vol. 54; no. 7; pp. 1985 - 1993
• Main Authors: Curran, J.A., Clyne, T.W.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.04.2006; Elsevier Science
• Subjects: Applied sciences; Coating; Crystalline oxides; Densitometry; Exact sciences and technology; Metals. Metallurgy; Microstructure; Modulus of elasticity; Origins; Oxide coatings; Porosity; Surface chemistry; Thermal conductivity; Coating; Crystalline oxides; Plasma; Electrodeposition; Porosity; Coatings; Surface treatment
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2005.12.029

Plasma electrolytic oxide coatings are generally assumed to be almost fully dense. However, evidence is presented here for the presence of sub-micrometre, surface-connected porosity in such coatings, on aluminium alloys, at levels of the order of 20%. This evidence comes from densitometry, mercury porosimetry, helium pycnometry, BET adsorption measurements and high-resolution scanning electron microscopy. The very fine scale of the porosity (pore diameter ∼10 to 100nm), coupled with severe difficulties in making unambiguous microstructural observations, may account for the failure to detect this feature previously. It is pointed out that various measured properties, such as Young’s modulus and thermal conductivity, are consistent with the presence of these relatively high porosity levels. Various other observed characteristics can also be explained on this basis. Finally, a possible mechanistic origin for the porosity is proposed.