Nano-mechanical and chemical analysis of hydrated anodic aluminum oxides

In this work, the effect of various Hot Water Sealing (HWS) times was determined on Anodic Aluminum Oxides (AAOs), that were formed on commercial Al-Mg-Si alloys. The evolving hydroxide layer causes significant alterations to the surface morphology and finishing. A mechanical softening and cracking of the formed aluminum hydroxide (AlOOH) were investigated with nanoindentation and scanning electron microscopy measurements. Cracks form in the vicinity of the surface or precipitates when exposed to sealing conditions commonly used in industry. Additionally, scanning transmission electron microscopy measurements were used to analyze the AlOOH and AAO layers. By analyzing the plasmon energy shift in both phases, electron-energy loss spectroscopy found a continuous transformation of AAO into AlOOH. Based on the nano-mechanical and chemical analyses, a cracking model was proposed. The interaction between AlOOH formation and precipitates incorporated in the AAO mainly contributed to the existence of various networks of cracks. • Hardness reduction of the surface layer affected by hot water sealing duration. • Change of electronic structure due to phase transformations of Al2O3 into AlOOH, measurable by STEM-EELS. • Model for the formation of voids and cracks due to different volume expansions and stresses at interfaces.