Measurement of Stress Changes during Growth and Dissolution of Anodic Oxide Films on Aluminum

Abstract

While stress is thought to play an important role in the development of self-organized porous films, mechanisms of stress generation during anodizing are not yet understood. In order to reveal depth distributions of stress in anodic films, phase-shifting curvature interferometry was used to monitor force transients (in-plane stress integrated through the sample thickness) during formation of anodic oxides on aluminum in phosphoric acid, as well as subsequent open-circuit dissolution. The measurements were not influenced significantly by electrostatic stress, internal stress in the metal samples, thermal stress, or stress induced by open-circuit dissolution. At typical current densities, the force became more compressive during anodizing, while a net tensile force change was measured after anodizing followed by complete oxide dissolution. Thus, it was revealed that anodizing generates both compressive stress in the oxide and tensile stress near the metal-oxide interface. Analysis of the open-circuit stress change revealed separate contributions from diffusional stress relaxations, and removal of residual oxide stress by dissolution. Residual stress distributions in the oxide, at nanometer depth resolution, were determined from measurements of dissolution rate and stress at open circuit, and validated through variations of the open-circuit dissolution rate.

Role of Oxide Stress in the Initial Growth of Self-Organized Porous Aluminum Oxide

“…A reflective gold coating was applied to the opposite side of the sample from that used for anodizing, in order to monitor the curvature change. Sample surfaces were prepared by etching in 10 wt% NaOH for 1 min at 60 o C, followed by immersion in 30 vol% HNO 3 [20,21]. Anodizing was performed at a constant current density of 5 mA/cm 2 in 0.4 M H 3 PO 4 at room temperature, using a two-electrode power supply (Keithley 2400) and a platinum wire counter electrode.…”

“…Details of this method are found in a previous communication, along with evidence validating the measurement of residual stress profiles [21]. After completion of anodizing, the current was set to zero to dissolve the anodic oxide at open circuit.…”

“…Recent work has introduced pore formation mechanisms in which oxide stress plays a prominent role. Since anodizing produces compressive stress in the oxide that increases with anodizing current density [16][17][18][19][20][21], stress should be elevated near the pore base where the current density is high. Thus, stress-driven ionic transport toward the pore walls is envisioned to assist pore formation.…”

“…Recently, we introduced measurements of residual stress distribution in anodic oxides, by monitoring stress changes during controlled open-circuit dissolution of the anodic film immediately following anodizing [21]. Using this method, we found that anodizing produces stress at both the metal-oxide or oxide-solution interfaces.…”

Fabrication of anodic porous alumina via anodizing in cyclic oxocarbon acids

“…Porous alumina consists of nano-scale hexagonal cells perpendicular to the substrate, and each cell possesses a nanopore at its center [1,2]. These cells are self-ordered by anodizing under appropriate electrochemical conditions, especially under a high electric field [3,4].…”