Factors Controlling Stress Generation during the Initial Growth of Porous Anodic Aluminum Oxide

Abstract

Keywords:anodizing stress oxide nanotube porous anodic oxide self-ordered porous oxide A B S T R A C T Anodic oxidation of reactive metals such as Al and Ti produces oxide films with self-organized arrangements of nanoscale pores. Stress-driven mass transport of oxide is considered to play an important role in pore formation and self-ordering. Using in situ stress monitoring during both anodizing and subsequent open-circuit oxide dissolution, distributions of in-plane residual stress were measured in anodic alumina films formed by galvanostatic anodizing in phosphoric acid. Anodizing produced significant stress both in the oxide and at the metal-oxide interface. For oxides grown to 20 nm thickness, the oxide stress was tensile below 3 mA/cm 2 and compressive above this current density, while the interface stress exhibited the opposite dependence. Stress generation correlated with interfacial volume change due to reactions and transport processes: oxide or interface stress was compressive when interfacial volume was created, and vice versa. Compressive stress buildup in the oxide is apparently required for self-ordered pore formation by flow-assisted mechanisms. From the present results, a simple criterion was derived specifying the conditions for compressive stress and pore formation in terms of parameters governing film composition, ionic transport and interfacial reaction kinetics.

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

“…The main panel of Figure 2 shows the force per width measured in a typical experiment, in which anodizing was followed by opencircuit dissolution of the oxide in the anodizing bath. Consistent with results in other papers, anodizing at 5 mA/cm 2 produces a compressive force that increases with time during barrier oxide growth [18,20,21,35]. The force per width change measured during anodizing will be referred to as the "anodizing force."…”

“…The interface force at the different anodizing potentials is compiled in Figure 3, The stress in this layer was tensile at small thickness, but became increasingly compressive at higher voltage, reaching levels of -2 to -4 GPa above 60 V. Elsewhere, we showed that near-surface tensile stress at low voltage is produced by the initial application of anodizing current, and seems to result from electric field-induced transfer of anions into the interior of the oxide [35]. As the voltage increases, this tensile nearsurface stress is increasingly masked by the compressive oxide stress introduced by anodizing [39].…”

“…The mechanisms of oxide and interface stress were discussed in detail earlier [35]. We showed that oxide and interface stress result from reactions at the oxide-solution and metal-oxide interfaces, respectively.…”

“…The force after oxide dissolution is attributable to elastic stress in either the Al substrate, or the residual oxide film of approximately 3 nm thickness remaining after dissolution. As noted elsewhere, the overall force change depends strongly on anodizing current density, showing that it is introduced during anodizing, as opposed to open-circuit dissolution itself [21,35]. The overall force per width change is referred to here as the "interface force.…”

“…Using this method, we found that anodizing produces stress at both the metal-oxide or oxide-solution interfaces. It was shown that compressive stress is produced at either interface when oxide is formed there, and tensile stress is generated at an interface when the oxide "lattice" is destroyed [35].…”