• Shashkov, Pavel
• Bousser, Etienne
• Yerokhin, Aleksey
• Withers, Pj
• Rogov, Aleksey B.
• Laugel, Nicolas
• Slater, Thomas J. A.
• Matthews, Allan
• Gholinia, Ali

Abstract

Formation of the high-temperature α-Al2O3 phase during Plasma Electrolytic Oxidation of aluminium at ambient bulk temperatures has been previously attributed to local microdischarge events providing multiple melting-solidification cycles in micro-volumes of the surface oxide layer. In this work, it is demonstrated that the α phase can be formed even if the microdischarge is fully suppressed under specific processing conditions. Oxide layers produced in the post-sparking anodising mode were studied by FIB, TEM, EBSD, EDS and GDOES techniques to reveal microstructural and chemical evolutions that accompany the γ to α alumina transition. Our results provide strong evidence that the α phase can form spontaneously in regions of oxide with the appropriate temperature, grain size and impurity distributions in the γ-Al2O3 matrix that allow sufficient mobility of α/γ grain boundaries. Ionic migration within the oxide and hydrothermal dissolution/precipitation in the associated microporous network that facilitate species mobility at the grain boundaries allow the critical temperature for activation of γ→α transition to be reduced. Overall, it is suggested that oxide layer growth can be considered in terms of a relatively simple Plug Flow Reactor model. This can help predict the phase transition kinetics depending on key processing parameters such as current density and frequency of pulse polarisation, thus enabling optimum control of coating microstructure for specific application requirements.

Topics

• density
• impedance spectroscopy
• surface
• grain
• grain size
• phase
• mobility
• aluminium
• phase transition
• transmission electron microscopy
• precipitation
• activation
• Energy-dispersive X-ray spectroscopy
• electron backscatter diffraction
• current density
• critical temperature