ORIGINAL PAPER Tetragonal to monoclinic phase transition observed during Zr anodisation Francisco Trivinho-Strixino & Donizete X. da Silva & Carlos O. Paiva-Santos & Ernesto C. Pereira Received: 25 September 2011 / Revised: 11 September 2012 / Accepted: 14 September 2012 # Springer-Verlag Berlin Heidelberg 2012 Abstract Plasma electrolytic oxidation (PEO) is a coating procedure that utilises anodic oxidation in aqueous electro- lytes above the dielectric breakdown voltage to produce oxide coatings that have specific properties. These condi- tions facilitate oxide formation under localised high temper- atures and pressures that originate from short-lived microdischarges at sites over the metal surface and have fast oxide volume expansion. Anodic ZrO 2 films were prepared by subjecting metallic zirconium to PEO in acid solutions (H 2 C 2 O 4 and H 3 PO 4 ) using a galvanostatic DC regime. The ZrO 2 microstructure was investigated in films that were prepared at different charge densities. During the anodic breakdown, an important change in the amplitude of the voltage oscillations at a specific charge density was ob- served (i.e., the transition charge density (Q T )). We verified that this transition charge is a monotonic function of both the current density and temperature applied during the anod- isation, which indicated that Q T is an intrinsic response of this system. The oxide morphology and microstructure were characterised using SEM and X-ray diffraction experi- ments (XRD) techniques. X-ray diffraction analysis revealed that the change in voltage oscillation was correlat- ed with oxide microstructure changes during the breakdown process. Keywords Valve metals . ZrO 2 . Anodic films . Anodic breakdown . Microstructure . Phase transformation . Plasma electrolytic oxidation (PEO) Introduction Anodic oxide films on valve metals have been investigated since the 1950s [1–5]. The electronic, electrochemical and optical properties of these materials motivated our study in this area due to the different technological applications and the interesting fundamental aspects of oxide growth for protective coatings. Recently, interest in these materials has increased due to the potential for morphological and composition control on anodic alumina [6–8] and other valve metal oxides [9–14] produced by plasma electrolytic oxidation (PEO). During the oxide growth at the galvanostatic DC regime, the film thickness increases until it reaches a critical value. At this stage, an oxide rupture can occur, which is known as an electrolytic breakdown. This process is characterised by a decrease in the oxide growth rate, potential oscillations caused by the destruction and healing processes in the oxide film, and visible sparks over the surface of the substrate [3, 15–17]. Experimental observations reveal that the electro- lytic breakdown phenomenon depends on various experi- mental parameters, such as electrolyte composition [18–20], applied current density [21–24] and solution temperature F. Trivinho-Strixino Departamento de Física, Química e Matemática, Universidade Federal de São Carlos, São Carlos, SP, Brazil C. O. Paiva-Santos Laboratório Computacional de Análises Cristalográficas e Cristalinas, Depto de Físico-Química, Instituto de Química, Universidade Estadual Paulista, Araraquara, SP, Brazil E. C. Pereira (*) Departamento de Química, Universidade Federal de São Carlos, Rod. Washington Luis Km 235, Cx 676, São Carlos, SP, Brazil e-mail: decp@power.ufscar.br Present Address: D. X. da Silva Universidade Federal de Tocantins, Palmas, TO, Brazil J Solid State Electrochem DOI 10.1007/s10008-012-1883-4