Microstructural evolution of ceramic nanocomposites coated on 7075 Al alloy by plasma electrolytic oxidation

Alumina-zirconia nanocomposite coatings were prepared on 7075 Al alloy by plasma electrolytic oxidation (PEO) at current densities 0.1–0.3 A/cm2 in a Zr-containing electrolyte under DC galvanostatic mode. Coatings were investigated by X-ray photoelectron spectroscopy (XPS), X-ray and electron diffraction, and high-resolution transmission electron microscopy (HRTEM). The coatings developed at current density higher than 0.2 A/cm2 consist of γ-, t- and α-alumina and tetragonal zirconia and were composed of four sub-layers: (i) interface nanocomposite layer close to the substrate with a dense structure of heterogeneously distributed α-Al2O3 and t-Al2O3 nanocrystals (2–5 nm in size) embedded in an amorphous matrix, followed by (ii) a pure amorphous Al2O3 sublayer, (iii) a dense zirconia encapsulated alumina sublayer and (iv) a top amorphous sublayer with dispersed globular alumina and zirconia nanoparticles. The zirconia encapsulated alumina sublayer is composed of large t-Al2O3 and γ-Al2O3 grains bordered by fine t-ZrO2 nanoparticles. The t-ZrO2 phase was nucleated in an atomically coherent manner at Al2O3 grain boundaries. The microstructure evolution in the coating revealed alumina formation at the early stages of the process while the oxidation, crystallization and incorporation of zirconia occurred at the last stages of the PEO process. The phenomenology of the PEO Al2O3/ZrO2 nanocomposite coating formation is presented to account for the present experimental observations.