Aluminum and its alloys are used widely in aerospace, automotive, architectural, lithographic, packaging, electrical, and electronic applications, attributed to their attractive properties, such as high electrical and thermal conductivity. Unfortunately, their extensive applications are restricted by the limitations in their corrosion resistance and tribological properties, especially in chloride environments. Depositing alumina-zirconia composite coatings is a well-known strategy for improving these properties. Alumina-zirconia benefits from the combination of the high fracture toughness of zirconia with the high hardness of alumina. To this end, alumina-zirconia composite coatings have drawn significant research attention due to their prominent properties.
This composite has been applied as a thermal barrier coating with remarkable oxidation and corrosion resistance and tribological properties using different spray and coating methods. Similarly, plasma electrolyte oxidation (PEO) is a well-known technique for coating valve metals like aluminum and their alloys to improve corrosion resistance and mechanical properties. It involves the application of high voltages on metal substrates immersed in electrolytes, followed by arching and plasma creation. The advantages of this technique are its environment-friendly nature, ability to form crystalline phases with high adhesion strength and applicability to a wide range of oxides, nitrides and carbide coatings.
The fabrication of nanocomposite coatings with enhanced corrosion and wear resistance through PEO have been reported. A good example is the coating produced by incorporating alumina-zirconia composites into magnesium alloy via PEO. Equipped with this knowledge, Dr. Nastaran Barati, Professor Jiechao Jiang and Professor Efstathios Meletis from the University of Texas at Arlington fabricated the alumina-zirconia nanocomposite coatings on 7075 Al alloy using PEO. The nanocomposite coatings were fabricated at current densities 0.1 – 0.3 A/cm2 in an electrolyte containing Zr. The porosity and discharge channel formation as well as the coating process, were discussed in detail. Their work is currently published in the journal, Surface and Coatings Technology.
The research team showed that incorporating the prepared alumina-zirconia composite coatings improved the aluminum alloys. Compared with uncoated 7075 Al, the wear resistance of the coated alloy significantly improved by 120 times while its corrosion rate decreased by 2.5 orders of magnitude. Detailed study of the voltage-time variations during coating, using X-ray and electron diffraction and X-ray photoelectron spectroscopy, revealed the formation of alumina during the initial stages of the PEO process while oxidation, crystallization and incorporation of zirconia occurred during the late stages.
The alumina-zirconia composite coatings prepared by the authors at current densities below 0.3 A/cm2, specifically 0.2 A/cm2, consisted of t-, γ- and α-alumina as well as tetragonal zirconia. Throughout the coating process, these coatings formed sub-layers: (1) non-porous nanocomposite sub-layer formed at the substrate interface consisting of heterogeneously distributed t-Al2O3 and α-Al2O3 nanocrystals; (2) pure amorphous Al2O3 sublayer consisting of a small amount of Zr; (3) dense nanocomposite sublayer composed of large t-Al2O3 and γ-Al2O3 grains whose boundaries were decorated with t-ZrO2 and (4) top amorphous sublayer with dispersed zirconia and globular alumina nanoparticles. Furthermore, the nucleation of the t-ZrO2 phase was atomically coherent at the Al2O3 grain boundaries.
In summary, University of Texas at Arlington scientists reported a in depth comprehensive study of the microstructural development of ceramic nanocomposites coated on 7075 Al via PEO. The presentation of the formation process of the PEO Al2O3/ZrO2 nanocomposite coating accounted for the present experimental observations. In a statement to Advances in Engineering, Professor Efstathios Meletis, the lead and corresponding author stated that the new surface treatment improves significantly the tribological and corrosion resistance properties of aluminum alloys, thereby expanding their application scope.
Barati, N., Jiang, J., & Meletis, E. (2022). Microstructural evolution of ceramic nanocomposites coated on 7075 Al alloy by plasma electrolytic oxidation. Surface and Coatings Technology, 437, 128345.