Segregation as a driving force in the formation of nanocomposite ZrO₂-Al₂O₃ coatings

ZrO₂-Al₂O₃ coatings were deposited on a floating substrate at a temperature of 370 °C. Various coating compositions, ranging from pure zirconia to 50 at.% alumina content, were deposited by reactive pulsed-DC magnetron sputtering using 51 mm diameter Zr and Al targets. The coating had a Zr-Al-O solid solution structure that was composed of nano-size cubic-ZrO₂ grains (20 ± 5 nm) with aluminum cations distributed inside them. The deposited coatings were annealed up to 1350 °C and their structural changes were studied using Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM) and Vickers hardness measurement. Two exothermic annealing events were observed. The first event appeared at 700–750 °C and was contributed to aluminum segregation to the grain boundaries, and the formation of a nanocomposite-ZrO₂/amorphous-Al₂O₃ structure. The second event appeared at 1000–1200 °C and was related to coarsening (“Ostwald ripening”) of the ZrO₂ grains. The coating hardness after the first event was stabilized to 19.5 ± 1.1 GPa because of the segregation, which hindered grain growth. However, after a second DSC cycle of the coating, only one sharp endothermic transformation peak was observed at onset temperature of 1060 ± 5 °C. This was associated with grain growth accompanied with zirconia transformation from the cubic to the monoclinic phase.