Study on the microstructure and defects of Al₂O₃/ZrO₂ nano-composites by positron lifetime spectroscopy

[Background] Al₂O₃/ZrO₂ nano-composites have excellent mechanical properties, high temperature stability and chemical stability, as well as good anti-irradiation effect. It is a potential nano-composites in practical industry. [Purpose] This study aims to investigate the microstructures and defects of Al₂O₃/ZrO₂ nano-composites at different annealing temperatures (room temperature-1000℃) by positron annihilation lifetime spectroscopy (PALS). [Methods] The Al₂O₃ and ZrO₂ nanoparticle powder with a mass ratio of 1:1 was ball-milled for 10 h and then pressed into pellets under a pressure of 5 MPa for 2 min. PALS was employed for detecting open-volume defects while X-ray diffraction (XRD) and transmission electron microscope (TEM) were used as auxiliary comparison tools for analyzing microstructures under various annealing temperatures. [Results] XRD results showed that the grain sizes of Al₂O₃, ZrO₂ and Al₂O₃/ZrO₂ nano-composites remained unchanged with annealing temperature below 500℃. With the increasing of annealing temperature from 500℃ to 1000℃, the grain sizes of ZrO₂ and Al₂O₃/ZrO₂ increased to 75 nm and 115 nm, respectively. PALS analysis showed that the main defects in the samples were vacancies, vacancy clusters and microvoids, and the defects mainly existed in the grain boundaries. When the annealing temperature was below 500℃, the total defect densities of the three samples were basically unchanged. Above 500℃, the open volume defects began to recover, and the change of ZrO₂ was more obvious, while the stability of Al₂O₃/ZrO₂ was higher because of inhibiting each other's grain growth. TEM images revealed that the grain size of nano-Al₂O₃ did not change with the annealing temperature below 500℃, but when the annealing temperature was higher than 500℃, the growth of nano-Al₂O₃/ZrO₂ was slow and agglomeration occurred. [Conclusions] Both grain size and defect density of Al₂O₃/ZrO₂ nano-composites remains unchanged with annealing below 500℃. Above 500℃, with the increase of annealing temperature, the grain of nano-Al₂O₃/ZrO₂ grew slower than nano-ZrO₂ and the openning volume defects began to recover which was confirmed by TEM observation. (authors)