The onset mechanism of flash sintering in Al ₂ O ₃ –8YSZ composites

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Understanding the onset mechanism of flash sintering is essential for advancing electric-field-assisted sintering technologies. Herein, the onset temperature of flash sintering was examined for alumina–8 mol% yttria-stabilized zirconia (Al₂O₃–8YSZ) composites with varying molar ratios of Al₂O₃ and 8YSZ under an applied electric field of 900 V/cm. The results show a composition-dependent variation in onset temperature, which can be divided into three different regions based on Al₂O₃ content, each reflecting a different onset mechanism. In region Ⅰ (0–62.5 mol%), the flash sintering behavior is dominated by 8YSZ owing to the internal electrochemical reaction driven by electric field. In region Ⅱ (62.5–80 mol%), flash sintering is determined by the percolation networks of 8YSZ, which offer conduction paths for current. In region Ⅲ (80–99 mol%), isolated 8YSZ particles catalyze the flash sintering of Al₂O₃ through interfacial dielectric breakdown. These results highlight a composition-dependent transition in onset mechanism of flash sintering: composites with low Al₂O₃ content exhibit a defect-dominated flash sintering associated with 8YSZ, while those with high Al₂O₃ content follow a thermally-controlled mechanism. Thus, Al₂O₃ and 8YSZ exhibit distinct onset mechanisms during flash sintering.