Impact of alumina powder bed on hardness and fracture toughness in the sintering process of NiO-GDC-Bi₂O₃ composite prepared by sol-gel method

This study explores the sintering process of NiO–Ce_0.9Gd_0.1O_1.95 + 35 wt% Bi₂O₃ ceramic composites synthesized via the sol-gel method, with a focus on the influence of an alumina powder bed during sintering. Uniaxially pressed disks were sintered at 890 °C for varying durations (0, 12, 24, and 61 h), with an additional disc sintered at 1450 °C for 6 h for comparative analysis. Sintering processes employed alumina as a powder bed, and subsequent evaluation included hardness and fracture toughness assessments. Relative densities of 53% and 48% were achieved for composites sintered at 890 and 1450 °C, respectively. The maximum hardness of 4.13 ± 0.13 GPa was attained by the composite sintered at 890 °C for 61 h, while the highest fracture toughness (2.98 ± 0.35 MPa.m^1/2) was observed in the composite sintered at 1450 °C for 6 h. Evidence indicates that an interaction between Bi₂O₃ and the alumina bed occurred, adversely affecting the mechanical properties by promoting the depletion of Bi₂O₃, especially at elevated sintering temperatures.