The impact of RE ₂ O ₃ ‒Al ₂ O ₃ additives on Si ₃ N ₄ ceramics prepared by the foam precursor method

Abstract This study examines how various rare‐earth oxides (RE 2 O 3 , where RE represents La, Gd, and Lu) combined with Al 2 O 3 as sintering aids affect the microstructure and mechanical properties of silicon nitride (Si 3 N 4 ) ceramics. The Si 3 N 4 ceramics were fabricated using the foam precursor method followed by hot‐press sintering. Results indicate that as the ionic radius of the rare‐earth dopant increases, the β‐phase content in the ceramic foams significantly rises. However, the impact on the size of β‐Si 3 N 4 grains is not significant. In dense ceramics, the average diameter and aspect ratio of β‐Si 3 N 4 grains increase with the decrease in the size of rare‐earth ions. The Si 3 N 4 ceramics doped with Lu (SNU) exhibited the highest flexural strength (1224 ± 163 MPa) and fracture toughness (10.5 ± 1.0 MPa·m 1/2 ), which is mainly attributed to their high aspect ratio and large‐sized β‐Si 3 N 4 grains, as well as the grain alignment perpendicular to the pressure direction formed during the hot‐press sintering process. The research indicates that the properties of Si 3 N 4 ceramics can be further enhanced by optimizing the types of RE 2 O 3 and the sintering process. Moreover, the foam precursor method holds, as a novel technique, significant potential for high‐performance enhancement of Si 3 N 4 ceramics.