| Summary: | The Si<sub>3</sub>N<sub>4</sub> ceramics were prepared in this study by gas pressure sintering (GPS) and spark plasma sintering (SPS) techniques, using 5 wt.% Yb<sub>2</sub>O<sub>3</sub>−2 wt.% Al<sub>2</sub>O<sub>3</sub> and 5 wt.% CeO<sub>2</sub>−2 wt.% Al<sub>2</sub>O<sub>3</sub> as sintering additives. Based on the difference in sintering methods and sintering additive systems, the relative density, phase composition, phase transition rate, microstructure, mechanical properties, and thermal conductivity were comparatively investigated and analyzed. SPS proved to be more efficient than GPS, producing higher relative density, bending strength, hardness, and thermal conductivity of Si<sub>3</sub>N<sub>4</sub> ceramic with both additive systems; however, the phase transition rate and fracture toughness were lower. Similarly, higher bending strength, hardness, and thermal conductivity were achieved with Yb<sub>2</sub>O<sub>3</sub>−Al<sub>2</sub>O<sub>3</sub> than CeO<sub>2</sub>−Al<sub>2</sub>O<sub>3</sub> in the case of GPS and SPS, and only the relative density, fracture toughness, and phase transition rate were lower.
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