Spark Plasma Sintering of Si3N4 Ceramics with Y2O3–Al2O3 (3%–10% wt.) as Sintering Additive

The ceramic samples fabricated by spark plasma sintering of powder mixtures based on silicon nitride (Si3N4) were investigated. The powder mixtures were made by wet chemical methods from commercial α-Si3N4 powder (the particle size <5 μm) and Y2O3-Al2O3 sintering additive (3% to 10% wt.). Sintering was carried out at the heating rate of 50 °C/min and the load of 70 MPa until the shrinkage end. The powder mixtures and ceramic samples were characterized by scanning electron microscopy and X-ray diffraction. The shrinkage of the powder mixtures during sintering was analyzed, and the activation energy of sintering was calculated according to the Young-Cutler model. The density, microhardness, and fracture toughness of the ceramic samples were also measured. All samples had high relative densities (98%–99%), Vickers microhardness 15.5–17.4 GPa, and Palmquist fracture toughness, 3.8–5.1 MPa∙m1/2. An increase in the amount of sintering additive led to a decrease in the shrinkage temperature of the powder mixtures. The amount of β-Si3N4 in the ceramics decreased monotonically with the increasing amount of sintering additive. The shrinkage rate did not decrease to zero when the maximum compaction was reached at 3% wt. of the sintering additive. On the contrary, it increased sharply due to the beginning of the Si3N4 decomposition.