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∙m^1/2. 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.

We have studied the thermal shock resistance of fine-grained Y_2.5Nd_0.5Al₅O₁₂ yttrium aluminum garnet (Nd:YAG) based composite ceramics reinforced with α-SiC particles (10–40 vol %), which are thought to be potentially attractive inert matrix fuel materials for nuclear power engineering. The ceramics containing 10, 20, and 30% SiC have high thermal shock resistance due to the compressive internal stress field formed during sintering. Raising the thermal shock temperature causes stable crack growth in the Nd:YAG/40% SiC composite ceramic. This is due to predominant crack propagation along boundaries between silicon carbide particles, which have lower strength in comparison with Nd:YAG/Nd:YAG and Nd:YAG/SiC boundaries.

The hydrolytic stability of ceramics based on Y_2.5Nd_0.5Al₅O₁₂ oxide with a garnet structure obtained by the spark plasma sintering (SPS) method has been studied. The tests were carried out in distilled water under hydrothermal conditions in an autoclave and, for comparison, in a static mode at room temperature. The mechanism of leaching of Y and Nd from the ceramics was investigated. It has been shown that at “low” temperatures (25 and 100 °C), the destruction of pores occured, and the intensity of the leaching process was limited by the diffusion of ions from the inner part of the sample to the surface. At “high” test temperatures (200 and 300 °C), intense destruction of the ceramic grain boundaries was observed. It was assumed that the accelerated leaching of neodymium is due to the formation of grain-boundary segregations of Nd³⁺ in sintered ceramics.

This article focuses on the process of producing ceramics based on the commercial Si₃N₄ powder of various dispersions (<5 μm and <1 μm) by spark plasma sintering (SPS). The powder mixtures of 97 wt % Si₃N₄ + 3 wt % additive of the Y₂O₃–Al₂O₃ composition are synthesized by the Pechini method. The SPS technology is used to obtain ceramic samples of ∅20 mm. The sintering is carried out in a vacuum at a heating rate of 50°C/min and a load of 70 MPa until the end of the shrinkage. The microstructure and phase composition of the ceramic samples are investigated. Mechanical properties are measured: Vickers hardness, Palmquist fracture toughness, and flexural strength according to the B3B (ball-on-three-balls test) method. Tribological tests are also carried out. It is established that the lower the dispersion of the powder mixtures based on Si₃N₄, the lower the end temperature of shrinkage. The relative density achieved is 96%. Ceramic materials based on Si₃N₄ powder with the dispersion of <1 μm are difficult to machine; they have the hardness of 19.0 ± 0.7 GPa and the crack resistance of 5.1 ± 0.4 MPa m^1/2. The flexural strength of the ceramics evaluated by the B3B method depends on the dispersion of Si₃N₄ powder; it is more than two times higher for the ceramics based on the commercial Si₃N₄ powder with the dispersion of <1 μm.

We have studied the thermophysical properties (specific heat, thermal diffusivity, and thermal conductivity) of fine-grained ceramic composites based on yttrium aluminum garnet, Y_2.5Nd_0.5Al₅O₁₂ (Nd:YAG), differing in silicon carbide (α-SiC) content. The thermal conductivity of the Nd:YAG/SiC composites has been shown to exceed that of CeO₂/SiC and Nd:YAG/Ni composites. The high thermal conductivity of Nd:YAG/SiC is due to the formation of a grain microstructure with a bimodal grain size distribution, in which large garnet grains are surrounded by regions enriched in the high-thermal-conductivity phase α-SiC.