The article presents the results of the investigation of the mechanism of the densification behavior of alumina-based ceramics during spark plasma sintering. The role of the heating rates and additives were investigated. The first (initial) stage of sintering was investigated by the Young–Cutler model. The second (intermediate) stage of sintering was investigated as a process of plastic deformation of a porous body under external pressure. It was shown that, at the initial stage, the formation of necks between the particles is controlled by grain boundary diffusion (the activation energy is Q_b ≈ 20 kT_m). At this stage, accommodation of the shape of the alumina particles is also occurring (an increase in the packing density). The accommodation process facilitates the shrinkage of the powder, which is reflected in a decrease in the effective activation energy of shrinkage at low heating rates (10 °C/min) to Q_b ≈ 17 kT_m. At heating rates exceeding 10 °C/min, the intensity of the processes of accommodation of alumina particles turns out to be much slower than the existing diffusion processes of growth of necks between the alumina particles. It was shown that the grain boundary sliding mechanism that occurs in the second stage of sintering can play a decisive role under conditions of spark plasma sintering with a high heating rate. The found value of the activation energy at the second stage of sintering is also close to the activation energy of grain–boundary diffusion of alumina (Q_b ≈ 20 kT_m). The influences of the second phase particles of MgO, TiO₂, and ZrO₂ on densification behavior of alumina-based ceramics were investigated. Since at the first stage of sintering the densification relates with the formation of necks between the particles of alumina, the additives (0.5% vol) have no noticeable effect on this process. It was also shown that the second phase particles which are located at the grain boundaries of alumina are not involved in the slip process during the second sintering stage. Analysis shows that additives act only in the final (third) stage of spark plasma sintering of alumina.

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.

Spark plasma sintering and a standard sintering method are used to manufacture ceramic plates with a diameter of 30 mm from aluminum oxide powder with a fraction size approximately equal to 3 µm. Dynamic strength of the ceramics obtained is studied. Dynamic loading is carried out by a hardened steel impactor. It is shown that energy dissipation in a material obtained using spark plasma sintering is substantially larger than that in a material produced using standard sintering, which means that it is more resistant to dynamic loads.

A new method of mechanical testing of high-strength ultrafine-grained ceramic materials by the method of spark plasma sintering (SPS) is proposed. A promising scheme for testing cylindrical SPS-samples is the “Ball on three balls test” (B3B) scheme. The B3B test method has advantages over the standard four-point bending test method, since it is not necessary to cut standard samples from disks, cylindrical samples are aligned on a special support. In this paper, we present a methodology for processing experimental B3B test data developed using the finite element method. The finite element method for estimating the parameters allows us to consider a larger number of parameters that affect the absolute value of the maximum principal stress. The experimental data processing technique takes into account the influence of the sample size, equipment geometry, physical and mechanical properties of the material, and friction coefficient.

The article shows the use of an experimental-calculation method for the refinement of thermoelectric constants of graphite materials and alumina ceramics by experimentally studying the heating of graphite equipment and refinement thermoelectric constants of materials using the finite element method (ANSYS WORKBENCH). To simulate the thermal fields of graphite equipment based of the data on electric current, voltage and heat exchange conditions, a coupled boundary electric and non-stationary temperature problem was realized. The application of the developed approach made it possible to clarify the thermoelectric constant of materials, which helped to reduce the difference between the experimental and simulated temperatures from 20 to 5%.