There are several types of sialons: α, β, X, O1, H, R [1] - [4] . They can be used in oxidizing environment up to 1300˚C and in protective environment up to 1800˚C [5] - [7] . Among these diverse types of sialons, they primarily exist in three phases: α-sialon, β-sialon, and amorphous or partially crystallized phases at the grain boundaries. α- and β-sialons are characterized by unique properties, exhibiting higher hardness and toughness compared to conventional silicon nitride. The α-sialon phase is characterized by higher hardness than the β-sialon phase. β-sialon, like conventional silicon nitride, is distinguished by its higher impact strength. Ceramics, in general, are characterized by high hardness and wear resistance but tend to be brittle. Therefore, our focus was more on the β-sialon phase to obtain a composite with relatively higher impact strength and crack resistance. This is precisely why the composite was chosen to be produced using aluminosilicate raw materials through reaction sintering in a nitrogen atmosphere by the alumino-thermic method, which ensures the formation of the β-sialon phase at a lower temperature compared to the other methods.

The material composition of the research object is provided in Table 1 .

The chemical composition of the Pologi clay (wt.%) is: SiO₂—47.92, Al₂O₃—35.20, Fe₂O₃—2.06, CaO—0.40, MgO—0.30, and firing loss—12.24. The refractoriness is 1710˚C - 1730˚C.

The chemical composition of kaolin (wt.%) is: SiO₂—46.45, TiO₂—0.33, Al₂O₃—38.70, Fe₂O₃—0.46, MgO—trace, CaO—0.36, Na₂O—0.45, K₂O—0.60, Loss on ignition—13.63. Refractoriness—1770˚C.

A detailed description of the composite preparation is provided in the works [8] - [23] . The phase analysis of the consolidated sample after hot pressing is presented in Figure 1 .

As shown in Figure 1 , the X-ray diffraction pattern of the CN-8 composite clearly exhibits the characteristic reflections of sialon—dhkl: 6.65; 5.45; 3.87; 3.67; 2.520; 2.21. Additionally, the diffraction maxima characteristic of corundum are observed—dhkl: 3.49; 2.52; 2.36; 2.09, which were introduced into the phase composition to reinforce the sialon phase.

The physical and technical properties of the obtained samples were studied. The results are presented in Table 2 .

The porosity and density of the samples were determined using the hydrostatic weighing method, while the compressive and bending strengths were measured on a 2054 P5 brand testing machine. The obtained results ( Table 2 ) indicate that the samples of the composite, with the selected composition, obtained by hot pressing at 1650˚C and 30 MPa pressure, are consolidated, and the phase composition aligns with the intended goal, as confirmed by the electron microscopy studies. Figure 2 presents the electron microscope image of the fracture surface of the CN-8 composite obtained at 1650˚C.

The compressive and flexural strength was measured using a German-made tensile testing machine, model R-100, which is equipped to determine the strength limits of samples under compression and three-point bending. The loading rate was 5 mm/min. The compression test was conducted in accordance with GOST 27034-86 (ISO 4506-79). Flexural testing of the ceramic materials was carried out according to GOST 20019-74. In determining the flexural strength limit, the maximum stress was calculated using the following formula:

σ_f = 3/2∙Pl/bh²

where:

The sample was a bar with a square cross-section, length l = 35 mm, width b = 5 mm, height h = 5 mm, and the distance between support points was 25 mm.

As seen from Figure 2 , the composite consists of two solid phases: fine corundum particles arranged in the sialon matrix, with the porous phase also represented by a small number of fine pores.

The fracture images clearly show that the material exhibits a plastic failure behavior, with cracks forming in the sialon phase. The propagation of these cracks is limited both by the sialon and the finest corundum nano-particles.

The pores are mainly rounded, and the average pore diameter has been calculated ( Table 3 ). The total volume of closed pores reaches 1.58%, which shows a slight difference from the pore volume determined by the hydrostatic method, which can be attributed to measurement inaccuracies inherent in each method. No penetrable or semi-penetrable pores are observed in the matrix. Based on the morphological images, the distribution of pores in the material is both uniform and non-uniform. It is assumed that the pore distribution factor in the matrix, according to Z. Kovziridze’s formula, is 0.9.

The formula of Z. Kovziridze was used to calculate the dependence of the macromechanical properties of the obtained composite on the porous phase in the matrix.

${\sigma }_{\text{m}/\text{p}}=\frac{\text{P}}{{\text{P}}_{\text{m}}\cdot {\text{F}}_{\text{p}}\cdot {\text{P}}_{\text{d}}\cdot {\text{P}}_{\text{vol}}}.$

Where P is the load, MPa; ${\text{F}}_{\text{p}}$—the form factor of the pores; ${\text{P}}_{\text{d}}$—the distribution factor of pores in the matrix. This value is equal to 1, and its value depends on the researcher, based on the morphological image. Depending on how the pores are distributed in the material and their size, the value of this factor can range from 1 to 0.8 If the pores are evenly distributed in the matrix and are approximately of the same size, the factor will be equal to 1. The factor is equal to 0.9 if the pore distribution is uneven, and finally, it is equal to 0.8 if the coalescence¹ process of the pores has started. P_vol.—the volumetric fraction of the porous phase in the matrix; P_m—the middle size of the pores.

${\sigma }_{\text{m}/\text{p}}=\frac{470}{0.4\times 1.58\times 0.9\times 13.3}=\frac{470}{7.56}=62.2\text{MPa}$

The average pore size is 0.855, taken from the sample fired at 1650˚C: 0.13, equal to the value obtained by the hydrostatic weighing method and the visual analysis of the electron microscopy images—1.58 mass%.

The composite in the Al₂O₃-SiAlON system was synthesized using a metallothermic and nitrogen-assisted reactive sintering method. To obtain a dense material, the porous (13% - 15%) composite was finely ground in an attritor, then hot-pressed at 1650˚C. Subsequent investigations were carried out using micro- and macromechanical methods, as well as structural-optical and electron microscopy techniques. The porous phase was examined, and its percentage content and size were determined, along with the percentage content and grain sizes of the crystalline components—silicon nitride and aluminum oxide.

The results of the study indicate that β-sialon with a silicon nitride structure has been obtained. The process is facilitated by the presence of an imperfect crystalline phase of newly formed silicon nitride at lower temperatures, which, due to its relatively large porosity, incorporates aluminum oxide and aluminum nitride into the structure. Subsequently, at higher temperatures of 1350˚C - 1450˚C, the β-sialon structure is formed.

The obtained material exhibits high operational properties. The bending strength is 470 MPa, while the compressive strength is 1923 MPa. Micromechanical analysis showed that no crack forms in the sialon matrix during the loading process.

The composite’s high operational properties were confirmed by calculating the mechanical modulus using Kovziridze’s method, with the obtained result exceeding expectations. This modulus allows for a realistic assessment of the material’s resistance to external loading. The material was obtained by solid-phase sintering.

The composite was also tested using Z. Kovziridze’s formula—the relationship between macromechanical properties and the content of the porous phase in the matrix.

To determine the micro- and macromechanical properties as a function of the crystalline phase content in the matrix, Kovziridze’s formula was also used. A high correlation of 0.811 was obtained.

¹Coalescence—the growth of pores in a solid body, accompanied by a reduction in their total surface area while maintaining the overall volume. The process of pore coalescence is observed in the final stages of sintering and is characterized by the increase in the size of larger pores due to the vacancy dissolution of smaller pores.

²Vphph—Volume of the porous phase.

³Correlation—A linear relationship between random events. The measure of correlation is the empirical correlation coefficient. In scientific literature, correlation refers to a statistically significant relationship between parameters of different processes. This definition of the term is not rigid.