Influence of the Morphology and Size of SiC Particles on the Mechanical Characteristics of SiC-Ceramics

The hot press method was used to obtain the SiC composite from different morphology and partial size powder. The starting SiC powder was: 1) fragmentation particle shape industrial charge of silicon carbide obtained by the Acheson method with sintering additive (9 wt.% Y2O3 − Al2O3) by Saint Gobain, 2) spherical particles SiC obtained by SHS in a laboratory. Bending strength, critical stress intensity factor, density, and the friction coefficient of samples of the obtained ceramics were determined. It has been established that the properties of ceramics obtained from SHS silicon carbide powder (spherical particles with sizes of 100–400 nm), due to better compaction, were at least 10% higher than samples from Saint Gobain powder: bending strength (400 ± 22 MPa), density (3.23 ± 0.01 g/cm3), critical stress intensity factor (К1С = 4.8 ± 0.3 MPa∙m1/2), friction coefficient (0.1126 ± 0.0031).

SiC ceramics has a high level of characteristics: low thermal coefficient of linear expansion, high Vickers hardness and bending strength, and low density.However, physical-chemical and mechanical properties of ceramic-based silicon carbide may vary depending on its preparation method, the morphology and particle size of the initial powder, and the presence and content of impurities [9].
Thus, the purpose of this work is to study the effect of the method of obtaining silicon carbide powder on the dispersion and morphology of silicon carbide powders, as well as the properties of ceramics obtained by hot pressing.

Materials and Methods
Silicon carbide powders from Saint Gobain (France) and powders obtained by the self-propagating hightemperature synthesis (SHS) were used to prepare SiC ceramics.
Silicon carbide powder from Saint Gobain (SiC SG ) is a mixture of silicon carbide powders and 9 wt.% oxide sintering additive (Y 2 O 3 -Al 2 O 3 , 3/5 ratio).Another silicon carbide powder was obtained under laboratory conditions by the SHS from Si + C elements (SiC SHS ).Since the thermal effect of the direct reaction of silicon with carbon is insufficient for carrying out the synthesis in the self-propagating combustion mode, the synthesis was carried out under nitrogen pressure.Nitrogen is an additional reagent that increases the energy of the process due to the additional Si + N 2 reaction.The combustion temperature increased to 1900-1950 °C, which ensured the primary reaction of silicon with carbon during the combustion of the Si + C charge in a nitrogen atmosphere because of the silicon and nitrogen interaction.SiC obtained in the reaction contains 6-8 wt.% silicon nitride.An excess of silicon (3-5 wt.%) from the stoichiometry for nitrogen fixation was introduced into the charge to minimize free carbon content in final SiC.A mixture of silicon KR-00 (GOST 2169-69) and carbon powder (soot P-804 T) was mixed in a ball mill for 1 hour.
Then, the initial mix was poured in portions into a paper cup and compacted with a pestle.The compaction of the charge is necessary to limit the filtration of nitrogen into the workpiece and thus create conditions for a complete interaction of silicon with carbon.The preform obtained was placed in the SHS reactor, and an ignition coil was connected.Then the reactor was closed and filled with nitrogen to a pressure of 30 atm and initiated combustion.Combustion proceeds at a rate of 0.3-0.4mm/sec.After the synthesis and cooling, one removed the resulting SiC sinter from the reactor, and the surface of the sinter was cleaned from unreacted charge components.After stripping, the sample was subjected to crushing and grinding.
Ceramic samples were obtained by firing silicon carbide powders by hot pressing (Thermal Technology Inc. press, model HP20-3560-20) at 1850 °C and 30 MPa in a protective argon atmosphere.This temperature was chosen based on preliminary experiments, during which the strength characteristics of ceramics obtained under various firing temperatures (1750-1900 °C) were measured.Hot pressing at temperatures of 1850 °C and 1900 °C makes it possible to obtain dense ceramic samples with fairly high strength parameters, which depend on the characteristics of the initial powders.It is not advisable to carry out hot pressing at temperatures above 1850 °C, since due to the intensification of recrystallization, the strength characteristics of ceramics are reduced.The strength properties of the studied samples during firing in the temperature range of 1700 °C-1750 °C are also lower due to the incomplete ceramic compaction process.Hot pressing (HP) is a process of obtaining products by sintering powder masses with simultaneous exposure to temperature and pressure.
Silicon carbide in its pure form does not have sufficient plasticity up to the decomposition temperature, and, accordingly, to achieve density values close to theoretical, it is necessary to apply high pressure and exposure to very high temperatures (up to 2300 °C) [10].In this regard, a small amount of sintering additives is added to the charge.The use of additives makes it possible to intensify the sintering process of silicon carbide by accelerating the phase transition β-SiC→α-SiC, and carrying it out at temperatures up to 2000 °C.Thus, the addition of suitable sintering aids leads to the formation of dense fine-grained microstructures, which leads to an increase in the strength of the sintered body.
Often, only aluminum oxide (Al 2 O 3 ) [11] is used as an activating additive, and its combination with other compounds, as a rule, oxides of rare earth elements.The sample is preliminarily molded by the method of uniaxial double-sided pressing, then placed in a graphite mold.The maximum realizable pressure during hot pressing is 50 MPa -this is due to the compressive strength of the graphite mold.The same sintering additive was used for SiC SHS powder as in the Saint Gobain charge (i.e., 10 wt.% Y 2 O 3 -Al 2 O 3 in the ratio of 3/5) to ensure the ceramic sample properties comparability.At a concentration of sintering additives above 10 wt.%, an excess of the resulting liquid phase is squeezed out of the sample, which leads to difficulty in extruding the sample, which can lead to damage to the mold and sample.SiC SHS powder was mixed with the sintering additive Y 2 O 3 -Al 2 O 3 (ration of 3/5) in a planetary mill in an isopropyl alcohol medium with zirconium dioxide balls for 20 minutes to prepare a batch.The resulting mixture of SiC SHS powder and Y 2 O 3 -Al 2 O 3 sintering additive was dried in an oven and then used to form workpieces by uniaxial one-sided pressing in a metal mold at 100 MPa pressure.The molding of preforms from SiC SG powder was carried out under the same conditions, without additional processing.The use of the process of hot pressing of silicon carbide samples with the oxide additive was able to reduce the firing temperature to 1850 °C (with liquid-phase sintering at 1930 °C [12]), as well as to limit grain growth during ceramic sintering, thereby reducing the likelihood of pores in the ceramic sample.
The phase composition of SiC was analyzed by an X-ray diffractometer (XRD, DRON-3) (CuKα radiation (λ = 1.5418Å), CоKα radiation (λ = 1.79020Å), scanning rate 2θ = 2 deg/min).The morphology of powder particles and the microstructure of ceramic samples were studied using a scanning electron microscope (SEM, Tescan Vega II SBU).The ceramics density was determined by the Archimedes method.The relative density was calculated based on the theoretical densities SiC 3.23 g/cm 3 and YAG 4.5 g/cm 3 .Ceramic specimens' bending strength values were determined on a mechanical testing machine using the three-point bending method (Instron 5581).The critical stress intensity factor was measured using the notch method.The wear resistance of ceramics was determined using a scanning nanohardness tester (NanoScan-3D).The NanoScan-3D scanning nano-hardness tester is designed to study the relief and structure of surfaces and measure the mechanical properties (including hardness and modulus of elasticity) of bulk materials and thin films on a submicron and nanometer scale..A piezo resonant sensor-cantilever of a tuning fork design with a higher bending stiffness of the cantilever is a sensitive element of the device on which the test tip is installed [13,14].The study used a diamond tip in a trihedral pyramid of the Berkovich type (the angle between the height and the adjacent face is 65°).Wear was measured by moving the tip into contact with the surface while maintaining a normal pressing force along a reciprocating path.The study was carried out at a load of 500 mN, the number of cycles was 500, and the length of the wear tracks was 3 mm.During the measurement, the relative depth of the tip into the surface of the test sample was recorded as a function of time.Figure 2 shows the SEM of silicon carbide powder obtained by the SHS.It can be seen that, in contrast to SiC SG , SiC SHS particles (both individual grains and their agglomerates) mainly have a shape close to spherical, and the particle sizes are 100-400 nm.Particles of an elongated shape, which is unusual for silicon carbide, are observed.X-ray phase analysis of the mixture was performed to establish the phase composition of the powders.The elongated particles in Fig. 2 (insert), according to the data available in the literature [15], most likely correspond to β-Si 3 N 4 , which can contribute to the formation of the reinforcement effect during sintering [16].

Results and Discussion
Samples were obtained by hot pressing at a temperature of 1850°С and a pressure of 30 MPa in a protective argon atmosphere for 30 min.SiC ceramic samples were ground on an automatic grinding and polishing machine.Then, samples were made from them for testing strength and crack resistance in the form of beams with dimensions of 5x5x25 mm using a cutting machine with a diamond disk.
Figure 4 show the diffraction patterns of the initial SHS silicon carbide powders.According to the data of X-ray phase analysis, the powder consists of two modifications of silicon carbide (α and β).
Since the SHS is an uncontrolled process during combustion, the temperature gradient in the preform leads to a partial transformation β ➔ α.In this regard, the SiC powder obtained by the SHS consists of a mixture of 3C-SiC (96-101-0996) and 6H-SiC (96-101-1054) polytypes with a phase ratio of 66.8 wt.% and 25.1 wt.%.The powder also contains a mixture of silicon nitrides -α-Si 3 N 4 (96-100-1239) and β-Si 3 N 4 (96-100-1246) with a phase ratio of 2.8 wt.% and 5.3 wt.%.The bending strength (σ ben ) of hot-pressed ceramics from SiC SHS powder is 8% higher than from SiC SG powder.
The critical stress intensity factor for ceramics (К 1С ) from Saint Gobain and SHS silicon carbide powders are 4.5 ± 0.2 MPa•m 1/2 and 4.8 ± 0.3 MPa•m 1/2 , respectively (i.e., approximately 7%).The SiC SHS ceramics density is higher than that of SiC SHS ceramics (Table 1).This is because the SHS silicon carbide powder particles, due to the spherical shape of the particles and the size of 100-400 nm, are compacted better than the SiC SG powder of the fragmented form of particles with a size of 1 μm.In addition, the presence of elongated silicon nitride grains in SiC SHS powder contributes to the creation of a reinforced structure since elongated grains are also observed after sintering (Fig. 5).Ceramics made from silicon carbide powder of the Saint Gobain brand is represented by fragmented grains, and has small pores (Fig. 6).
Figure 7 shows 3D models of grooves formed on ceramic samples from Saint Gobain and SHS silicon   carbide powder after wear testing, respectively Fig. 7a  and b.The studies were carried out according to the sclerometer method -the application of microgrooves to study wear resistance.The friction coefficient (f) for ceramics was calculated according to the described procedure [13,14].The ceramics friction coefficients are f = 0.5035 ± 0.0029 from Saint Gobain powders and f = 0.1126 ± 0.0031 form SHS powders.Therefore, the wear of ceramics from Saint Gobain powders is significantly higher than the wear of ceramics from SHS silicon carbide powder, according to the experimental data.

Conclusion
Thus, in the research, samples of silicon carbide ceramics were obtained by hot pressing from initial powders differing in particle size and morphology -Saint Gobain commercial charge (fragmentation particle shape, average size 1 μm) and SHS silicon carbide (spherical particle size with dimensions up to 400 nm).
The influence of the method of obtaining initial silicon carbide powders (different in size and morphology of grains) on the physicochemical and mechanical properties of ceramic samples is presented.Ceramics obtained from SHS silicon carbide powder is compacted better, it provides an increase in the properties of ceramics by 10%, relative to ceramics from Saint Gobain powder due to the spherical shape and significantly smaller particle size (up to 400 nm).
In addition, the presence of elongated silicon nitride grains exhibits the effect of reinforcing the structure, which, in turn, can favorably affect the strength characteristics of ceramics.
It has been established that samples obtained by hot pressing from SHS silicon carbide powders have a bending strength of 400 ± 22 MPa, a density of 3.23 ± 0.01 g/cm 3 , a critical stress intensity factor for ceramics K 1C = 4.8 ± 0.3 MPa•m 1/2 , friction coefficient f = 0.1126 ± 0.0031.Konstantin A. Kim − investigation properties of the material.Yury F. Kargin − planning an experiment, processing measurement results.
Funding The reported study was funded by the scholarship of the President of the Russian Federation SP-472.2022.3.The methodical part of the work (XRD, SEM, flexural strength, density) was carried out according to the state task № 075-01176-23-00.

Figures 1
Figures1 and 2show the morphology of both initial silicon carbide powders.Figure1is the SEM of SiC SG powder.The bright areas are components of the oxide additive

Table 1
Properties of SiC ceramics obtain from SiC SG and SiC SHS powder