The effect of vacuum annealing (1750 °C, 3 h) on the phase composition, structure, and properties of the surface layer of fine-grained α-WC + W2C ceramics has been studied. Ceramics of the α-WC + 10% W2C composition with a high relative density and an average grain size of 0,1–0,2 μm were produced using the Spark Plasma Sintering (SPS) method. Vacuum annealing of the ceramic samples was carried out in two modes: (i) annealing of samples in contact with graphite; (ii) annealing of samples without contact with graphite. Changes in the phase composition and microstructure of the surface layers of ceramics after annealing have been studied using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Microhardness, nanohardness, and Young's modulus of the ceramics were measured after annealing. It has been shown that contact between the ceramic sample and graphite leads to carbon diffusion and transformation of W2C into α-WC. Vacuum annealing of the samples not in contact with graphite results in the transformation of α-WC into W2C and tungsten. Microstructure areas containing only tungsten are characterized by abnormally large grains up to ∼80 μm in size, elongated perpendicular to the surface. After annealing, areas with increased microhardness up to ∼40 GPa were found on the surface of ceramics.

We report on the fabrication and optical properties of erbium-doped Y2O3-MgO and Gd2O3-MgO nanocomposite ceramics, with a particular focus on their mid-infrared emission. The ceramics were fabricated by a self-propagating high-temperature synthesis followed by hot pressing. The transmittance of 1.7 mm-thick 5 at.% Er:Y2O3-MgO and 7 at.% Er:Gd2O3-MgO ceramics at 3 mu m amounted to 79.4% and 21.5%, respectively. This difference in transmission, as revealed by microstructural analysis, is due to variation in the distribution of residual pore sizes, while the average grain size is almost the same for both ceramics, being similar to 200 nm. The composites exhibited luminescence in the visible, near-infrared, and mid-infrared spectral ranges attributed to electronic transitions of Er3+ ions in the cubic sesquioxide phase. Peculiarities in the Raman and luminescence spectra were identified in comparison to single-phase Er:Y2O3 and Er:Gd2O3 ceramics, which may indicate certain solubility of MgO in the sesquioxide phase of the composites.

The features of spark plasma sintering (SPS) of plasma-chemical nanopowders WC – (0.3, 0.6, 1) wt.% Co were studied. The SPS process of ultralow-cobalt hard alloys can be sequentially represented as a change of the following stages: rearrangement of particles at lower temperatures (Stage I) → sintering of WC – Co particles due to Coble diffusion creep of cobalt, the intensity of which is determined by the grain boundary diffusion rate (Stage II ) → sintering due to diffusion creep, the rate of which is limited by the bulk diffusion in cobalt (Stage III-1) → sintering of tungsten carbide particles along the intergranular boundaries of WC/WC under conditions of intensive grain growth (Stage III-2). Samples with a high density (96.4 – 98.4 %) and high mechanical properties were obtained (for the WC – 0.3 % Co hard alloy: Hv ~ 20.5 GPa, K1C = 7.1 MPa·m1/2).

The features of high-speed spark plasma sintering (SPS) of plasma-chemical nanopowders WC – (0.3, 0.6, 1) wt. % Co with the addition of 0.3 and 0.5 wt. % graphite were studied. The structural features of the ultralow-cobalt hard alloys with graphite addition during SPS are due to the simultaneous influence of an increased concentration of oxygen adsorbed on the surface of plasma-chemical WC – Co nanoparticles during mixing with graphite, and the effect of graphite, which leads to a decrease in activation energy of sintering due to a decrease in the intensity of formation of η-phase particles in “oxidized” WC – Co nanopowders, as well as the formation of a fairly uniform fine-grained structure. Samples of fine-grained ultralow-cobalt hard alloys with increased hardness and fracture toughness were obtained (for a WC – 0.6 wt. % Co – 0.3 wt. % C hard alloy with an average grain size of ~ (1 – 1.5) mm, the hardness is Hv = 20.2 – 20.5 GPa with a minimum crack resistance coefficient KIC = 9.2 – 10.4 MPa·m1/2).

α-Si3N4-based powder composites containing 3 wt

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

The study investigates the density, phase composition, microstructure and mechanical properties (microhardness, fracture toughness) of binderless WC + SiC and WC + SiC + C ceramics obtained by Spark Plasma Sintering (SPS). Nanopowders of a-WC produced by DC arc plasma chemical synthesis were used as raw materials. Powder compositions for sintering contained graphite (0.3, 0.5% wt.) or b-SiC (1, 3, 5% wt.) with 0.3% wt. graphite. It was shown that WC + 1% wt. SiC + 0.3% wt.C ceramics have a homogeneous fine-grained microstructure, high relative density, increased microhardness and Palmquist fracture toughness (Indentation Fracture Resistance). The kinetics of the initial sintering stage of WC + C and WC + C + SiC powder compositions was also analyzed using high-temperature dilatometry at the conventional pressureless sintering (CPS) conditions. The CPS and SPS activation energies of WC + SiC powder at the intensive shrinkage stage were determined using the Young-Cutler model. The CPS activation energies of WC, WC + C and WC + C + SiC powder compositions are close to the activation energy of diffusion of the carbon C along the a-WC grain boundaries. The SPS activation energies of WC + C and WC+ C + SiC powder compositions turn out to be lower than the activation energy of the C of a-WC grain boundary.

This article focuses on the process of producing ceramics based on the commercial Si3N4 powder of the various dispersions (< 5 μm and < 1 μm) by spark plasma sintering (SPS). The powder mixtures of 97 wt. % Si3N4 + 3 wt. % additives of the Y2O3 – Al2O3 composition were synthesized by the Pechini method. The SPS technology was used to obtain the ceramic samples of ∅ 20 mm. Sintering was carried out in a vacuum at the heating rate of 50 °C/min and the load of 70 MPa until the shrinkage end. The microstructure and the phase composition of the ceramic samples were investigated. Mechanical properties were measured: Vickers hardness, Palmquist fracture toughness, flexural strength according to the B3B (Ballon-Three-Balls-Test) method. Tribological tests were also carried out. It was established that the lower the dispersion of the powder mixtures based on Si3N4, the lower the shrinkage end temperature. The relative density achieved is 96 %. The ceramics based on Si3N4 powder with the dispersion of <1 µm are difficult to machine, characterized by the hardness of 19.0 ± 0.7 GPa and the crack resistance of 5.1 ± 0.4 MPa·m1/2. The flexural strength of the ceramics evaluated by the B3B method depends on the dispersion of Si3N4 powder and is more than 2 times higher for the ceramics based on the commercial Si3N4 powder with the dispersion of <1 μm.

This review aimed to determine and analyze the current understanding of the origins and mechanisms of carbonization in different materials: 1) metallic and hard alloys, 2) oxide, carbide, and nitride ceramics, and 3) transparent ceramics during spark plasma sintering (SPS). The carbon sources include the graphite mold, graphite punches, and graphite foil. These are used to reduce the gap between the sample and the inner surface of the mold. It has been shown that the carbon diffusion can strongly affect the chemical and phase composition, microstructure, and physical and mechanical properties of samples produced using SPS technology. The researchers consider two main mechanisms of carbon interaction with the materials being sintering. The diffusion of carbon into the surface layers of the material is suggested to be the primary mechanism. The characteristic scale of carbon diffusion is tens to hundreds of microns. The carbide layer is often formed on the surface of the material. The second mechanism that promotes the carbon contamination of the samples is suggested to be the condensation of carbon-containing gases, such as CO and/or CO2, inside the pores of the sample. It is proposed that the carbon-containing gases are mainly formed as a result of the interaction between graphite components of the mold and residual oxygen. The presence of СО and/or СО2 gases in the work chamber of the setup is possible during SPS. It has been shown that the addition of carbon can enhance the mechanical and performance properties of structural materials. That opens up new prospects for the fabrication of functionally gradient materials by SPS. At the same time, carbon contamination leads to the deterioration of the dielectric and optical properties of functional ceramics, which necessitates the improvement of the SPS modes.

Методами Печини и распылительной сушки были изготовлены порошковые смеси на основе α-Si 3 N 4 с 3 мас. % спекающей добавки состава Y 2 O 3 –Al 2 O 3 в стехиометрии 2 : 1, 1 : 1 и 3 : 5. Изучены четыре образца керамики, изготовленных методом электроимпульсного плазменного спекания порошковых смесей. Спекание проводилось до температуры окончания усадки порошковых смесей со скоростью нагрева 50°C/мин и при нагрузке 70 МПа. Проведен анализ усадки порошковых смесей в процессе спекания, рассчитана энергия активация спекания, измерены плотность, микротвердость и трещиностойкость (вязкость разрушения) образцов керамики. Наибольшая твердость H V = 16.5 ГПа при трещиностойкости K I c = 3.8 МПа м 1/2 была достигнута для образца с мольным соотношением оксидов Y 2 O 3 : Al 2 O 3 = 3 : 5, спеченного при 1860°C, уплотнение образца при этом составило 99.0%. Шихта для данного образца была синтезирована методом распылительной сушки.

A solid-phase diffusion welding of coarse-grained and ultrafine-grained (UFG) specimens of titanium near-α alloy Ti-5Al-2V used in nuclear power engineering was made by Spark Plasma Sintering. The failure of the welded specimens in the conditions of hot salt corrosion and electrochemical corrosion was shown to have a preferentially intercrystalline character. In the case of the presence of macrodefects, crevice corrosion of the welded joints was observed. The resistance of the alloys against the intercrystalline corrosion was found to be determined by the concentration of vanadium at the titanium grain boundaries, by the size and volume fraction of the β-phase particles, and by the presence of micro- and macropores in the welded joints. The specimens of the welded joints of the UFG alloy are harder and have a higher resistance to hot salt corrosion and electrochemical corrosion.

The process of spray drying synthesis of the charge compositions based on silicon nitride α-Si3N4 with organic compounds of aluminum and yttrium in the molar ratio of 3:5 (stoichiometry of yttrium-aluminum garnet) as the sintering additive is considered. The sintered compositions 91.5 % wt. Si3N4 + 8.5 % wt. additive (in terms of garnet) were investigated by X-ray diffraction analysis and scanning electron microscopy as well as by the methods of thermal analysis. The charge compositions were annealed in four stages up to a temperature of 1000℃ in order to decompose organics and form the oxide phase of the sintering additive. High-speed (100 °C/min) spark plasma sintering (SPS) technology was used to produce 10 mm ceramic samples in vacuum, under uniaxial pressure of 70 MPa. The microstructure, mechanical properties and phase composition of ceramics were investigated. Influence of preliminary annealing of charge compositions on structure, phase composition and physical-mechanical properties of ceramics were studied. It is established that preliminary multistage annealing of charge compositions influences the SPS kinetics as well as the density and phase composition of the ceramic. It has been established that the kinetics of SPS of the pre-annealed powders has two-stage character of the shrinkage. In this case denser ceramic microstructure is formed than in the case of reaction synthesis of sintering additive (for charge composition without pre annealing) during the SPS, but pre annealing slows down the growth of elongated β-Si3N4 grains and the volume of sintering additive phase increases. It is shown that in the case of sintering ceramics from unannealed charge compositions the material has lower density but higher hardness. Based on the Yang-Kutler model, the activation energy of the SPS process is determined and it is shown that the compaction kinetics of Si3N4 with sintering additive powders is determined by the intensity of viscous flow of the oxide phase on the grain boundaries of ceramics.

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.

The reaction of triphenylantimony with ROOH peroxides (R = t -Bu, H) and unsaturated carboxylic acids afforded triphenylantimony dicarboxylates Ph3Sb(O2CCH=CH2)2, Ph3Sb(O2CCMe=CH2)2, Ph3Sb(O2CCH=CHMe)2, Ph3Sb(O2CCH=CHPh)2, Ph3Sb(O2CCH=CHC6H4NO2-3)2, Ph3Sb(O2CCH=CHC4H3O)2, Ph3Sb(O2CCH=CH-C6H4OMe-4)2, Ph3Sb(O2CCH=CHCH=CHMe)2, Ph3Sb(O2CCH2CH=CH2)2, p -Tol3Sb(O2CCH=CHPh)2 with yields of 35-86%. The comparison of the obtained compounds structure using the structural parameter τ (from the X-ray diffraction data), as well as the difference in the values of νas(COO) and νs(COO) (from the IR data) was investigated. In all compounds, the coordination of the antimony atom occupies an intermediate position between the trigonal-bipyramidal and tetragonal-pyramidal. In all compounds, there is an additional coordination of the antimony atom on carbonyl oxygen atoms. Intermolecular interactions in acrylate, methacrylate, crotonate and triphenylantimony sorbate with the participation of C=C double bonds of unsaturated carboxylate fragments of neighboring molecules located on top of each other were revealed. Antimony-containing transparent polymers - polymethyl methacrylate and polystyrene, which may be of practical value for the creation of new composite materials, have been synthesized on the basis of triphenylantimony dicrotonate. The molecular mass characteristics of polymers, the absorption of X-rays, UV light by the obtained polymers were measured.

alpha-Si3N4-based powder composites containing 3 wt % Y2O3-Al2O3 sintering aids with 2 : 1, 1 : 1, and 3 : 5 stoichiometries have been prepared by the Pechini process and spray drying. We have studied four ceramic samples produced by spark plasma sintering of the powder composites. The sintering process was run under a load of 70 MPa at a heating rate of 50 degrees C/min as long as shrinkage of the powder composites continued. We have analyzed the shrinkage behavior of the powder composites during the sintering process; evaluated the activation energy for sintering; and measured the density, microhardness, and fracture toughness (cracking resistance) of the ceramic samples. The highest hardness, H-V = 16.5 GPa, in combination with a fracture toughness K-Ic = 3.8 MPa m(1/2), was offered by the material with an Y2O3 : Al2O3 molar ratio of 3 : 5 sintered at 1860 degrees C. The corresponding densification factor was 99.0%. The starting mixture for this material was synthesized using spray drying.

— α-Si 3 N 4 -based powder composites containing 3 wt % Y 2 O 3 –Al 2 O 3 sintering aids with 2 : 1, 1 : 1, and 3 : 5 stoichiometries have been prepared by the Pechini process and spray drying. We have studied four ceramic samples produced by spark plasma sintering of the powder composites. The sintering process was run under a load of 70 MPa at a heating rate of 50°C/min as long as shrinkage of the powder composites continued. We have analyzed the shrinkage behavior of the powder composites during the sintering process; evaluated the activation energy for sintering; and measured the density, microhardness, and fracture toughness (cracking resistance) of the ceramic samples. The highest hardness, H V = 16.5 GPa, in combination with a fracture toughness K Ic = 3.8 MPa m 1/2 , was offered by the material with an Y 2 O 3 : Al 2 O 3 molar ratio of 3 : 5 sintered at 1860°C. The corresponding densification factor was 99.0%. The starting mixture for this material was synthesized using spray drying.

The research results conducted on binderless tungsten carbide (WC) ceramics obtained by spark plasma sintering (SPS) of WC powders with different average particle sizes (95, 800, 3000 nm) are presented. Nonuniform distribution of crystalline phases and microstructure of the WC ceramics was studied using layer-by-layer X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Surface layers of the WC-based ceramics are characterized by nonuniform distribution of W2C crystalline phase and grain sizes, including the appearance of abnormally large grains. Thickness of the nonuniform layer was at least 50 μm. The effect under study is associated with an intense carbon diffusion from graphite foil. On the one hand, this contributed to a decrease in the intensity of W2C phase particle formation, which is transformed into α-WC phase due to the carbon. On the other hand, it caused abnormal grain growth in the layer where the carbon diffused. The obtained value of the carbon diffusion depth (50 μm) exceeds the values known from the literature (up to 1 μm in the case of volume diffusion even at temperature of 2370 °C and exposure time of ~60 h). The use of boron nitride (BN) as a protective coating on graphite mold parts did not prevent the formation of nonuniform layer on the ceramic surface.

There were examined four ways of applying sintering additives (Y2O3 : Al2O3 = 3 : 5) to particles of commercial Si3N4 powders differing in particle size composition: amorphous nanopowder and microcrystalline α-Si3N4 powder. The oxide sintering additive coating was produced in two steps: wet chemical preparation of the starting Si3N4 powder with compounds containing yttrium and aluminum ions and annealing of the resultant powder composites at a temperature of 1000°C in air. The way the sintering addition is applied and the particle size composition of the starting Si3N4 powder have been shown to influence the composition of the crystalline phases forming during annealing. The optimal process for obtaining Si3N4 ceramics with improved physical and mechanical properties is spray drying, which makes it possible to obtain spherical agglomerates.