Abstract—The possibility of low-temperature in situ synthesis of (Ti, W)C using plasma-chemical WC nanopowders and industrial micron TiC powders is demonstrated. Sintering/synthesis of WC–(25, 50, and 75) wt

The temperature and heating rate affecting the shrinkage kinetics are studied for cylindrical workpieces obtained from submicron and fine aluminum oxide powder. The studies involve the powder from three batches: (1) submicron ( 0.15 µm) α-Al2O3 powder, (2) submicron ( 0.2 µm) α-Al2O3 powder having an amorphous layer deposited on the particle surface, and (3) fine ( 1 µm) α-Al2O3 powder. It is established that the powder particles in all batches has a monocrystalline structure. The powder workpieces are sintered using the electric pulse (spark) plasma sintering (SPS) technique. The shrinkage curves are analyzed using the Young–Cutler and the Coble models. The kinetics of sintering workpieces is shown to depend on diffusion developing between the powder particles. The sintering kinetics of workpieces made from submicron powder depends on intensity of the grain-boundary diffusion. In the sintering workpieces made of finely dispersed powder, the kinetics is additionally dependent on simultaneously developing volumetric and grain-boundary diffusion. It is established that the presence of an amorphous layer on the surface of particulate α-Al2O3 having submicron size affects the rate of migration of grain interfaces and the parameters of the Coble equation at the final SPS stage. It is assumed that the accelerated growth of grains and an increase in the microhardness of samples obtained through sintering workpieces made from submicron powder with an amorphous layer on the particle surface is caused by a higher density of defects at the grain interfaces. The elevated density of defects at grain interfaces can result from crystallization of the amorphous layer.

The mechanisms of high-speed sintering of tungsten and W+5wt%Ni nanopowders obtained by high-energy ball milling (HEBM) have been studied. The phase composition, microstructure parameters, hardness and fracture resistance of the obtained samples were investigated. It is shown that the samples have high relative density, small grain size and increased hardness. It is established that the formation of strong intermetallic phases MexWyCz and MexWy, as well as MeO oxides occurs at SPS of mechanically activated nanopowders. The simultaneous increase in the content of intermetallic phases and reduction of the grain size leads to a non-monotone character of the dependence of the SPS activation energy on the HEBM time. It is shown that the main mechanism of SPS of tungsten W+5wt%Ni nanopowders is Coble creep.

α-Si3N4-based powder composites containing 3 wt

The mechanisms of high-speed sintering of tungsten and W+5wt

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.

The sintering mechanisms of WC – Al2O3 nanopowder compositions with different contents of aluminum oxide particles (1, 3, 5 wt.%) were investigated. Samples of WC – Al2O3 ceramics were produced by Spark Plasma Sintering method (SPS) in vacuum, by heating to a temperature of 1450 °C at a rate of 50 °C/min under uniaxial stress 70 MPa. Plasma-chemical nanopowders of tungsten monocarbide and submicron powders of aluminum oxide were used to make the ceramics. The density, microstructure, phase composition, microhardness (Hv) and fracture toughness (KIC) of the ceramics were investigated. It was shown that the use of the SPS method makes it possible to obtain WC-Al2O3 ceramics with good relative density (95.4-98.1%) and a homogeneous microstructure with ultrafine grain size (0.1 – 0.2 μm). By the method of the X-ray phase analysis, it was established that in the process of SPS of WC-Al2O3 ceramics the formation of an undesirable W2C phase takes place, leading to decrease in the fracture toughness KIC. To reduce the intensity of W2C particle formation, colloidal graphite (0.1, 0.2, 0.3 wt.%) was added to WC – Al2O3 ceramics. Using the Young-Cutler model and the model of diffusion resorption of pores it was shown that the main mechanism of SPS of WC – Al2O3 ceramics is grain boundary diffusion. It is shown that the introduction of graphite leads to a decrease in the activation energy of SPS of WC – Al2O3 ceramics, which is probably due to a decrease in the W2C particle content to 0.5 wt.%.

The effect of the lithium chloride and lithium fluoride additives (2 wt. %) on the kinetics of Spark Plasma Sintering of industrial fine alumina powder was investigated. The Al2O3 + 2 % LiF powder was obtained by mixing the α-Al2O3 fine powders with the aqueous solution of LiF. The Al2O3 + 2 % LiCl fine powder was obtained by joint grinding of the components in a planetary mill. The sintering of the powders was performed with the heating rates of 10 and 50 °C/min up to the temperature corresponding to the end of the shrinkage. The ceramics sintered with the heating rate of 10 °C/min had the relative density of 97.4 – 98.7 %. The addition of lithium fluoride into the alumina powder was found to allow reducing the temperature of the beginning of the intensive powder shrinkage from 1400 – 1500 °C down to 1255 – 1335 °C. LiCl was shown to evaporate at low heating temperatures and not to affect the compaction intensity of the Al2O3 powder. The presence of the overheated LiF melt (not having enough time to evaporate from the specimen volume completely) in the ceramics leads to the appearance of the residual porosity and to the reduction of the hardness of the ceramics. Using the Young – Cutler equation, the powder compaction mechanisms were determined for pure alumina α-Al2O3 and for the powder compositions with the LiCl and LiF additives in the rapid heating conditions. The sintering activation energy was shown to be close to the one of the grain boundary diffusion. The LiF melt was found to promote the sliding of the Al2O3 fine particles during the low-temperature compaction stage.

Results of investigations of the kinetics of solid solution decomposition during annealing of fine-grained (FG) Al–0.5

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.

The possibility of low-temperature in-situ synthesis of (Ti, W)C ceramics from plasma-chemical nanopowders of tungsten carbide and commercial micron-sized titanium carbide powders has been demonstrated. Sintering/synthesis of WC – (25, 50, 75) wt. % TiC ceramics was carried out by spark plasma sintering (SPS) by heating powders at a rate of 50 °C/min in vacuum under conditions of application of a voltage of 70 MPa, to a temperature of more than 1200 °C. It has been established that the synthesis proceeds most efficiently in nanopowders with the addition of 50 and 75 wt. % TiC. It is shown that the joint application of plasma-chemical synthesis of nanopowders and the SPS method makes it possible to obtain fine-grained ceramics with increased density and satisfactory mechanical properties (Vickers hardness 17 – 18 GPa, minimum crack resistance coefficient according to Palmquist ~ 3 MPa·m1/2).

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 structure, microstructure, coefficient of thermal expansion (CTE), and mechanical properties of Na1+2xZr2−xCox(PO4)3 ceramics (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were studied. Na1+2xZr2−xCox(PO4)3 submicron powders with the NaZr2(PO4)3 structure (NZP, kosnarite type) were obtained by the solid-phase method. The starting reagents (NaNO3, ZrOCl2·8H2O, NH4H2PO4, CoCl2·6H2O, ethanol) were mixed with the addition of ethyl alcohol. The resulting mixtures were annealed at 600 °C (20 h) and 700 °C (20 h). The obtained phosphates crystallized in the expected structure of the NaZr2(PO4)3 type (trigonal system, space group R3¯c). Thermal expansion of the powders was studied with high-temperature X-ray diffraction at temperatures ranging from 25 to 700 °C. CTEs were calculated, and their dependence on the cobalt content was analyzed. Na1+2xZr2−xCox(PO4)3 ceramics with high relative density (93.67–99.70%) were obtained by Spark Plasma Sintering (SPS). Ceramics poor in cobalt (x = 0.1) were found to have a high relative density (98.87%) and a uniform fine-grained microstructure with a grain size of 0.5–1 µm. Bigger cobalt content leads to a smaller relative density of ceramics. During the sintering of ceramics with high cobalt content, anomalous grain growth was observed. The powder compaction rate was shown to be determined by creep and diffusion intensity in the Na1+2xZr2−xCox(PO4)3 crystal lattice. SPS activation energy in ceramics increased as the cobalt content grew. The microhardness and fracture toughness of ceramics did not depend on their cobalt content.

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.

Powder mixtures based on Si3N4 nanoparticles and yttrium and aluminum oxides were prepared. Ceramic samples with a relative density of 95–96% have been produced by spark plasma sintering at a pressure of 80 MPa and a heating rate of 50°C/min. The microstructure of the samples has been shown to be formed by Si2N2O grains ~0.7 μm in size, with an amorphous oxide phase in between. The highest hardness (13.3 GPa) was reached by the sample containing the Y2O3 and Al2O3 oxides in the molar ratio 2 : 1 and obtained at a sintering temperature of 1770°C.

We have studied the effect of monoclinic ZrO2 additions (1.5, 5, and 10 vol