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

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.

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 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.

— 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.

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.

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.

Ceramics obtained by spark plasma sintering of powder compositions of Si3N4 was studied by the method of layer-by-layer X-ray diffraction analysis. The effect of carbon diffusion in the surface layers of the sintered ceramics from the graphite mold was observed. The homogeneity of the ceramic phase composition along the depth of the sample was shown. Therefore, a uniform distribution of the temperature field inside the sintered sample was concluded.

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.

X-ray diffraction methods are indispensable for studying crystalline materials and provide determination of the phase composition, internal stresses and the parameters of unit cells of crystalline phases, as well as the crystalline size, which is related to the average grain size and affects the width of the diffraction peaks. We present the results of evaluating the repeatability of the X-ray diffraction data obtained for silicon nitride powders Si 3 N 4 of different particle size and different initial phase ratios α-, β-Si 3 N 4 . The relative error of measuring the diffraction peaks was determined using the intensity detector and the absolute error of calculating the unit cell parameters was determined by the Rietveld method. The average particle size of the initial powders was analyzed using scanning electron microscopy. X-ray diffraction studies were performed according to the Bragg – Brentano scheme using CuKα radiation (λ = 1.5406 Å). A series of three experiments was performed for each powder. It is shown that the relative error of intensity measurements with a detector does not exceed 2% for peaks corresponding to 3σ criterion, and the absolute error of the determination of the unit cell phase parameters by the Rietveld method is 0.001 Å. The results obtained can be used to assess the stability of the diffractometer both for the samples based on silicon nitride and materials of different composition and structure, especially for submicron samples. In the latter case, the error of the parameters obtained by X-ray phase analysis can be taken into account without resorting to statistical estimates, as in the method of least squares.

Ceramics obtained by spark plasma sintering of powder compositions of Si 3 N 4 was studied by the method of layer-by-layer X-ray diffraction analysis. The effect of carbon diffusion in the surface layers of the sintered ceramics from the graphite mold was observed. The homogeneity of the ceramic phase composition along the depth of the sample was shown. Therefore, a uniform distribution of the temperature field inside the sintered sample was concluded. Keywords: silicon nitride, ceramics, spark plasma sintering, X-ray diffraction.

In order to obtain ceramics based on Si 3 N 4 with improved physical and mechanical properties, methods of creating composites for their subsequent consolidation by the spark plasma sintering method were investigated. Four different methods of producing mixtures based on the Si 3 N 4 and Al 5 Y 3 O 12 (YAG) precursor were considered in order to obtain a homogeneous distribution of the YAG sintering additive on the surface of the Si 3 N 4 particles. It is shown that the YAG and Y-Si-Al-O-N phases are formed by co-precipitation, deposition in a gelatin matrix and Pecini methods. The spray drying method does not lead to the formation of the YAG phase. This may indicate a high homogeneity of the YAG coating on the surface of the Si 3 N 4 particles, preventing the growth of crystalline particles.

The interaction of zirconium (IV) chloride and hafnium (IV) cloride with tetraorganylphosphonium chlorides in solutions of acetonitrile synthesized the following complexes: [Ph 3 PCH=CHCH 3 ][ZrCl 6 ] ( 1 ), [Ph 3 PCH 2 C(O)CH 3 ][HfCl 6 ] ( 2 ). The structures 1 and 2 were determined by XRDA and IR. The X-ray diffraction patterns of crystals 1 and 2 were obtained at 293 K on an automatic diffractometer D8 Quest Broker (MoK α radiation, λ = 0.71073 Å, graphite monochromator) the phosphorus atoms of complexes 1 and 2 have a distorted tetrahedral configuration in the cations. [C 42 H 40 P 2 Cl 6 Zr ( 1 ), M = 910.60; the triclinic syngony, the symmetry group P ; cell parameters: a = 10.189(8), b = 14.428(7), c = 15.229(8) Å; a = 83.31(2) degrees, β = 73.77(3) degrees, g = 87.75(3) degrees; V = 2135(2) Å 3 ; the crystal size is 0.72 × 0.36 × 0.3 mm; intervals of reflection indexes –12 ≤ h ≤ 12, –18 ≤ k ≤ 18, –19 ≤ l ≤ 19; total reflections 36133; independent disclosures 7367; R int = 0.0326; GOOF = 1.180; R 1 = 0.0907; w R 2 = 0.2675; residual electron density ­­–0.91/ 0.827 e/Å 3 , C 42 H 40 P 2 Cl 6 O 1 Hf ( 2 ), M = 1029.87; the triclinic syngony, the symmetry group P ; cell parameters: a = 10.323(3), b = 10.721(3), c = 11.122(3) Å; a = 67.634(13) degrees, β = 78.219(17) degrees, g = 73.041(14) degrees; V = 1082.7(5) Å 3 ; the crystal size is 0.57 × 0.39 × 0.22 mm; intervals of reflection indexes –19 ≤ h ≤ 19, –20 ≤ k ≤ 20, –21 ≤ l ≤ 21; total reflections 118390; independent disclosures 16166; R int = 0.0486; GOOF 1.009; R 1 = 0.0447; w R 2 = 0.0772; residual electron density ­–1,013/0,910 e/Å 3 ]. The СРС valence angles are 107.95(16)°–110.94(15)° and 106.72(17)°–113.51(17)° for 1 , 105.85(15)°–110.97(15)° for 2 , distance P-С 1.771(6)–1.801(6) Å и 1.781(6)–1.801(6) Å in 1 ; 1.790(2)–1.821(2) Å in 2 . In octahedral anions [ZrCl 6 ] 2− and [HfCl 6 ] 2– trans-angles ClZrCl and ClHfCl equal 180.0º, distance 2.462(3)–2.476(3) Å и 2.462(2)– 2.468(2) Å in a crystal solvate in 1 , 2.4513(10)–2.462(2) Å in 2 . Complete tables of coordinates of atoms, bond lengths and valence angles are deposited at the Cambridge Structural Data (No. 1913593 for 1 , 1919938 for 2 , deposit@ccdc.cam.ac.uk; http://www.ccdc. cam.ac.uk).