arXiv subjects

N. V. Isaeva

Publications and source records attributed to N. V. Isaeva.

3 recordsLinked to original sources

arXiv2026

Low-temperature Spark Plasma Sintering of fine refractory composite powders core-shell: A case of the powders W@Ni

The mechanisms of fast low-temperature Spark Plasma Sintering (SPS) of W + 10% wt. Ni powders were investigated. The powder compositions were obtained in two methods: (i) by mixing W and Ni powders in a specified ratio (hereinafter referred to as W + Ni); (ii) by Ni deposition on the surface of submicron W particles allowing the formation of particles with a core W - shell Ni structure (hereinafter referred to as W@Ni). To reduce the concentrations of oxygen and oxides, the powders were annealed in hydrogen. The solid-phase sintering was performed at various temperatures (1000-1150°C), pressures (40-80 MPa), heating rates (50-500°C/min), and isothermal holding times (0-20 min). The sintering temperatures corresponded to the onset of intense dissolution of W in Ni. The samples had high relative density and small grain sizes. The activation energy of SPS of the mixed powders was close to the one of the grain boundary diffusion. The key mechanism for the compaction of W@Ni particles in the SPS process is Coble creep. The increasing of the sintering temperature was shown to lead to an increase in the solubility of W in Ni and, consequently, to an increase in the number of secondary Ni4W particles formed during cooling down. The grain growth has a minor effect on the mechanical properties of the W alloy compared to the changes in its phase composition.

cond-mat.mtrl-sci↗

arXiv2023

Combined effect of SiC and carbon on sintering kinetics, microstructure and mechanical properties of fine-grained binderless tungsten carbide

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.

cond-mat.mtrl-sci↗

arXiv2022

Low-temperature creep of binderless tungsten carbide with different grain sizes

The creep mechanism in the compression testing of the tungsten carbide with different grain sizes has been studied. The WC samples with high density (96.1-99.2%) were obtained by SPS from nano-, submicron, and micron-grade WC powders. The samples had a coarse-grained (CG) surface layers of ~0.3 mm in thickness and ultrafine-grained (UFG) central parts consisting of WC with a small fraction of W2C. The creep tests were conducted in two regimes: (Mode #1) holding at different temperatures (1300-1375C) at 70 MPa; (Mode #2) tests at different stresses (50, 70, 90 MPa) at 1325C. Tests in Mode #1 were done to determine the effective creep activation energy Qcr while tests in Mode #2 - to determine the coefficient n in the power law creep equation. The increasing of the fraction of the W2C particles from 1.7 up to 4% was found to result in a decrease in the Qcr from 17.5 down to 13 kTm. The coefficient n equals to 3.1-3.7. The Qcr in the WC sintered from nanopowders was shown to be 31 kTm. This value is 1.5-2 times greater than the Qcr in the UFG samples obtained by SPS from commercial powders. The increased fraction of the W2C formed when sintering the nanopowders with increased adsorbed oxygen concentration was suggested to be one of the origins of the increase in the Qcr when testing the UFG samples. The mechanical removing of the CG layers from the surfaces of the tungsten carbide sample was shown to result in an accelerated creep, insufficient decrease in the Qcr and coefficient n to 2.5-2.6. The creep rate of the samples was suggested to be determined simultaneously by the creep process in the CG surface layers and the creep process in the UFG central parts of the samples. The creep rate in the surface CG layers is determined by intensity of carbon diffusion in the WC crystal lattice while the creep rate in the UFG central parts - by the intensity of grain boundary diffusion.

cond-mat.mtrl-sci↗