Kinetics Of Spark Plasma Sintering Research Articles

Investigation of the Effect of LiCl and LiF Additives on the Kinetics of Spark Plasma Sintering of Fine-Grained Alumina

Investigation of the Effect of LiCl and LiF Additives on the Kinetics of Spark Plasma Sintering of Fine-Grained Alumina

Effect of High-Energy Ball Milling Time on the Density and Mechanical Properties of W-7%Ni-3%Fe Alloy

The present work was aimed at the investigation of the effect of high-energy ball milling (HEBM) time on the sintering kinetics, structure, and properties of the heavy tungsten alloy (HTA) W-7%Ni-3%Fe. The HTA samples were obtained from nanopowders (20–80 nm) using conventional liquid-phase sintering (LPS) in hydrogen and using spark plasma sintering (SPS) in vacuum. The HTA density was shown to depend non-monotonously on the HEBM time that originates from the formation of nonequilibrium solid solutions in the W-Ni-Fe systems during HEBM. The SPS kinetics of the HTA nanopowders was shown to have a two-stage character, the intensity of which depends on the Coble diffusion creep rate and on the intensity of diffusion of the tungsten atoms in the crystal lattice of the γ-phase. The kinetics of sintering of the initial submicron powders has a single-stage character originating from the intensity of the grain boundary diffusion in the γ-phase. The dependencies of the hardness and of the yield strength on the grain sizes were found to obey the Hall–Petch relation. The hardness, strength, and dynamic strength in the compression tests of the fine-grained tungsten alloys obtained using SPS and LPS were studied.

Исследование влияния добавок LiCl и LiF на кинетику электроимпульсного плазменного спекания мелкозернистого оксида алюминия

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.

Effect of grain boundary state and grain size on the microstructure and mechanical properties of alumina obtained by SPS: A case of the amorphous layer on particle surface

The work was aimed at the investigation of kinetics of Spark Plasma Sintering (SPS) of the α-Al2O3 particles with amorphous surface layers and investigation of the effect of the amorphous layers on the grain growth and on the mechanical properties of alumina. The objects of investigations comprised:(i) submicron α-Al2O3 powder, (ii) submicron α-Al2O3 powder with the amorphous layers on the particles' surfaces, and (iii) the fine-grained α-Al2O3 powder. The submicron powders (i) and (ii) were used to analyze the effect of the amorphous layers on the sintering kinetics. Powders (i) and (iii) were used to analyze the effect of the initial particle sizes on the shrinkage kinetics. The effect of the temperature regime and of the rate (Vh) on the shrinkage kinetics of the submicron and fine alumina powders has been studied. The shrinkage curves were analyzed using the Young–Cutler and Coble models. The sintering kinetics was shown to be determined by the intensity of grain boundary diffusion for the submicron powders and by simultaneous lattice diffusion and grain boundary one for the fine powders. The amorphous layers on the surfaces of the submicron α-Al2O3 particles were found to affect the grain boundary migration rate and the Coble equation parameters at the final stages of SPS. The abnormal characteristics of the alumina ceramics sintered from the submicron powder with the amorphous layers on the particles’ surfaces were suggested to originate from the increased concentration of the defects and of the excess free volume at the grain boundaries formed during crystallization of the amorphous layers.

Изготовление керамических композитов на основе порошка нитрида кремния с осажденной спекающей добавкой

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.

A Study of the Impact of Graphite on the Kinetics of SPS in Nano- and Submicron WC-10%Co Powder Compositions

This study investigates the impact of carbon on the kinetics of the spark plasma sintering (SPS) of nano- and submicron powders WC-10 wt.%Co. Carbon, in the form of graphite, was introduced into powders by mixing. The activation energy of solid-phase sintering was determined for the conditions of isothermal and continuous heating. It has been demonstrated that increasing the carbon content leads to a decrease in the fraction of η-phase particles and a shift of the shrinkage curve towards lower heating temperatures. It has been established that increasing the graphite content in nano- and submicron powders has no significant effect on the SPS activation energy for “mid-range” heating temperatures, QS(I). The value of QS(I) is close to the activation energy of grain-boundary diffusion in cobalt. It has been demonstrated that increasing the content of graphite leads to a significant decrease in the SPS activation energy, QS(II), for “higher-range” heating temperatures due to lower concentration of tungsten atoms in cobalt-based γ-phase. It has been established that the sintering kinetics of fine-grained WC-Co hard alloys is limited by the intensity of diffusion creep of cobalt (Coble creep).

Comparison of Conventional and Flash Spark Plasma Sintering of Cu–Cr Pseudo-Alloys: Kinetics, Structure, Properties

Spark plasma sintering (SPS) is widely used for the consolidation of different materials. Copper-based pseudo alloys have found a variety of applications including as electrodes in vacuum interrupters of high-voltage electric circuits. How does the kinetics of SPS consolidation for such alloys depend on the heating rate? Do SPS kinetics depend on the microstructure of the media to be sintered? These questions were addressed by the investigation of SPS kinetics in the heating rate range of 0.1 to 50 K/s. The latter conditions were achieved through flash spark plasma sintering (FSPS). We also compared the sintering kinetics for the conventional copper–chromium mixture and for the mechanically induced copper/chromium nanostructured particles. It was shown that, under FSPS conditions, the observed maximum consolidation rates were 20–30 times higher than that for conventional SPS with a heating rate of 100 K/min. Under the investigated conditions, the sintering rate for mechanically induced composite Cu/Cr particles was 2–4 times higher compared to the conventional Cu + Cr mixtures. The apparent sintering activation energy for the Cu/Cr powder was twice less than that for Cu–Cr mixture. It was concluded that the FSPS of nanostructured powders is an efficient approach for the fabrication of pseudo-alloys.

Binderless tungsten carbides with an increased oxygen content obtained by spark plasma sintering

The features of spark plasma sintering (SPS) of plasma-chemical nanopowders with an increased oxygen content are investigated. It is shown that the process of shrinkage of nanopowders during SPS is limited by the rate of grain-boundary diffusion with anomalously low values of the activation energy. It has been established that a decrease in the activation energy of SPS may be due to the effect of oxygen on the diffusion permeability of grain boundaries of tungsten carbide at the stage of intense compaction, as well as anomalous grain growth at the stage of high-temperature sintering. The SPS kinetics of WC-W2C-WO3-W nanopowder compositions at the stage of intensive compaction is controlled by the rate of sintering of oxide particles with their simultaneous transformation into W2C particles, and then, at the stage of high-temperature sintering, by the process of plastic flow of W2C particles in the presence of tungsten particles. Ceramic samples with high density (98-99%), ultrafine-grained structure (average grain size less than 0.3 μm), having increased hardness were obtained by the SPS method. Hv = 30.5 GPa at Palmquist crack resistance ∼ 6.5 MPa · m1/2.

Influence of oxygen on densification kinetics of WC nanopowders during SPS

The shrinkage of the WC nanopowders with increased oxygen content during Spark Plasma Sintering (SPS) was shown to be limited by the grain boundary diffusion with abnormally low activation energy. It was found that the reduction of the SPS activation energy may originate from the effect of oxygen on the diffusion permeability of the grain boundaries in tungsten carbide at the intensive compaction stage as well as from an abnormal grain growth at the high-temperature sintering stage. The kinetics of SPS of the WC-W2C–WO3–W nanopowders at the intense compaction stage is controlled by the sintering rate of the oxide particles with simultaneous transformation of these ones into the W2C particles and then, at the high-temperature sintering stage – by the plastic flow of the W2C particles in the presence of tungsten ones. The ceramic specimens with high density (98–99%) and ultrafine grained structure (mean grain size < 0.3 μm) having an increased hardness Hv = 30.5 GPa at minimum Palmquist fracture toughness ~ 6.5 MPa m1/2 were obtained by SPS.

Fabrication of fine-grained CeO2-SiC ceramics for inert fuel matrices by Spark Plasma Sintering

The possibility to fabricate highly dense (95.3–95.8% of theoretical density) composite ceramics CeO2 – x α-SiC (x = 10, 20 vol %) as a promising mineral-like material for the inert fuel matrices (IFM) by Spark Plasma Sintering (SPS) has been explored. The kinetics of SPS of the CeO2 – SiC composites in the heating-up conditions was found to have a multi-stage character. The introduction of the α-SiC particles resulted in a shift of the characteristic sintering temperatures towards higher values, allows forming the highly dense fine-grained structure, and leads to increasing of the thermal conductivity of the cerium dioxide – based ceramics.

Kinetics of Spark Plasma Sintering of WC–10% Co Ultrafine-Grained Hard Alloy

The effect of the carbon content on the kinetics of spark plasma sintering of nanopowder and submicron powder of WC–10% Co compositions was studied. Free carbon in the graphite form was introduced into nanopowders and submicron powders by mixing. The activation energy of solid-phase sintering at isothermal holding and continuous heating was determined. It is shown that increase in the carbon content leads to decrease in the volume fraction of the η phase and shift of the shrinkage curve into the region of low temperatures. It is established that an increase in the carbon content does not have a significant effect on the sintering activation energy of nanopowders and submicron powders in the region of “average” heating temperatures, the value of which is close to the activation energy of grain boundary diffusion in cobalt. It is shown that an increase in the carbon content leads to a significant decrease in the activation energy of consolidation of WC–Co powders in the region of “high” sintering temperatures owing to a decrease in the concentration of tungsten atoms in the γ phase based on cobalt. The sintering kinetics of fine-grained WC–Co alloys is limited by the rate of Coble diffusion creep of cobalt.

Study of the kinetics of spark plasma sintering of ultrafine-grained hard alloys WC-10%Co

The effect of carbon on the kinetics of spark plasma sintering (SPS) of submicron powder compositions WC-10wt.%Co is investigated. Free carbon in the form of graphite was added into submicron powders by mixing. The activation energy of solid-phase sintering under the conditions of isothermal hold and continuous heating rate sintering was determined. It was shown that an increase in the carbon content leads to a decrease in the volume fraction of particles of the η-phase and a shift of the shrinkage curve to the region of lower heating temperatures. It was established that an increase in the graphite content does not significantly affect the activation energy of sintering of submicron powders in the region of “average” heating temperatures, the value of which is close to the activation energy of grain boundary diffusion in cobalt. It has been shown that an increase in graphite content leads to a significant decrease in the activation energy of compaction of WC-Co powders in the region of “high” sintering temperatures due to a decrease in the concentration of tungsten atoms in the γ phase based on cobalt. The kinetics of sintering of fine-grained WC-Co hard alloys is limited by the intensity of the cobalt diffusion creep by Coble.

Исследование особенностей высокоскоростного спекания плазмохимических нанопорошков карбида вольфрама с повышенным содержанием кислорода

Spark plasma sintering (SPS) of plasma-chemical nanopowders tungsten carbide with a high oxygen content are studied. The results show that the nanopowder WC shrinkages during SPS is limited by grain boundary diffusion with abnormally low activation energies. It is established that the decrease in the activation energy can be caused by the influence of oxygen on the diffusion permeability of the tungsten carbide grain boundaries at the stage of intense compaction, as well as by abnormal grain growth at the stage of high-temperature sintering. The SPS kinetics of WC-W2C-WO3-W nanopowder compositions at the stage of intense compaction is controlled by the rate of oxide particle sintering with their simultaneous transformation into W2C particles, and then, by the plastic flow of W2C particles in the presence of W particles at the stage of high-temperature sintering. Using the SPS method, we obtained ceramics with a high density (98-99%), ultrafine-grained structure (the average grain size is less than 0.3 mm), having an increased hardness HV = 30.5 GPa with a Palmquist crack resistance of ~6.5 MPa·m1/2.

Investigation of the kinetics of spark plasma sintering of alumina ceramics. Part 1. The initial stage of sintering

The article presents the results of the investigation of the initial sintering stage of alumina. The authors determined the activation energy of shrinkage at the initial stage of sintering. The study shows that the additives do not change the kinetics of shrinkage at the initial stage of SPS.

Investigation of the kinetics of spark plasma sintering of alumina. Part 2. Intermediate and final stages of sintering

The article presents the results of the investigation of the initial sintering stage of alumina. The authors determined the activation energy of shrinkage at the initial. The study shows that the effect of additives is observed only at the final stage of sintering.

Effect of graphite addition on the spark-plasma sinterability of ZrB2 and ZrB2–SiC ultra-high-temperature ceramics

The effect of a 2wt% addition of particulate graphite on the spark-plasma sintering (SPS) kinetics of both ZrB2 and ZrB2+30vol% SiC was investigated using to that end two broad sets of powder mixtures prepared by high-energy co-ball-milling. It was observed that the particulate graphite addition enhances the SPS kinetics of both systems, and it was identified that, contrary to the case in conventional sintering, this improvement in sinterability is not due to the carbothermal reduction of oxides in situ during SPS but to the lubrication imposed by the graphite flakes inducing a greater green-body densification. Consequently, the graphite is not consumed during SPS but remains in the microstructure of the resulting ultra-high-temperature ceramics (UHTCs) as flaky particles dispersed homogeneously at grain boundaries. Implications of interest for the UHTC community are discussed.

Spark-plasma-sintering kinetics of ZrC–SiC powder mixtures subjected to high-energy co-ball-milling

Abstract The effect of SiC addition (5, 17.5, or 30 vol%) on the spark-plasma sinterability of ZrC ultra-high-temperature ceramics (UHTC) was investigated as a function of the intensity of high-energy co-ball-milling. It was found that the spark-plasma-sintering (SPS) kinetics of the ZrC–SiC powder mixtures is enhanced progressively with increasing co-milling time, which is due to the progressive refinement of the crystallite sizes. It was also found that in practice the SiC addition is beneficial for the SPS kinetics of ZrC if the co-milling time is short enough so as to refine the SiC crystallite size only to the submicrometre range, although the improvement in sinterability does not correlate with the SiC content. On the contrary, the SiC addition is increasingly detrimental for those co-milling times that promote SiC refinement to the nanoscale. This unexpected trend is due to slower ZrC crystallite size refinement in the presence of SiC and, especially, to the complex role played during SPS by the passivating SiO 2 films formed on the SiC particles, and is different from what has been observed in ZrB 2 –SiC. Finally, implications of interest for the UHTC community are discussed.

On the enhancement of the spark-plasma sintering kinetics of ZrB2–SiC powder mixtures subjected to high-energy co-ball-milling

The spark-plasma sintering (SPS) kinetics of ZrB 2 –SiC powder mixtures was investigated as a function of the degree of high-energy co-ball-milling and of the SiC content (5, 17.5, or 30 vol%). As in ZrB 2 without SiC, it was found that the crystal size refinement induced by the continued milling progressively enhances the SPS kinetics of ZrB 2 –SiC, again only moderately if the refinement is to the ultra-fine range, but very marked if the refinement is to the nanoscale. It was also found that the SiC addition further enhances the SPS kinetics of ZrB 2 , although the improvement did not scale directly with the SiC content, and became less relevant with the refinement of the ZrB 2 crystal sizes to the nanoscale. The improved kinetics induced by the SiC addition was identified as being due to the formation of amorphous borosilicate from the oxide passivating layers on the ZrB 2 and SiC particles. This not only speeds up the interparticle diffusion, but also it is segregated under the application of pressure into the multi-grain joints, filling pores. The enhanced kinetics induced by the progressive milling is due to the continuous reduction of the diffusion distances and to the development of a greater density of grain boundaries available as faster diffusion paths, together with the greater formation of amorphous borosilicate. Implications of interest for the ultra-high-temperature ceramics community are discussed.

Improvement of the Spark‐Plasma‐Sintering Kinetics of ZrC by High‐Energy Ball‐Milling

The effect of the duration of high‐energy ball‐milling on the spark‐plasma sintering (SPS) kinetics of additive‐free ZrC ultra‐high‐temperature ceramics (UHTCs) was investigated. It was found that the crystallite size of ZrC is refined progressively from the as‐purchased condition down to a few nanometers with increasing high‐energy ball‐milling time, and that this crystallite size refinement progressively enhances the SPS kinetics, reducing the characteristic temperatures of the sintering process. While this enhancement was found to be only moderate with the refinement to the ultra‐fine range, it was highly marked with the refinement to the nanometer range. Implications for the lower‐temperature processing of ZrC UHTCs are discussed.

Crystal-size dependence of the spark-plasma-sintering kinetics of ZrB2 ultra-high-temperature ceramics

The crystal-size dependence of the spark-plasma-sintering (SPS) kinetics of ZrB2 ultra-high-temperature ceramics (UHTCs) was investigated. It was found that refining the starting powder enhances the SPS kinetics, reducing the onset temperatures of sintering and of the intermediate and final sintering regimes, as well as promoting a greater maximum shrinkage rate at lower temperatures. This enhancement was only relevant with reduction in crystal size to the nanoscale. Finally, the implications for low-temperature sintering of ZrB2 UHTCs are discussed.