Spark Plasma Sintering Densification Research Articles

Model of SPS Two-Stadium Synthesis and Densification Reactor Applied for Ultrafine Zirconium Nitride Powder

The volumetric changes and variable porosity due to the concentration expansion of the solid phase in the synthesis of zirconium nitride (ZrN) are studied. The model of two-stage reactor based on spark plasma sintering (SPS) is proposed. At the first stage the synthesis for the given kinetics is simulated. At the second stage the densification of ZrN using the Olevsky’s sintering model [1-5] is applied. The synthesis and densification processes using the prescribed heat sources, at the given positions inside the reactor is simulated. The generalization of the two-temperature model [6] and the formula of the porosity in the densification using calculation of the solid concentration expansion and thermal dispersion is proposed. The concentration expansion coefficients in the process of zirconium nitrogenating at a given initial density values and coefficients of expansion of reagents .is studied The temperature at the stage of ZrN synthesis and porosity variation at the stage of densification are in satisfactory agreement with experimental results [2,7,8]

SPS densification and ablative performance of ZrB2–SiC–ZrC composites derived from the ternary powders by sol-gel

ZrB2–SiC–ZrC (ZSZ) composite ceramics were successfully prepared by sol-gel method combined with spark plasma sintering (SPS) technology. ZrB2–SiC–ZrC multiphase powders with a scale of about 100 nm were firstly synthesized by sol-gel method at 1500 °C. The sintering performance of the sol-gel powders was compared with compositionally similar commercial powders mixture by SPS. The relative densities of the corresponding target composite (ZSZ1) and reference composite (ZSZ2) were reached to 96.2% and 92% under 2000 °C/40 MPa, respectively. The densification curves indicated that the residual SiO2 and B2O3 oxides derived from the precursor triggered the liquid phase sintering mechanism of sol-gel powders above 750 °C. The removal of oxide impurities through carbothermal reduction, and the diffusion and attachment of the liquid phase at the grain boundary jointly promote the densification of ZSZ1. ZSZ1 presented better oxidation/ablation resistance than ZSZ2 during the oxyacetylene ablation tests for 200 s. The mass and line ablation rate of ZSZ1 were only −4.268 × 10−4 g/s and −8.32 × 10−3 mm/s, respectively, which is attributed to be the structure of ZrO2 fine particles uniform distributed in SiO2 melt.

Effect of High Pressure Spark Plasma Sintering on the Densification of a Nb-Doped TiO2 Nanopowder

Sintering under pressure by means of the spark plasma sintering (SPS) technique is a common route to reduce the sintering temperature and to achieve ceramics with a fine-grained microstructure. In this work, high-density bulk TiO2 was sintered by high pressure SPS. It is shown that by applying high pressure during the SPS process (76 to 400 MPa), densification and phase transition start at lower temperature and are accelerated. Thus, it is possible to dissociate the two densification steps (anatase then rutile) and the transition phase during the sintering cycle. Regardless of the applied pressure, grain growth occurs during the final stage of the sintering process. However, twinning of the grains induced by the phase transition is enhanced under high pressure resulting in a reduction in the crystallite size.

Effect of high-energy ball-milling on the spark plasma sinterability of ZrB2 with transition metal disilicides

A wide suite of powder mixtures of ZrB2 with five different additions of transition metal disilicides (MeSi2; 5, 17.5, or 30 vol% MoSi2; 17.5 vol% TaSi2; or 17.5 vol% ZrSi2) were prepared by high-energy ball-milling (HEBM) for different times, and their optimal densification temperatures were evaluated by dilatometric spark-plasma-sintering (SPS) tests to compare their sinterability. SPS densification tests at the so-determined optimal densification temperatures were also performed in selected cases. It was found that HEBM progressively enhanced the sinterability, especially once the ZrB2 particle sizes were refined to the ultrafine range or below (<250 nm). It was also observed that the sinterability was enhanced (i) with the greater addition of MeSi2, and (ii) with the lower refractoriness of the MeSi2. Nonetheless, under long-time HEBM, the addition of harder, more refractory MeSi2 or of softer, less refractory MeSi2 is equally effective in terms of sinterability, both enabling low-temperature densification (SPS at ∼1200 °C).

The Influence of Porous Structure on the Interdiffusion Kinetics of Cu-Ni System During Spark Plasma Sintering

The interdiffusion kinetics of Cu-Ni system and the influence of porous structure on atomic diffusion kinetics during spark plasma sintering (SPS) were investigated. The interdiffusion coefficient of the Cu-Ni system annealed under SPS is much higher than that without current when the temperature exceeds 700 °C. By comparing the interdiffusion behaviors between Ni powder/Cu foil interface and Ni foil/Cu foil interface, the interdiffusion rate at the foil/powder interface was found to be significantly higher than that at the foil/foil interface during SPS. The diffusion process at the Ni powder/Cu foil interface shows two clearly identified stages: a high diffusion rate at the initial stage with high porosity and a slower diffusion rate at the mid-late stage with low porosity. The diffusion coefficient at the initial stage is nearly 5.2 times higher than that of the mid-late stage, demonstrating that the diffusion rate at the foil/powder interface decreases with the gradual reduction of porosity during the SPS densification process. The porous structure leads to an extremely high local current density at the neck area, which results in a high density of crystal defects at the diffusion interface and subsequently accelerates atomic diffusion.

Densification of complex shape ceramics parts by SPS

Spark Plasma Sintering (SPS) equipments, with higher productivity and hybrid heating modes, allow reducing thermal gradients in large samples and broaden their application potential. Furthermore, strong shape limitations remain: only simple shapes can be obtained because of the use of uniaxial pressure.The aim of this paper is to propose a SPS densification method for complex shape ceramic or powder metallurgy parts, without any modification of the tools and equipment. The samples to be sintered are placed in a powder bed in the classical die used in SPS. Different powder beds have been tested.A numerical model was developed in order to allow a rapid simulation of the force field around a part surrounded by a powder bed in a SPS tool. The optimization of the operating conditions was carried out, first, on small simple-shape samples, then, on complex geometry parts which were densified to demonstrate the feasibility of the proposed sintering process.

Mechanical and Microstructural Characterization of Powder Medium Entropy Alloy

High and medium entropy alloys are an intriguing new material class, revered for possessing an exceptional combination of primarily mechanical properties. In this scientific contribution, the CoCrNi medium entropy alloy has been produced by a combination of mechanical alloying (MA) and spark plasma sintering (SPS). The properties have been characterized by means of X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and tensile testing. The powder after the MA process exhibited nano-grained microstructures with fine chemical homogeneity. After the SPS densification, a full density of the bulk products has been achieved. The microstructure was composed of major FCC phase and minor secondary precipitates. The materials possessed high strength values coupled by reasonable ductility levels, therefore overall satisfactory results.

Estimation of diffusivity from densification data obtained during spark plasma sintering

Evaluation of the diffusion coefficient of metal powders was attempted by using the power-law creep model in conjunction with the isothermal densification kinetics during spark plasma sintering (SPS). The diffusion coefficients obtained from the densification data of elemental Fe, Ni and Al powders are found to be higher than those reported in the literature. The higher values of diffusivity can be attributed to electric current effects. Our analysis demonstrates that it is possible to evaluate diffusion coefficients from experimental SPS densification data.

Densification Kinetics of CeO2 Reinforced 8 Mol.% Y2O3 Stabilized ZrO2 Ceramics

The grain growth kinetics of 8YSZ ceramics processed using spark plasma sintering (SPS) has been investigated in the temperature ranging from 1100°C to 1500°C. The activation energy during SPS densification was obtained as 332 kJ/mol with grain boundary diffusion as a dominant mechanism. Further, the effect of CeO2 on the densification kinetics of 8YSZ ceramic processed via SPS and conventional sintering (CS) has been delineated. The lower grain boundary mobility of CS-processed composites (an order of magnitude lower than SPS) is attributed to the solute drag and lattice distortion mechanism. However, no significant change in the grain boundary mobility was observed with CeO2 addition (~ 14.7–43.9 × 10−18 m3/N/s for CS and 107.2–116.7 × 10−18 m3/N/s for SPS) revealing that the defect concentration is nearly constant in 8YSZ. The study highlights the effect of sintering techniques (SPS and CS) and reinforcement (CeO2) on engineering the desired microstructure of 8YSZ ceramic.

Creation of individual few-layer graphene incorporated in an aluminum matrix

3D-networks of few-layer graphene (FLG) platelets at grain boundaries, sandwiched between thin amorphous Al2O3 layers, were fabricated by spark plasma sintering (SPS) of graphene oxide (GO)/Al mixed powders. The GO was prepared by a modified Hummers’ method, and was thermally reduced to FLG simultaneously during SPS densification. Subsequent plastic flow of the Al matrix during the hot extrusion process caused the destruction of this structure, rearranged the FLG platelets individually into the uniaxial direction, and made them incorporate in the Al matrix. Observations by high-resolution transmission electron microscopy proved the existence of a direct-contact interface between the FLG and the Al matrix without any interfacial compounds, and revealed that the Al matrix featured a fairly low dislocation density. Consequently, the mechanical strength of Al matrix was noticeably enhanced by FLG incorporation, agreeing with the potential strengthening effect predicted by the load transfer mechanism.

Understanding the combustion process for the synthesis of mechanically robust SnSe thermoelectrics

Recently, very good thermoelectric performance in both n-type and p-type SnSe has been achieved but was accompanied by poor mechanical properties. Moreover, the synthesis routes used, either the Bridgeman method for single crystal growth or conventional melting methods for polycrystalline samples, are highly energy and time intensive. In the present study, single phase SnSe was synthesized by the Self-Propagating High Temperature Synthesis (SHS) combined with Spark Plasma Sintering (SPS) in a very short time of less than 1h, and with minimum consumed energy. The thermodynamic and kinetic parameters, as well as the role of a pre-synthesized powder of SnSe added to the elemental Se and Sn powders prior to the SHS process, were systematically investigated for the first time. The presence of the pre-synthesized SnSe powder in the mixture of elemental Se and Sn powders promotes multipoint nucleation and inhibits the grain growth, resulting in a weak preferential orientation after SHS that is maintained in the bulk samples after SPS. Thermoelectric and mechanical properties of such samples are superior in comparison to the samples that do not contain the pre-synthesized powder of SnSe in the starting material. The compressive strength, bending strength and fracture toughness of the sample containing 30% of the pre-synthesized SnSe were 74.7MPa, 40.6MPa and 0.32MPam1/2, respectively. A maximum ZT value of 0.5 was achieved for this sample at 773K, which is comparable with polycrystalline samples synthesized by the conventional melting method. This work shows that the SHS reaction combined with SPS densification is an exceptionally rapid and cost effective synthesis process that yields mechanically robust bulk SnSe, a crucial step for large scale commercial applications of this thermoelectric material.

Effect of electric current on densification behavior of conductive ceramic powders consolidated by spark plasma sintering

The densification mechanism of conductive powders is revealed by comparing the electric current-assisted spark plasma sintering (SPS) of ZrN powder with conventional hot pressing (HP) carried out with the same powder and under the same pressure and temperature. To determine the actual temperature inside ZrN powder, a sacrificial thermocouple is directly inserted into the powder during the SPS process. The spatial distribution of the electric current passing through the powder is calculated using the finite element modeling. The porosity-interparticle neck area geometrical relationship is applied to estimate the electric current density inside the powder volume subjected to SPS. For the first time, by taking into account the explicit influence of the electric current effect on the SPS densification mechanism, the constitutive equations describing the electric current-assisted hot pressing of powders are developed. The densification mechanism of ZrN is determined by the inverse regression of the new SPS constitutive equations and by utilizing the experimental results on ZrN powder consolidation with and without the participation of the electric current effect.

Comparison of densification kinetics of a TiAl powder by spark plasma sintering and hot pressing

Densification kinetics by spark plasma sintering (SPS) and hot pressing (HP) have been compared, under isothermal conditions and with heating rates of 20 °C/min. Careful calibration of sample temperature has been carried out to obtain comparable results. In all cases, densification kinetics did not exhibit significant differences, ruling out any influence of the SPS current. The stress exponent n and the activation energy Q of the Norton law describing deformation at high temperature of the powder particles have been determined by isothermal experiments at different stresses and temperatures, respectively. The values obtained, n = 1.9 ± 0.3 and Q = 308 ± 20 kJ/mol for SPS, n = 1.5 ± 0.3 and Q = 276 ± 40 kJ/mol for HP, come close in both techniques. Using these values, anisothermal densification kinetics at heating rates of 20°C/min and 100 °C/min, typical of the SPS, could be analytically reproduced, using literature models. The activation parameters suggest that SPS densification kinetics occurs by dislocation climb controlled by Al bulk diffusion, that is, by classical metallurgical mechanisms.

Densification and microstructural evolution of a high niobium containing TiAl alloy consolidated by spark plasma sintering

Gas-atomized Ti–45Al–7Nb–0.3W alloy powders were consolidated by the spark plasma sintering (SPS) process. The densification course and the microstructural evolution of the as-atomized powders during SPS were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) techniques. As a result of SPS densification, special (α + γ) precipitation zones are formed in the initial stage of sintering, and the residual β phases in the microstructure of the powders are fragmentated. During the following SPS course, α2/γ lamellar colonies at the edge of the precipitation zone, α2 and B2 phase as well as dynamic recrystallized γ grains are found to form. For the as-atomized powders sintered at 1000 °C, the densification is preceded by the early rearrangement of the powder particles and the following formation of sintering necks. For the powders sintered at 1200 °C, plastic deformation plays an important role in densification. Local melting and surface bulging between two adjacent particles can also serve as one of the densification mechanisms. In the later stage of sintering, the growth of sintering necks controlled by diffusion and the pore closure would make important contributions to the densification.

SPS densification and microstructure of ZrB2 composites derived from sol–gel ZrC coating

A technique for densifying ultra high temperature ceramic composites while minimising grain growth is reported. As-purchased ZrB2 powder was treated with a zirconia-carbon sol–gel coating. Carbothermal reduction at 1450 ◦ C produced 100–200 nm crystalline ZrC particles attached on the surface of ZrB2 powders. The densification behaviour of the sol–gel coated powder was compared with both the as-purchased ZrB2 and a compositionally similar ZrB2–ZrC mixture. All three samples were densified by spark plasma sintering (SPS). The ZrB2 reference sample was slow to densify until 1800 ◦ C and was not fully dense even at 2000 ◦ C, while the sol–gel modified ZrB2 powder completed densification by 1800 ◦ C. The process was studied by ram displacement data, gas evolution, SEM, and XRD. The sol–gel coated nanoparticles on the ZrB2 powder played a number of important roles in sintering, facilitating superior densification by carbothermal reduction, nanoparticle coalescence and solid-state diffusion, and controlling grain growth and pore removal by Zener pinning. The sol–gel surface modification is a promising technique to develop ultra-high temperature ceramic composites with high density and minimum grain growth. © 2014 Elsevier Ltd. All rights reserved.

Enhancement in Performance of the Tubular Thermoelectric Generator (TTEG)

For use in a tubular thermoelectric generator (TTEG), we fabricated tubular Bi0.5Sb1.5Te3/Ni composite using a melt-spinning technique combined with the spark plasma sintering (SPS) process. With this method, powder sintering, joining of two different materials, and tubular shaping can be achieved simultaneously. The tilted laminate structure which is crucial for the transverse thermoelectric effect was successfully achieved in the sample after SPS densification. The sintered samples showed better mechanical stability and thermoelectric properties compared with the previously studied melt-cast sample. We confirmed larger open-circuit voltage of 240 mV and generating power of 2.5 W with a 100-mm-long TTEG under the small temperature difference of 83 K, and the corresponding power density for a unit heat transfer surface area was approximately 800 W m−2.

SPS densification behavior of W-5.6Ni-1.4Fe heavy alloy powders

Spark plasma sintering (SPS) method was used to sinter two different types of 93W-5.6Ni-1.4Fe (wt pct) heavy alloy powders which were respectively prepared by simple mixing and high-energy ball milling (HEBM) for 40 h. In SPS processing, the powder compacts were heated at 100 °C/min to the desired sintering temperature with 5 min holding. The SPS densification behavior of the two types of powders was investigated. For the simple mixed powders, the compacts start to shrink around 850 °C, and the densification is enhanced by diffusion and plastic deformation with the increase of temperature. Consequently the compacts shrink effectively and obtain nearly full density around 1230 °C. While for the as-milled powders, recovery and recrystallization occur between 700–850 °C due to the crystal lattice distortion and defects resulted from HEBM, causing the soften of particles and a slight shrink of compacts. When increasing temperature, nanosintering of W crystallines in the matrix occurs in the inside of the composite powders, and the densification rate accelerates and maximizes at about 1050 °C. Owing to the improved activity of powders from HEBM, the tungsten grains coarsen rapidly in the sintering, simultaneously, the harmful intermetallic phases Ni2W4C and Fe6W6C are generated.

Spark Plasma Sintering Behavior of 93W-5.28Ni-1.32Fe-0.4Y&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Heavy Alloy Powders

Spark plasma sintering (SPS) method was used to consolidate ultra-fine and mixed 93W-5.28Ni-1.32Fe-0.4Y2O3 (wt pct) powders, and the effects of sintering parameters on the densification degree and the performance of the as-sintered materials were investigated. Results showed that the SPS densification process could be divided into four stages. Compacts were densified rapidly at the stage III and a proper holding time promoted W diffusing into fcc-type Ni-Fe based solution. Heating rate and sintering time had a great influence on densification. When the powders were heated at a rate of 100°C/min to 1230°C and then held for 5min, the density, hardness and transverse rupture strength of the as-sintered material reached the optimum, being 17.18×103kg/m3, 41.4HRC and 935.1MPa, respectively. Its corresponding fracture morphology was characterized as intergranular rupture of W-W, and accompanied with some transgranular cleavage of tungsten grain.

Spark Plasma Sintering Densification Mechanism for Cemented Carbides with Different WC Particle Sizes

The paper is focused on understanding the densification characteristics of WC–Co composite powders by the spark plasma sintering (SPS) technique, especially the essential mechanisms for the distinct differences of the effects of WC particle size on the densification behavior between the SPS and the conventional sintering technologies. For the particular combination and contacting state between WC and Co after ball milling in which Co forms thin films coating the WC particles, a model that quantitatively describes the densification process of SPS WC–Co powders with different WC particle sizes has been developed. The calculated results show that both the actual temperature in the Co film and the melting temperature of the Co film increase with the increase of the WC particle size. As a result, the formation and growth of the sintering necks due to the rapid melting and solidification of the Co films turn to be weakly influenced by the WC particle size, and hence the SPS densification is almost independent of the WC particle size. The model calculations are consistent with the experimental findings that in the SPS processes the temperatures corresponding to the start of the densification and the peak of the displacement rate, respectively, are nearly the same for the WC–Co powders with different WC particle sizes.

Effects of WC particle size on densification and properties of spark plasma sintered WC–Co cermet

Abstract The WC–Co cermet bulks were prepared by spark plasma sintering (SPS) using powder mixtures with different-scaled WC particles. The SPS densification process was studied by calculating the current distribution between the powder sample and the die in the SPS system. The microstructures were characterized and compared for different samples by the WC grain size, Co mean free path and contiguity of WC grains. In spite of a weak effect of WC particle size on the SPS densification stages, the WC particle size plays a significant role in the homogeneity of the cermet microstructure. Good mechanical properties of the SPSed cermet were obtained with an optimized WC and Co particle-size combination. The effects of scale combination of WC and Co particles on the microstructure hence the properties of the SPSed cermet were discussed.