Spark Plasma Sintering Pressure Research Articles

A relationship of porosity and mechanical properties of spark plasma sintered scandia stabilized zirconia thermal barrier coating

Porous ceramic materials are popularly accepted as thermal barrier coatings (TBCs) in insulating gas turbine parts working at high temperatures. In this research, three different types of Scandia stabilized zirconia (ScSZ) systems, a nanometric 10 mol% ScSZ (10-ScSZ), a micrometric 8 mol% ScSZ (8-ScSZ) and a combination of these two powders (10-8-ScSZ) have been developed by Spark Plasma Sintering (SPS) process. Varying SPS parameters, for instance, temperature, pressure and dwell time were applied to develop the different volumes of porosity in the materials. Subsequently, the microstructure of the materials has been studied and mechanical properties have been evaluated. All three materials demonstrate a reduced porosity level at high sintering temperature and pressure. However, the nanometric 10-ScSZ material shows a higher reduction of porosity from 51.8% to 11.1% at 30 MPa pressure and 40.2%–8.5% at 60 MPa pressure within the temperature range of 1000–1200 °C. Besides, the 10-8-ScSZ composite exhibits substantially increased porosity in comparison to its constituent parts. The results also show that the nanometric 10-ScSZ material exhibits a greater mechanical strength including Vickers microhardness of 81 HV, flexural strength of 361 MPa and elastic modulus of 187 GPa at 5% porosity level, as compared to the other two materials. Additionally, it is observed that all the mechanical properties for all three materials consistently decrease with the increase in porosity levels. While compared with the traditional atmospheric plasma spray (APS) processed ceramic coating, the porous ScSZ coating materials exhibit a larger elastic modulus. Therefore, the porous ScSZ developed by the SPS process could be a prospective alternative thermal barrier coating (TBC).

Synergy of grain size and texture effect for high-performance Mg3Sb2-based thermoelectric materials

High-performance n-type Mg3Sb2-based thermoelectric materials have attracted much attention due to the six-fold degenerate carrier pockets in the conduction band. This study uncovers the electrical and thermal transport behaviors in different directions with respect to the spark plasma sintering (SPS) pressure that is rarely investigated. Results indicate that the grain size in the ⊥P direction (perpendicular to the SPS pressure direction) is slightly larger than that in the ∥P direction. In addition, a moderate texture is observed, reflecting a preferential orientation of the (001)-plane in the ⊥P direction. The synergy of both grain size and the texture leads to an improved electrical conductivity for Mg3.24Sb1.5Bi0.49Te0.01 compounds with lamellar crystal structures. Consequently, the highest thermoelectric figure of merit ZTpeak ∼1.71 and ZTavg ∼1.12 (almost the best ZT values ever reported for Te-doped Mg3Sb2-based materials) are achieved in the ⊥P direction compared to the ∥P direction.

Phase transformation kinetics and grain orientation of multi-cation doped (Yb, Sm) α/β-SiAlON ceramic tool materials

The densification behavior, phase transformation kinetics, and grain orientation of multi-cation doped (Yb, Sm) α/β-SiAlON ceramic tool materials prepared via spark plasma sintering (SPS) were studied. The results indicated that the (Yb, Sm) α/β-SiAlON ceramics were rapidly densified at 1540 °C. Various intergranular phases were formed due to the doping of rare earth oxides with different ionic radii. β-SiAlON precipitated ahead of α-SiAlON and elongated rapidly above 1600 °C. The β-SiAlON grains were preferred to orient in the direction perpendicular to the SPS pressure due to the dissolution/precipitation and grain rotation mechanism. The tendency of orientation decreased with the growth of β-SiAlON grains and the increase of liquid phase. The orientation of β-SiAlON grains resulted in anisotropic mechanical properties of (Yb, Sm) α/β-SiAlON ceramics, which fracture toughness measured in the perpendicular plane (⊥) was higher than that in the parallel plane (//).

Effect of Ultra-High Pressure Sintering and Spark Plasma Sintering and Subsequent Heat Treatment on the Properties of Si3N4 Ceramics.

In this study, coarse Beta silicon nitride (β-Si3N4) powder was used as the raw material to fabricate dense Si3N4 ceramics using two different methods of ultra-high pressure sintering and spark plasma sintering at 1550 °C, followed by heat treatment at 1750 °C. The densification, microstructure, mechanical properties, and thermal conductivity of samples were investigated comparatively. The results indicate that spark plasma sintering can fabricate dense Si3N4 ceramics with a relative density of 99.2% in a shorter time and promote α-to-β phase transition. Coarse β-Si3N4 grains were partially fragmented during ultra-high pressure sintering under high pressure of 5 GPa, thereby reducing the number of the nucleus, which is conducive to the growth of elongated grains. The UHP sample with no fine α-Si3N4 powder addition achieved the highest fracture strength (822 MPa) and fracture toughness (6.6 MPa·m1/2). The addition of partial fine α-Si3N4 powder facilitated the densification of the SPS samples and promoted the growth of elongated grains. The β-Si3N4 ceramics SPS sintered with fine α-Si3N4 powder addition obtained the best comprehensive performance, including the highest density of 99.8%, hardness of 1890 HV, fracture strength of 817 MPa, fracture toughness of 6.2 MPa·m1/2, and thermal conductivity of 71 W·m−1·K−1.

High Pressure (HP) in Spark Plasma Sintering (SPS) Processes: Application to the Polycrystalline Diamond.

High-Pressure (HP) technology allows new possibilities of processing by Spark Plasma Synthesis (SPS). This process is mainly involved in the sintering process and for bonding, growing and reaction. High-Pressure tools combined with SPS is applied for processing polycrystalline diamond without binder (binderless PCD) in this current work. Our described innovative Ultra High Pressure Spark Plasma Sintering (UHP-SPS) equipment shows the combination of our high-pressure apparatus (Belt-type) with conventional pulse electric current generator (Fuji). Our UHP-SPS equipment allows the processing up to 6 GPa, higher pressure than HP-SPS equipment, based on a conventional SPS equipment in which a non-graphite mold (metals, ceramics, composite and hybrid) with better mechanical properties (capable of 1 GPa) than graphite. The equipment of UHP-SPS and HP-SPS elements (pistons + die) conductivity of the non-graphite mold define a Hot-Pressing process. This study presents the results showing the ability of sintering diamond powder without additives at 4–5 GPa and 1300–1400 °C for duration between 5 and 30 min. Our described UHP-SPS innovative cell design allows the consolidation of diamond particles validated by the formation of grain boundaries on two different grain size powders, i.e., 0.75–1.25 μm and 8–12 μm. The phenomena explanation is proposed by comparison with the High Pressure High Temperature (HP-HT) (Belt, toroidal-Bridgman, multi-anvils (cubic)) process conventionally used for processing binderless polycrystalline diamond (binderless PCD). It is shown that using UHP-SPS, binderless diamond can be sintered at very unexpected P-T conditions, typically ~10 GPa and 500–1000 °C lower in typical HP-HT setups. This makes UHP-SPS a promising tool for the sintering of other high-pressure materials at non-equilibrium conditions and a potential industrial transfer with low environmental fingerprints could be considered.

Interface configuration effect on mechanical and tribological properties of three-dimension network architectural titanium alloy matrix nanocomposites

Strong interfacial bonding is the primarily crucial issue in titanium matrix composites (TMCs). In this study, three-dimensional (3D) network structured Ti6Al4V (TC4) matrix TMCs with 0.15 wt% few-layer graphene (FLG) were fabricated by high pressure spark plasma sintering (SPS, 200–300 MPa) at 850 ℃, 900 ℃ and 950 ℃, correspondingly producing three types of interface configuration with in-situ TiC to FLG ratio of 4:6, 7:3 and 9:1. Composites sintered under 900 ℃ displayed outstanding integrated properties, including high micro-hardness (345.2 HV), excellent ultimate tensile strength (UTS, 1041 MPa) with acceptable tensile ductility (ε, 9.3%) and wear resistance with the lower wear rate (ω, 5.3 × 10−4 mm3/Nm). The synergistic effect of moderately generated TiC reinforcements on FLG (ratio of 7:3) in 3D network boundary under 900 ℃ leads to efficient load-bearing capacity, acting as the dominant strengthening mechanism. Self-lubricating effect of FLG and debris strengthening of TiC and TiO2 are also responsible for improved wear resistance.

Textured Bi4Ti3O12 Ceramics: One‐Step Spark Plasma Sintering and Their Single‐Crystal‐Like Polar Anisotropy

Texturing is the most effective way to engineer the performance of bismuth layer–structured ferroelectric (BLSF) ceramics by utilizing the strong anisotropy of grains, but the existing processing approaches are still complex. Herein, a one‐step spark plasma sintering (SPS) approach is demonstrated to prepare textured Bi4Ti3O12 (BiT) ceramics with high performance. The degree of orientation can be controlled from 57% to 95% by only adjusting the sintering temperature from 840 to 1000 °C. The largest spontaneous polarization (Ps) of ≈32.2 μC cm−2 is obtained in 95% <001> textured ceramics along the direction perpendicular to the SPS pressure, accompanied with the maximum electro‐strain of ≈0.14% and piezoelectric coefficient of 14.3 pC N−1. Moreover, under the same conditions of poling and illumination, the textured ceramics exhibit unique anisotropic photoresponse in which the photocurrent polarity is dependent on the testing orientations. In particular, a short‐circuit photocurrent density of 8.76 nA cm−2 is achieved along the direction of SPS pressure after positive poling, while the value goes to −79.4 nA cm−2 along direction perpendicular to the SPS pressure. These results are expected to provide a paradigm to achieve highly textured BLSF ceramics for electric and photoelectric applications.

Synthesis and behavior of bulk iron nitride soft magnets via high-pressure spark plasma sintering

In this study, dense bulk iron nitrides (FexN) were synthesized for the first time ever using spark plasma sintering (SPS) of FexN powders. The Fe4N phase of iron nitride in particular has significant potential to serve as a new soft magnetic material in both transformer and inductor cores and electrical machines. The density of SPSed FexN increased with SPS temperature and pressure. The microstructure of the consolidated bulk FexN was characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry. XRD revealed a primary phase of Fe4N with secondary phases of Fe3N and metallic iron. Finite element analysis (FEA) was also applied to investigate and explain localized heating and temperature distribution during SPS. The effects of processing on interface bonding formation and phase evolution were investigated and discussed in detail to provide insight into fundamental phenomena and microstructural evolution in SPSed FexN.Graphic abstract

Al excess extends Hall‐Petch relation in nanocrystalline zinc aluminate

AbstractThe increase in hardness with decreasing grain size is a well‐established size effect known as the Hall‐Petch relationship. In ceramics, there has been controversy surrounding the existence of a low size limit below which size‐induced hardening no longer occurs and softening is observed instead. Here, this so‐called inverse Hall‐Petch relationship was observed in quasi‐stoichiometric dense nanocrystalline zinc aluminate while an extension of the normal Hall‐Petch behavior was demonstrated by Al‐rich zinc aluminate nanoceramics. Vickers hardness increased with grain refinement for quasi‐stoichiometric samples prepared by High Pressure Spark Plasma Sintering, exhibiting a maximum of 18.6 GPa at a grain size of 21.4 nm. Conversely, Al‐rich zinc aluminate produced by the same technique strengthened up to 19.2 GPa at 12.6 nm grain sizes. Cross‐sections of Vickers indentation imprints showed that while quasi‐stoichiometric zinc aluminate showed a change in sub‐surface cracking pattern from larger to smaller grain sizes (before and after the inverse Hall‐Petch), the Al‐rich samples had sub‐surface cracking similar to those found in large grain sizes in quasi‐stoichiometric samples. These results suggest that softening at small grain sizes is driven by the activation of shear and fracture at weak grain boundaries which can be mitigated by Al enrichment.

Effect of sintering pressure and temperature on structure and properties of Ni Al metal-intermetallic composites produced by SPS

In recent decades, spark plasma sintering (SPS) has become a widespread technique in the production of high-quality intermetallic-based materials. In this study, we used SPS to fabricate multilayer metal-intermetallic (MIL) composites from pure nickel and aluminum foils. The effect of sintering temperature and pressure on the structure and properties of the fabricated MIL composites is discussed. The sintering temperature was changed from 900 to 1100 °C, the pressure varied from 10 to 40 MPa. The structure of the materials was investigated using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray energy-dispersive spectroscopy (EDX), and X-ray diffraction (XRD). Depending on the sintering regimes the intermetallic layers in the composites consisted of Ni 2 Al 3 , Ni 3 Al, and NiAl of cubic and tetragonal modifications. Tetragonal NiAl appeared in the form of microtwinned lamellar martensite. A change in SPS temperature from 900 to 1100 °C led to an increase in the volume fraction of nickel-enriched intermetallic compounds, which was accompanied by improving composites strength. With an increase of the SPS pressure from 10 to 30 MPa, the number of pores and cracks in composites decreased. At the same time, their bending strength increased by 2 times (from 450 to 900 MPa). Fractographic studies showed that the fracture mechanism of the intermetallic layers depends on their elemental composition and varies from brittle transcrystalline to wavy “feathery”. • Multilayer metal-intermetallic composites were produced by spark plasma sintering. • Ni 2 Al 3 , NiAl, and Ni 3 Al compounds were formed in the intermetallic layers. • Increase in temperature led to an increase in the volume fraction of Ni-rich phases. • NiAl twinned lamellar martensite was formed after sintering at 1100 °C. • Increase in pressure and temperature increased tensile and bending strength.

Single-step, high pressure, and two-step spark plasma sintering of UO2 nanopowders

Three different spark plasma sintering (SPS) treatments were applied to highly sinteractive, near-stoichiometric UO2.04 nanocrystalline (5 nm) powders produced by U(IV) oxalate hydrothermal decomposition at 170 °C. The sintering conditions for reaching 95 % theoretical density (TD) in regular SPS, high pressure SPS (HP-SPS), and, for the first time, two-step SPS (2S-SPS), were determined. Densification to 95 % TD was achieved at 1000 °C in regular SPS (70 MPa applied pressure), 660 °C in HP-SPS (500 MPa), and 650−550 °C in 2S-SPS (70 MPa). With the goal of minimising the grain growth during densification, the sintering treatments were optimised to favour densification over coarsening, and the final microstructures thus obtained are compared. Equally dense UO2 samples of different grain sizes, ranging from 3.08 μm to 163 nm, were produced. Room-temperature oxidation of the powders could not be avoided due to their nanometric dimensions, and a final annealing treatment was designed to reduce hyperstoichiometric samples to UO2.00.

Pore/skeleton structure and compressive strength of porous Mo3Si-Mo5Si3-Mo5SiB2 intermetallic compounds prepared by spark plasma sintering and homogenization treatment

Next to aluminum-based porous alloys, porous Mo-Si-B intermetallic compounds are the most promising filtration and separation material to handle harsh service conditions of ultra-high-temperature corrosive environment. Porous Mo3Si-Mo5Si3-Mo5SiB2 intermetallics were fabricated by a two-step method of spark plasma sintering (SPS) and homogenization treatment (HT) using Mo, Si, and B as the starting powders. In addition, the evolution of the pore/skeleton structure and compressive behavior of porous Mo3Si-Mo5Si3-Mo5SiB2 intermetallic compounds by the two-step method was investigated. In SPS, the phase transformation included a solid–liquid reaction and solid–solid diffusion, and the pore structure changed significantly with the increase of sintering temperature. By HT, smooth skeletons and size centralized pores were obtained. With the increase in the HT time, the precise control of pore/skeleton structure parameters was achieved while simultaneously maintaining stable porosity. Combined SPS+HT led to a wide range of porosities, and pore/skeleton parameters were controlled by the sintering temperature and SPS pressure. Compression test results indicate that the porous Mo3Si-Mo5Si3-Mo5SiB2 intermetallics do not exhibit a plastic deformation ability at room temperature and that the transgranular cleavage fracture dominates the crack propagation in the skeletons. The quantitative relationship between porosity and compressive strength was established by σc = σc0 × (1 – ρ)1/J. The effect of HT on the pore structure factor J was revealed: at constant porosity, the strength can be increased by the increase in the J value via HT.

Evolution of microstructure, mechanical, and optical properties of Y2O3-MgO nanocomposites fabricated by high pressure spark plasma sintering

A high-pressure spark plasma sintering (SPS) process was applied for consolidating Y2O3–MgO nanocomposites. This approach enabled to fabricate a fully dense infrared (IR) transparent nanocomposites, which possess an average grain size of ∼70 nm and high hardness, at a relatively low sintering temperature of 1130 °C under a high pressure of 300 MPa. The light transmittance was improved with increasing pressure and reached to the maximum transmittance of 64.5% at a wavelength of 0.2–1.6 μm owing to the fine-grained microstructure. The Vickers hardness exhibited 16.6 ± 0.7 GPa for the grain size of 74 nm, which is significantly higher than that of the sub-micro grains obtained at a conventional sintering pressure of 70 MPa (11.9 ± 0.8 GPa). The hardness rigorously followed the Hall–Petch relationship, that is, it is enhanced with a reduction of the grain size. Successful fabrication of the high-performance Y2O3–MgO nanocomposites indicates that the nanopowder processing followed by the high-pressure sintering process can be applied for fabricating fully dense fine-grained nanocomposites with excellent optical and mechanical properties.

Sintering highly dense ultra-high temperature ceramics with suppressed grain growth

Grain coarsening normally occurs at the final stage of sintering, resulting in trapped pores within grains, which deteriorates the density and the performance of ceramics, especially for ultra-high temperature ceramics (UHTCs). Here, we propose to sinter this class of ceramics in a specific temperature range and coupled with a relatively high pressure. The retarded grain boundary migration and pressure-enhanced diffusion ensure the proceeding of densification even at final stage. A highly dense TaC ceramic (98.6 %) with the average grain size of 1.48 μm was prepared under 250 MPa via high pressure spark plasma sintering using a Cf/C die at 1850 °C. It was suggested that the final-stage densification is mainly attributed to grain boundary plastic deformation-involved mechanisms. Compared to the usual sintering route using a high temperature (>2000 °C) and normal pressure (<100 MPa), this work provides a useful strategy to acquire highly dense and fine-grained UHTCs.

Study of the Comparative Effect of Sintering Methods and Sintering Additives on the Microstructure and Performance of Si3N4 Ceramic.

The Si3N4 ceramics were prepared in this study by gas pressure sintering (GPS) and spark plasma sintering (SPS) techniques, using 5 wt.% Yb2O3–2 wt.% Al2O3 and 5 wt.% CeO2–2 wt.% Al2O3 as sintering additives. Based on the difference in sintering methods and sintering additive systems, the relative density, phase composition, phase transition rate, microstructure, mechanical properties, and thermal conductivity were comparatively investigated and analyzed. SPS proved to be more efficient than GPS, producing higher relative density, bending strength, hardness, and thermal conductivity of Si3N4 ceramic with both additive systems; however, the phase transition rate and fracture toughness were lower. Similarly, higher bending strength, hardness, and thermal conductivity were achieved with Yb2O3–Al2O3 than CeO2–Al2O3 in the case of GPS and SPS, and only the relative density, fracture toughness, and phase transition rate were lower.

The Effect of SiC Content on Microstructure and Microwave Heating Rate of h-BN/SiC Ceramics Fabricated by Spark Plasma Sintering.

Hexagonal boron nitride/silicon carbide (h-BN/SiC) ceramics were fabricated by a spark plasma sintering (SPS) method. Phase and microstructure of ceramics were characterized and observed, respectively, using the X-ray diffraction, scanning electron microscope and electron probe microanalysis. The effect of molar ratios of SiC to h-BN on the microstructure, relative density, hardness, thermal conductivity, and the heating rate by microwaves on the ceramics were investigated. The results showed that the orientation of flake-like h-BN was significantly influenced by SiC content in h-BN/SiC ceramics. With the increasing of SiC content, the h-BN flakes gradually became an isotropic distribution from the preferred orientation aligning in a SPS pressure direction. The relative density of h-BN/SiC ceramics was 97.6 ± 0.9% at a molar ratio of SiC to h-BN of 40/60 mol%. The preferential orientation of h-BN flakes contributed to a relatively high thermal conductivity along the SPS pressure direction, which was beneficial to increasing the heating rate of h-BN/SiC ceramics in microwave fields.

Highly-doped Nd:YAG ceramics fabricated by conventional and high pressure SPS

Abstract Spark plasma sintering (SPS) is an effective process for the fabrication of highly transparent oxide ceramics for photonic applications. In the present study, Nd-doped yttrium aluminum garnet (Nd:YAG) ceramics with various dopant concentrations (0.5–5 at.%) were fabricated at 1300–1400 °C using conventional SPS (60 MPa) and high pressure (300 MPa) conditions. The appearance, X-ray diffraction pattern, densification regime, microstructure, mechanical and optical properties were compared; the dependency on Nd concentration and sintering pressure is discussed. The pressure applied during the sintering process seemed to have only a minor effect on the luminescent properties, while the mechanical properties were superior for the samples sintered under high pressure conditions. The known concentration quenching phenomenon was observed and an equation for estimation of the Nd concentration, based on phosphorescence lifetime, is suggested.

Stress-enhanced dynamic grain growth during high-pressure spark plasma sintering of alumina

Applying high pressure during the sintering of ceramic materials is a common practice that allows for a reduction of the sintering temperature and the obtaining of fine-grained microstructures. In this work, we show that the final grain size of submicron alumina increased consistently with applied pressure during low temperature (1000–1050 °C), high pressure (500–800 MPa) spark plasma sintering. Grain size trajectories and microstructural observations indicated that stress-enhanced grain growth occurred during the final stage of the sintering process, whereas thermally controlled grain boundary migration was negligible. We suggest that this dynamic, stress-enhanced grain growth is controlled by grain-boundary sliding, grain rotation and coalescence. A strong correlation was found between calculated creep strain rates and grain growth rates, such as during superplastic deformation.

Enhancement of Thermoelectric Performance in CuSbSe2 Nanoplate‐Based Pellets by Texture Engineering and Carrier Concentration Optimization

This work reports the thermoelectric properties of the CuSbSe2 -x mol% PtTe2 (x = 0, 0.5, 1.0, 1.5, and 2.0) pellets composed of highly oriented single crystalline nanoplates. CuSbSe2 -PtTe2 single crystalline nanoplates are prepared by a wet-chemical process, and the pellets are prepared through a bottom-up self-assembly of the CuSbSe2 -PtTe2 nanoplates and spark plasma sintering (SPS) process. X-ray diffraction and field emission scanning electron microscopic analyses show a highly textured nature with an orientation factor of ≈0.8 for (00l) facets along the primary surface of the pellets (in-plane, perpendicular to the SPS pressure). By this way, bulk-single-crystal-like electrical and thermal transport properties with a strong anisotropy are obtained, which results in an effective optimization on thermoelectric performance. The maximum in-plane thermoelectric figure-of-merit ZT value reaches 0.50 at 673 K for CuSbSe2 -2.0 mol% PtTe2 pellet, which is about five times higher than the in-plane ZT (0.10) for pure CuSbSe2 .

Low pressure spark plasma sintered hydroxyapatite and Bioglass® composite scaffolds for bone tissue repair

Hydroxyapatite (HA) based composite scaffold materials were prepared by combining it with Bioglass® (BG) to overcome the major limitations of HA such as the lack of osseointegration, inadequate bioactivity and biodegradation. Earlier attempts to prepare BG reinforced HA composites via the conventional sintering process resulted excessive reactions between the constituents and/or crystallization of BG. Excessive reaction between the constituents and crystallization of BG are known to lengthen the bioactivity response. In this work, low pressure spark plasma sintering (SPS) was utilized to prepare HA-BG composite scaffold materials with BG content until 30 wt%. Compared to the conventional sintering practice, the milder processing conditions during the SPS process such as the compaction pressure, sintering time and temperature produced HA-BG composite scaffolds without the excessive reactions between the constituents and prevented the crystallization of BG. All of the developed composites were composed of calcium phosphate (HA and β-Tricalcium Phosphate phases) and glassy phases only. The effect of BG addition on the physical properties such as the bulk density, relative density and hardness are well in line with the XRD and FESEM analyses. The in vitro bioactivity investigation from the immersion of samples in simulated body fluid (SBF) confirmed the improved bioactivity of the composite samples with increased BG content.