Two-step Spark Plasma Sintering Research Articles

Tuning mechanical and electrical performances of B 4C–TiB 2 ceramics in a two-step spark plasma sintering process

Tuning mechanical and electrical performances of B ₄C–TiB ₂ ceramics in a two-step spark plasma sintering process

Preparation of tungsten-particle-reinforced Zr-based bulk metallic glass composites by two-step spark plasma sintering: microstructure evolution, densification mechanism and mechanical properties

Preparation of tungsten-particle-reinforced Zr-based bulk metallic glass composites by two-step spark plasma sintering: microstructure evolution, densification mechanism and mechanical properties

Developing high performance in titanium carbide nanocomposites containing hybrid SiC nanowire and CNT

Lightweight titanium carbide (TiC) has garnered considerable engineering applications in various advanced manufacturing industries. However, the intrinsic brittleness of TiC significantly limited its further applications. High-toughness and damage-tolerance TiC is always highly desirable for industry. Herein, we developed high-performance TiC nanocomposites reinforced by hybrid carbon nanotube (CNT) and SiC nanowire (SiCnw) through two-step spark plasma sintering, highlighting the synergic role of CNT and SiCnw on the microstructure and properties of the TiC matrix. Specifically, CNT was helpful in maintaining the high aspect ratio of SiCnw during the sintering process, while SiCnw contributed to the homogeneous distribution of CNT throughout the TiC matrix. The flexural strength and fracture toughness were simultaneously enhanced by 48.1% and 56.9% in case of CNT/SiCnw ratio of 1:2 comparing with pure TiC, respectively. This study provided the new insight on developing high-performance TiC materials.

Microstructure and mechanical properties of Ti-Zr alloys fabricated by two-step spark plasma sintering from TiH2 and ZrH2 powders

ABSTRACT Ti is well-known for its high strength-to-weight ratio and biocompatibility. It can be alloyed with Zr to improve mechanical properties. Currently, Ti-Zr alloys can be fabricated by powder metallurgy using Ti and Zr powders, although high oxidation reactivity and cost could still be an issue. In this study, premixed TiH2 and ZrH2 powders were used to prepare Ti-Zr binary alloys with different Zr contents of 0–30 mass% ZrH2 via dehydrogenation and sintering by spark plasma sintering (SPS) process. α-Ti and δ-TiH2 phases coexisted in the sintered Ti-Zr alloys, and a characteristic lamellar microstructure was formed. The tensile strength of the Ti-Zr alloys increased with increasing Zr content due to the solid solution effect, grain refinement and the appearance of δ-TiH2 phases, although the elongation was reduced. This study shows that the fabricated Ti-Zr alloys possess controllable mechanical properties, which can be beneficial for biomedical and other engineering applications.

Microstructure and Mechanical Properties of Y4Zr3O12-Added Fe-13.5Cr-2W Oxide-Dispersion-Strengthened Steels, Containing High Contents of C and N, Prepared by Mechanical Alloying and Two-Step Spark Plasma Sintering.

Oxide-dispersion-strengthened (ODS) steel is considered as a promising candidate structural material for nuclear applications. In this study, the microstructure and mechanical properties of Y4Zr3O12-added Fe-13.5Cr-2W ODS steels, containing high contents of C and N, prepared by mechanical alloying (MA) and two-step spark plasma sintering (SPS), were investigated. The results showed that pure Y4Zr3O12 powders, with a grain size of 3.5 nm, were well prepared with NH3·H2O addition by the sol-gel method in advance, in order to avoid the formation of some coarse or undesired oxides. W was completely dissolved into the matrix after 48 h of ball milling at 300 rpm, and the main elements were uniformly distributed on the surface of the milled powders. The unexpected face-centered cubic (FCC, γ)/body-centered cubic (BCC, α) dual-phase structure of the sintered specimens, could be explained by the unexpectedly high contents of C and N from the raw powder production process, fast-sintering characteristic of SPS, and inhibitory effect of W on the diffusion of C. The experimental results were approximately consistent with the simulation results from the Thermo Calc software. The temperature combination of 800 °C and 1100 °C during the SPS process, provided a relatively more homogeneous microstructure, while the combination of 750 °C and 1150 °C, provided the highest ultimate tensile strength (UTS), of 1038 MPa, with the highest uniform elongation (UE), of 6.2%. M23C6, Cr2O3, M2(C,N), and other precipitates, were mainly distributed at grain boundaries, especially at the triple junctions, which led to Cr depletion at grain boundaries.

Scintillation properties of (Ga[formula omitted]Al[formula omitted])[formula omitted]O[formula omitted] ceramics

In this communication scintillation properties of (GaxAl1−x)2O3 (x = 1, 0.75, 0.5, and 0.2) ceramics are reported. The investigated samples have been fabricated through two-step spark plasma sintering. Their properties have been studied by means of pulse height spectra, scintillation time profiles, radio- and thermoluminescence. Although room temperature scintillation yields of the samples do not exceed 1000 ph/MeV, their scintillation decays appear relatively fast as compared to single crystals.

Effect of NiTi content on the mechanical properties and microstructure of TiB2–TiN-based composite ceramic material prepared using spark plasma sintering

In this study, TiB2–TiN-based composite ceramic materials modified with NiTi were prepared by a two-step spark plasma sintering process (900 °C/3 min + 1650 °C/5 min). The effects of adding different volume fractions of NiTi (0–12 vol%) on the mechanical properties and microstructure of the ceramic material were investigated. The results showed that the optimal mechanical properties of the composite ceramic were obtained when the NiTi content was 9 vol%. Its flexural strength, fracture toughness and hardness were 942.5 MPa, 5.96 MPa m1/2 and 19.8 GPa, respectively. The addition of NiTi promoted densification during the sintering process, which improved the sinterability and refined the grain size of the materials. The toughening mechanisms were attributed to branching, deflection and bridging of cracks. Furthermore, a new toughening mechanism, phase change toughening was observed due to the addition of NiTi. Its specific manifestation in TiB2–TiN–NiTi cermet materials was the limitation of crack extension, which caused NiTi to transform from austenite to martensite when the crack extended into the region containing NiTi, and this consumed most of the fracture energy. The fracture toughness of the TiB2–TiN–NiTi composite ceramic materials was improved by the ability of NiTi to hinder crack growth.

Characterization of ZnO thermoelectric ceramics and their microstructures consolidated by two-step spark plasma sintering

Characterization of ZnO thermoelectric ceramics and their microstructures consolidated by two-step spark plasma sintering

Fabrication of transparent Ce3+-doped (Gd,Lu)3Al5O12 ceramics by two-step spark plasma sintering

Transparent Ce3+:(Gd,Lu)3Al5O12 with microstructure control was fabricated by two-step spark plasma sintering. In the two-step profile, the heating rate was changed from 50 to 5°C/min at the first step temperatures. During the initial stage of shrinkage, the holding time of the first step sintering could induce densification by suppressing the microstructure coarsening. As compared to the single-step profile, the two-step profile showed a smaller grain size, which decreased with a decrease in the first step temperature. The porosity of the two-step profile was lower than that of the single-step profile, and the lowest porosity was obtained at the first step temperature of 1000°C, which was the starting point of shrinkage. The TS-1000 specimen showed the highest transmittance among all specimens because of the microstructure control offered by the two-step profile. Thus, by employing the two-step profile, the transmittance could be increased from 50.1% (SS-1250) to 56.5% (TS-1000).

Microstructure of Ultrafine-Grained Al2O3–ZrO2 Ceramics Produced by Two-Step Spark Plasma Sintering

Microstructure of Ultrafine-Grained Al2O3–ZrO2 Ceramics Produced by Two-Step Spark Plasma Sintering

Band gap tuning of Ga2O3–Al2O3 ceramics

Translucent ceramics comprising monoclinic Ga 2 O 3 and non-cubic Ga 2 O 3 –Al 2 O 3 solid solution with a composition of Ga 1.5 Al 0.5 O 3 , GaAlO 3 and Ga 0.4 Al 1.6 O 3 were fabricated through two-step spark plasma sintering by using calcinated commercial mixture oxide ceramic powders. All sintered ceramics were nearly fully densified, with a relative density exceeding 99.2%. The average grain sizes decreased with increasing Al content. Band gap was tuned from 4.08 to 6.37 eV. The highest total transmittance (73.6% at 1100 nm) was measured in Ga 0.4 Al 1.6 O 3 ceramics with both hexagonal (rhombohedral) and monoclinic crystal structures and a minimum average grain size of 0.288 ± 0.067 μm. Defect related photoluminescence, measured under excitation by a 254 nm UV lamp in the GaAlO 3 and Ga 0.4 Al 1.6 O 3 ceramics, lasted more than 15 s after removal of the UV source. These translucent ceramics from Ga 2 O 3 –Al 2 O 3 solid solution provide an alternative form of a potential transparent conducting material.

Tribological performance of binderless tungsten carbide reinforced by multilayer graphene and SiC whisker

Binderless tungsten carbides (BTC) have attracted the attention of both academia and industry due to their excellent hardness, high-temperature strength as well as corrosion resistance. Herein, a set of binderless WC-based composites were developed employing two-step spark plasma sintering. The mechanical response together with tribological performance was investigated as a function of various reinforcements including SiC whisker (SiCw) and multilayer graphene (MLG). The results suggested that the coupling and synergy of MLG and SiCw played a critical role in microstructure refinement and homogenization, consequently advancing the mechanical and tribological properties of BTC. The MLG-SiCw/BTC displayed the optimal comprehensive performances, and the associated toughening and anti-friction mechanisms were determined and analyzed. Such material design offers an effective approach to enhance the mechanical and tribological responses of ceramic materials.

Refractory WMoNbVTa high-entropy alloy as a diffusion barrier between a molybdenum substrate and MoSi2 ceramic coating

Coating a MoSi2 ceramic layer on Mo substrates is an excellent strategy to enhance their oxidation resistance at elevated temperatures. Suppressing elemental interdiffusion across the Mo/MoSi2 interface is very important for extending the service life of Mo substrates. The present study attempted to develop a WMoNbVTa high-entropy alloy (HEA) layer as a diffusion barrier at the Mo/MoSi2 interface. A new double-layer WMoNbVTa/MoSi2 coating was fabricated on Mo substrates through a two-step spark plasma sintering (SPS) method. Then, the interdiffusion behaviors of Mo/WMoNbVTa/MoSi2 and Mo/MoSi2 systems at 1200–1500 °C were investigated comparatively. A WMoNbVTa HEA powder with a single body-centered cubic (BCC) structure was prepared through mechanical alloying and the intermediate WMoNbVTa HEA layer was successfully fabricated using the milled HEA powder. After long-term oxidation, two diffusion layers of Mo5Si3 and (WMoNbVTa)5Si3 appeared at the WMoNbVTa/MoSi2 interface, while no apparent diffusion zone was found at the Mo/WMoNbVTa interface. The growth rate of the thickness of the diffusion layer at the Mo/WMoNbVTa/MoSi2 interface was much lower than that at the Mo/MoSi2 interface. The WMoNbVTa HEA layer effectively suppressed the interdiffusion of alloying elements between the Mo substrate and MoSi2 ceramic coating. This is attributed to the sluggish diffusion effect of the WMoNbVTa HEA during high-temperature oxidation. The results suggest that WMoNbVTa HEA can be a suitable diffusion barrier between Mo substrates and MoSi2 ceramic coating.

Hardness anisotropy and active slip systems in a (Hf-Ta-Zr-Nb)C high-entropy carbide during nanoindentation

Hardness anisotropy and deformation-induced dislocations in (Hf-Ta-Zr-Nb)C high-entropy carbide grains of low-index facets were investigated using nanoindentation and transmission electron microscopy (TEM). The (Hf-Ta-Zr-Nb)C sample was prepared by a two-step spark plasma sintering (SPS) and nanoindentation was performed on grains mapped previously by electron backscatter diffraction. To investigate the deformation zones under the indents, thin lamellas were extracted from grains of {001}, {101} and {111} orientations using a focused ion beam (FIB) technique and were studied by TEM. Nanoindentation revealed that grains of {100} orientation exhibit about 7% lower hardness (~35 GPa) compared to the {110} and {111} facets (~38 GPa) while there was no anisotropy found for the indentation modulus. TEM analysis of the lamellas revealed heavily deformed zones under the indents with dislocations of a/211¯0 type Burgers vector. TEM-based trace analysis of dislocations revealed the activation of the 1101¯11 and 1101¯10 slip systems in grains indented on the {001} and {101} facets, respectively. The hardness anisotropy of (Hf-Ta-Zr-Nb)C was attributed to the different operating slip systems, similar to that was reported for the component monocarbides in the literature.

Fabrication of in situ elongated β-Sialon grains bonded to tungsten carbide via two-step spark plasma sintering

In order to reduce the difficulty of preparing binder-less cemented carbide and further broaden its application prospects, tungsten carbide toughened by in situ elongated β-Sialon grains was developed via sintering ball-milled WC and α-Si3N4 powders using Al2O3–ZrO2 as a sintering aid and transformation additive. The two-step spark plasma sintering of the mixture at 1650 °C with dwelling at 1500 °C for 10 min was conducted under 30 MPa uniaxial pressure, and the densification behaviors, phase transformations, mechanical properties, and microstructures of the produced composites were investigated. The addition of Al2O3–ZrO2 reduced the initial temperature of the densification process by approximately 100 °C and its final temperature by 200 °C (compared with the densification temperatures of pure WC and Si3N4 materials) and fully transformed α-Si3N4 to Sialon (Si–Al–O–N) phases. Microstructural characterization data showed that the WC matrix contained homogeneously distributed equiaxed and elongated β-Si5AlON7 grains. The WC composites containing in situ elongated β-Sialon grains exhibited an optimal hardness of 18.93 ± 0.03 GPa and enhanced fracture toughness of 10.43 ± 0.27 MPa m1/2. The toughening mechanism of the β-Sialon phase involved the pull-out of elongated grains and crack bridging.

Nanoindentation and tribology of a (Hf-Ta-Zr-Nb-Ti)C high-entropy carbide

A (Hf-Ta-Zr-Nb-Ti)C high-entropy carbide was prepared by ball milling and a two-step Spark Plasma Sintering process, achieving a single-phase ceramic sample with a high relative density of 99.4 %. The wear resistance of the sample was measured by tribology and micro-scale mechanical behaviour was studied by nanoindentation on both the non-deformed and worn surfaces. Grains and the vicinity of grain boundaries exhibited high hardness values of 38.5 ± 0.5 GPa and 35.5 ± 1.0 GPa with similar Young’s moduli of 562 ± 11 GPa and 547 ± 16 GPa, respectively. The dominant wear mechanism was mechanical wear with limited grain pull-out and fracture, and with a localized and thin tribo-layer formation. The specific wear rate exhibited an increase with the increasing load from 2.53·10−6 mm3/Nm at 5 N to 9.03·10−6 mm3/Nm at 50 N. This was correlated to the decrease of nanohardness of the worn surfaces with increasing wear load, which is attributed to the increased number of microcracks.

Nanostructured multicomponent Nd-Fe-B magnets prepared by a spark-plasma-sintering approach

The magnetic properties of an Nd-Fe-B-type permanent magnet depend on the microstructure and the chemistry of the material. To compensate for unfavorable microstructural features, such as micron-sized grains of the hard-magnetic phase, the intrinsic coercivity is often enhanced by the addition of heavy-rare-earth elements, but these additions also reduce the magnetization. In some applications, like electro-mechanical devices, a high intrinsic coercivity is only required in certain regions of a magnet. In such cases, using magnets with locally tailored magnetic properties, i.e., multicomponent magnets, would enhance the magnet-containing device’s performance. Here, we propose a spark-plasma-sintering (SPS) approach to the manufacture of multicomponent Nd-Fe-B magnets. By exploiting the SPS-specific processing conditions, namely fast heating rates (100 °C/min) and low consolidation temperatures (≈670 °C), such magnets can be prepared directly from nanostructured melt-spun powders (the one-step SPS approach) or SPS-processed precursor magnets (the two-step SPS approach). Optimizing the SPS processing conditions prevents the grain coarsening related to the pre-existing microstructural inhomogeneities of the powders. The magnetic characterization reveals reliable performance over a wide range of operating temperatures. The high remanent magnetization of a heavy-rare-earth-free powder (Br = 0.82 T) and the high intrinsic coercivity of a powder containing 1.5 at. % Dy (Hci = 2075 kA/m) are preserved in a multicomponent magnet characterized by an abrupt change in magnetic properties close to the interface between the respective magnet parts.

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.

Low temperature densification mechanism and properties of Ta1-xHfxC solid solutions with decarbonization and phase transition of Cr3C2

As a novel member of the ultra-high temperature ceramic family, which have extremely high melting points and remarkable hardness, Ta1-xHfxC solid solution ceramics are promising for applications in thermal protection systems. Ta1-xHfxC (x = 0, 0.2, 0.5, 0.8, and 1.0) with 2 vol% Cr3C2, were densified up to 98.8% at 2000 °C using a two-step spark plasma sintering process. Effect of Cr3C2 on the linear shrinkage of Ta1-xHfxC was investigated. Possible ‘eutectic’ reaction within TaCCr3C2 ceramic was inferred to contribute to the shrinkage at 1462 °C. High-angle annular dark-field scanning transmission electron microscopy combined with energy-dispersive spectroscopy was employed to further confirm the mutual diffusion between rock-salt structured ‘CrCx’ and TaC. Flexural strength, fracture toughness and Vicker’s hardness of Ta1-xHfxC ceramics were in the range of 439–492 MPa, 4.0–5.8 MPa∙m1/2 and 14.9–19.9 GPa respectively. The coefficient of thermal expansion (in the temperature range of 25–1000 °C) and thermal conductivity (at 1000 °C) of Ta1-xHfxC varied from 7.17 to 7.51 × 10−6 K−1 and 31.9–42.9 W/m·K, respectively. The high-temperature strength of Ta0.5Hf0.5C decreased to 165 MPa up to 1600 °C, approximately 34% of room-temperature strength, and a ‘zig-zag’ load-displacement curve was observed.

Effect of Nb content on the phase composition, densification, microstructure, and mechanical properties of high-entropy boride ceramics

Dense high-entropy (Hf,Zr,Ti,Ta,Nb)B2 ceramics with Nb contents ranging from 0 to 20 at% were produced by a two-step spark plasma sintering process. X-ray diffraction indicated that a single-phase with hexagonal structure was detected in the composition without Nb. In contrast, two phases with the same hexagonal structure, but slightly different lattice parameters were present in compositions containing Nb. The addition of Nb resulted in the presence of a Nb-rich second phase and the amount of the second phase increased as the Nb content increased. The relative densities were all >99.5 %, but decreased from ∼100 % to ∼99.5 % as the Nb content increased from 0 to 20 at%. The average grain size decreased from 13.9 ± 5.5 μm for the composition without Nb additions to 5.2 ± 2.0 μm for the composition containing 20 at% Nb. The reduction of grain size with increasing Nb content was due to the suppression of grain growth by the Nb-rich second phase. The addition of Nb increased Young’s modulus and Vickers hardness, but decreased shear modulus. While some Nb dissolved into the main phase, a Nb-rich second phase was formed in all Nb-containing compositions.