Reactive Spark Plasma Sintering Research Articles

Influence of Elemental Carbon (EC) Coating Covering nc-(Ti,Mo)C Particles on the Microstructure and Properties of Titanium Matrix Composites Prepared by Reactive Spark Plasma Sintering.

This paper describes the microstructure and properties of titanium-based composites obtained as a result of a reactive spark plasma sintering of a mixture of titanium and nanostructured (Ti,Mo)C-type carbide in a carbon shell. Composites with different ceramic addition mass percentage (10 and 20 wt %) were produced. Effect of content of elemental carbon covering nc-(Ti,Mo)C reinforcing phase particles on the microstructure, mechanical, tribological, and corrosion properties of the titanium-based composites was investigated. The microstructural evolution, mechanical properties, and tribological behavior of the Ti + (Ti,Mo)C/C composites were evaluated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), electron backscatter diffraction analysis (EBSD), X-ray photoelectron spectroscopy (XPS), 3D confocal laser scanning microscopy, nanoindentation, and ball-on-disk wear test. Moreover, corrosion resistance in a 3.5 wt % NaCl solution at RT were also investigated. It was found that the carbon content affected the tested properties. With the increase of carbon content from ca. 3 to 40 wt % in the (Ti,Mo)C/C reinforcing phase, an increase in the Young’s modulus, hardness, and fracture toughness of spark plasma sintered composites was observed. The results of abrasive and corrosive resistance tests were presented and compared with experimental data obtained for cp-Ti and Ti-6Al-4V alloy without the reinforcing phase. Moreover, it was found that an increase in the percentage of carbon increased the resistance to abrasive wear and to electrochemical corrosion of composites, measured by the relatively lower values of the friction coefficient and volume of wear and higher values of resistance polarization. This resistance results from the fact that a stable of TiO2 layer doped with MoO3 is formed on the surface of the composites. The results of experimental studies on the composites were compared with those obtained for cp-Ti and Ti-6Al-4V alloy without the reinforcing phase.

Study of phase formation and properties of high-entropy carbide HfTaTiNbZrC5 obtained by selfpropagating high-temperature synthesis

This work is devoted to the study of the combustion synthesis mechanism in complex multi-component systems using the quenching of the combustion front in a copper block. Bulk specimens were produced by reactive spark plasma sintering (R-SPS) of mechanically activated mixtures as well as by SPS of high-entropy ceramics obtained by combustion synthesis. The resistance of high-entropy carbides towards oxidation was studied in relation to the number of metallic constituents.

High-entropy rare earth tetraborides

Six high-entropy rare earth tetraborides of the tetragonal UB4-prototyped structure have been successfully synthesized for the first time. The specimens are prepared from elemental precursors via high-energy ball mill and in-situ reactive spark plasma sintering. The sintered specimens are >98 % in relative densities without detectable oxide impurities (albeit the presence of minor hexaborides in some compositions). No detectable secondary phase is observed in the composition (Y0.2Nd0.2Sm0.2Gd0.2Tb0.2)B4, which is proven homogeneous at both microscale and nanoscale. The Vickers microhardness are determined to be ∼13−15 GPa at a standard indentation load of 9.8 N. A scientifically interesting observation is represented by the anisotropic lattice distortion from the rule-of-mixture averages. This work expands the family of high-entropy ceramics via fabricating a new class of high-entropy borides with a unique tetragonal quasi-layered crystal structure.

Influence of sintering parameters on transparency of reactive SPSed Nd3+:YAG ceramics

Fine-grained 4 at% Nd3+:YAG transparent ceramics were prepared by reactive spark plasma sintering using a two-step heating profile. The effect of key sintering parameters (heating rate, external pressure, sintering and post-annealing temperatures) on densification peculiarities, optical and mechanical properties was investigated. Special attention was paid to the behavior and nature of carbon contaminations, dispersed in the yttrium aluminum garnet matrix. It was established that the optimal heating rate to the isothermal exposure temperature was 15 °C min−1. In this case, supporting the uniformity of temperature field distribution between the punches ensures the synthesis of the single-phase product and positive densification dynamics throughout the sintering trajectory for the disk geometry of samples. It was shown that an increase in applied pressure from 30 to 70 MPa changed the color of ceramics: the samples darkened and became dark brown. This may be due to an increase in the concentration of oxygen vacancy defects (color centers). The formation of silicon oxycarbides SiOxCy (where x + y = 4; x,y ≥ 1) in ceramics at sintering temperatures T > 1400 °C was revealed by X-ray photoelectron spectroscopy. It was found that these impurity phases demonstrated a thermal stability and resistance to decomposition during post-annealing. The study showed that 4 at% Nd3+:YAG ceramics obtained by reactive spark plasma sintering at 1350°С//10 min//30 MPa//15 °C min−1 and post-annealed at 900 °C for 1 h possessed a mean grain size of 0.74 μm, a microhardness of 13.2 GPa, a residual porosity of 0.0192 vol% and in-line optical transmittance 73.7% at λ = 1064 nm, which is 0.87 from the theoretical value for a single crystal.

Microstructure and mechanical behaviour of transient liquid phase spark plasma sintered B4C–SiC–TiB2 composites from a B4C–TiSi2 system

Almost fully-dense B4C–SiC–TiB2 composites with a high combination of strength and toughness were prepared through in situ reactive spark plasma sintering using B4C and TiSi2 as raw materials. The densification, microstructure, mechanical properties, reaction, and toughening mechanisms were explored. TiSi2 was confirmed as a reactive sintering additive to promote densification via transient liquid-phase sintering. Specifically, Si formed via the reaction between B4C and TiSi2 that served as a transient component contributed to densification when it melted and then reacted with C to yield more SiC. Toughening mechanisms, including crack deflection, branching and bridging, could be observed due to the residual stresses induced by the thermoelastic mismatches. Particularly, the introduced SiC–TiB2 agglomerates composed of interlocked SiC and TiB2 played a critical role in improving toughness. Accordingly, the B4C–SiC–TiB2 composite created with B4C-16 wt% TiSi2 achieved excellent mechanical performance, containing a Vickers hardness of 33.5 GPa, a flexural strength of 608.7 MPa and a fracture toughness of 6.43 MPa m1/2.

Ablation resistance of HfC(Si, O)-HfB2(Si, O) composites fabricated by one-step reactive spark plasma sintering

A dense HfC(Si, O)-HfB2(Si, O) composite was fabricated by reactive spark plasma sintering using HfC and SiB6 as starting reactants. The best ablation resistance was obtained with the composite fabricated with the addition of 15 vol.% SiB6. After ablation under an oxyacetylene flame for 60 s, the mass and linear ablation rates of this composite were −0.007 mg cm−2 s−1 and −0.233 μm s−1, respectively. The negative ablation rates are the result of a slight mass gain/thickness increase, which indicate that the oxidation process was stable and mechanical scouring was limited during ablation. This enhanced ablation resistance was attributed to a unique double-layered oxide formation, which possessed lower oxygen permeability and better mechanical strength. The solid solution nature of the composite and its appropriate phase composition were responsible for the stable oxide structure formation.

A new class of high-entropy M3B4 borides

A new class of high-entropy M3B4 borides of the Ta3B4-prototyped orthorhombic structure has been synthesized in the bulk form for the first time. Specimens with compositions of (V0.2Cr0.2Nb0.2Mo0.2Ta0.2)3B4 and (V0.2Cr0.2Nb0.2Ta0.2W0.2)3B4 were fabricated via reactive spark plasma sintering of high-energy-ball-milled elemental boron and metal precursors. The sintered specimens were ∼98.7% in relative densities with virtually no oxide contamination, albeit the presence of minor (4–5 vol%) secondary high-entropy M5B6 phases. Despite that Mo3B4 or W3B4 are not stable phase, 20% of Mo3B4 and W3B4 can be stabilized into the high-entropy M3B4 borides. Vickers hardness was measured to be 18.6 and 19.8 GPa at a standard load of 9.8 N. This work has further expanded the family of different structures of high-entropy ceramics reported to date.

Role of carbon morphology on the synthesizability of ZrC during spark plasma sintering of ZrB2–Zr–C composites

ZrB2-based composites were consolidated through reactive spark plasma sintering of the powder mixtures containing relatively 5 vol.% of metallic Zr particles, and graphite flakes (GF) or carbon nanotubes (CNT) as the carbon source, with the aim of ZrC in-situ synthesis in ZrB2 matrix. Microstructural and phase analyses indicated that GFs cannot result in detectable ZrC in the final microstructure. Contrarily, it was found that CNTs promote the in-situ synthesizability of zirconium carbide in the composite system. However, the sample containing graphite flakes presented better densification outcomes. The mechanism of ZrC formation in the composites, based on the carbon source, was finally discussed and illustrated.

Synthesis of YB2C2 by high‐energy ball milling and reactive spark plasma sintering

AbstractX‐ray pure yttrium diborodicarbide (YB2C2) was synthesized for the first time via modified spark plasma sintering (SPS) using Y2O3, B4C, and carbon as starting materials. Homogeneous intermixing of the raw powder by high energy ball milling (HEBM) promoted the formation of YB2C2 powder with layered structure by boro/carbo‐thermal reduction. The average particle size of the synthesized powder at an optimum synthesis temperature (1625℃) was 568 nm. Small amount of YB2C was formed by consuming YB4 at and above 1625°C. The metal basis purity of the synthesized YB2C2 powder was 99.84%. X‐ray pure YB2C2 was obtained by densifying the synthesized YB2C2 powder at 1900°C for 60 minutes at a pressure of 80 MPa using SPS. This study reports a simple method for the fabrication of X‐ray pure rare earth metal diborodicarbides (ReB2C2).

Cu-Matrix Composites by Reactive Spark Plasma Sintering of Mechanoactivated Cu–Si–C Powder Mixtures

Cu-Matrix Composites by Reactive Spark Plasma Sintering of Mechanoactivated Cu–Si–C Powder Mixtures

Dense HfB2 ceramics fabricated by high-energy ball milling and spark plasma sintering

The effects of high-energy ball milling (HEBM) on the sintering ability and microstructure of hafnium diboride (HfB2) ultrahigh-temperature ceramics (UHTCs) were investigated. After HEBM, the average particle size range of the HfB2 ceramics decreased to 10–20 nm with a slight alteration in the crystal structure. The microscopic grains of the sintered samples were homogeneous and fine in size. WC removed oxide impurities on the surface of the HfB2 particles by reacting with the oxide impurities, which further improved the sintering ability of the HfB2 ceramics. The densification mechanism of the HfB2 ceramics was also investigated, and nearly fully dense HfB2 (99.1%) was fabricated by reactive spark plasma sintering (R–SPS) at 2100 °C for 35 min under a pressure of 40 MPa. HEBM increased the surface energy of the fine particles, which provided a sufficient driving force to improve the sintering ability and significantly increase the densification of HfB2.

Synthesize of V4AlC3 Based MAX Phase Composites by Reactive Spark Plasma Sintering of V2O5:Al:C

In this research V4AlC3 based composites were synthesized by reactive spark plasma sintering (RSPS) method. V2O5:Al:C starting materials with molar ratios of 1:6:1.5 and 1:7:1.5 were heat treated at 1400 °C. Final composition, microstructural, physical, mechanical and tribological properties of the prepared composites were compared. The X-ray diffraction analysis results showed that the formation of V4AlC3 as main phase alongside Al2O3 and V2AlC by-products. Increasing the Al content from 6 moles to 7 moles led to formation of VC0.845 and Al2O3 main phases as well as V4AlC3 and V2AlC minor phases in this sample due to extraction of Al from die during RSPS operation. Microstructural studies revealed that the uniform distribution of the phases with low amount of porosities. The layered microstructure of the composites confirmed the formation of MAX phases. Higher bending strength and is achieved for the synthesized composite prepared by 7 moles Al (490 ± 5 MPa) than the composite prepared with 6 moles Al (375 ± 11 MPa). The composite prepared with 7 moles Al also exhibited lower wear rate (3.7 × 10− 5 mm3/Nm) than that of prepared with 6 moles Al (2.4 × 10− 4 mm3/Nm). Microstructural studies of the worn surfaces indicated that tribo oxidation is dominant mechanism for both fabricated composites.

High-entropy monoborides: Towards superhard materials

Single-phase high-entropy monoborides (HEMBs) of the CrB prototype structure have been synthesized for the first time. Reactive spark plasma sintering of ball milled mixtures of elemental precursor powders produced bulk (V0.2Cr0.2Nb0.2Mo0.2Ta0.2)B, (V0.2Cr0.2Nb0.2Mo0.2W0.2)B, and (V0.2Cr0.2Nb0.2Ta0.2W0.2)B HEMB specimens of ~98.3–99.5% relative densities. Vickers hardness was measured to be ~22–26 GPa at an indentation load of 9.8 N and ~32–37 GPa at 0.98 N. In particular, the load-dependent hardness of (V0.2Cr0.2Nb0.2Ta0.2W0.2)B is higher than those of ternary (Ta0.5W0.5)B (already considered as superhard) and hardest reported high-entropy metal diborides, and on a par with the classical superhard boride WB4.

Phase Formation and Densification Peculiarities of Hf–C–N Solid Solution Ceramics during Reactive Spark Plasma Sintering

Herein, phase formation and sintering of hafnium carbonitride ceramics from a 4HfC–3HfN powder mixture using reactive spark plasma sintering are studied. X‐ray diffraction (XRD) data show that the mutual heterodiffusion of carbon C and nitrogen N increases with sintering temperature within the studied temperature range 1500–2100 °C. 4HfC–3HfN powder sintering at 1900 °C for 5 min under 60 MPa pressure yields HfCxN1−x solid solution ceramics. The ceramic sample prepared under optimal conditions (2100 °C, 5 min, 60 MPa) is characterized by an average grain size of 10 μm, microhardness of 19.6–22.5 GPa, and relative density above ≈97%.

Direct synthesis of p-type bulk BiCuSeO oxyselenides by reactive spark plasma sintering and related thermoelectric properties

Herein, we demonstrate that BiCuSeO compound can be formed in bulk directly from the raw materials through reactive spark plasma sintering (RSPS) followed by ball milling and a second short spark plasma sintering step. Compared to BiCuSeO samples obtained by a conventional solid-state reaction, the electrical transport properties of the RSPS bulk were moderately affected by the sintering technique, while the lattice thermal conductivity was almost unaffected, and the figure of merit zT attained a value at 773 K comparable to state-of-the-art BiCuSeO. The results indicate a new scalable method for the preparation of oxyselenides.

Microstructure and mechanical properties of B4C-TiB2 composite ceramic fabricated by reactive spark plasma sintering

B4C-TiB2 composite ceramic was prepared by reactive spark plasma sintering, using amorphous B, Ti, and graphite as the raw materials. The reaction process and the phase composition in the process of sintering were studied. The effects of the ratio of raw materials and sintering process on the microstructure and mechanical properties of B4C-TiB2 composite ceramic were investigated. The composition of the sintered sample was B4C, TiB2, and bits of residual unreacted graphite. B and Ti preferentially reacted to form TiB2 at 800 °C, and then B and graphite reacted to form B4C at 1250 °C. The 75 vol% B4C-25 vol% TiB2 composite ceramic synthesized with 60.6 wt% B, 25.8 wt% Ti, and 13.6 wt% graphite and sintered at 1900 °C for 15 min resulted in nearly full densification and optimal mechanical properties. The relative density, Vickers hardness, fracture toughness, and flexural strength were 98.6 ± 0.01%, 26.6 ± 0.01 GPa, 5.9 ± 0.13 MPa·m1/2, and 605 MPa, respectively.

Pressure induced enhancement in the thermoelectric and mechanical properties of Ni-doped skutterudites during spark plasma sintering

ABSTRACT Effect of applied pressure during reactive spark plasma sintering (RSPS) of undoped and Ni-doped CoSb3skutterudite samples on their thermoelectric and mechanical properties is reported. Undoped and Ni-doped CoSb3 samples (CoSb3, Ni0.1Co0.9Sb3 and Ni0.2Co0.8Sb3) were processed using ball milling followed by RSPS at two different applied pressures of 50 MPa and 80 MPa. Electrical conductivity and thermopower of the samples processed at 80 MPa are enhanced substantially than those processed at 50 MPa. As a result, the thermoelectric power factor at 773 K for Ni0.2Co0.8Sb3 processed at 50 MPa is increased from 2 mW/mK2 to 3.3 mW/mK2 processed at 80 MPa, the highest value of unfilled Ni-doped CoSb3skutterudites reported so far. Nanomechanical testing reveals the significant improvement of the Young’s modulus and mechanical properties of the samples processed at 80 MPa. Our results show that the materials can be used in the thermoelectric device for waste heat recovery applications.

New Synthesis Route for Complex Borides; Rapid Synthesis of Thermoelectric Yttrium Aluminoboride via Liquid-Phase Assisted Reactive Spark Plasma Sintering

YxAlyB14 ceramics are of high interest as high temperature thermoelectric materials with excellent p, n control. In this study, direct synthesis of dense polycrystalline YxAlyB14 (x ~0.64, 0.52 ≤ y ≤ 0.67) ceramics was successfully carried out by spark plasma sintering using commercially available precursors. YB4, AlB2 and B powders were reactively sintered with an additive AlF3 at 1773 K for 5–60 min in reduced Ar atmosphere. The sinterability was remarkably enhanced by liquid phase sintering comparing to conventional synthesis techniques. Phase composition analysis by X-ray diffraction showed that main peaks belong to YxAlyB14 with the MgAlB14 structure type and no peaks of AlF3 were detected. The thermoelectric behavior was changed from p-type to n-type with increasing Al occupancy. Power factor and ZT values measured in this study were found to be in the same range as the best values previously reported. This original synthesis process is found to be less precursor-consuming as compared to previous synthesis processes, and strikingly, less time-consuming, as the synthesis time, is shortened from 8 h to 5 min for p-type and to 1 h for n-type. The total process time is shortened from ≥3 days to ~4–5 h. This discovery opens the door for more accessible synthesis of complex borides.

Synthesis and characterisation of Ca1-xCexZrTi2-2xCr2xO7: Analogue zirconolite wasteform for the immobilisation of stockpiled UK plutonium

A series of Ca1-xCexZrTi2-2xCr2xO7 zirconolite ceramics (0 ≤ x ≤ 0.35) were reactively sintered in air at 1350 °C for 20 h. Single phase zirconolite-2M was formed for x ≤ 0.15, with Cr2O3 and an undesirable Ce-bearing perovskite phase present above x = 0.20. Electron diffraction analysis confirmed that the zirconolite-2M polytype was maintained over the solid solution. X-ray absorption near edge structure (XANES) data determined that between 10–20% Ce was speciated as Ce3+, and Cr was present uniformly as Cr3+ with near edge features consistent with occupation of octahedral sites within the zirconolite-2M structure. A sample corresponding to x = 0.20 was processed by reactive spark plasma sintering (RSPS), with a rapid processing time of less than 1 h. XANES data confirmed complete reduction to Ce3+ during RSPS, promoting the formation of a Ce-bearing perovskite, comprising 19.3 ± 0.4 wt. % of the phase assemblage.

Dissolving and stabilizing soft WB2 and MoB2 phases into high-entropy borides via boron-metals reactive sintering to attain higher hardness

Five single-phase WB2- and MoB2-containing high-entropy borides (HEBs) have been made via reactive spark plasma sintering of elemental boron and metals. A large reactive driving force enables the full dissolution of 10−20 mol. % WB2 to form dense, single-phase HEBs, including (Ti0.2Zr0.2Hf0.2Mo0.2W0.2)B2, (Ti0.2Ta0.2Cr0.2Mo0.2W0.2)B2, (Zr0.2Hf0.2Nb0.2Ta0.2W0.2)B2, and (Zr0.225Hf0.225Ta0.225Mo0.225W0.1)B2; the successful fabrication of such single-phase WB2-containing HEBs has not been reported before. In the processing science, this result serves perhaps the best example demonstrating that the phase formation in high-entropy ceramics can strongly depend on the kinetic route. A scientifically interesting finding is that HEBs containing softer WB2 and/or MoB2 components are significantly harder than (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 (with harder binary boride components). This exemplifies that high-entropy ceramics can achieve unexpected properties.