Effect of Al addition on SiC–B4C cermet prepared by pressureless sintering and spark plasma sintering methods

SiC–B₄C–Al cermets containing 5, 10 and 20 wt.% of Al were fabricated by high-energy planetary milling followed by conventional sintering and spark plasma sintering (SPS) techniques separately. The average particle size reduced to ~ 3 μm from an initial size of 45 μm after 10 h of milling. The as-milled powders were conventionally sintered at 1950 °C for 30 min under argon atmosphere and SPS was carried out at 1300 °C for 5 min under 50 MPa applied pressure. The formation of Al₈B₄C₇ and AlB₁₂ phases during conventional sintering and SPS were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The formation of Al₈B₄C₇ at 700 °C and AlB₁₂ at 1000 °C was well supported by XRD and differential scanning calorimetry (DSC). The maximum relative density, microhardness and indentation fracture resistance of SiC–B₄C–10Al consolidated by SPS are 97%, 23.80 GPa and 3.28 MPa·m^1/2, respectively.     

Influence of mechanically milled powder and high pressure on spark plasma sintering of Mg–Cu–Gd metallic glasses

In this study a gas atomized Mg₆₅Cu₂₅Gd₁₀ amorphous powder was mechanically milled and consolidated by spark plasma sintering (SPS) at a high pressure (500 MPa) at different temperatures (443, 433, and 423 K) to provide an insight into the consolidation and mechanical behavior of Mg-based metallic glasses. Microstructural evolution in the vicinity of the interfaces of SPSed Mg–Cu–Gd BMG powders was characterized using scanning and transmission electron microscopy, and the associated amorphous phase transformation and thermal stability were analyzed using X-ray diffraction and differential scanning calorimetry. The results show that interfacial bonding between amorphous particles was enhanced by the disruption of surface oxides as well as by structural relaxation of the milled powder and the presence of high pressure during SPS. A finite element method with COMSOL software was also applied to investigate and explain the powder packing density, localized heating, and temperature distribution during SPS.     

Formation and characterization of mechanically alloyed Ti–Cu–Ni–Sn bulk metallic glass composites

In the present study, amorphous Ti₅₀Cu₂₈Ni₁₅Sn₇ and its composite powders reinforced with 4, 8, and 12 vol.% of W additions were prepared by mechanical alloying. After 5 h of milling, amorphous powders with homogeneously dispersed W nanoparticles were synthesized. The as-milled Ti₅₀Cu₂₈Ni₁₅Sn₇ and composite powders were then consolidated by vacuum hot pressing into disc compacts with a diameter and thickness of 4 and 10 mm, respectively.The structure of the as-milled powders and consolidated compacts was characterized by X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy. While the thermal stability was examined by differential scanning calorimeter (DSC). In addition, the mechanical property of the consolidated BMGs was evaluated by Vickers microhardness tests. The experimental results showed that W nanoparticles ranged from 20 to 200 nm were embedded within the amorphous matrix. The presence of W nanoparticles did not dramatically change the glass formation ability of amorphous Ti₅₀Cu₂₈Ni₁₅Sn₇ powders. While the thermal stability of amorphous powders differed from those of its composites. A significant hardness increase with W additions was noticed for consolidated composite compacts.     

Pressure effect on crystallization kinetics of an Al–La–Ni amorphous alloy

The pressure effect on the crystallization kinetics of an amorphous Al₈₉La₆Ni₅ alloy has been investigated by means of piston–cylinder measurements within a pressure range of 0–1.0 GPa. It was found that an applied pressure enhances the first primary crystallization, i.e. the precipitation of f.c.c.-Al from the amorphous phase. The crystallization temperature decreases from 495 to 440 K when the applied pressure increases from ambient to 0.9 GPa. Meanwhile, the thermal stability of the residual amorphous phase with the Al dispersion is elevated at higher pressures. The crystallization temperature of the residual amorphous phase increases by 35 K/GPa when the pressure increases. The observed pressure effect on the crystallization kinetics in the amorphous alloy, which cannot be interpreted by means of the pressure effect on atomic diffusion, may be well understood in terms of the volume change effect at the early stage of crystallization.     

Effect of reinforcement NbC phase on the mechanical properties of Al2O3-NbC nanocomposites obtained by spark plasma sintering

The sintering behavior of Al₂O₃-NbC nanocomposites fabricated via conventional and spark plasma sintering (SPS) was investigated. The nanometric powders of NbC were prepared by reactive high-energy milling, deagglomerated, leached with acid, added to the Al₂O₃ matrix in the proportion of 5 vol% and dried under airflow. Then, the nanocomposite powders were densified at different temperatures, 1450–1600 °C. Effect of sintering temperature on the microstructure and mechanical properties such as hardness, toughness and bending strength were analyzed. The Al₂O₃-NbC nanocomposites obtained by SPS show full density and maximum hardness value > 25 GPa and bending strength of 532 MPa at 1500 °C. Microstructure observations indicate that NbC nanoparticles are dispersed homogeneously within Al₂O₃ matrix and limit their grain growth. Scanning electron microscopy examination of the fracture surfaces of dense samples obtained at 1600 °C by SPS revealed partial melting of the particle surfaces due to the discharge effect.     

Fabrication and soft-magnetic properties of Fe–B–Nb–Y glassy powder compacts by spark plasma sintering technique

Magnetic properties of (Fe_0.72B_0.24Nb_0.04)_95.5Y_4.5 sample were investigated. The sample was produced from glassy powders made by the gas-atomization and consolidation using the spark plasma sintering (SPS) technique. Maximum relative density of 99.5% was achieved in the spark plasma sintered (SPSed) compact due to the viscous flow enhanced by the applied stress even under the glass transition temperature. X-ray diffraction pattern of the compact indicates that the glassy structure was maintained through the SPS process. However, the results of differential scanning calorimetry (DSC) showed that the glass transition temperature and crystallization temperature of the SPSed glassy compact shift to a higher and lower temperature, respectively, that is, a smaller ΔT_x. Saturation magnetization of the SPSed glassy compact became 10% higher than that of the initial glassy powder. The Curie point was enhanced from 522 K for the glassy powder to 548 K for the SPSed glassy compact. Spin-exchange interaction is expected to be enhanced by a short-range scale atomic rearrangement caused by the high applied stress and temperature during the SPS process.     

Investigation of thermoelectric properties of Cu2GaxSn1 − xSe3 diamond-like compounds by hot pressing and spark plasma sintering

P-type compounds Cu₂Ga_xSn_1 ? xSe₃ (x = 0.025, 0.05, 0.075) with a diamond-like structure were consolidated using hot pressing sintering (HP) and spark plasma sintering (SPS) techniques. High-temperature thermoelectric properties as well as low-temperature Hall data are reported. Microstructural analysis shows that the distribution of Ga is homogeneous in the samples sintered by HP but inhomogeneous in the samples sintered by SPS, even with an electrically isolating and thermally conducting BN layer during the sintering. The Seebeck coefficients of the samples sintered by HP and SPS show similar dependence on the carrier concentration and are insensitive to the composition inhomogeneity. In contrast, the composition inhomogeneity results in lower carrier mobility and thus lower electrical conductivity in the samples sintered by SPS than those sintered by HP. Lattice thermal conductivity is further reduced through Ga doping; however, this effect is weakened by the inhomogeneous distribution of Ga. Due to their larger carrier mobility and lower lattice thermal conductivity, the samples sintered by HP exhibit 15–35% higher thermoelectric figure of merits (ZT) than those SPS samples with a high Ga doping level and without the coated BN layer, in which the composition homogeneity is worse. A ZT value of 0.43 is obtained for the HP Cu₂Ga_0.075Sn_0.925Se₃ sample at 700 K.     

Nano-sized zirconium carbide powder: Synthesis and densification using a spark plasma sintering apparatus

Nano-sized zirconium carbide powder was synthesized at 1600 °C by the carbothermal reduction of ZrO₂ using a modified spark plasma sintering (SPS) apparatus. The synthesized ZrC powder had a fine particle size of approximately 189 nm and a low oxygen content of 0.88 wt%. The metal basis purity of the synthesized powder was 99.87%. The low synthesis temperature, fast heating/cooling rate and the effect of current during the modified SPS process effectively suppressed the particle growth. Using the synthesized powder, monolithic ZrC ceramics with high relative density (97.14%) were obtained after the densification at 2100 °C for 30 min at a pressure of 80 MPa by SPS. The average grain size of the densified ZrC ceramics was approximately 9.12 μm.     

Fabrication and characterization of bulk glassy Co40Fe22Ta8B30 alloy with high thermal stability and excellent soft magnetic properties

In this work, Co₄₀Fe₂₂Ta₈B₃₀ alloy as a new bulk metallic glass with a wide supercooled liquid region of 74 K and excellent soft magnetic properties was prepared by the powder metallurgy method. Glassy Co₄₀Fe₂₂Ta₈B₃₀ powders were obtained by ball milling of melt-spun glassy ribbons at a cryogenic temperature and subsequently consolidated by hot pressing into disk-shaped specimens 10 mm in diameter and 2 mm thick. It was found that the new glassy alloy exhibited the largest diameter compared with the other Co-based bulk metallic glasses produced in the well-known Co–Fe–Ta–B alloying system up to now. The influence of the consolidation time on the microstructure and magnetic properties of the bulk samples was investigated by X-ray diffraction, differential scanning calorimetry, vibrating sample magnetometry and Faraday magnetometry. The results indicate that the new alloy exhibits a long incubation time before crystallization upon annealing above the glass transition temperature: noticeably longer than for other known (Co,Fe)-based amorphous alloys. The glassy bulk sample consolidated for 600 s at 923 K had a relative density of 99.2%, a saturation magnetization of 46.6 A m² kg^?1, a Curie temperature of 425 K and a low coercivity of 6 A m^?1. In addition, the new bulk glassy alloy exhibited ultra-high hardness of 13.48 GPa. The mechanisms by which the thermal stability and incubation time prior to crystallization increase are explained in accordance with pair correlation function analysis.     

Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-CuO powder mixtures

Understanding the microstructure of W–Cu nanocomposite powder is essential for elucidating its sintering mechanism. In this study, the effect of milling time on the structural characteristics and densification behavior of W-Cu composite powders synthesized from WO₃-CuO powder mixtures was investigated. The mixture of WO₃ and CuO powders was ball-milled in a bead mill for 1 h and 10 h followed by reduction by heat-treating the mixture at 800 °C in H₂ atmosphere with a heating rate of 2 °C/min to produce W-Cu composite powder. The microstructure analysis of the reduced powder obtained by milling for 1 h revealed the formation of W–Cu powder consisting of W nanoparticle-attached Cu microparticles. However, Cu-coated W nanocomposite powder consisting of W nanoparticles coated with a Cu layer was formed when the mixture was milled for 10 h. Cu-coated W nanopowder exhibited an excellent sinterability not only in the solid-phase sintering stage (SPS) but also in the liquid-phase sintering stage (LPS). A high relative sintered density of 96.0% was obtained at 1050 °C with a full densification occurring on sintering the sample at 1100 °C. The 1 h-milled W-Cu powder exhibited a high sinterability only in the LPS stage to achieve a nearly full densification at 1200 °C.     

Spark plasma sintering of (Ti0.9W0.1) C powders prepared by mechanical alloying

Nanocrystalline (Ti_0.9W_0.1)C powder with a diffraction crystallite size of about 10 nm was synthesized by mechanical alloying. The formation of (Ti_0.9W_0.1)C carbide was detected by XRD measurements and microscopic observation. The sintering of these powders by a spark plasma sintering (SPS) at different temperatures were also studied. The results show that the maximum hardness was obtained for more relative density materials, meanwhile, the grain size is large. The micro-hardness and the relative density of the powder milled for 10 h and sintered at 1200 °C for 5 min under 100 MPa reach, respectively, 2978 HV and 98.35%.     

Electric insulation of a FeSiBC soft magnetic amorphous powder by a wet chemical method: Identification of the oxide layer and its thickness control

An electrical insulating layer formed on the surface of an Fe_79.3Si_5.7B_13.3C_1.7 soft magnetic amorphous powder was characterized and its layer thickness was evaluated. The insulating layer was prepared by chemical oxidation of the powder in 10% nitric acid–ethanol solution. The insulating layer mainly consisted of magnetite and/or maghemite. Moreover, the insulating layer avoided further oxidation up to T ≈ 940 K, which is above the crystallization temperature of the amorphous powder. A correlation of the layer thickness with density and saturation magnetization is presented and discussed. Layer thicknesses calculated either from density or saturation magnetization of either magnetite or maghemite were in good agreement with each other.     

Synthesis and properties of nanostructured dense LaB6 cathodes by arc plasma and reactive spark plasma sintering

Nanostructured polycrystalline LaB₆ ceramics were prepared by the reactive spark plasma sintering method, using boron nanopowders and LaH₂ powders with a particle size of about 30 nm synthesized by hydrogen dc arc plasma. The reaction mechanism of sintering, crystal structure, microstructure, grain orientations and properties of the materials were investigated using differential scanning calorimetry, X-ray diffraction, Neutron powder diffraction, Raman spectroscopy, transmission electron microscopy and electron backscattered diffraction. It is shown that nanostructured dense LaB₆ with a fibrous texture can be fabricated by SPS at a pressure of 80 MPa and temperature of 1300 °C for 5 min. Compared with the coarse polycrystalline LaB₆ prepared by traditional methods, the nanostructured LaB₆ bulk possesses both higher mechanical and higher thermionic emission properties. The Vickers hardness was 22.3 GPa, the flexural strength was 271.2 MPa and the maximum emission current density was 56.81 A cm^?2 at a cathode temperature of 1600 °C.     

Bulk amorphous Al85Fe15 alloy and Al85Fe15-B composites with amorphous or nanocrystalline-matrix produced by consolidation of mechanically alloyed powders

An Al80Fe14B6 powder mixture was subjected to mechanical alloying. Presence of an amorphous structure in the milling product was revealed by XRD investigations. The calorimetric study showed that the amorphous phase crystallised above 370 °C. The milled Al₈₀Fe₁₄B₆ powder was consolidated under a pressure of 7.7 GPa in different conditions: at 350 °C and at 1000 °C. Besides, the mechanically alloyed amorphous Al₈₅Fe₁₅ powder was consolidated at 360 °C. The amorphous structure was retained after consolidation applied at 350 °C and 360 °C. Compaction at 1000 °C caused crystallisation of the amorphous phase and appearance of metastable nanocrystalline phases. Structural investigations revealed that both bulk Al₈₀Fe₁₄B₆ samples are composites with boron particles embedded in amorphous or nanocrystalline matrix. The hardness of the nanocrystalline-matrix composite and of the amorphous-matrix one is equal to 707 HV1 and 641 HV1 respectively, whereas that of bulk amorphous Al₈₅Fe₁₅ alloy is 504 HV1. The specific yield strength of amorphous-matrix and nanocrystalline-matrix composites, estimated using the Tabor relationship, is 625 and 650 kNm/kg respectively, while that of amorphous Al₈₅Fe₁₅ alloy is 492 kNm/kg. We also suppose that application of high pressure affected crystallisation of amorphous phase, influencing the phase composition of the products of this process.     

Effect of sintering temperature on the structure and properties of polycrystalline cubic boron nitride prepared by SPS

The polycrystalline cubic boron nitride (PcBN) with Si₃N₄–AlN–Al₂O₃–Y₂O₃ ceramic system as binding agents was prepared by spark plasma sintering (SPS). The starting materials Si₃N₄, AlN, Al₂O₃, Y₂O₃, and cBN in the ratio of 22:14:10:4:50 were heated to a sintering temperature between 1250 °C and 1450 °C at a heating rate of 300 °C/min, with a holding time of 5 min in nitrogen atmosphere. The microstructure, phase constitution, microhardness and fracture toughness of the prepared PcBN were then studied. It was shown that the Si₃N₄–AlN–Al₂O₃–Y₂O₃–cBN polycrystalline materials were densified in a very short sintering time resulting in materials with relative densities of more than 95%. When the sintering temperature increased, the microhardness and fracture toughness of prepared PcBN were also increased. The microhardness of PcBN prepared at 1250–1450 °C was between 28.0 ± 0.5 GPa and 48.0 ± 0.9 GPa, and its fracture toughness K_IC was from 7.5 ± 0.2 MPa m^1/2 to 11.5 ± 0.3 MPa m^1/2. Microstructure study showed that the ceramic-binding agents bonded with cubic boron nitride particles firmly. Our work demonstrated that spark plasma sintering technology could become a novel method for the preparation of PcBN cutting materials.     

Spark plasma sintering of TiC particle-reinforced molybdenum composites

In order to improve the recrystallization resistance and the mechanical properties of molybdenum, TiC particle-reinforcement composites were sintered by SPS. Powders with TiC contents between 6 and 25 vol.% were prepared by high energy ball milling. All powders were sintered both at 1600 and 1800 °C, some of sintered composites were annealed in hydrogen for 10 h at 1100 up to 1500 °C. The powders and the composites were investigated by scanning electron microscopy and XRD. The microhardness and the density of composites were measured, and the densification behavior was investigated. It turns out that SPS produces Mo–TiC composites, with relative densities higher than 97%.The densification behavior and the microhardness of all bulk specimens depend on both the ball milling conditions of powder preparation and the TiC content. The highest microhardness was obtained in composites containing 25 vol.% TiC sintered from the strongest milled powders. The TiC particles prevent recrystallization and grain growth of molybdenum during sintering and also during annealing up to 10 h at 1300 °C. Interdiffusion between molybdenum and carbide particles leads to a solid solution transition zone consisting of (Ti_1 −x Mo_x)C_y carbide. This diffusion zone improves the bonding between molybdenum matrix and TiC particles. A new phase, the hexagonal Mo₂C carbide, was detected by XRD measurements after sintering. Obviously, this phase precipitates during cooling from sintering temperature, if (Ti_1 −x Mo_x)C_y or molybdenum, are supersaturated with carbon.     

Spark plasma sintering of pure TiCN: Densification mechanism,grain growth and mechanical properties

This study aims to disclose the densification mechanism and grain growth behaviors during the spark plasma sintering (SPS) of undoped TiCN powder. The SPS experiments were performed under temperatures ranging from 1600 °C to 2200 °C and a fixed pressure of 50 MPa. The sintering mechanisms were described in different models according to two grain growth behaviors: densification without grain growth at low temperatures (1600–1700 °C) and grain growth without apparent densification at higher temperatures (1800–2200 °C). At the constant grain stage, a creep model is applied to describe the densification process. In addition, the effective stress exponents, n, are calculated, indicating that the densification can be attributed to both grain boundary sliding (n = 1.5) and dislocation climbing (n = 3.13 or n = 4.29). During the second stage of sintering, the grain growth model reveals that the grain-growth is controlled by grain boundary diffusion. In addition, the Vickers hardness varies from 4326 Hv to 6762 Hv when the density ranges from 90% to 96.3%.     

Fe–B–C composites produced using spark plasma sintering

Fe–B–C composites were produced using iron and boron carbide powders. The powders were mixed to produce various compositions, ranging from 1 vol.% Fe to 80.1 vol.% Fe. Spark plasma sintering (SPS) was used to densify the composite powder green compacts. The sintering temperatures used ranged from 900 °C for the composites with a high iron content to 2000 °C for those with a high boron carbide content. It was evident that during the sintering process the iron reacted with the boron carbide. XRD analysis showed the presence of FeB, Fe₂B, Fe₃C, Fe₃(B_0.6C_0.4), Fe₂₃(B,C)₆ and residual carbon as reaction products. The composites were found to have hardness values between 9.8 and 33.1 GPa with the higher hardness being associated with the higher boron carbide contents. The fracture toughness values determined were in the range of 2.8–5.3 MPa m^0.5. With increasing iron content from 1 to 5 vol.%, it is evident that the FeB formed begins to embrittle the material rather than increase the fracture toughness as a result of the high residual stresses between the B₄C and FeB phases.     

Formation of porous metallic glass compacts by electro-discharge sintering

A single pulse of 0.1–0.9 kJ/0.45 g atomized amorphous Cu₅₄Zr₂₂Ti₁₈Ni₆ powders in size range of 90–150 μm was applied to fabricate porous metallic glass compacts using electro-discharge sintering (EDS) with 3 and 4 mm in diameter. The structural and thermal analysis of the samples indicated that formation of the porous metallic glass compacts occurs only when low electrical input energy was induced on the amorphous powders. Furthermore, the critical input energy inducing crystallization of the amorphous phase during EDS is strongly dependent on the size of the sample.     

Reaction spark plasma sintering of niobium diboride

Niobium diboride (NbB₂) is synthesized and consolidated by the spark plasma sintering technique. Elemental reactants such as niobium (Nb) and boron (B) were subjected to two stage heat treatment, initially at 1200 °C for synthesis and followed by densification at the temperatures in the range of 1700 °C to 1900 °C. High dense NbB₂ (~ 97.7%ρ_th) is obtained at 1900 °C after 15 min holding period. Load application during heat treatment stage is found to improve the sinterability of the niobium diboride compacts. Hardness, elastic modulus and indentation fracture toughness of the high dense NbB₂ are measured as 20.25 GPa, 539 GPa and 4 MPa m^1/2 respectively.