Synthesis and microstructure evolution of WCoB based cermets during spark plasma sintering

WCoB based cermets were prepared by spark plasma sintering at sintering temperature among 600°C-1200 °C. The phase evolution was investigated by detecting density behavior, phase composition, microstructure and mechanical properties during sintering process. The sintering process can be divided into three stages: powder densification, solid phase reaction and liquid phase sintering. WCoB hard phase forms at 1000 °C during solid phase sintering, showing better mechanical properties than Co₂B, especially on Vicker's hardness. WCoB hard phase forms on the basis of Co₂B binary boride and its content increases in liquid phase sintering stage with high density. The Vicker's hardness and transverse rupture strength (TRS) reach the maximum value of 1262 Hv and 1212 MPa at 1200 °C and 1170 °C, respectively. The fracture toughness reaches the maximum value of 21.8 MPa m^1/2 at 1050 °C, and the inter-granular fracture is the main fracture mechanism.     

Microstructure evolution during spark plasma sintering of FJS-1 lunar soil simulant

A spark plasma sintering (SPS) process has been explored to densify FJS-lunar soil simulants for structural applications in space explorations. The effect of SPS conditions, such as temperature and pressure, on the densification behavior, phase transformation, microstructural evolution, and mechanical properties of FJS-1 have been examined by conducting the X-ray diffraction analysis, electron microscopy imaging, and nano/micro indentation testing. Test analysis results were also compared to results from the FJS-1 powder and sintered samples without pressure. The FJS-1 powder was composed of sodian anorthite, augite, pigeonite, and iron titanium oxide. When FJS-lunar soil simulants were sintered without pressure, the main phase evolved from sodian anorthite to the intermediate sodian anorthite, jadeite and glass, and iron titanium oxide at 1000°C, which were further transformed into filiform and feather-shaped augite and schorlomite at 1100°C. Most densification processes in pressureless sintering occurred at 1050°C-1100°C. During the SPS process, the main phases were sodian anorthite, pigeonite, and iron titanium oxide at 900°C. These phases were transformed to sodian anorthite, glass, and feather-shaped augite at 1000°C and 1050°C, with the nucleation of dendritic schorlomite at 1050°C. Significant densification by SPS can be observed as low as 900°C, which indicates that the application of pressure can substantially lower the sintering temperature. The SPSed samples showed higher Vickers microhardness than the pressureless sintered samples. The mechanical properties of the local phases were represented by the contour maps of elastic modulus and nanohardness. Multiscale mechanical test results along with the microstructural characteristics further imply that the SPS can be considered a promising in-situ resource utilization (ISRU) method to densify lunar soils.     

Densification mechanisms and microstructural evolution during spark plasma sintering of boron carbide powders

Micron-sized boron carbide (B₄C) powders were subjected to spark plasma sintering (SPS) under temperature ranging from 1700 °C to 2100 °C for a soaking time of 5, 10 and 20 min and their densification kinetics was determined using a creep deformation model. The densification mechanism was interpreted on the basis of the stress exponent n and the apparent activation energy Q_d from Harrenius plots. Results showed that within the temperature range 1700–2000 °C, creep deformation which was controlled by grain-boundary sliding or by interface reaction contributed to the densification mechanism at low effective stress regime (n = 2,Q_d = 459.36 kJ/mol). While at temperature higher than 2000 °C or at high stress regime, the dominant mechanism appears to be the dislocation climb (n = 6.11).     

Microstructural evolution of ZnO via hybrid cold sintering/spark plasma sintering

In this work, we demonstrate a hybrid cold sintering/spark plasma sintering (CSP-SPS) process to densify ZnO ceramic with controlled grain growth. The densification of ZnO is initially activated at 85 °C, and high densities (>98%) are achieved at 200–300 °C in only 5 min with a low assisted pressure of 3.8–50 MPa. The microstructure of ZnO grains experiences a mild coarsening from ~205–680 nm during the CSP-SPS. In comparison, a much higher temperature (>770 °C) is required to sinter ZnO ceramic via SPS, and the grain size exhibits an obvious overgrowth to ~10 µm. The calculated apparent activation energy of grain growth using CSP-SPS is 69.3 ± 6 kJ/mol, which is much lower than that of SPS samples with 296.8 ± 59 kJ/mol. In addition, the conduction mechanism of the CSP-SPS and SPS samples is investigated using impedance spectroscopy. Overall, CSP-SPS is promising for the fabrication of fine ceramics with mild sintering conditions.     

High field ZnO varistors prepared by spark plasma sintering

Abstract

ZnO varistors with submicrometre and nanoscaled microstructures and enhanced electrical properties were prepared by spark plasma sintering (SPS). The densification, grain size and switch field of the varistors were compared with those of hot pressed material. The switching field increased with decreasing grain size, and very rapidly below 500 nm. Switching fields up to 180 kV cm^?1 were obtained for ceramics with submicrometre grain sizes (380 nm). This is nearly two orders of magnitude higher than those currently reported for commercial ZnO varistors. A nano powder, prepared by high energy milling, was sintered to a high density at much lower temperatures compared with the submicron powders and had a nanoscale grain size (45 nm). The nanoceramic broke down dielectrically under very high fields (>260 kV cm^?1) before a varistive response was apparent.     

Microstructural evolution,mechanical and thermal properties of LaB6 embedded in Si-B-C-N prepared by spark plasma sintering

Si-B-C-N monoliths with 5 wt% LaB₆ additives were prepared by spark plasma sintering at 1250–2000 °C and 50 MPa using a mechanically alloyed mixture of graphite, c-Si, h-BN and LaB₆ powders as the starting materials. Microstructural evolution, mechanical and thermal properties of the as-prepared La/Si-B-C-N monoliths were investigated. The densification of the ceramics starts at 1160° and ends at 1800 °C with the formation of La-containing compounds coupled with SiC and BN(C) phases. La-containing BN(C) grains develop into a lamellar structure at 1900 °C offering improved fracture toughness and decreased Vickers hardness, flexural strength and elastic modulus. The formation of lamellar BN(C) is also responsible for a high thermal expansion coefficient of 4.2×10⁻⁶ /°C.     

Pressure-enhanced densification of TaC ceramics during flash spark plasma sintering

Flash spark plasma sintering (FSPS) offers extremely high heating rates to consolidate ceramics at a short time. However, significant grain growth sometimes occurs accompanied by rapid densification. In this work, a FSPS apparatus available for applying pressure was used to sinter TaC ceramics from powder compacts without preheating. It is found that the use of a higher pressure can efficiently promote densification and retard significant grain growth. Dense bulk TaC ceramics (95.18%) with average grain size of 4.09 μm were obtained in 90 seconds under 80 MPa. Such a process should facilitate the fast preparation of refractory ceramics with fine-grained microstructure.     

Fabrication and thermal conductivity of AlN/BN ceramics by spark plasma sintering

Aluminum nitride/boron nitride (AlN/BN) ceramics with 15–30 vol.% BN as secondary phase were fabricated by spark plasma sintering (SPS), using Yttrium oxide (Y₂O₃) as sintering aid. Effects of Y₂O₃ content and the SPS temperature on the density, phase composition, microstructure and thermal conductivity of the ceramics were investigated. The results revealed that with increasing the amount of starting Y₂O₃ in AlN/BN, Yttrium-contained compounds were significantly removed after SPS process, which caused decreasing of the residual grain boundary phase in the sintered samples. As a result, thermal conductivity of AlN/BN ceramics was remarkably improved. By addition of Y₂O₃ content from 3 wt.% to 8 wt.% into AlN/15 vol.% BN ceramics, the thermal conductivity increased from 110 W/m K to 141 W/m K.     

Reactive sintering cBN-Ti-Al composites by spark plasma sintering

When synthesizing polycrystalline cubic boron nitride (PcBN) at normal pressure, cBN had a trend of hexagonal transformation, which reduces the hardness and strength of PcBN. The cBN-Ti-Al composite was prepared by spark plasma sintering with introducing Ti and Al to absorb hexagonal boron nitride (hBN) transformed from cBN. By the results of X-ray diffraction (XRD), Ti and Al reacted with BN and forming TiN, TiB₂, and AlN, which combined cBN as the binder by chemical bonding. The mechanical properties of the prepared composite increased as the increment of sintering temperature. The threshold temperature for preparing composite without hBN phase was at 1400 °C. The composite with optimal mechanical properties was prepared at 1400 °C, and the relative density, the bending strength, hardness, and fracture toughness were 98.9 ± 0.1%, 390.7 ± 4.4 MPa, 14.1 ± 0.5 GPa, and 7.6 ± 0.1 MPa·m^0.5, respectively.     

Electrical and thermal response of silicon oxycarbide materials obtained by spark plasma sintering

In order to obtain dense silicon oxycarbide (SiOC) materials that maintain the properties of glass, non-conventional spark plasma sintering was used to sinter SiOC powders from 1300 to 1700 °C and with 40 MPa of pressure. The concurrence of electrical current, high pressure and low vacuum while the material is being heating produces a dense SiOC-derived material composed of a SiO₂ glassy matrix reinforced with SiC nanowires grown in situ, graphene-like carbon and turbostratic graphite. SiOC materials with high electrical and thermal response are obtained as a result of this new processing technique. Electrical resistivity undergoes an extraordinary decrease of five orders of magnitude from 1300 (1.0 × 10⁵ Ω m) to 1700 °C (0.78 Ω m), ranging from insulate to semiconductor material; and thermal conductivity increases by 30%, for these sintering temperatures.     

Heating mechanism of spark plasma sintering

According to the plasma sintering process and our theoretical calculation, we analyzed the heating mechanism of spark plasma sintering (SPS). It is proposed that the sintering temperature of conductive materials rises faster mainly due to the contribution of the direct current (DC) component in pulse current, current skin effect and eddy current in the mould and blank, Joule's heat generated by eddy current in grains, small heat capacity of the heated system and direct-contact heating. For non-conductive materials, the similar reasons can be found except for the heat effect of eddy current. By analyzing the heating mechanism of SPS, it is concluded that the temperature on the surface of a conductive green body should be higher than that on the inner surface of the die, whereas the opposite case holds for a non-conductive blank, that has already been verified by Nagae et al. [Journal of the Japan Society of Powder and Powder Metallurgy 44 (1997) 945–950] and by Sumi et al. [Journal of the Japan Society of Powder and Powder Metallurgy 45 (1998) 153–157].     

Microstructure and thermal conductivity of fully ceramic microencapsulated fuel fabricated by spark plasma sintering

Fully ceramic microencapsulated pellet (FCM), consisting of tristructural isotropic (TRISO) particles embedded in silicon carbide (SiC) matrix, was fabricated using spark plasma sintering. The parameters affecting the densification of SiC matrix were first investigated, and then FCM pellets were prepared using TRISO particles with/without outer pyrolytic carbon (OPyC) layer. Effects of thermal exposure on the TRISO particles during SPS were evaluated. In addition, the thermal condcutvitities of FCM pellet, as well as the SiC matrix, were measured using laser flash. It was revealed that the TRISO particles with OPyC layers significantly lower the thermal conductivity of FCM pellet. Based on Maxwell‐Eucken model, the predicted effective thermal conductivities of TRISO particles with/without OPyC layers were 14.4 W/m K and 25.2 W/m K, respectively. Finite elements simulation indicated that the SiC layer in TRISO particle plays a dominant role on the thermal conductivity of FCM. The presence of OPyC layers would generate gaps/porous SiC near the interface and resist the heat flows, leading to a lower thermal conductivity of FCM.     

Mechanical and thermal properties of ZrC/ZTA composites prepared by spark plasma sintering

In the present work, the influence of sintering temperature and particle size of pristine ZrC particles on the microstructure, mechanical properties, and thermal properties of ZrC/ZTA ceramic composites are investigated. Specimens consolidated by spark plasma sintering at different sintering temperatures from 1500 °C to 1800 °C. XRD results revealed that α-Al₂O₃, t-ZrO₂, ZrC, and a small quantity of m-ZrO₂ phases are present in the composites. The microstructure of μm-ZrC/ZTA is found to be more compact than nm-ZrC/ZTA composites. There is an apparent increase in the average grain size with the increase in temperature. From the micrographs of fracture surfaces, step-wise transgranular fracture structures are observed. Relative densities and Vickers hardness are in proportion to sintering temperature from 1500 °C to 1700 °C. The maximum Vickers hardness of 1919 HV1 is obtained for μm-ZrC/ZTA composites. Indentation fracture toughness displays a gradual rise when the temperature rises from 1500 °C to 1700 °C, then deteriorates at 1800 °C for both nm-ZrC/ZTA and μm-ZrC/ZTA ceramic composites. The maximum fracture toughness values for nm-ZrC/ZTA and μm-ZrC/ZTA are 6.75 MPa m^1/2 and 6.83 MPa m^1/2, respectively. The thermal conductivity of the specimens decreased gradually as the temperature increases from 100 °C to 1000 °C. The obtained results indicated that the 1700 °C is the optimized sintering temperature where μm-ZrC/ZTA composites have excellent performance on microstructure, mechanical properties, and thermal properties than nm-ZrC/ZTA composites.     

Flash spark plasma sintering of 3YSZ

Flash SPS (FSPS) consolidation of 3YSZ cold pressed pellets was investigated. The results show that FSPS allows ultra-rapid consolidation of 3YSZ samples in 30–90 s under an applied electric field. The DC field induces electrochemical blackening. The partial reduction process starts from the cathode (-) and propagates toward the anode (+). This phenomenon, not previously discussed in FSPS literature, induces the development of internal temperature gradients resulting in a polarity dependent grain size/densification.     

Numerical modeling of heat transfer during spark plasma sintering of titanium carbide

Spark plasma sintering (SPS) is an efficient manufacturing method especially for ultra-high temperature ceramics (UHTCs) such as titanium carbides. Heating mechanism in SPS is a result of high electric current in the device including die, punch, and sample powder. Because the temperature distribution in the sintering process has considerable effect on the microstructure of the final sintered sample, in the present work, SPS of a cylindrical sample consist of a titanium carbide was investigated numerically. The governing equations of heat diffusion and electricity distribution in the whole device was solved using finite element method. In the heat diffusion equation, heat generation per volume was considered as a result of electric current in the device. Boundary conditions including radiation heat transfer and convective cooling by water flow were modelled by Stefan-boltzman and Newton cooling laws, respectively. The maximum temperature was observed at the center of the TiC sample. The radial temperature distribution in the sample showed considerable gradient as the minimum and maximum temperatures were 2000 °C and 1920 °C, respectively. Despite the radial direction, vertical temperature gradient was negligible in TiC sintering. Although the highest current density and consequent heat generation were observed at the die/punch interface with the minimum cross section, the maximum temperature of the whole apparatus was at the punch location.     

Simple method for the identification of electrical and thermal contact resistances in spark plasma sintering

The electrical and thermal contact resistances are key parameters for obtaining an accurate electro-thermal model of the spark plasma sintering (SPS) process. However, due to the lack of a general expression, these parameters are usually determined empirically. Thus, they are only valid for a specific material and SPS configuration. A simple method based on a limited amount of experiments as well as a new formulation of the electrical and thermal contact resistances are developed. First, the evolution of those resistances is optimized on simple shapes (pellets) experiments. They are then transferred into the electro-thermal simulation of complex shapes configurations, which showed a good agreement between the experimental and computed data.     

Micromechanical properties and microstructure evolution of magnesia partially stabilized zirconia prepared by spark plasma sintering

Magnesia partially stabilized zirconia (Mg-PSZ) is a widely used engineering ceramic owing to its high hardness and exceptional toughness. It is usually processed by conventional firing followed by subeutectoid aging. In this work, Mg-PSZ was prepared by spark plasma sintering (SPS) followed by sequential subeutectoid aging to fine-tune its mechanical properties. Mg-PSZ prepared by SPS with the rapid heating capability presents much smaller grains than conventionally prepared counterparts. After aging, a significant fraction of the matrix cubic phase transforms into tetragonal, orthorhombic, and monoclinic zirconia. Microindentation and in-situ microcompression tests reveal that aging Mg-PSZ for 4 h leads to maximum fracture toughness and fracture strain due to the tetragonal-to-monoclinic transformation toughening. Post compression TEM analyses show dominant monoclinic ZrO₂ decorated by a high density of twin boundaries and stacking faults formed to accommodate the shear deformation. Preparation of Mg-PSZ by SPS offers rapid and effective approaches in finetuning the phases and mechanical properties.     

Two-step pressure sintering of transparent lutetium oxide by spark plasma sintering

Transparent lutetium oxide (Lu₂O₃) body was prepared by spark plasma sintering using a two-step pressure profile combined with a low heating rate. The effects of pre-load pressures from 10 to 100 MPa and heating rates from 0.03 to 1.67 K s⁻¹ on the microstructures and optical properties were investigated. With increasing pre-load pressures from 10 to 100 MPa, the grains became smaller with a narrower distribution, whereas the transmittance showed maxima at 30 MPa. The average grain size slightly increased from 0.67 to 0.86 μm as the heating rate increased from 0.03 to 1.67 K s⁻¹, while the transmittance decreased. Transmittances of 60% at 550 nm and 79% at 2000 nm were obtained under a pre-load pressure of 30 MPa at a heating rate of 0.17 K s⁻¹.     

Correlation between crystallinity and thermal expansion in LiAlSiO4 ceramics prepared by spark plasma sintering

LiAlSiO₄ (LAS) ceramics are prepared by using the sol-gel method followed by spark plasma sintering. XRD patterns and SEM images verify that the ceramics contain amorphous and LAS phases and that microcracks appear in the sample prepared at 900 °C due to its larger grain size. Compared with applied pressure and soaking time, sintering temperature has a greater impact on the crystallinity and density of the ceramics during sintering. High-temperature XRD results reveal that the LAS phase exhibits its intrinsic negative thermal expansion independently in all samples regardless of crystallinity. The coefficients of thermal expansion (CTE) measured by the dilatometric method change from positive values in samples prepared at 600 and 650 °C to near zero in samples prepared at 700 and 800 °C and then to a negative value in the sample prepared at 900 °C. The combined effects of an amorphous phase with a positive CTE and the LAS phase with a negative CTE are responsible for the observed transformation of thermal expansion in the samples. The calculated total CTEs of the glass-ceramic bulks are in agreement with the results measured through the dilatometric method in samples prepared at 650–800 °C. Microcracks in the sample prepared at 900 °C cause a more negative bulk CTE than the calculated CTE.     

Carbon nanotube toughened hydroxyapatite by spark plasma sintering: Microstructural evolution and multiscale tribological properties

Carbon nanotube (CNT) reinforced hydroxyapatite (HA) composite synthesized using spark plasma sintering is investigated in this study. Quantitative microstructural analysis suggests that CNTs play a role in grain boundary pinning and are responsible for the improved densification and retention of nanostructure throughout the thickness of the sintered pellet. HA crystal forms coherent interface with the CNT, resulting in a strong interfacial bond. The uniform distribution of 4 wt.% CNTs in the HA matrix, good interfacial bonding and fine HA grain size help to improve the fracture toughness by 92% and elastic modulus by 25% as compared to the HA matrix without CNT. Toughening mechanisms have been explained in terms of interfacial shear strength and pull-out energy of CNT from the HA matrix. CNT plays a major role in improving the wear resistance of HA matrix at both macro- and nano-scale. It is concluded that graphene layer removal from the CNT surface occurs during macro-wear, but not for nano-wear. Thus, the coefficient of friction (CoF) in HA-CNT decreases in macro-wear due to lubrication available through delaminated graphene layers.