Microstructure-mechanical-tribological property correlation of multistage spark plasma sintered tetragonal ZrO2

We report here the development of dense yttria-stabilized t-ZrO₂ ceramics with more uniform and finer grain sizes and concomitantly better mechanical and tribological properties via multistage spark plasma sintering (SPS). The dense tetragonal ZrO₂ ceramics were obtained by adopting three different SPS heating cycles, designed on the basis of fundamental sintering theory. The suppression of grain growth to nanosize regime (～100–150 nm), along with the development of more uniform grain size distribution was achieved with multistage sintering (MSS), as compared to normal single stage sintering (SSS). Finer microstructural scale, along with superior hardness also led to improved fretting wear resistance for the ZrO₂ samples processed via MSS. Based on the experimental results and analysis, a correlation has been established between the SPS processing schemes, microstructural development and mechanical as well as tribological properties of the tetragonal ZrO₂. The effectiveness of MSS to produce tetragonal ZrO₂ ceramics with better mechanical and tribological properties was confirmed at two different levels of yttria content (3 and 2 mol%).     

KNbTeO6 transparent ceramics prepared by the combination of pressure-less sintering and pseudo hot isostatic pressing

KNbTeO₆ transparent ceramics were prepared by combining pressure-less sintering and pseudo-hot isostatic pressing (PHIP) of the synthesized submicron single-phase powder. The PHIP was conducted by wrapping coarse magnesium aluminate powders around the pre-sintered body in the spark plasma sintering (SPS) furnace. With an average grain size of 412 ± 23 nm, the in-line transmittance of transparent KNbTeO₆ ceramics reaches 80.25% at 2677 nm. By contrast, the density of the samples prepared by conventional SPS with the same sintering procedure is only 98.73%, and the highest in-line transmittance 64.25% occurs at 4976 nm. In particular, by investigating the sintering mechanism of PHIP, the improvement of microstructure and optical transmittance could be attributed to the plastic deformation caused by shear stress. The obtained ceramics exhibited excellent mechanical and dielectric properties, which was benefited from the novel sintering technology.     

Electromechanical properties of Ba(Zr0.2Ti0.8)O3 ceramics prepared by spark plasma sintering

Ba(Zr_0.2Ti_0.8)O₃ (BZT20) ceramics were prepared by spark plasma sintering (SPS) and conventional sintering. The dynamic field-induced displacement and small-signal remnant piezoelectric constant measured by a resonant–antiresonant frequency method were evaluated. By normal sintering, the density, grain size, and dielectric constant of the ceramics increased with sintering temperature. The BZT20 ceramics prepared by SPS were characterized by linear field-induced strain. In response to the application of post-annealing at 1300 °C, BZT20 ceramics exhibited linear strain loop and high field-induced strain corresponding to dynamic strain/field d₃₃ at 20 kV/cm of 290 pm/V. The remnant piezoelectric properties of the BZT20 ceramics were found to largely depend on the preparation conditions, including the sintering temperature and annealing temperature. The BZT20 ceramics prepared by SPS and post-annealed at 1300 °C showed Q_m and k_p values of 325 and 25.1 (%), respectively.     

Effect of temperature on the structure and mechanical properties of SiC–TiB2 composite ceramics by solid-phase spark plasma sintering

SiC composite ceramics have good mechanical properties. In this study, the effect of temperature on the microstructure and mechanical properties of SiC–TiB₂ composite ceramics by solid-phase spark plasma sintering (SPS) was investigated. SiC–TiB₂ composite ceramics were prepared by SPS method with graphite powder as sintering additive and kept at 1700 °C, 1750 °C, 1800 °C and 50 MPa for 10min.The experimental results show that the proper TiB₂ addition can obviously increase the mechanical properties of SiC–TiB₂ composite ceramics. Higher sintering temperature results in the aggregation and growth of second-phase TiB₂ grains, which decreases the mechanical properties of SiC–TiB₂ composite ceramics. Good mechanical properties were obtained at 1750 °C, with a density of 97.3%, Vickers hardness of 26.68 GPa, bending strength of 380 MPa and fracture toughness of 5.16 MPa m^1/2.     

Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering

Spark plasma sintering (SPS) is a powerful technique to produce fine grain dense ferrite at low temperature. This work was undertaken to study the effect of sintering temperature on the densification, microstructures and magnetic properties of magnesium ferrite (MgFe₂O₄). MgFe₂O₄ nanoparticles were synthesized via sol–gel self-combustion method. The powders were pressed into pellets which were sintered by spark plasma sintering at 700–900 °C for 5 min under 40 MPa. A densification of 95% of the theoretical density of Mg ferrite was achieved in the spark plasma sintered (SPSed) ceramics. The density, grain size and saturation magnetization of SPSed ceramics were found to increase with an increase in sintering temperature. Infrared (IR) spectra exhibit two important vibration bands of tetrahedral and octahedral metal-oxygen sites. The investigations of microstructures and magnetic properties reveal that the unique sintering mechanism in the SPS process is responsible for the enhancement of magnetic properties of SPSed compacts.     

On the fabrication and mechanical properties of Si3N4 ceramics with low content sintering additives

Si₃N₄ ceramics were prepared by hot pressing (HP) and spark plasma sintering (SPS) methods using low content (5 mol%) Al₂O₃–RE₂O₃(RE = Y, Yb, and La)–SiO₂/TiN as sintering additives/secondary additives. The effects of sintering additives and sintering methods on the composition, microstructures, and mechanical properties (hardness and fracture toughness) were investigated. The results show that fully density Si₃N₄ ceramics could be fabricated by rational tailoring of sintering additives and sintering method, and TiN secondary additive could promote the density during HP and SPS. Besides, SN-AYS-SPS possesses the most competitive mechanical properties among all the as-prepared ceramics with the Vickers hardness as 17.31 ± .43 GPa and fracture toughness as 11.07 ± .48 MPa m^1/2.     

In situ X-ray diffraction study of a TiO2 nanopowder Spark Plasma Sintering under very high pressure

We investigated the effect of very high pressure on the sintering temperature, phase transition and the grain growth during Spark Plasma Sintering (SPS) of a 15 nm TiO₂ nanopowder. Using in situ synchrotron X-ray diffraction during sintering at 1.5 and 3.5 GPa, we followed the evolution of the crystalline phases and the crystallite size as a function of temperature. In comparison, in the laboratory, SPS experiments were performed on two original facilities: A Paris-Edinburgh press and a high-pressure module adapted to standard SPS equipment. We studied the effect of the pressure on the sintering in the range 76 MPa to 3.5 GPa. We have shown that highly dense nanostructured ceramics can be prepared under very high pressure at low sintering temperatures. At 1 GPa, we limited the grain growth to an average size of 233 nm by heating at only 560 °C, and achieved a relative density of 98 %.     

Synthesis and characterization of (HfMoTiWZr)C high entropy carbide ceramics

(HfMoTiWZr)C high entropy carbides (HEC) were prepared from the commercial carbide powders of IVB (Hf, Ti, Zr) and VIB (Mo, W) group metals of the periodic table via high energy ball milling (HEBM) and spark plasma sintering (SPS). Metal carbide powders (HfC, TiC, ZrC, Mo₂C and WC) were HEBM’d for 3 h in a vibratory ball mill, and then SPS’d at different temperatures (1800, 1900, 2000 and 2100 °C). The HEBM’d powders and SPS’d ceramics were characterized in composition, density and microstructure using an X-ray diffractometer (XRD), a scanning electron microscope/energy dispersive spectrometer (SEM/EDS), a particle size analyzer and a pycnometer. Also, microhardness and sliding wear tests were conducted on the SPS’d ceramics. Based on the performed characterization, a single-phase FCC structure was observed at all sintering temperatures indicating a high entropy carbide ceramic, and they all have high hardness and wear resistance values.     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

Reactive,nonreactive, and flash spark plasma sintering of Al2O3/SiC composites—A comparative study

The influence of different SPS-based methods, that is, conventional spark plasma sintering (SPS), flash SPS (FSPS), and reactive SPS (RSPS) on the properties of Al₂O₃/SiC composite was investigated. It was shown that the application of preliminary high energy ball milling of the powders significantly enhances the sinterability of the ceramics. It was also demonstrated that FSPS provides unique conditions for rapid, that is, less than a minute, consolidation of refractory ceramics. The Al₂O₃-20 wt% SiC composite produced by FSPS possesses the highest relative density (~99%), fracture toughness (7.5 MPa m^1/2), hardness (20.3 GPa) and wear resistance among all ceramics produced by other SPS-based approaches with dwelling time 10 minutes. The RSPS ceramics hold the highest Young's modulus (390 GPa). Substitution of micron-sized Al₂O₃ particles by nano alumina does not lead to measurable enhancement of the mechanical properties.     

Effects of phase constitution and microstructure on energy storage properties of barium strontium titanate ceramics

Barium strontium titanate (Ba_0.3Sr_0.7TiO₃, BST) ceramics have been prepared by conventional sintering (CS) and spark plasma sintering (SPS). The effects of phase constitution and microstructure on dielectric properties, electrical breakdown process and energy storage properties of the BST ceramics were investigated. The X-ray diffraction analysis and dielectric properties measurements showed that the cubic and tetragonal phase coexisted in the SPS sample while the CS sample contained only tetragonal phase. Much smaller grain size, lower porosity, fewer defects and dislocation were observed in SPS samples, which greatly improved the electrical breakdown strength of the Ba_0.3Sr_0.7TiO₃ ceramics. The enhanced breakdown strength of the SPS samples resulted in an improved maximum electrical energy storage density of 1.13 J/cm³ which was twice as large as that of the CS sample (0.57 J/cm³). Meanwhile, the energy storage efficiency was improved from 69.3% to 86.8% by using spark plasma sintering.     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.     

Influence of the Spark Plasma Sintering temperature on the structure and dielectric properties of BaTi(1-x)ZrxO3 ceramics

In this work, structural and dielectric properties of BaTi_(1-x)Zr_xO₃ (BTZ) ceramics prepared by Spark Plasma Sintering (SPS) from powders obtained via Self-propagating High-temperature Synthesis (SHS) are shown to be strongly affected by the sintering temperature. In addition, a post-annealing treatment in air of the as-prepared ceramics leads to a transition from the hexagonal to the tetragonal and cubic phases. The SPS ceramics corresponding to compositions 0.05 ≤ x ≤ 0.20 and obtained at a sintering temperature of 1200 °C exhibit a standard ferroelectric behavior. In contrast, a diffuse phase transition is observed for the case of ceramics sintered at higher temperatures. Finally, the BTZ ceramic containing 5 at.% of Zr displays the best dielectric permittivity and piezoelectric properties as compared to the other compositions taken into account.     

Static and dynamic mechanical properties of boron carbide processed by spark plasma sintering

Spark plasma sintering (SPS) has become a popular technique for the densification of covalent ceramics. The present investigation is focused on the static mechanical properties and dynamic compressive behavior of SPS consolidated boron carbide powder without any sintering additives. Fully dense boron carbide bodies were obtained by a short high temperature SPS treatment. The mechanical properties of the SPS-processed material, namely hardness (32 GPa), Young modulus (470 GPa), fracture toughness K_C (3.9–4.9 MPa m^0.5), flexural strength (430 MPa) and Hugoniot elastic limit (17–19 GPa) are close or even better than those of hot-pressed boron carbide.     

Low-temperature reactive spark plasma sintering of dense SiC-Ti3SiC2 ceramics

SiC-based ceramics are of great interest for various advanced applications. However, its fabrication requires high-temperature treatment at ～2000 – 2100 °С. In this study, we developed an approach based on low-temperature reactive spark plasma sintering to produce dense SiC-based ceramics with superior mechanical properties. It was found that an SPS temperature of 1600 °C and introduction of 10 – 15 wt% of mechanically activated non-oxide Ti–Si–C additive is required to manufacture ceramics with a theoretical density of higher than 90%. Nonetheless, employing 5 – 15 wt% of the additive mixture and an SPS temperature of 1700 °C, the maximum density of ～ 98% was achieved. The controlled formation and decomposition of the in-situ Ti₃SiC₂ MAX phase enables the fabrication of the engineering ceramics with enhanced compressive strength (550 MPa), elastic modulus (485 GPa), and microhardness (32 GPa), which are comparable to the best-reported SiC ceramics. The study has a significant potential for practical application in the production of advanced SiC-based ceramics for various purposes and could be used for further understanding and development of the high-temperature sintering methods.     

Toughening mechanisms of ZTA ceramics at cryogenic temperature (77 K)

ZTA ceramics containing 20 wt% ZrO₂ were fabricated at different sintering temperatures (1450, 1500 and 1550 °C) by SPS and HP processes, respectively. The influence of sintering process on the mechanical properties of ZTA ceramics at 298 K and 77 K was investigated. It can be seen that the bending strength and fracture toughness of samples prepared by the two processes both improved at cryogenic temperature. The stress-induced martensitic transformation toughening mechanism was confirmed by the in-situ Raman technique. The tetragonal ZrO₂ would be even more easy to transform because of the residual stress generated when temperature decreased from 298 K to 77 K. Therefore, the transformation toughening effect would become stronger, result in the increase of mechanical properties.     

Monoclinic phase-free,low temperature spark plasma sintering of CeO2-doped ZrO2 ceramics and its associated benefits on mechanical properties

Consolidating a CeO₂-doped ZrO₂ ceramics, free from monoclinic phase using spark plasma sintering (SPS) is a major challenge faced by previous researchers; Ce⁺⁴ → Ce⁺³ conversion under reducing environments was assigned as the prime factor. We report dense (> 95 % of theoretical density) 20 mol. % CeO₂-doped ZrO₂ ceramics, free from monoclinic phase and any of micro/ macro-cracks via SPS. The sintering temperature (1175 ℃) used for the present work was the lowest compared to previous reports on the same system. Phase analysis revealed a mixture of tetragonal (major phase) and cubic phase (minor). No depletion of cerium (Ce) from the ZrO₂ matrix and no additional/impurity phases were noted after SPS; a common issue that has been observed in most of the previous works. Sintered ceramics showed appreciably high hardness (>11 GPa); the obtained toughness was in-between of tetragonal and cubic CeO₂-ZrO₂ ceramics.     

Reactive FAST/SPS sintering of strontium titanate as a tool for grain boundary engineering

A high-pressure FAST/Spark Plasma Sintering method was used to produce dense SrTiO₃ ceramics at temperatures of 1050 °C, more than 250 °C below typical sintering temperatures. Combining SPS with solid-state reactive sintering further improves densification. The process resulted in fine-grained microstructures with grain sizes of ∼300 nm. STEM-EDS was utilized for analyzing cationic segregation at grain boundaries, revealing no cationic segregation at the GBs after SPS. Electrochemical impedance spectroscopy indicates the presence of a space charge layer. Space charge thicknesses were calculated according to the plate capacitor equation and the Mott-Schottky model. They fit the expected size range, yet the corresponding space charge potentials are lower than typical values of conventionally processed SrTiO₃. The low space charge potential was associated to low cationic GB segregation after SPS and likely results in better grain boundary conductivity. The findings offer a path to tailor grain boundary segregation and conductivity in perovskite ceramics.     

Yb doped MgO transparent ceramics generated through the SPS method

Yb doped (0, 0.02, 0.1 and 0.5 at%) MgO transparent ceramics were synthesized through spark plasma sintering (SPS) at the relatively low temperature of 1100 °C for 5–60 min under a pressure of 105 MPa. The effects of dopant concentration and sintering holding time on the densification and microstructure evolution of MgO ceramics were investigated. All ceramics reached a relative density greater than 99.20%. The 0.02% Yb-doped MgO ceramic sintered at 1100 °C for 60 min showed the highest in-line transmittance, of 80% at 1030 nm, a value close to that of MgO single crystals. Yb dopant improved the transmittance, degree of densification and control of grain growth. Herein, the influence of Yb doping on the crystalline phase and microstructure was explored, and the photoluminescence properties of Yb in transparent MgO ceramics were investigated.     

Preparation of TiB2–SiC composites toughened with interlocking microstructure by self-assembled TiB2 plates

The spark plasma sintering (SPS) technique was found to effectively improve the mechanical properties of TiB₂–SiC ceramic by forming a unique interlocking structure. This study investigated the phase transition process of the hexagonal micro-platelets TiB₂ powders with self-assembled structure during the molten-salt-mediated carbothermal reduction and its effect on promoting the mechanical properties of TiB₂-based ceramics. It was found that the SPS approach ensured a highly densified TiB₂–SiC ceramics with enhanced Vickers hardness of 21.0 ± 1.3 GPa and fracture resistance of 7.8 ± 0.3 MPa m^1/2. The performance enhancement of the resultant TiB₂–SiC composite was attributed to the interlocking structure from the original anisotropic TiB₂ powders, which could effectively absorb the energy and facilitate the crack deflection.