High-pressure Sintering Process Research Articles

Densification mechanisms during high-pressure sintering of nanocrystalline Gd2Zr2O7 ceramic

High-pressure sintering (HPS) is a promising technique for producing nanocrystalline ceramics with unique properties. However, the densification mechanisms of HPS for nanocrystalline ceramics at different sintering stages remain controversial. This study focuses on Gd2Zr2O7 (GZO) nanocrystalline ceramics, investigating their microstructure evolution and densification behavior under varying pressures, temperatures, and dwelling times. The HPS process involves distinct stages: cold compaction, hot compaction, and isothermal progression. During cold compaction, densification is driven by the breakdown of aggregates, particle rearrangement, and local plastic deformation, resulting in a relative density of 84.8 %. The intermediate stage achieves an increase to ∼93.7 % in relative density by grain boundary-mediated plastic deformation mechanism. The final isothermal stage, held at 500 °C/5 GPa, achieves nearly full density (∼99.2 %) through diffusion creep, eliminating microstructural defects. This comprehensive understanding of HPS densification mechanisms, exemplified by GZO ceramics, contributes to advancing nanocrystalline ceramic applications.

Microstructure and mechanical properties of high-pressure sintered B6O-SiC nanocomposites

Boron suboxide (B6O) is recognized as a superhard material with a low mass density, high resistance to chemical wear, and excellent wear resistance. Despite its desirable properties, the limited fracture toughness of B6O restricts its application in industrial contexts. This study presents the structural and mechanical characterization of B6O-SiC nanocomposites, which were synthesized via a high-pressure high-temperature sintering process of B6O powders and SiC whiskers. The sintering process induced fragmentation of SiC whiskers, resulting in the homogenous distribution of SiC fragments within the B6O matrix. An increase in SiC content was observed to decrease the composite's hardness, while initially reducing then enhancing its toughness. The nanocomposites containing 20 wt% and 30 wt% SiC whiskers exhibited significant improvements in fracture toughness, averaging 6.5 MPa m1/2 and 7.0 MPa m1/2, respectively—approximately threefold the toughness of polycrystalline B6O—while sustaining high hardness values of 36.3 GPa and 35.6 GPa on average. Microstructural analyses revealed that the composites’ superior mechanical performance is due to the presence of strong grain boundaries, as well as a high density of nanotwins and stacking faults. The findings demonstrate a viable method for producing B6O-based nanocomposites with enhanced hardness and toughness, potentially expanding their industrial applicability.

Heterogeneous diamond–TiC composites with high fracture toughness and electrical conductivity

The concurrent enhancement of toughness, electrical conductivity, and thermal stability in polycrystalline diamond composites represents a formidable challenge. Here, a novel diamond-TiC composites (referred to as DTC) with high fracture toughness and conductivity were synthesized through a high-temperature and high-pressure reaction sintering process utilizing two precursors: diamond mixtures containing TiH2 and Ti-coated diamonds. On the one hand, the inherent bonding and encasing of TiC onto the diamond matrix form a heterogeneous structure, providing high strength and excellent fracture toughness up to 11.6 MPa·m1/2. On the other hand, the established oxygen barrier and current pathway offer a thermal stability of 765 °C and high conductivity up to 837 S·m−1. Remarkably, this rare combination of mechanical, electrical, and thermal properties has been realized within a single material under relatively moderate sintering conditions, thus positioning diamond-TiC composites as a promising ceramic for electrical applications in high-stress environments.

Fabrication of nanocrystalline SiO2–ZrO2 glass-ceramic via a high-pressure cold sintering process

The nanocrystalline 0.35SiO2-0.65ZrO2 (65 mol% monoclinic ZrO2) glass-ceramic was successfully fabricated via a high-pressure cold sintering process (CSP), displaying remarkable nano hardness and elastic modulus of 12.5 and 154.8 GPa, respectively. We conducted a thorough investigation of the influence of NaOH solution concentration, liquid-to-solid mass ratio, sintering temperature, and pressure on the phase compositions, microstructural evolution, and densification. Notably, excessive NaOH concentration (>0.7 M) results in the transformation of SiO2 from an amorphous phase to α-quartz, hindering densification. However, below 0.7 M, increasing NaOH concentration enhances the densification processes. Furthermore, higher temperature and pressure contribute to improved densification. It is worth mentioning that there is no grain growth for the ZrO2 phases during the CSP, indicating that the densification of the nanocrystalline 0.35SiO2-0.65ZrO2 glass-ceramic is primarily governed by the glass phase.

Fabrication of dense YSZ ceramic via vat photopolymerization with low ceramic particles loading

In this work, dense Yttria stabilized zirconia (YSZ) ceramic was printed by vat photopolymerization. We first formulated a slurry with good ceramic particle dispersion and modified the sintering process of the printed part. We improved the dispersion of the yttria stabilized zirconia (YSZ) particles by mixing with dispersant before preparing the slurry. After printing with the slurry, the printed part was sintered under a pressure of 20 KPa. No pores could be observed in the cross-section view indicating we successfully fabricated ultra-dense YSZ ceramic by vat polymerization and high-pressure sintering process.

Influence of Sintering Conditions and Nanosilicon Carbide Concentration on the Mechanical and Thermal Properties of Si3N4-Based Materials.

In the work, silicon nitride ceramics (Si3N4) and silicon nitride reinforced by nano silicon carbide particles (Si3N4-nSiC) in amounts of 1-10 wt.% were investigated. The materials were obtained using two sintering regimes: under conditions of ambient and high isostatic pressure. The influence of the sintering conditions and the concentration of nanosilicon carbide particles on the thermal and mechanical properties was studied. The presence of highly conductive silicon carbide particles caused an increase in thermal conductivity only in the case of the composites containing 1 wt.% of the carbide phase (15.6 W·m-1·K-1) in comparison with silicon nitride ceramics (11.4 W·m-1·K-1) obtained under the same conditions. With the increase in the carbide phase, a decrease in the densification efficiency during sintering was observed, which caused a decrease in thermal and mechanical performance. The sintering performed using a hot isostatic press (HIP) proved to be beneficial in terms of mechanical properties. The one-step high-pressure assisted sintering process in the HIP minimizes the formation of defects at the sample surface.

Development of TbCu7-type Sm-Fe-N anisotropic magnet powder and its sintered magnets

Anisotropic TbCu7-type Sm-Fe-N magnet powder and its sintered magnets were developed using TbCu7-type Sm-Fe alloy powder synthesized by a modified low-temperature reduction diffusion process. Using Sm metal powder instead of SmCl3 and Ca and using the eutectic salt LiCl-CaCl2 to lower the synthesis temperature, an almost single-phase TbCu7-type Sm-Fe powder was obtained. By nitriding the powder with an NH3-H2 mixture gas, a TbCu7-type Sm-Fe-N anisotropic magnet powder was demonstrated for the first time. Furthermore, by sintering the anisotropic powder with a high-pressure current sintering process, a TbCu7-type Sm-Fe-N anisotropic sintered magnet with a relative density of 86.8% was produced. The magnetization ratio σr/σ90 kOe was decreased by the formation of the α-Fe phase as a result of the decomposition of the TbCu7-type Sm-Fe-N phase in the sintering process, whereas the coercivity was slightly increased. This result indicates that the TbCu7-type Sm-Fe-N magnet is less affected by the existence of α-Fe compared with a Sm2Fe17N3 magnet.

The Influence of Sintering Pressure on the Preparation, Friction Properties, and Magnetic Properties of Ti2AlC-TiC and Ti3AlC2-TiC Composites Under High-Pressure and High-Temperature

Due to their high melting point, low density, and affordable price, TiC ceramics have emerged as one of the most promising ultrahigh temperature structural materials. However, its development and usage are severely constrained by its weak mechanical characteristics. The Ti2AlC-TiC and Ti3AlC2-TiC composites were synthesized by using the laminated metal-ceramic Ti2AlC/Ti3AlC2 as the binder, combined with high-temperature and high-pressure sintering technology (GPa order of magnitude confinement system). The thermal analysis shows that the exothermic heat of Ti/1.2Al/2TiC is less than that of 3Ti/1.2Al/2C, 2Ti/1.2Al/C, and 3Ti/1.5Al/C. The ratio of Ti/1.2Al/2TiC can suppress the thermal explosion phenomenon during the sintering process and improve the safety of the high-pressure sintering process. XRD results showed that the main composition of the sintered products was Ti2AlC-TiC at 2∼3.5 GPa and Ti3AlC2-TiC at 4∼5 GPa. As the sintering pressure increased from 2 GPa to 5 GPa, the bonding phase changed from Ti2AlC to Ti3AlC2. The thickness of the synthetic Ti3AlC2 layer is near 0.1 µm under 5 GPa. The average friction coefficient of Ti2AlC-TiC composites sintered 2 GPa with POM is as low as 0.1767, while the average friction coefficient of Ti3AlC2-TiC composites with POM increased to 0.3468∼0.3797. The wear width of Ti2AlC-TiC is lower than that of Ti3AlC2-TiC composites sintered under 4–4.5 GPa. The wear width of Ti3AlC2-TiC composites decreases following the rise of sintering pressure and is reduced to 0.3504 mm under 5 GPa. Ti2AlC-TiC and Ti3AlC2-TiC composites with PP ball counterparts have friction coefficients of 0.375 to 0.45 and worse wear resistance than that of POM balls. Ti3AlC2-TiC composites synthesized at 5 GPa with a Cu ball had the smallest friction coefficient (0.4558) and the smallest wear scar width (0.6289 mm). The friction coefficients and wear widths of synthetics with Al pairs ranged from 0.48–0.697 and 0.851–1.087 mm, respectively, and varied irregularly with increasing sintering pressure. Ti2AlC-TiC composites synthesized at 2 GPa with agate and glass pairs have coefficients of friction of 0.5 and 0.61, respectively. Abrasion widths of Ti2AlC-TiC composites synthesized at 2 GPa with agate and glass pairs were 1.3 mm and 0.72 mm, respectively. The friction coefficient of synthetics with agate and glass pairs increases with increasing sintering pressure. The magnetization and magnetization intensity of the Ti3AlC2-TiC composites are stronger than those of Ti3AlC2. Ti2AlC-TiC and Ti3AlC2-TiC composites held promise to serve in the field of ceramic grinding processing applications.

Observation of room-temperature ferroelectricity in spark-plasma sintered GdCrO3

A spark-plasma sintered ${\mathrm{GdCrO}}_{3}$ (SPS-GCO) is found to stabilize in its ferroelectric phase beyond room temperature. The intrinsic nature of this room-temperature ferroelectricity is established using ferroelectric positive-up--negative-down measurements and supported through piezoresponce force microscopy measurements. The SPS-GCO undergoes antiferromagnetic ordering at much lower temperatures, only below $\ensuremath{\approx}170\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Thus, any role of magnetism to the observed room temperature ferroelectricity in SPS-GCO can be ruled out. This is contrast to the concomitant antiferromagnetic and ferroelectric ordering observed below $\ensuremath{\approx}170\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in ${\mathrm{GdCrO}}_{3}$ (GCO) (prepared using standard solid-state synthesis technique). Using detailed Rietveld refinements of room-temperature x-ray diffraction patterns, SPS-GCO is found to stabilize in the noncentrosymmetric orthorhombic $Pna{2}_{1}$ space group (the reported low-temperature ferroelectric phase in GCO), while GCO stabilizes in the centrosymmetric $Pbnm$ space group at room temperature. Using first-principles calculations, we investigated the relative energies among various possible structures of ${\mathrm{GdCrO}}_{3}$ and found that the orthorhombic $Pna{2}_{1}$ and $Pbnm$ space groups are the most stable structures. The ferroelectric $Pna{2}_{1}$ phase of SPS-GCO (stabilized at room temperature using the high-pressure and high-temperature spark-plasma sintering process) undergoes transition to the paraelectric centrosymmetric phase upon heating beyond $\ensuremath{\approx}450\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ (as confirmed using dielectric and calorimetric measurements), which on subsequent cooling to room temperature does not undergo a transition back to the ferroelectric phase and remains in the centrosymmetric $Pbnm$ phase.

Locally ordered nano-domains as novel microstructure defects suppressing the phonon transport in SnTe thermoelectrics

The phonon transport is intensively related to microstructure defects. Herein, a unique high-pressure sintering technology (GPa-level) was applied to modify the microstructure of pristine SnTe. There are many nano-domains exist in the high pressured NaCl-type SnTe binary compounds. The fast Fourier transform images show the domain phases have two sets of reflections with comparable intensity and exhibit doublet periodicities of special lattice planes, which demonstrates the domain phases are the CuPt and CuAu-I-type variants evolved from the NaCl-type matrix phase. An in-depth investigation indicates the constructions of the domain phase are related to locally off-stoichiometric of SnTe and the introduced strong inner-stress during high pressure sintering process. Additionally, a calculation of the phonon transport discloses the nano-domains restrict the phonon relaxation time by orders of magnitude over a wide phonon frequency range. As a result, a wide frequency phonon scattering network can be established to lower the lattice thermal conductivity.

High pressure densification behavior of alumina particles

Pressure is an important thermodynamic parameter in addition to temperature and chemical composition during material sintering process. However, few experimental images intuitively describing the densification process of grains during compression, especially for the high melting point ceramics. Here we observed deformation evolution and mechanical response of spherical alumina particles after high pressure treatment. By analyzing the plastic deformation and grain boundary characters of spherical alumina particles, and the boundary stress and yield stress variation of grains under various pressures and temperatures, a clear picture of the high-pressure powder densification and sintering process is depicted.

High Pressure Densification Behavior of Alumina Particles

Pressure is an important thermodynamic parameter in addition to temperature and chemical composition during material sintering process. However, few experimental images intuitively describing the densification process of grains during compression, especially for the high melting point ceramics. Here we observed deformation evolution and mechanical response of spherical alumina particles after high pressure treatment. By analyzing the plastic deformation and grain boundary characters of spherical alumina particles, and the boundary stress and yield stress variation of grains under various pressure temperature, a clear picture of the high-pressure powder densification and sintering process is depicted.

Effect of Co Leaching on the Vertical Turning Lathe Wear Properties of Polycrystalline Diamond Compact Manufactured by High Temperature and High Pressure Sintering Process

Polycrystalline diamond compact (PDC) has excellent wear resistance, high impact resistance, superior fatigue properties, and has been used in the oil and gas drilling industries. This study investigated the effect of Co leaching on the microstructure, vertical turning lathe (VTL) wear properties of PDCs manufactured by high-temperature and high-pressure (HTHP) sintering. The VTL wear test has the advantage of simulating the actual oil drilling environment by using granite as the workpiece. PDC sintered material that did not receive Co leaching was named HTHP sintered PDC-A, and the material subjected to Co leaching was called Co-leached PDC-B in this study. As a result of XRD analysis of both PDCs, diamond and WC peaks were detected, and only the HTHP sintered PDC-A exhibited some Co peaks. In the HTHP sintered PDC-A, the binder WC and Co were evenly distributed at the diamond interface. However, in the Co-leached PDC-B, some empty spaces were observed at the diamond interface. The HTHP sintered PDCA exhibited a similar or slightly higher VTL wear resistance than the Co-leached PDC-B, but only in the short sliding distance. In the long sliding distance after 9 km, Co-leached PDC-B showed significantly superior wear resistance compared to the HTHP sintered PDC-A. The HTHP sintered PDC-A exhibited both abrasive and adhesive wear behaviors, while the Co-leached PDC-B showed only abrasive wear. Based on the above results, the VTL wear mechanism of PDCs, and ways of improving wear resistance were also discussed.

Production of silicon carbide reinforced molybdenum disilicide composites using high-pressure sintering

Studying the mechanical and thermal properties as well as the relationship between microstructure evolution and strengthening mechanisms is crucial for obtaining superior high-temperature refractory molybdenum disilicide-silicon carbide composites. In this study, molybdenum disilicide and silicon carbide powders with different volume ratios were used to fabricate molybdenum disilicide-silicon carbide composites via high pressure and high temperature at 5.5 GPa/1773 K. The introduction of the second phase improved the materials' mechanical properties. Transmission electron microscopy demonstrated that severe plastic deformation generated numerous micro-defects and grain refinement during the high pressure sintering process. The second phase enhancement also improved the samples’ performances. Under high pressure and high temperature sintering process, the thermal and mechanical properties of the materials increased dramatically, including a well Vickers hardness (~20.6 GPa), fracture toughness (~10.3 MPa·m1/2) and thermal stability (>1673 K).

Evolution of microstructure, mechanical, and optical properties of Y2O3-MgO nanocomposites fabricated by high pressure spark plasma sintering

A high-pressure spark plasma sintering (SPS) process was applied for consolidating Y2O3–MgO nanocomposites. This approach enabled to fabricate a fully dense infrared (IR) transparent nanocomposites, which possess an average grain size of ∼70 nm and high hardness, at a relatively low sintering temperature of 1130 °C under a high pressure of 300 MPa. The light transmittance was improved with increasing pressure and reached to the maximum transmittance of 64.5% at a wavelength of 0.2–1.6 μm owing to the fine-grained microstructure. The Vickers hardness exhibited 16.6 ± 0.7 GPa for the grain size of 74 nm, which is significantly higher than that of the sub-micro grains obtained at a conventional sintering pressure of 70 MPa (11.9 ± 0.8 GPa). The hardness rigorously followed the Hall–Petch relationship, that is, it is enhanced with a reduction of the grain size. Successful fabrication of the high-performance Y2O3–MgO nanocomposites indicates that the nanopowder processing followed by the high-pressure sintering process can be applied for fabricating fully dense fine-grained nanocomposites with excellent optical and mechanical properties.

Self-bonding mechanism of pure polycrystalline cubic boron nitride under high pressure and temperature

Cubic boron nitride (cBN) powders with different grain size were used as a starting material, and the pure polycrystalline cubic boron nitride (PcBN) samples were sintered under the same conditions at 7GPa, 1700 °C, 270 s. Abrasion resistance and compressive strength of sintered pure PcBN samples were tested, and the microstructure was observed and analyzed by SEM, HRTEM, XRD and Raman spectrum. The results show that the particle size of cBN has not only a great influence on microstructure and property of the sintered pure PcBN un-der the same conditions but also influence on the bonding mechanism of the cBN particles. The inner stress of the pure PcBN sintered with the cBN particle size of 10 μm during the high pressure sintering process is about 199–222% higher than that of the sample with the cBN size of 1 μm. The wear ratio and the compressive strength of PcBN samples sintered with 10 μm cBN was increased by 47.7% and 39.7% than that of the sintered sample with 1 μm cBN respectively. The coarser-grained PcBN contains serious deformation and crushing, which can be attributed to the twin boundary and the stacking faults in the samples. As a consequence, the abrasion ratio and compressive strength of the pure PcBN are dramatically increased. It is assumed that self-bonding mechanism generated from plasticity deformation, which is a main bonding mechanism for the high pressure sintering of pure cubic boron nitride.

Crystal structure of the λ-Al13Fe4-type intermetallic (Al,Cu)13(Fe,Cu)4

An (Al,Cu)13(Fe,Cu)4phase of [trideca(aluminium/copper) tetra(iron/copper)] was obtained by the high-pressure sintering (HPS) process of a mixture with composition Al78Cu15Fe7. Structure analysis suggests that Cu atoms can replace both Al and Fe atoms. Such findings support previous anomalous X-ray scattering (AXS) results although deviate from other reports. The new phase adopts the λ-Al13Fe4structure type where Cu atoms partially occupy the 2cand 4isites for Al and Fe atoms, respectively, with occupancies of 0.233 (5) and 0.173 (16).

Crystal structure of ω-Al4Cr

The new polymorph of tetraaluminium chromium, Al4Cr, designated as the ω-phase, was obtained as the product from a high-pressure sintering (HPS) process of a stoichiometric Al4Cr mixture. The crystal structure is isotypic with Al4Mo and Al4W. The unit cell of ω-Al4Cr is much smaller than any of the other reported Al4Cr polymorphs, containing 30 rather than several hundred atoms.

Crystal structure of Al45Cr7C0.32

During a high-pressure sintering (HPS) process of a stoichiometric Al7Cr mixture, the ternary phase Al45Cr7C0.32 (pentatetracontaaluminium heptachromium carbide) was obtained serendipitously. The C atom (occupancy 0.32) is irregularly surrounded by six Al atoms.

On the densification mechanism of nano grained Yttrium aluminum garnet transparent ceramic during high pressure sintering process

The sintering densification mechanism of nano grained Yttrium aluminum garnet transparent ceramic during high pressure sintering process can be divided into three period. The first sintering stage can be described as plastic deformation after yield point. The intermediate stage is conducted by grain boundary viscous sliding. The grain boundary diffusion is the significant contribution for the final sintering densification period.