High-pressure Sintering Research Articles

Ultrafine-grained LaB 6–ZrB 2 composite ceramics with superior mechanical and thermionic emission properties prepared by high-pressure sintering

Ultrafine-grained LaB ₆–ZrB ₂ composite ceramics with superior mechanical and thermionic emission properties prepared by high-pressure sintering

Effect of pressure on the sintering mechanisms of tantalum carbide ceramics

AbstractHigh pressure sintering shows great superiority in promoting the densification of the hard‐to‐densify refractory ceramics, and the underlying mechanisms are thus of great interest. In the present work, the densification process of tantalum carbide (TaC) ceramics affected by pressure was discussed through comparing the microstructural characteristics and sintering kinetics. The TaC ceramics sintered at 30–250 MPa show dense and uniform microstructure without oxide impurities and high‐density dislocations. With the increase of sintering pressure, particle rearrangement becomes prominent at initial stage, whereas grain boundary diffusion rather than lattice diffusion dominants the final stage mechanism. Grain growth is enhanced under high pressure through the plastic deformation process. Our work should facilitate to understand the mass transport mechanisms during the high pressure sintering or deformation process of carbide ceramics.

Sintering polycrystalline silicon carbide composite ceramics with ultra-high hardness under high pressure

Silicon carbide (SiC) is an important structural ceramic material, exhibiting exceptional comprehensive properties that are unmatched by metals and other structural materials. In this study, a combination of α-SiC micron powder and β-SiC nanopowder was utilized as precursor materials for high pressure and high temperature (HPHT) sintering. Under a pressure of 5.0 GPa, polycrystalline SiC samples with mixed grain size were sintered within a temperature range from 1000 to 1700 °C, and compared with SiC samples sintered from single micron powder under identical temperature and pressure conditions. The microstructures of the two sets of SiC samples were observed using scanning electron microscopy. Additionally, the stress states and strengthening mechanisms among SiC grains with mixed grain size under HPHT were further analyzed in conjunction with X-ray diffraction results. The polycrystalline SiC composite ceramics sintered at 1700 °C exhibited superior mechanical and thermal properties, achieving a Vickers hardness of 35.2 GPa that is 23.5 % higher than that obtained by conventional spark plasma sintering and even surpassing the hardness of single crystal SiC, demonstrating thermal stability up to 1405 °C in air environment. Transmission electron microscopy was employed to analyze defects and plastic deformation in these samples. The study suggests that the primary strengthening mechanisms of the sintered polycrystalline SiC composite ceramics under HPHT include the increase in micro defects induced by in-situ plastic deformation at elevated temperatures and the effects of high-temperature creep. This study provides new insights into the HPHT sintering of hard materials.

Achieving excellent thermal transport in diamond/Cu composites by breaking bonding strength-heat transfer trade-off dilemma at the interface

The heat transport enhancement of diamond/Cu composites, a new generation of thermal management materials, is trapped in the bonding strength-heat transfer trade-off dilemma at the interface due to the noticeable difference in physical and chemical properties between Cu and diamond. Herein, we propose a new strategy combining ultrathin interface modification and low-temperature high-pressure (LTHP) sintering process to prepare the diamond/Cu composites. With a suitable coefficient of thermal expansion (CTE) of <10 ppm/K, the obtained diamond/Cu composites exhibit an outstanding thermal conductivity (k) value of 763 W/m K, over 90 % of the theoretical prediction of the differential effective medium (DEM) model. Meanwhile, using a lower diamond volume fraction (45 % vs. 50%–70 %), the k value is higher than those by conventional powder metallurgy, meaning a substantial reduction in the cost by reducing diamond filler content. For such a highly mismatched diamond/Cu interface, we maintain a high bonding strength by lowering the thermal stress damage while achieve a high thermal conductance (G) of 93.5 MW/m2 K by minimizing the heat transfer obstacles. The prepared interface structure is a diamond/TiC/CuTi2/Cu configuration, where the two possible heat transfer bottlenecks (the diamond/TiC interface and the TiC/CuTi2 interlayer) are no longer limiting factors on the overall interface. The successful resolution to the interfacial heat transfer problem is responsible for the superior thermal transport performance of the composites. This work deals with the critical challenge for the diamond/Cu composites and offers deep insight into the improvement mechanisms of thermal transfer. The proposed strategy can be generalized to the integration of highly mismatched interfaces widely present in other composites or thermal management systems.

Ti4Fe2C0.82O0.18.

The phase with composition Ti4Fe2C0.82O0.18, tetra-titanium diiron carbide oxide, was unexpectedly synthesized by high-pressure sinter-ing (HPS) of a stoichiometric mixture with nominal composition Ti2Fe. The Ti4Fe2C0.82O0.18 phase crystallizes in the Fd m space group and can be considered as the Ti2Fe structure filled with C and O atoms co-occupying the same octa-hedral void [occupancy ratio 0.82 (7):0.18 (7)]. The Ti4Fe2C0.82O0.18 phase is isotypic with Ti4Ni2C and Ti4Fe2O0.407, and is the first example where C and O atoms co-occupy the same site in filled Ti2Fe structures.

Abnormal sintering behaviors of chromium carbide under high pressure and high temperature

Controlling grain behavior during high pressure and high temperature (HPHT) sintering is crucial for synthesizing high-performance bulk ceramic materials. Chromium carbide (Cr3C2), a representative transition metal carbide ceramic, finds extensive industrial applications owing to its exceptional combination of properties. In this study, the intricate grain behaviors in Cr3C2 under 15 GPa/25–1700 °C are revealed by HPHT methods. It is found that the mechanical and elastic properties of Cr3C2 exhibit a significant dependence on temperature-induced grain behavior. Particularly, the mechanical properties of Cr3C2 exhibit abnormal softening at 15 GPa/1200 °C and 15 GPa/1500 °C. Through characterization and analysis of the microstructure, it is confirmed that the significant change in mechanical properties stems from the abnormal sintering behaviors of distinctive competitive behavior of grain refinement and growth. The findings highlight the significant influence of the competitive behavior on microstructural evolution, which exhibits strong correlations with the mechanical properties.

Mechanical properties of high-entropy carbides prepared by high-pressure sintering: Role of carbon stoichiometry

The scarcity of carbon in high-entropy carbides is widely recognized to significantly affect their fabrication and performance. However, achieving both high hardness and plasticity in these carbides remains a formidable challenge. This study investigates the impact of carbon vacancies on the phase structure and mechanical properties of high-entropy carbides, particularly (Hf0.25Zr0.25Nb0.25Ta0.25)Cx (x = 1, 0.8, 0.6, or 0.5), utilizing density functional theory calculations and experimental analysis. Theoretical computations indicate a rock-salt structure for the carbides, with reduced cell parameters as carbon vacancies increase. Notably, carbides with lower carbon content exhibit softer characteristics, as evidenced by Pugh's ratio. Experimental fabrication and testing reveal a decrease in Vickers hardness with decreasing carbon content, attributed to changes in bonding types. Certain compositions, particularly (Hf0.25Zr0.25Nb0.25Ta0.25)C0.6 and (Hf0.25Zr0.25Nb0.25Ta0.25)C0.5, display plastic behavior alongside high hardness, offering promising avenues for future research and design of high-entropy ceramics.

Superior uniform deformation ability of intragranular nano TiB reinforced Ti-6.5Al–2Zr–Mo–V fabricated by hot isostatic pressing

In order to improve the uniform deformation ability of titanium matrix composites, the intragranular nano-TiB whiskers (TiBnw) reinforced Ti-6.5Al–2Zr–1Mo–1V (TA15) composite was designed and fabricated by hot isostatic pressing (HIP) at the temperature of 1000 °C. The TiBnw was successfully planted into the pre-alloyed powders before HIP. The composite sintered for 6h (HIP-6h) realized a complete densification process benefiting from a high sintering pressure of 150 MPa. While the 2h sintering process (HIP-2h) was insufficient to densify. The HIP-6h exhibited an unparalleled uniform elongation of 9.5 %, which was superior to the survey TA15 matrix titanium composites. The excellent uniform deformation ability was demonstrated to be associated with the equiaxed structure and the blocking and multiplication effect on dislocations from the intragranular TiBnw.

High-pressure synthesis of fully dense polymer-derived SiCN ceramics: Structural evolution and mechanical properties

This study explored the microstructural evolution and mechanical behavior of dense amorphous SiCN ceramics by consolidating the PDCs-SiCN powder through high-pressure sintering at different temperatures. The results indicate that the SiCN ceramics remain in an amorphous state under pressures of 5 GPa and temperatures below 1200 ℃. As the temperature increases, the density increases steadily while the porosity decreases, exhibiting excellent properties with a porosity of 0.208 %, hardness and elastic modulus of 26.04±1.29 GPa and 223.8±6 GPa, respectively. As the temperature increases further, the amorphous three-dimensional network deteriorates, leading to a phase transition from amorphous to crystalline. This simultaneous presence of amorphous and crystalline phases can result in an increased formation of pores in ceramics. Precipitation of the soft phase “graphite” and the appearance of high porosity significantly diminish the mechanical properties of bulk ceramics. The high-pressure method represents an attractive avenue for the development of high-performance amorphous Si-based composite ceramics.

Simple and efficient synthesis of [Ca24Al28O64]4+(4e−) electronic compounds and preliminary exploration of its electron emission as a cathode emitter

[Ca24Al28O64]4+(4e−) electronic compound (C12A7:e−) is a ceramic with extremely low work function, and it exhibits outstanding potential in cathode electron emission. However, the existing preparation methods for this material are cumbersome, time-consuming, and have limited electron density. In this paper, C12A7:e− were prepared using high-pressure hydrothermal reaction and plasma activation sintering. The free O2− in the cages of the sample achieved accelerated diffusion and transition under the high temperature, strong electric field, high-energy pulse current, and plasma activation. Meanwhile, a high concentration of electrons (2 × 1021 cm−3) was rapidly injected into the sample. In addition, the C12A7:e− blocks have fewer defects, such as pores, cracks, and impurities, and their relative density reaches 99.6 %. The electron emission density and effective work function of the C12A7:e− are 1.49 A/cm2 and 2.25 eV, respectively, at 1100 °C. This article provides a reference for preparing C12A7:e− and their applications in electronic emission.

Ultra-fast preparation of UO2 fuel pellets by high pressure spark plasma sintering

A new sintering technique as a combination of ultra-fast spark plasma sintering and high-pressure sintering was employed to densify UO2 fuel pellets. UO2 pellets with a relative density of 97.5 % and inhibited grain growth were fabricated at 1000 °C under 150 MPa within only 4.5 min. Both short sintering time and high pressure might explain the inhibited grain growth and hence finer microstructures. UO2 fuel pellets prepared in this work exhibit excellent mechanical properties.

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.

Densification, hydrogen retention, microstructure, and mechanical properties of ε-ZrH2-x monolith fabricated by high-pressure spark plasma sintering

The sintering of zirconium hydride (ZrH2-x) without hydrogen atmosphere faces the challenge of thermal dehydrogenation problem, which is overcome by the high-pressure spark plasma sintering (HPSPS) technique in this study. The application of high pressure remarkably reduces the initial densification temperature and effectively restrains the thermal dehydrogenation of ZrH2-x in SPS. Under 500 MPa loading, ε-ZrH1.72 monolith with high densification (96.62 %) and high hydrogen density (6.04 × 10 22 atoms H/cm3) is sintered at a relatively low temperature of 700 °C for only 5 min. Meanwhile, the grain growth of ZrH2-x is very limited. The Young’s modulus and hardness of the sintered ε-ZrH1.72 are 58.68 GPa and 2.61 GPa respectively. Compared with the directly hydrided ε-ZrH2-x, the sintered ε-ZrH1.72 has a more uniform hardness distribution and improved fracture toughness of 3.19 MPa m1/2. This study provides a valuable reference for the sintering of other easily decomposable materials.

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.

Orientation Relationship of the Intergrowth Al13Fe3 and Al13Fe4 Intermetallics Determined by Single-Crystal X-ray Diffraction

In the Al-Fe binary system, the Al13Fe3 phase as well as the Al13Fe4 phase has similar icosahedral building blocks like those appearing in quasicrystals. Therefore, it is of vital importance to clarify the formation process of these two phases. Coexistence of the Al13Fe3 and Al13Fe4 phases was discovered from the educts obtained with a nominal atomic ratio of Al/Fe of 9:2 by high-pressure sintering for the first time. Firstly, single crystal X-ray diffraction (SXRD) combined with a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX) measurement capabilities were adopted to determine the detailed crystal structures of both phases, which were sharply refined with regard to Al13Fe3 and Al13Fe4. Secondly, the orientation relationship between Al13Fe3 and Al13Fe4 was directly deduced from the SXRD datasets and the coexistence structure model was consequently constructed. Finally, seven pairs of parallel atomic planes and their unique orientation relations were determined from the reconstructed reciprocal space precession images. In addition, the real space structure model of the intergrowth crystal along with one kind of interfacial atomic structure were constructed from the determined orientation relations between two phases.

Spark plasma sintering and mechanical properties of two grades of PEKK presenting different Tere/Iso ratios

AbstractTwo grades of PolyEtherKetoneKetone with different Tere/Iso ratios (PEKK 60/40 and PEKK 80/20) were consolidated using spark plasma sintering. Processing parameters were varied and their influence on mechanical properties was assessed. The study reveals that sintering at a temperature close to the melting temperature along with an applied pressure of 18 MPa leads to better mechanical properties, while the effect of dwell time remains limited. A high sintering pressure is shown to promote the cohesion of PEKK 60/40 samples when the temperature is closer to the glass transition temperature, while pressure does not play a significant role for PEKK 80/20. Post‐mortem fractographic analyses provide explanations of the underlying phenomena. PEKK samples sintered with optimal processing parameters display higher compressive yield strength and stiffness than PEKK samples elaborated from conventional processing routes. They however also exhibit a fragile behavior which can be attributed to the incomplete healing between the powder particles.

The influence of interface products on the mechanical and electrical properties of graphene aluminum composites

Graphene has been identified as an efficient filler to increase the strength and conductivity of aluminum but the reaction between aluminum and graphene at elevated temperature is inevitable. In this research, diverse interfacial products between aluminum and graphene were generated by modulating the sintering temperature, and their influences on the mechanical and electrical characteristics of graphene-aluminum composites were examined. Copper encapsulation on graphene has been utilized to isolate graphene from contacting aluminum directly as well as to doping graphene for conduction purposes. The synthesis of copper-coated graphene-reinforced aluminum composites involved several steps, including electrochemical exfoliation, electroless chemical deposition, mechanical ball milling, and high-pressure sintering. The results demonstrate that the formation of a solid solution between copper nanoparticles and aluminum promotes strong interfacial bonding with graphene, leading to a 22.76 % improvement in the conductivity of the composite compared to the pristine aluminum matrix. Additionally, by minimizing the formation of aluminum carbide, the addition of graphene results in a tensile strength of 404 MPa for the composite material, representing a significant 88.8 % increase over the aluminum matrix.

The comparison of densification mechanisms and microstructure evolution among TiB2 ceramics sintered under different levels of high pressure

TiB2 shows significant advantages in high-tech industry but it is hard to fulfill densification. High pressure sintering is an effective method to obtain TiB2 ceramics. This study reports the sintering condition of TiB2 ceramics under different levels of high pressure from 150 MPa to 15 GPa. The results show that the densification temperature reduced with the applied pressure. The densification mechanism of TiB2 ceramics sintered under 150 MPa during the final holding time at 1900℃ was dominated by the mixture of dislocation motion and grain boundary sliding. When the applied pressure achieves at 1.5 GPa, the densification mechanism was transformed into the plastic deformation of grains and grain boundary diffusion, a number of small angle grain boundaries, dislocation cells, substructures and straight grain boundaries were observed. Finally, the densification mechanism was transformed into plastic deformation of grains and grain boundary while the driving force increased to 15 GPa. The dislocation and substructures density increased with pressure, leading to the grain refinement. Our study compared the densification and microstructure evolution of TiB2 ceramics sintered under high pressure, proving the high pressure sintering could regulate the microstructure and refine grains.

Binderless nanocrystalline tungsten carbide with enhanced hardness induced by high-pressure sintering

In this study, binderless WC ceramics with grain size of 210 nm were obtained under a high-pressure of 200 MPa at a relatively low temperature of 1550 °C. The effects of high pressure on the densification behavior and grain growth were investigated. Increasing the sintering pressure accelerated the densification rate, decreased the sintering temperature, and inhibited grain growth. HRTEM observations revealed highly stable Σ 2 grain boundaries and numerous microscopic defects (stacking faults and Lomer-Cottrell locks), with dislocation density up to 4.26×1015 m−2. High-pressure strengthening effect enhanced the hardness and wear resistance by inducing plastic deformation. The high-pressure sintered WC ceramics exhibited enhanced hardness of 32.19 GPa and excellent tribological properties. Compared with the low-pressure samples, the hardness increased by 23%, the friction coefficient reduced by 30%, and the wear resistance improved by 63%, which were mainly related to the fine-grained microstructure, stable Σ 2 grain boundaries, and high-density defects.

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.