Pressure Sintering Research Articles

Physical properties of radiation dense reactive sintered borides

Reactive Sintered Borides (RSBs) were fabricated to investigate the effects of carbon and chromium content producing a series of samples referred to as Gen 2 RSBs. These RSBs were tested and evaluated for various mechanical (bulk moduli, fracture toughness, nano and micro-hardness) and microstructural properties. It is the first time when the correlation between different RSB phases and mechanical properties are understood in detail, along with the CALPHAD assessment of RSBs. CALPHAD was used to investigate the effect of sintering pressure and composition on melting point and liquid volume at sintering temperature and was critical in understanding of phase differences in Gen 1 and Gen 2 RSBs. Effects of carbon and chromium addition on mechanical and microstructural properties of RSBs were also investigated. Optimal second generation RSBs have fracture toughness of 7–10.1 MPa m0.5 and hardness of 15.9–19.1 GPa. RSBs with relative sintered density >98 % were achieved in this work. Combined X-ray diffraction and EBSD-EDX techniques were used to determine and confirm phase presence in RSBs. This work has demonstrated tractability and reproducibility of RSB materials with suitable mechanical properties for potential applications in spherical tokamaks alongside existing cemented tungsten carbides.

Fabrication and improved properties of ZrC-SiC-ZrB2 ceramics by ultra-high pressure sintering

ZrC-SiC-ZrB2 ceramics were prepared via ultra-high pressure sintering under 15 GPa at 1200 °C. The densification mechanism, microstructures, and properties of the prepared ceramics were investigated. The sintered ceramics with ZrB2 content of 15 vol% showed the improved comprehensive mechanical properties, including Vickers hardness of 26.6 GPa, Young's modulus of 469.1 GPa, and fracture toughness of 5.0 MPa·m1/2, which was 86.7% higher than that of pure ZrC ceramics. In the sintered sample, the ultra-high pressure induced new microstructure, such as L-C lock in ZrC phase, stacking faults and nanotwins in SiC phase, and severe lattice distortion in ZrB2 phase, jointly contributed to the improvement in Vickers hardness. Deflection and bridging during crack propagation were ascribed to the uniform combination of various phases.

Investigation of the Machined Surface Integrity of WC-High-Entropy Alloy Cemented Carbide

A fine-grained WC-15wt%Al0.5CoCrFeNi cemented carbide was prepared through a vacuum and gas pressure sintering. For achieving high surface integrity, diamond wheel grinding serves as the primary molding process for the machining of WC cemented carbide. To reveal the influence of grinding on the surface integrity of fine-grained WC-HEA cemented carbide, studies were conducted on grinding force, surface microstructure, surface roughness, residual stress, microhardness, and bending strength. The morphological analysis of the ground surface indicated a transition in the material removal mechanism of WC-HEA cemented carbide from ductile removal to brittle removal, with brittle removal becoming predominant as the depth of grinding increases. With the increasing depth of grinding, the grinding force increases, and the grinding force increases while the surface roughness decreases. Correspondingly, there is an improvement in both hardness and bending strength. Additionally, grinding induces high residual compressive stress on the surface, with a maximum compressive stress of 1795 MPa. The bending strength of the material is found to be dependent on the residual stress.

Sintering densification behavior of nano-ITO powder with high oxygen vacancy induced via plasma stimulation

With the growing electronics display industry, indium tin oxide (ITO) powder plays a crucial role in target preparation. To enhance the sintering performance of ITO powder, a high-activity nano ITO powder was prepared using plasma spraying, and compared to a commercial powder. This study aimed to investigate the effects of different preparation methods on powder morphology, dispersibility, oxygen vacancy concentration, and the physical properties of powders on target density, surface morphology, grain size, element distribution, and solid solubility. The results indicate that the plasma spraying high-activity nano ITO powder exhibits better dispersibility, higher oxygen vacancy concentration, and a specific surface area of up to 88.39 m2/g compared to the commercial powder. Under normal pressure sintering conditions, plasma spraying nanoscale ITO powder exhibits higher sintering activity, leading to easier densification of ceramic targets with reduced porosity, more uniform surface grains, and significant stepped growth between surface grains. High-resolution transmission electron microscopy (HRTEM) analysis suggests that targets prepared with high-activity powder have greater tin solubility in indium oxide compared to traditional commercial ceramic targets. This article further explores the relationship between powder specific surface area, oxygen vacancy, and sintering activity, along with the formation mechanism of high oxygen vacancy.

Fabrication, microstructure, and mechanical properties of novel three‐layer structured bionic ceramic cutting tools

AbstractCeramic cutting tools have low fracture toughness owing to brittleness. To improve the fracture toughness and flexural strength of ceramic cutting tools, the growth model of SiCw during the sintering process was established by the laminated structure and anisotropy of growth of bionic shells. Furthermore, a theoretical model of sintering parameters and total fracture toughness was achieved. Then, the dispersion process of SiCw was improved to enhance the homogeneity of the mixed powders further. Finally, a novel three‐layer structured bionic ceramic cutting tool was fabricated by hot‐pressing. The effects of sintering temperature, holding time, and sintering pressure on the mechanical properties and microstructure of bionic ceramic cutting tools were investigated. The results showed that the flexural strength (three point bending), fracture toughness (indentation method), and Vickers hardness of the three‐layer structured bionic ceramic cutting tool with excellent dispersed were 735 ± 34 MPa, 5.68 ± 0.06 MPa·m1/2, and 19.66 ± 0.12 GPa, respectively. The microstructure of the fracture revealed that the fracture mechanism of the bionic ceramic cutting tool was a mixture of intergranular fracture, transgranular fracture, whisker pull‐out, whisker fracture, forked crack, crack deflection, crack bridging, zig‐zag crack, and crack termination. The accuracy of the total fracture toughness model was reliable.

Low temperature fabrication of highly dense α‐Si3N4 ceramics

AbstractHighly dense α‐silicon nitride (Si3N4) ceramics were densified at a lower temperature of 1450°C under a moderate mechanical pressure of 50 MPa from commercial α‐Si3N4 powder and with conventional Al2O3–Y2O3 sintering additives. The fabricated α‐Si3N4 ceramics exhibited a hardness of 19 GPa, which is higher than that of the β‐Si3N4 ceramics usually obtained. The effect of sintering temperature and sintering pressure in the liquid phase sintering of the α‐Si3N4 ceramics is studied. The role of the dissolution–reprecipitation process and the particle rearrangement during densification of the α‐Si3N4 ceramics is discussed.

Effect of pressure on sintering behavior of polycrystalline diamond

Polycrystalline diamond composites (PDC) have been used as a cutting material in a multitude of fields, from mechanical processing to oil and gas drilling. In this study, PDC samples were prepared under high pressure and high temperature (HPHT) conditions within a pressure interval of 5.5 to 10.5 GPa by sintering diamond powder (average grain size ~3 μm) with WC-10 wt% Co substrate. Samples obtained with varying sintering pressures and temperatures were characterized using Vickers hardness tester, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results revealed that: (1) within a specified material system, each distinct sintering pressure condition corresponds to a unique optimal sintering temperature that yields the pinnacle of consolidation efficiency in samples. This optimal temperature exhibits a linearly increasing trend with rising sintering pressure; (2) at the intersection where the sintering pressure attains its optimal match with temperature, an increase in pressure fosters more intimate contact and stronger inter-grain bonding among diamond particles, thus amplifying the DD bond formation effect, which consequently leads to a linear enhancement in the Vickers hardness values of PDC with increasing sintering pressure; (3) simultaneously, under conditions of optimal temperature matching, elevating the sintering pressure notably decreases the cobalt content within the polycrystalline diamond layer of PDC samples. These findings furnish crucial experimental evidence for optimizing PDC material properties, significantly contributing to the advancement of both theoretical exploration and technological innovation within the field, carrying profound academic value and practical guidance implications.

Synergistic strategy for enhancing the thermoelectric properties of Bi0.5Sb1.5Te3 with excess Te through low-temperature liquid phase sintering method

The thermoelectric properties of Bi0.5Sb1.5Te3 synthesized through the low-temperature liquid phase sintering (LPS) method can be significantly improved by employing a synergistic approach. During the heat treatment, excess Te undergoes self-swelling under optimal sintering pressure, facilitating the slip of matrix grains perpendicular to the applied pressure and subsequent recrystallization, leading to enhanced electrical properties. Building upon this basis, the heightened Seebeck coefficient and power factor of the thermoelectric matrix stem from the incorporation of 0.6 vol% B4C nanoparticles. Simultaneously, lattice thermal conductivity experiences notable suppression due to enhanced phonon scattering induced by additional interfaces, defects, and second-phase. Lastly, a markedly improved maximum ZT value of 1.30 at 333 K is achieved when the volume percentage of B4C nanoparticles is 0.6 vol%, representing a remarkable 23% enhancement compared to the untreated sample. These results indicate that the LPS method, coupled with the enhanced second nanophase scattering mechanism, is beneficial to enhancing the thermoelectric properties of Bi2Te3-based materials.

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.

Ultrafast high-temperature sintering (UHS) of WC and WC-containing ZrB2

WC and ZrB2 are refractory ceramics with excellent thermophysical properties and melting temperatures exceeding 2800°C. Both materials require the application of external pressure and long sintering times for their consolidation. In particular, commercially available ZrB2 powders are intrinsically difficult to sinter and usually need long pre-processing steps such as high-energy ball milling. Ultrafast high-temperature sintering (UHS) is a recently developed technique that enables the consolidation of bulk ceramics within minutes. In the present work, pure WC was efficiently densified to above 98% in just 3 min by UHS. Moreover, small WC additions enhanced ZrB2 densification by activating liquid phase sintering. Samples containing 5 and 10 vol% WC were sintered to 95 and 96%, respectively, in 2 min. All the WC initially present in the blend reacts to form a liquid phase during sintering and solidifies as WB and (Zr,W)C upon cooling. The formation of a ZrB2-(Zr,W)B2 core-shell structure was detected in all the sintered composites. The hardness of UHS samples reaches 15 GPa (WC - ZrB2 composites) and 21 GPa (pure WC), similar to that measured in materials obtained by slower and more sophisticated pressure-assisted sintering techniques.

Novel Low Temperature and Low Pressure Sintering of ADAS Radar Sensor Antenna Stack

A low temperature and low pressure sintering of antenna stacks with the promising prospect of achieving a single step bonding process of multi-layer wave guide antennas for automotive radar sensors is demonstrated. Silver nanoparticles (Ag-NP) paste with the ability to sinter at 200°C in air allows for easy integration into standard surface mount device reflow process. The paste drying and sintering process of the Ag-NP paste is first optimized using dummy test chips to reduce the typical voiding and channeling in the interconnect when sintered without pressure. In especially, a slow ramp rate with an integrated predrying step is applied after placement of the test chips into the wet paste. Afterwards, the process is transferred to the antenna. However, pre-existing bending in the large antenna layers challenges the sintering process without pressure, e.g. larger amounts of paste need to be applied for compensation of the bending. Therefore, the use of a low bonding force in observed to improve the quality of the sintered interfaces. Near bulk like Ag sintered interconnects were realized.

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.

The investigation of synthesis and textured properties of in situ formed hBN with spark plasma sintering

Hexagonal boron nitride (hBN) exhibits high thermal conductivity within the in-plane direction and significantly lower thermal conductivity in the cross-plane direction because of its anisotropic nature. When the texture properties of bulk hBN are strong, it is a potentially promising application area for the insulating heat spreaders and heat sinks. In this study, hBN was synthesized in situ with the spark plasma sintering (SPS) technique and the effects of sintering temperature on the orientation of hBN grains were investigated. The precursor was prepared by chemically reacting urea and boric acid in heat treatment at 850 °C. The hBN was produced using the previous precursor in a SPS at 1500, 1700, and 1900 °C. The samples were characterized using XRD, FTIR, SEM and Raman spectroscopy. The properties of the samples were further determined through measurements of density, DSC and thermal diffusivity. The hBN samples obtained through the SPS process have nano and micron-size grains. The hBN was highly crystalline and homogeneous, as determined by the XRD pattern, Raman spectra and Raman maps. The orientation of hBN grains has been revealed in the c-axis of hBN crystallites and was preferably arranged within a plane parallel to the SPS pressing direction. The hBN orientation preference index (IOP) value of −19925.5 at 1700 °C indicates the highest quality texture orientation. Increasing the temperature to 1900 °C decreased the orientation in the direction parallel to the pressing direction. The highest thermal conductivity calculated according to the thermal diffusivity perpendicular to the sintering pressure direction was 8.86 W m−1K−1 at 1900 °C.

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.

Quantitative analysis and comparison on densification kinetics and grain growth mechanisms using reactively and directly densified TiB2-SiC-hBN ceramics

Compared to samples sintered from directly mixed powders, reactive sintering enables the production of denser ceramic materials with finer grains at even lower temperatures. However, there is a lack of quantitative comparisons between the two sintering approaches in terms of densification mechanisms and grain growth kinetics. In this work, taking two types of TiB2-SiC-hBN ceramics sintered from raw powders containing TiB2, SiC and hBN (TSB-M) and TiN, Si and B4C (TSB-R) as model materials, above mechanisms were quantitatively investigated using spark plasma sintering. Effects of dwell temperature and sintering pressure on the microstructure evolutions, densification kinetics as well as grain growth of TSB-R and TSB-M were thoroughly examined. The densification mechanisms for both samples under different conditions were revealed by calculating the effective stress exponent and apparent activation energy. According to regression coefficients for different grain growth exponents, the dominated growth mechanisms for SiC and TiB2 grains were also explored.

Fabrication of Si3N4 ceramics substrates with high thermal conductivity by tape casting and gas pressure sintering

AbstractIn this paper, high thermal conductivity Si3N4 ceramics were successfully fabricated through exploring and optimizing the tape casting process. The impact of various organic additives on the rheological characteristics of Si3N4 slurry was explored, and the pore size distribution and microstructure of the green tapes at different solid loadings were investigated, as well as the microstructure of Si3N4 ceramics. Green tapes with a narrow pore size distribution, a small average pore size, and a high density of 1.88 g cm−3 were prepared by the investigation and optimization of the Si3N4 slurry formulation. After gas pressure sintering, Si3N4 ceramics with a density of 3.23 g cm−3, dimensions of 78 mm × 78 mm, and a thickness of 0.55 mm were obtained. The microstructure of the Si3N4 ceramics showed a bimodal distribution and a low content of glassy phases. The thermal conductivity of the Si3N4 ceramics was 100.5 W m−1 K−1, the flexural strength was 735 ± 24 MPa, and the fracture toughness was 7.17 MPa m1/2.

Cold Sintering Mediated Engineering of Polycrystalline SnSe with High Thermoelectric Efficiency.

Despite the attractive thermoelectric properties in single crystals, the fabrication of high-performance polycrystalline SnSe by a cost-effective strategy remains challenging. In this study, we prepare the undoped SnSe ceramic with remarkable thermoelectric efficiency by the combination of a cold sintering process (CSP) and thermal annealing. The high sintering pressure during CSP induces not only highly oriented grains but also a high concentration of lattice dislocations and stacking faults, which leads to large lattice strain that can shorten the phonon relaxation time. Meanwhile, the thermal annealing breaks the highly resistive SnOx layers at grain boundaries, which improves the electrical conductivity and power factor. In addition, the grain growth during annealing further turns the broken SnOx layers into nanoparticles, which further lowers the thermal conductivity by enhanced scattering. As a result, a peak ZT of 1.3 at 890 K and a high average ZT of 0.69 are achieved in the polycrystalline SnSe, suggesting great potential in mid-temperature power generation. This work may pave the way for the mass production of SnSe-based ceramics for thermoelectric devices.

Design and optimization of TiB2‐based ceramic tool materials based on fuzzy theory

AbstractBased on the material properties and fuzzy theory, a new design method of TiB2‐based composite ceramic tool material was proposed, and the TiB2‐based composite ceramic tool material with excellent friction and wear resistance was designed. Initially, the fuzzy evaluation method was used to establish the matrix of the friction and wear resistance of the material, and the TiB2‐based material component with excellent friction and wear resistance was determined. Ultimately, based on the principle of fuzzy cognitive map, the correlation mapping of “sintering process–microstructure–mechanical properties” was established, and the composition ratio and sintering process were optimized. The results show that the TiB2–TaC–TiC ceramic tool material had excellent friction and wear resistance. When the volume content of TaC was 8 vol.%, the volume content of TiC was 20 vol.%, the heating rate was 100°C/min, the holding time was 8 min, the sintering temperature was 1600°C, and the sintering pressure was 50 MPa, the mechanical properties were hardness 23.5 GPa, bending strength 438 MPa, and fracture toughness 10.26 MPa∙m1/2.

Preparation of extra coarse-grained WC-Co cemented carbides by doping sodium in the ammonium tungstate solution during evaporation crystallization process

Coarse tungsten powders are usually used as raw material for preparing coarse grained WC-Co cemented carbides. Sodium can effectively promote the growth of tungsten powder during the reduction of tungsten oxide with hydrogen. Conventional addition of sodium salt into tungsten oxide powders by mechanical milling has the disadvantage of uneven distribution of sodium, which leads to abnormal growth of tungsten powders and affects the performance of WC-Co cemented carbides. In this study, uniform sodium doped ammonium paratungstate (APT) was prepared by adding sodium carbonate into ammonium tungstate solution during the evaporation and crystallization process based on the formation of ammonium-sodium paratungstate. Experimental results showed that sodium was uniformly distributed in the APT and promoted the growth of APT crystal. The sodium-doped APT was then used to produce coarse tungsten powders. Compared with tungsten powders obtained from tungsten trioxide with addition of sodium by mechanical milling, the coarse tungsten powders had larger average particle size and more uniform particle size distribution. The coarse tungsten powders were subsequently prepared into tungsten carbide powders and WC-Co cemented carbides by carburization and pressure sintering, respectively. Results showed that the obtained WC-Co cemented carbides had larger average WC grain size (6.8–7.2 μm), higher fracture toughness and transverse ruptures strength, and more uniform microstructure than that of WC-Co cemented carbides obtained by addition of sodium with mechanical milling. Moreover, sodium thoroughly volatilized during hydrogen reduction and carburization process, so that the purity of WC-Co cemented carbides was not reduced. Therefore, this study provided a new route for preparation of extra coarse-grained WC-Co cemented carbides.

Ultrasensitive quantum capacitance detector at the edge of graphene

Graphene, with its outstanding electrical properties, has appealed to the attention of researchers all over the world for biosensing applications. So far, much of the research has been conducted on the field-effect modulations of carriers in the basal plane of graphene after adsorbing charged biomolecules. However, another essential aspect--the graphene edge--has been largely ignored due to the difficulties in both manufacture and characterization. Here, we propose a facile intercalation and pressure sintering approach that enables the fabrication and exposure of only graphene edges. The quantum capacitance of the exposed edges is proportional to the local density of state (DOS) and can be harvested for biochemical sensing. Notably, due to fringe electric field enhancement and biomolecular convergence at the one-dimensional graphene edges, we are able to detect four representative amino acids at 0.01 fg/mL concentrations within several minutes. These achievements in innovative quantum capacitance measurements of graphene edges, combined with simple and robust device fabrication by eliminating complex micro-nano processing, offer a new avenue for the next generation of biochemical sensors with ever-demanding sensitivities.