Pressureless Sintering Research Articles

Pressureless sintering kinetics analysis of Ti3SiC2 and Ti2AlC powdered MAX phases

This paper reports the pressureless sintering behavior and the activation energy of the powdered MAX phases Ti3SiC2 and Ti2AlC. A non-isothermal technique was used to determine the sintering kinetic parameter. The Ti3SiC2 and Ti2AlC MAX phases showed the maximum sintering rate at 1723 K, 0.14 and 0.10 µm/s, respectively. The sintering rate of the sample at different temperatures followed a cubic equation which was determined. The sintering activation energy (Ea) for the Ti3SiC2 and Ti2AlC samples was 362.1 kJ/mol and 640.3 kJ/mol, respectively.

Iron-enhanced simulated lunar regolith sintered blocks: Preparation, sintering and mechanical properties

Lunar surface constructions face a hostile service environment, the objective of this study is to develop a lunar base construction material with excellent mechanical properties and superior durability. Based on the abundant iron (Fe) resources on the moon, simulated lunar regolith sintered blocks (CUG-1A) with varying Fe additions (0–10 wt%) were prepared by pressureless sintering, and their sintering characteristics, phase compositions and mechanical performance before and after high-low temperature treatment were studied. The results show that the introduction of the Fe phase noticeably improve the sintering quality and increase the relative density. Furthermore, the Fe phase significantly fortified the sintered blocks, when the Fe addition reached at 10 wt%, the blocks could achieve a compressive strength of 150.92 MPa and a tensile strength of 21.01 MPa. The Fe phase's reinforcement could stem from the mechanisms of promoting sintering, crack deflection and Fe phase bridging. Meanwhile, the high-low temperature treatment does not cause any substantial alterations in the blocks' mechanical strength nor their failure modes. Lastly, large-sized sintered blocks with high dimensional regularity were produced through process optimization, providing a reference for the construction of lunar bases.

Enhancing mechanical properties and density of WC-Co with pressureless sintering via shell structure binder jetting additive manufacturing

The binder jetting (BJT) additive manufacturing (AM) process is suitable for producing complex WC-Co cemented carbide parts. To achieve near-full-density in the final parts, the hot isostatic pressing (HIP) sintering process is typically employed to enhance densification. However, HIP sintering requires high equipment specifications and is relatively more costly compared to conventional pressureless sintering. To improve the density of WC-Co parts printed by BJT AM under the pressureless sintering environment, the shell structure printing method was used in this study. The shell structure printing only jets binder to the model's outer profile, creating a shell that encapsulates the central powder, mitigates binder pyrolysis and residual effects on particle densification, and enhances post-sintering density. In this work, the effect of shell thickness, binder type, and printing layer thickness on the density and mechanical properties of the WC-12Co parts under pressureless sintering conditions were investigated. The results show that the density and mechanical properties of the WC-12Co parts significantly improved with the decreasing shell thickness. Specifically, the WC-12Co printed using shell structure achieves a relative density of over 98% and a transverse rupture strength (TRS) of 1954 MPa after pressureless sintering, a result comparable to BJT printed samples treated with HIP sintering. Moreover, the results indicate using a binder with lower residual carbon content and smaller printing layer thickness has a more positive effect on improving density and mechanical strength when employing the shell structure printing method. Microstructure morphology and phase analysis results indicate that the shell structure printing method does not affect the growth of WC grains in the shell and center regions, nor does it influence its phase composition. The shell structure printing method offers a new approach to the manufacturing of near-full-dense WC-Co materials using the BJT process.

Characterization of Cu-Graphene composites synthesized through pressure-less sintering for application in electrical contacts

Characterization of Cu-Graphene composites synthesized through pressure-less sintering for application in electrical contacts

Pushing the grain size limit of pressureless-sintered alumina nanocrystalline ceramics by non-oxidizing atmospheres

Nanocrystalline ceramics are of interest due to their refined grains and expected ductility, yet their pressureless sintering is challenging considering the large coarsening driving force and the coupled densification-coarsening kinetics. Of special interest is the pressureless sintering of alumina nanocrystalline ceramics, with fundamental significance to ceramic processing science and technology. Previously reported efforts offered dense alumina nanocrystalline ceramics with average grain size Gavg down to 34 nm and with a coarsening ratio β (defined as the ratio of average grain size after and before sintering) of 7.2 from nanoparticles to dense alumina ceramics, by synergistically optimized α-Al2O3 nanoparticles and pressureless two-step sintering in air. Here we further pushed Gavg down to 29 nm and β down to 5.8, by changing air to non-oxidizing atmosphere (e.g., N2, Ar, and vacuum). We identified pronounced difference in sintering kinetics in air versus in N2, Ar, or vacuum, which is unexpected for wide-bandgap, redox-insensitive oxide materials of alumina. With smaller initial nanoparticles (e.g., <3.5 nm) and better optimized sintering conditions, it may be possible to pressureless-sinter dense alumina ceramics with Gavg below 20 nm.

Ultrafast pressureless sintering of high conductivity Ni-based cermets inert anode for Carbon‐free electrolytic aluminum

Cermet is a promising inert anode material to solve the problem of high carbon emission and bearing C-F harmful gas generation of primary aluminum produced from bauxite using molten electrolysis with carbon anode. However, a major challenge for traditional methods is to ensure that the cermet inert anode simultaneously has good conductivity, corrosion resistance, and strength. Herein, a new ultrafast pressureless sintering (UPS) method is developed to greatly improve the conductivity of metal ceramic inert anode materials while ensuring good corrosion resistance and strength. In a departure from conventional practice that metal phase undergoes local enrichment accompanied by irregular formation of ceramic particles at low heating rates, here UPS achieves rapid diffusion and uniform distribution of metal phase at regular ceramic particle interfaces during liquid phase sintering. The new method is applicable to Ni-based cermet and the production of Ni-based cermet inert anode with excellent conductivity in a few tens of seconds has been demonstrated, which is able to improve the quality of cermet inert anode requiring a low energy input. The conductivity of cermet produced by the applied UPS process is 15-fold higher than that of other pressureless sintering processes with low heating rates. These Ni-cermet inert anodes exhibit high corrosion resistance and density with excellent mechanical properties. This rapid sintering technique facilitates the structural design of inert anodes and develops a pathway of new systems with scalable, efficient, and high-quality sintering of composite materials.

Effect of sintering temperature and doping content on phase evolution and microstructure of doped UO2 ceramics

ZrO2-doped UO2 ceramics have been considered as a promising high performance nuclear fuel in LWR due to its desirable properties. ZrO2-doped UO2 ceramic fuel samples with different contents of ZrO2 (20 wt%, 25 wt% and 30 wt% ZrO2, equivalent to 35.4 mol%, 42.3 mol% and 48.4 mol%, respectively) were prepared by pressureless sintering at various temperatures (1550 ℃, 1650 ℃ and 1750℃, respectively) for 4 h in a hydrogen reducing environment. Phase evolution and microstructure of as-sintered ZrO2-doped UO2 ceramic samples were investigated by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), respectively. Results show that there exists coexistence of uranium-rich U1-xZrxO2 phase with face-centered cubic (FCC) structure and zirconium-rich Zr1-yUyO2 phase with tetragonal structure in the 20 wt% ZrO2-doped UO2 ceramic sample when the sintering temperature is lower than 1650 ℃, while the UO2 and ZrO2 will form a single U1-xZrxO2 solid solution after sintering at 1750℃ for 4 h with the ZrO2-doped UO2 ceramic samples containing 25 or 30 wt% ZrO2, respectively. The significant differences of phase structure and ionic radius between ZrO2 and UO2 and the high atomic diffusion barrier are the main reasons that it is difficult to form a single solid solution for UO2-ZrO2 ceramic samples sintered at low temperature.

Stabilization of phase‐pure tetragonal zirconia ceramics with minimum CeO2 doping and associated benefits

AbstractIn the quest of finding the minimum ceria doping for a stabilized zirconia matrix, we report a remarkable structure–property correlation for a high (&gt; 99%) density, fine grained (∼100 nm), fully phase‐pure (confirmed via both slow scan X‐ray diffraction and Raman spectroscopy) tetragonal 10 mol.% CeO2‐doped zirconia ceramics (10Ce‐TZP). A starting near spherical, ultrafine (∼7 nm) powders with high surface area (∼170 m2/g) ensured the conventional pressureless sintering at 1150°C without the need of any additives, binders, and/or sintering aid to achieve this high density. No monoclinic phase was traced at the end of the low‐temperature degradation test for 100 h in boiling water; negligible (∼5% monoclinic phase) amounts showed up in the thermal quenching tests. The sintered 10Ce‐TZP ceramic recorded comparable/better hardness and indentation toughness value with respect to higher (i.e., 12 mol.%) ceria doped‐zirconia systems. For the first time, nanograined translucent 10Ce‐TZP ceramic portrayed a remarkable transmittance of ∼38% at 600 nm wavelength for 0.6 mm thick pellet.

Preparation and microwave dielectric properties of calcite (CaCO3) ceramics

Calcite, as the thermodynamically stable phase of calcium carbonate, is a promising low-cost candidate as low-εr microwave dielectric ceramics because of its large natural reserve and the strong covalency of C–O bond, while it is difficult to be densified due to the decomposition at about 627 °C. In the present work, calcite ceramics with the relative density of 87.8 % and low Qf value of 5,600 GHz were prepared by the room-temperature cold sintering, and the pressure-less sintering at 400–600 °C was conducted subsequently. The pressure-less sintering only increases the relative density slightly, while the microstructural evolution including the formation of uniform grains and significant grain growth were observed with increasing the sintering temperature, leading to a dramatic increase in Qf value. The best microwave dielectric properties with εr = 7.60, Qf = 20,200 GHz, and τf = −154 ppm/°C at around 12 GHz were obtained in the calcite ceramic sintered at 600 °C, indicating the applicable possibility of carbonate ceramics in microwave communications.

Properties of a Pressureless Sintered 2Y-TZP Material Combining High Strength and Toughness

Yttria stabilized zirconia materials are frequently used in mechanical engineering and biomedical applications. Demanding loading conditions require materials combining a high level of strength and fracture toughness. A ready-to-press alumina doped 2 mol% yttria-stabilized zirconia powder was shaped by axial pressing and sintering in air at 1250–1500 °C for 2 h. At 1350 °C the best combination of strength (1450 MPa) and toughness (7.8 MPa√m) was achieved. Materials sintered in the middle of the chosen temperature range combine full density, high transformability and small grain size. Toughness measurements by direct crack length measurements delivered unrealistically high fracture toughness values.

Mechanical and optical properties of porous black Al2O3: Experimental, DFT, and wave optics investigation

Porous black Al2O3 was fabricated by gel casting and pressureless sintering (PS). MnO2 and Fe2O3 incorporated in Al2O3 led to the formation of MnAl2O4 and FeAl2O4 at ≥1200 °C, enhancing its opacity. The influence of various MnO2 content and sintering temperature on the mechanical and optical properties of porous black Al2O3 was investigated. The results showed that as MnO2 content and sintering temperature increased, porosity decreased, bulk density increased, compressive strength increased, and opacity increased. Using density functional theory (DFT), the intrinsic properties of MnAl2O4 and FeAl2O4 were investigated, including band structure (BS), electron distribution, partial density of states (PDOS), and optical properties such as refractivity, absorption, dielectric constant, conductivity, and dissipation factor across various wavelengths. These DFT calculations were incorporated into the wave optics model to assess the response and impedance matching of porous black Al2O3 to transverse electric (TE) and transverse magnetic (TM) waves with various MnAl2O4 content.

Preparation and mechanical properties of (MoCrVMn)2FeB2 based high-entropy cermets

A novel (MoCrVMn)2FeB2 based high-entropy cermets were prepared by two-steps method and the microstructure and mechanical properties were investigated. The single-phase (MoCrVMn)2FeB2 powders were synthesized at 1400 °C for 2 h, and subsequently the (MoCrVMn)2FeB2 based high-entropy cermets were prepared via pressureless sintering technique. The results revealed that the phase evolution was as follows: (Mo, Cr, V, Mn, Fe, B) → (Fe2B, V2B3, MoB) → ((FeVMn)2B, (FeVMnCr)2B, Mo2FeB2) → (MoCrVMn)2FeB2. The (MoCrVMn)2FeB2 based high-entropy cermets sintered at 1250 °C showed good denseness and the ceramic phases were uniformly distributed in the metal bond, with the optimal mechanical properties of hardness 1355 HV30 and fracture toughness 19.09 MPa m1/2. With the increase of temperature, the hardness increased first and then decreased, and the fracture toughness was basically unchanged first and then increased, and then decreased. This study provides a new route to further optimize the performance of Mo2FeB2 based cermets.

The impact of Si3N4 incorporation on the mechanical characteristics of ZrB2–SiC nanocomposite sintered via pressureless method

Due to their unique properties, ultra-high-temperature ceramics possess have great potential for aerospace and industrial applications. In this study, we utilized ZrB2 and Si3N4 powders in micron scale, along with SiC powder in both nano and micron scale. A nanocomposite comprising ZrB2-20 vol% SiC (15 % nano - 5 % micron)-10 % ZrC was incorporated into the base composition by adding Si3N4 in volumes of 1-2-3 volume and 4 %. The process is carried out by pressure-less sintering at 2150 °C. The evaluation in this study involved X-ray diffraction (XRD) analysis, hardness measurement, density determination, fracture toughness assessment, and bending test. The results showed that addition of Si3N4 enhances the relative density, hardness, and fracture toughness of this nanocomposite. The sample with Si3N4volume fraction of 3 % has the best performance, with a relative density of 99.4 %, a hardness of 20.33 GPa, and a fracture toughness of 3.12 MPa M1/2.

Driving force for enhancing the cold silver sinter joining using time domain-dependent Ag–Au epitaxy and interfacial stress

We reveal the mechanism of the cold silver sinter joining process using time-dependent characteristics of Ag–Au interactions grown in the highly (111) orientation. In the initial (∼15 min) state at 175 °C, the Ag–Au inter-diffusion layers developed strongly in the (001) direction, and with increasing time (∼60 min), they switched epitaxial growth in the (111) orientation. The die-bonding strength was significantly improved from 14.9 MPa to 37.8 MPa when the sintering time increased from 15 min to 60 min. The gradual Ag–Au epitaxial growth behavior with a higher stacking-fault energy (SFE) at 175 °C was the dominant factor linearly increasing the cold silver sinter joining strength and ultimately maximizing energy absorption leading to fracture. The surface finish of Au, which can contribute to the time-dependent growth of Ag–Au inter-diffusion layers, underwent a dramatic transformation from a Cube texture to a γ-fiber texture over time. This change was attributed to the micro/nanoscale shear deformation, and reduction of the twin boundary fraction. This study unveiled the in-depth fundamental mechanism for strengthening and pressureless cold sinter joining of Ag–Au epitaxial layers by comprehensively considering this texture behavior, changes in X-ray residual stress, and mechanical properties.

High-temperature stability of wave absorption properties for SiC enhanced ZrB2–Al2O3 ceramics

To address the challenge of high-temperature microwave absorbing materials (MAM) for the hot-end components of aircraft, SiC enhanced ZrB2–Al2O3 (ZSA) ceramics are prepared by pressureless sintering. In ZSA ceramics, ZrB2 and SiC are the absorbers and Al2O3 is the matrix. The effect of the content of SiC on microwave absorption properties for the ZSA ceramics are studied. The results show that the addition of SiC enhances the MA properties and high-temperature stability of ZrB2–Al2O3 ceramics. Ceramics with an effective absorption bandwidth (EAB) of 3.9 GHz at a thickness of 1.56 mm; and a minimum reflection loss (RLmin) of −15 dB is obtained. After treatment at 1000 °C for 20 h in an air atmosphere, the EAB reduces to 3.5 GHz at the thickness of 1.56 mm, however, RLmin reaches −21 dB. ZSA ceramics consume microwaves through effect including interfacial polarization relaxation, conductive loss and multiple reflection loss. This work is expected to provide new ideas for the design and development of high-temperature microwave absorbing materials.

Effect of TiC addition on microstructure and properties of network structured TiC–ZrO2 composite conductive ceramics

Zirconia (ZrO2) ceramics have excellent mechanical properties and high-temperature conductivities. However, ZrO2 ceramics do not possess room-temperature conductivity, which significantly limits their application as functional components in smart communication devices and wearable smart products. To achieve room-temperature conductivity in ZrO2 ceramics, this study prepared a network-structured TiC–ZrO2 composite conductive ceramic with a low titanium carbide (TiC) addition using pressureless sintering. The influence of the TiC addition on the microstructure, mechanical properties, and electrical properties of the TiC–ZrO2 composite material was investigated. The results showed that the composite material consists of cubic zirconia (c-ZrO2), tetragonal zirconia (t-ZrO2), and titanium carbide (TiC) phases. When the TiC content was 11 vol %, the relative density, open porosity, flexural strength, and fracture toughness of the sample were 98.6 ± 0.1 %, 0.16 ± 0.06 %, 383 ± 38 MPa, and 4.66 ± 0.26 MPa m1/2, respectively. The electrical percolation threshold of the TiC–ZrO2 composite was between 8.0 and 11.0 vol %. When the TiC content is 18.8 vol %, the sample exhibited the lowest room-temperature resistivity of 3.00 × 10−5 Ω m. The electrical resistance of the TiC–ZrO2 composite material is mainly attributed to the grain boundary effect. With increasing TiC content, the resistivity of the composite material decreased, exhibiting linear conductivity and achieving conductive percolation characteristics.

Effect of the paste spreadability on ultrasonic transducer fabricated by pressureless sintering of nano-Ag

Paste spreadability is critical to paste spread layer and thus the quality of final product in pressureless sintering of nano-Ag, but the metrics and the underlying mechanics have not yet been well acknowledged. In this work, various ultrasonic transducers are fabricated by pressureless sintering of various nano-Ag pastes. Particle dynamics mechanism during the paste spreading process and the relationship between paste viscosity and spreadability are clarified. The influences of paste spreadability on dynamic performances of transducer are analysed, and the underlying mechanisms are also investigated. The results show that the normalized mass of Ag NPs is basically positively correlated with the flatness of the coupling layer, and the flatness of the coupling layer is also positively correlated with the peak-to-peak voltage and SNR of the transducer. The attenuation in the far-field scattering process mainly depends on the porosity of the coupling layer.

The effect of printing parameters on the properties of 17-4 PH stainless steel fabricated by material extrusion additive manufacturing

The 17-4PH stainless steel filament was characterised and utilised to study the effect of printing parameters, i.e. printing temperature, layer thickness, infill pattern and extrusion multiplier on the physical properties. The as-printed and as-sintered internal structures were analysed. The results showed that the as-printed density increases with increasing printing temperature and extrusion multiplier and decreasing layer thickness. The use of the line infill pattern also provided slightly higher as-printed density than the concentric infill pattern due to the low fraction of void between deposited paths. After sintering, the trace of these voids can be observed together with smaller-size residual pores from the spaces between powders, which is the nature of the pressureless sintering process. The microstructure of the as-sintered specimens was similar to the typical microstructure of the 17-4PH alloy fabricated by metal injection moulding process, which contains delta ferrite, martensite and Si-rich phases. In additions, the internal void generated during debinding and sintering results in unexpectedly low tensile properties and results in the difference in tensile properties between the concentric and line infill patterns.

High-throughput composition screening of high-entropy rare-earth monosilicates for superior CMAS corrosion resistance up to 1873 K

Superior calcium-magnesium-aluminum-silicate (CMAS) corrosion resistance is the key to the applications of high-entropy rare-earth monosilicates (HEREMs) as environmental barrier coatings (EBCs) for next-generation gas turbine engines. Here, we report a high-throughput composition screening strategy to achieve superior CMAS corrosion resistance in HEREMs for the first time. Specifically, we first fabricate ten kinds of (Ho0.25Yb0.25Lu0.25X0.25)2SiO5 (X = Sc, Tm, Er, Y, Dy, Gd, Eu, Sm, Nd, and La) HEREM samples using a high-throughput pressure-less sintering technique, and subsequently screen their CMAS corrosion resistance at 1673 K via a high-throughput testing apparatus. Among all the as-fabricated samples, the HEREM-Er samples are found to have the best CMAS corrosion resistance, even up to 1873 K, which outperforms the EBC materials that have been reported. Such superior CMAS corrosion resistance of the HEREM-Er samples is primarily attributed to the formation of a dense apatite barrier layer, which results from its decreased melting point, as confirmed by first-principles calculations and molecular dynamic simulations. Our work provides a guide for designing exceptional anti-CMAS corrosion high-entropy rare-earth silicates.

Analysis of fiber-reinforced silicon carbide formed via material extrusion

Here, we report a method for fabrication of robust and near-net shaped carbon-fiber reinforced silicon carbide (Cf/SiC) through the additive manufacturing method of material extrusion. The addition of fibers allows for enhanced damage tolerance and thermal shock resistance but little is known about the resulting affects to resulting ink rheology, printability and resulting fiber alignment in the printed components. As such, three separate inks were developed with carbon fiber loadings of 0 %, 5 % and 15 % by volume. All inks had requisite rheology for material extrusion to fabricate near-net and dense components via pressureless sintering with densities of 93.95 %, 92.11 %, and 85.65 %, respectively. Micro X-ray computed tomography and deep learning auto-segmentation was utilized to fully describe the fiber orientation in 3D, revealing highly oriented fibers. 4-point flexure testing was performed on as-printed bend bars for all 3 ink compositions. The bars had robust average flexural strengths of 114–260 MPa, with a decrease in strength observed in fiber-reinforced samples as attributed to a decrease in sample density. However, an increase in Weibull modulus (18.0 compared to 7.0–7.3) and observed fiber pull out for the 15 % Cf samples suggest promise in toughening and reliability of fiber reinforcements. Oxyacetylene torch testing was performed on all 3 types of printed parts to study the oxidation and ablation resistance of the silicon carbide samples with fibers. Only samples with fiber reinforcements survived thermal shock of testing further emphasizing the need and benefit of fiber inclusions.