Pressureless Sintering Research Articles

In vitro degradation behavior, cytocompatibility and hemocompatibility of topologically ordered functionally graded iron-hydroxyapatite-zinc composite biodegradable biomaterial fabricated using 3D printing and pressureless microwave sintering

In vitro degradation behavior, cytocompatibility and hemocompatibility of topologically ordered functionally graded iron-hydroxyapatite-zinc composite biodegradable biomaterial fabricated using 3D printing and pressureless microwave sintering

Near-fully dense nanocrystalline hafnium via pressureless two-step sintering

Grain refinement in hafnium (Hf), a typical refractory metal, encounters significant challenges due to its inherent high sintering temperature requirements and uneven sintered capillary force that lead to rapid grain coarsening and inhomogeneity. Herein, we present a novel approach of the pressureless two-step sintering to prepare near-fully dense nanocrystalline Hf with an average grain size of around 40 nm, which represents the smallest grain and highest density for nanocrystalline pure Hf to the date. Such achievements not only broaden the applicability of pressureless two-step sintering but also offer insights to produce large size of finer-grained refractory metals and their alloys.

Pressureless sintering and properties of additively manufactured ZrB2–SiC

Pressureless sintering and properties of additively manufactured ZrB₂–SiC

Hot Uniaxial Pressing and Pressureless Sintering of AlCrCuFeMnNi Complex Concentrated Alloy-A Comparative Study.

External pressure is often applied during sintering to obtain materials with improved properties. For complex concentrated alloys (CCAs), this processing step is commonly performed in vacuum. However, this can promote the evaporation of elements and increase the oxide content, thereby degrading the properties of the alloy. In this study, we compared the microstructures and properties of AlCrCuFeMnNi CCA samples obtained by hot uniaxial pressing sintering (HPS) and pressureless sintering (PLS) using a helium atmosphere purified by an oxygen getter system. The powders were prepared from mixtures of CrFeMn, AlNi and Cu and sintered by HPS at 900 °C for 1 h with an applied pressure of 30 MPa and by PLS at 1050 °C for 1 h. The samples were characterised using X-ray diffraction, scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, electron backscattering diffraction, density measurements and hardness tests. It was found that the oxygen getter system promoted oxygen partial pressure values at sintering temperatures similar to those of a mixture of 90% helium and 10% hydrogen. The HPS allowed us to obtain almost fully dense samples with a smaller average grain size and finer distribution of aluminium oxides than PLS. These differences increased the hardness of the samples sintered under pressure.

Achieving enhanced high-temperature strength in Ti-48Al-1Fe alloy sheets by direct hot pack-rolling of powder-sintered billets without cogging

The TiAl alloy is a novel lightweight high-temperature structural material that exhibits exceptional performance. The brittleness and mechanical properties of the material can be enhanced by improving the microstructure via rolling. The Ti-48Al-1Fe alloy with high density was produced using powder compaction and pressure-less sintering. Subsequently, the TiAl alloy sheet was formed via hot pack rolling. This study examined the sheet formability of PM Ti-48Al-1Fe alloy sheets at various temperatures, as well as the microstructure and mechanical properties at varied levels of rolling deformations. The microstructure of the powder metallurgy (PM) TiAl alloy sheet has a unique duplex structure, consisting of α2/γ lamellar colonies and a composite structure. The rolling deformation process generates spherical recrystallized grains, which effectively reduce stress concentration. The enhanced composite structure is mostly localized at the interfaces between grains, creating a robust obstacle for the movement of dislocations at high temperatures. This results in the desired outcome of reinforcing the mechanical properties of the material at high temperatures through grain boundary strengthening. This study demonstrates that the ultimate tensile strength (UTS) of PM TiAl sheet tensile specimens in the rolling direction at room temperature is 443 MPa with 1 % elongation, whereas at 800 °C, the UTS rises to 548 MPa with 2.5 % elongation. This study proposes a novel process for the efficient production of Ti48Al1Fe sheets with good high-temperature mechanical properties. This technique entails the hot rolling of high-density sintered powder metallurgy billets, offering an innovative approach for the economical and swift production of TiAl alloy sheets during practical manufacturing process.

Preparation of 99.6 % alumina ceramic substrates with high thermal conductivity by tape casting and warm pressing process

High-purity alumina ceramic substrates have drawn immense attention because of its high strength and hardness, as well as good chemical stability. However, the high sintering temperature and low product yield are significant barriers to the preparation of high-purity alumina ceramic substrates. In this study, modified alumina nanoparticles are used to fabricate high-purity alumina ceramic substrates under pressureless sintering via a tape casting–warm pressing process. The influences of powder modification, forming processes, sintering temperature, and powder particle size on the microstructure, mechanical properties, and thermal properties of the fabricated alumina ceramic substrates are investigated. The experimental results show that tape casting–warm pressing significantly improves the microstructure and density of the green tape and ceramic substrate. As the particle size decreases from 1 μm to 100 nm, the grain arrangement of the ceramic substrate gradually becomes more compact. The smooth ceramic substrate prepared with 100 nm modified powders achieves a relative density of 98.85 % when sintered at 1550 °C, while the average roughness is only 20.0 nm. Especially, the 99.6 % alumina ceramic substrate exhibits excellent physical and mechanical properties, with a thermal conductivity of 37.39 W/(m·K) and fracture toughness of 4.68 MPa m1/2, making it highly suitable for application in the power integrated-circuit field.

Effect of Porosity on High Temperature Compressive Behavior of Textured Ti<sub>3</sub>SiC<sub>2</sub> Bodies Prepared by Pressureless Sintering

Effect of Porosity on High Temperature Compressive Behavior of Textured Ti<sub>3</sub>SiC<sub>2</sub> Bodies Prepared by Pressureless Sintering

Effect of ball milling on densification and alloying in SiCp/Al powder metallurgy processes

In this study, unprocessed raw Al, Cu, Mg powders, and SiC particles were mixed and subjected to high-energy ball milling (HEBM) to mitigate the negative effects of the oxide film on the quality of the pressureless sintering billets. Comparative experiments including pores and intermetallics examination were performed on both the mixed and HEBM powders across various preparation stages, including cold isostatic pressing, sintering, and hot extrusion of the 15vol.%SiC/Al-Cu-Mg. The pore structure and degree of alloying were analyzed using X-ray synchrotron radiation tomography. The results indicate that the brief HEBM process led to excellent mechanical properties in aluminum matrix composites after pressureless sintering and hot extrusion. The composites using ball-milled powder was relatively denser and complete alloying after sintering and extrusion, resulting in a density of up to 0.995 and a uniform distribution of Cu elements. The HEBM SiCp/Al had a tensile strength of 573 MPa and a fracture strain of 5.1 % which were higher than that of the mixed SiCp/Al. The results indicate that diffusion of alloy elements such as Cu was hindered by the interfaces between powders, led to Cu segregation and lower density of only 0.982 even after extrusion. Therefore, the composites using mixed powder had a lower tensile strength of 483 MPa and a fracture strain of 3.5 %. The study concluded that reducing the number and types of inter-particle interfaces could enhance the alloying and pore bridging processes.

Compatibility of the mechanical properties and thermal conductivity by the formation of a secondary phase in AlN ceramics: application of multifractal analysis for the microstructure evaluation

Aluminum nitride (AlN) ceramics exhibiting different secondary phase morphologies were fabricated using pressureless sintering at varying temperatures ranging from 1700-1850 °C, with a 5 mass% Y2O3 sintering additive. Although the thermal conductivity of the AlN ceramics increased as the sintering temperature increased, the bending strength increased below 1800 °C and decreased above 1800 °C. The morphology of the secondary phase in the AlN ceramics demonstrated granular, reticulate, and linear forms in that order of the low sintering temperature. The value of generalized dimensions Dq obtained by multifractal analysis increased below 1800°C and decreased above 1800°C. Higher values of Dq indicated more complex morphology and higher dispersion, corresponding to the observed morphological changes in secondary phase and the increase or decrease in the value of Dq. The sample sintered at 1800 °C had the highest Dq value and the reticulate secondary phase, with a thermal conductivity of 160 W m-1 K-1 and a maximum bending strength of 324 MPa. These results suggest that the morphology of the secondary phase affects the bending strength of AlN ceramics and that the value of Dq may be an indicator of the microstructure that achieves both thermal conductivity and mechanical properties.

Impact of Nb content on the morphology and properties of Ti (C0.5N0.5)-FeCrMo-based green cermets

A series of TiCN-FeCrMo-xNb (x= 2,4,6,8 wt.%) was prepared through pressureless vacuum sintering at a sintering temperature of 1500°C. This study aims to fabricate TiCN-high chromium Fe-based cermets and investigate the effect of incorporating strong carbide-forming elements such as Nb on the microstructure, phase, and mechanical properties. The microstructure and composition of the sintered cermets were investigated by Scanning electron microscope (SEM) in combination with an energy dispersive spectrometer (EDS) respectively, and detailed investigation of phase constitution was carried out using thermodynamic calculations and X-ray diffraction (XRD) measurements. Microstructure tends to become finer with increasing Nb additions (up to 4 wt.%); however, when Nb additions increase to 6-8 wt.%, ceramic particles become much bigger with a bimodal distribution of Ti(C,N) particle size. Microstructure study reveals that the addition of Nb inhibits the grain growth by diffusion of Nb in the Ti(C,N) during solid-state sintering and leads to refining the microstructure. Based on these findings, optimal Nb addition (up to 4 wt.%) in the Ti(C,N)-FeCrMo -based cermets plays the role of an inhibitor in the dissolution-reprecipitation process, inhibiting the precipitation of brittle (Ti, M) C, N rim phases. The mechanical properties of the Ti(C,N)-FeCrMo -based cermet increased with 2-4 wt.% Nb addition and decreased sharply with further increasing the Nb content. The maximum hardness and fracture toughness have been achieved for the cermet with 4 wt.% Nb with 1322 ± 61 HV30 and 8.56 ± 0.17 MPa.m1/2 respectively.

A combined stereolithography and a pressureless sintering method to prepare SiC ceramics

The use of submicron silicon carbide powder as a raw material is crucial for preparing high-performance silicon carbide ceramics. However, its strong absorption of ultraviolet light makes stereolithography of submicron silicon carbide powder challenging. In this study, submicron SiC particles, photosensitive resin, and photoinitiator were used as raw materials to prepare SiC ceramics via stereolithography combined with a pressureless sintering method. The effects of ultraviolet wavelength, type, and content of photoinitiator on the curing properties of submicron SiC ceramic slurry were investigated. The results showed that the curing thickness of the SiC ceramic slurry significantly increased with the increasing ultraviolet wavelength. Compared to photoinitiators TPO and 396, photoinitiator 819 exhibited a better curing effect. Additionally, increasing the photoinitiator content facilitated the generation of more free radicals in the SiC ceramic slurry at low light intensity, thereby improving its ultraviolet absorption characteristics. When the dosage of photoinitiator 819 was 8.8 %, the SiC slurry achieved the best curing performance, with a single-layer curing thickness of 50 μm and a sintering density of 95 % for SiC ceramics.

Composition engineering of high-entropy rare-earth monosilicates enables remarkable CMAS corrosion resistance

Exploring superior calcium-magnesium-aluminosilicate (CMAS) corrosion resistance is crucial for high-entropy rare-earth monosilicates (HEREMs) as the next-generation environmental barrier coating (EBC) materials. However, related studies are rarely reported. This work presents the exploration of HEREMs with remarkable CMAS corrosion resistance by engineering their compositions. The equimolar 3-to-9 cation high-entropy rare-earth monosilicate (3-9HEREM) specimens were initially prepared using a pressure-less sintering technique; subsequently, their resistance to CMAS corrosion was evaluated at temperatures up to 1,600 °C. The results demonstrate that the 5HEREM specimens possess the best CMAS corrosion resistance among all the as-fabricated specimens, surpassing other reported EBC materials. Such remarkable CMAS corrosion resistance results from the generation of a dense apatite protective layer originating from its low dissolution rate at elevated temperatures.

The role of magnesium oxide addition in densification of AlON transparent ceramics by pressureless sintering

AbstractPressureless sintering processing was adopted to fabricate transparent aluminum oxynitride (AlON) ceramics by using magnesium oxide (MgO) as a sintering additive into synthetic single phased aluminum oxynitride powder. Under the condition of adding 0.6 wt% magnesium oxide, the relative density of aluminum oxynitride ceramics increased from 98.67% with no addition to 99.9% after being held at 1800°C for 24 h, coupled by a reducing of average grain size from 190 µm to 130 µm. The optical linear transmittance of the AlON ceramic sample (thickness 1.2 mm) reached 84.2% at the wavelength of 1100 nm. The microstructural characterizations by energy dispersive spectroscopy and atom probe tomography indicated that the doping Mg element was uniformly dispersed inside aluminum oxynitride grains of as‐sintered ceramics. It was demonstrated that magnesium oxide played a key role in inhibiting the movement of grain boundaries, eliminating pores, leading to the fully densification of aluminum oxynitride ceramics.

Highly Robust, Pressureless Silver Sinter-Bonding Technology Using PMMA Combustion for Power Semiconductor Applications.

This study presents the development of a highly robust, pressureless, and void-free silver sinter-bonding technology for power semiconductor packaging. A bimodal silver paste containing silver nanoparticles and sub-micron particles was used, with polymethyl methacrylate (PMMA) as an additive to provide additional thermal energy during sintering. This enabled rapid sintering and the formation of a dense, void-free bonding joint. The effects of sintering temperature and PMMA content on shear strength and microstructure were systematically investigated. The results showed that the shear strength increased with rising sintering temperatures, achieving a maximum of 41 MPa at 300 °C, with minimal void formation due to enhanced particle necking facilitated by PMMA combustion. However, at 350 °C, the shear strength decreased to 35 MPa due to cracks and voids at the copper substrate-copper oxide interface caused by thermal expansion mismatch. The optimal PMMA content was found to be 5 wt.%, balancing sufficient thermal energy and void reduction. This pressureless sintering technology demonstrates significant potential for high-reliability applications in power semiconductor modules operating under high-temperature and high-stress conditions.

In situ construction of one-dimensional structures induced by Ba doping in Al2O3-TiO2 ceramics derived from quenched powders

In this work, we propose a strategy for in situ construction of rod-like grains in Al2O3-TiO2 ceramics through Ba doping. Ba-doped Al2O3-13 wt% TiO2 composite powder was initially synthesized using combustion synthesis-air atomization, where feedstocks were melted by the Al-O2 combustion reaction and subsequently atomized and rapidly cooled in air, producing quenched powders rich in metastable phases. Upon heat treatment at 1100℃, the metastable phases transformed into stable forms, accompanied by phase separation and solid solution precipitation. Notably, Ba doping induced the formation of rod-like hollandite grains with preferential growth along the [001] direction. Bulk ceramics were prepared through pressureless sintering, and the sample sintered at 1350℃ exhibited a hardness of 14.14±0.61 GPa and a fracture toughness of 5.17±0.49 MPa∙m1/2. The toughening behavior of rod-like grains, including crack deflection and pull-out/bridging effects, was observed. This study provides insights into the evolution of quenched oxides and the design of ceramic microstructures.

Mn2+ doped aluminum-rich spinel transparent ceramic phosphor with higher performance for wide color-gamut backlight display

Due to the great potential of narrow-band green-emitting phosphors for wide color-gamut white light-emitting diodes (WLED) backlight displays, there is great interest in finding host materials with the desired luminescent properties and high quantum efficiency. Herein, a series of Mg0.98-xAl2(1+x/3)O4: 0.02Mn2+ (x = 0, 0.125, 0.25) powders were synthesized using high-temperature solid-phase reactions, followed by a combination of pressureless sintering and hot isostatic pressure sintering to fabricate transparent ceramic phosphors. The crystal structure refinement, NMR spectroscopy, and photoluminescence results show tetrahedral coordination of Mn2+ ions and an increase in cation vacancies and disorder with increasing x. These transparent ceramics exhibit narrow-band green emission, which shifts from 523 to 520 nm with increasing x. The Mg0.855Al2.08O4: 0.02Mn2+ ceramic sample has an in-line transmittance of over 80 % above 500 nm and internal quantum efficiencies as high as 96.2 %. The luminous intensity of the Mg0.855Al2.08O4: 0.02Mn2+ ceramic remains above 88.6 % at 425 K compared to room temperature, with a thermal conductivity of 12.4 W·m−1·K−1. A WLED device fabricated using the Mg0.855Al2.08O4: 0.02Mn2+ ceramic, K2SiF6:Mn4+ phosphors, and a blue chip show a wide color gamut of 126 % National Television System Committee standard. The results show that Mn2+-doped aluminum-rich spinel transparent ceramics have great potential for application in wide color gamut WLED backlight devices.

Digital Light Processing Followed by Pressureless Sintering of Metal-Reinforced Ceramics: Adjustment of Process Parameters and Correlation with Composites Properties

AbstractDigital light processing (DLP) belongs to additive manufacturing techniques and is frequently used in shaping ceramics. The paper concerns the adjustment of the DLP method to metal-reinforced ceramics, especially dispersions containing high concentrations of Al2O3 (45 vol%) and molybdenum or nickel particles (0.5 vol%). Different glycol acrylates, deflocculants (polyelectrolytes and diammonium hydrogen citrate), and photoinitiators (Omnirad group) were examined regarding their influence on the rheological properties of the dispersions and the cure depth under the external halide UV lamp and LED projector built into the 3D printer. In the examined systems, the cationic polyelectrolyte KD1 dissolved in 2-butanone allowed to obtain dispersions of the lowest viscosity. Printing parameters (light exposure time, single layer height) were matched, and the properties of the materials were examined. The Vickers hardness of the sintered bodies equalled 19.4 GPa, 14.5 GPa and 17.3 GPa for Al2O3, Al2O3-Ni and Al2O3-Mo samples, respectively. The microstructure was analyzed using SEM, followed by EDS and XRD. The addition of only 0.5 vol% of Ni has improved the fracture toughness of alumina by up to 36–40% (according to Niihara and Anstis equations). The exemplary objects in the form of cog wheels were printed and densified at 1550 °C in a reductive atmosphere of Ar/H2.

Electrodynamic Properties of AlN–C and AlN–C–Mo Composites Produced by Pressureless Sintering

Electrodynamic Properties of AlN–C and AlN–C–Mo Composites Produced by Pressureless Sintering

A rapid pressureless sintering strategy for LLZTO ceramic solid electrolyte sheets prepared by tape casting

Garnet-type electrolytes are regarded as one of the most promising solid-state electrolytes (SSEs) for lithium-ion batteries due to their potential advantages in terms of energy density, electrochemical stability and safety. To achieve the maximum energy density, it is necessary to ensure that the electrolyte layer is as thin as possible. Nevertheless, thin sheet SSE is more challenging to sinter than pellet due to the greater lithium volatilization from the high surface/volume ratio. Garnet-type SSE (Li6.5La3Zr1.5Ta0.5O12, LLZTO) green tape was prepared by the tape-casting technique. The effects of supporter, sintering temperature and dwell time on the relative density, microstructure and ionic conductivity of thin sheet were investigated. A ceramic SSE sheet with a thickness of 173 μm, a relative density of 97.2 %, an ionic conductivity of 2.02 × 10−4 S/cm at 25 °C and an activation energy of 0.25 eV, was achieved using a rapid pressureless sintering at 1250 °C for 25 min with a MgO supporter. This work offers insights into the practical production of LLZTO sheets.

Powder Metallurgical Processing of Al–5 wt% Cu Matrix Composites Reinforced with MoSi2 and WSi2 Particulates

Herein, investigations on the microstructural, physical, and mechanical properties of molybdenum disilicide (MoSi2)‐ and tungsten disilicide (WSi2)‐reinforced aluminum (Al)–copper (Cu) matrix composites are reported. Powder metallurgy methods such as mechanochemical synthesis (MCS), mechanical alloying (MA), cold pressing, and pressureless sintering are combined to produce composites. First of all, MoSi2 and WSi2 nanoparticles are synthesized by MCS and selective acid leaching, yielding reinforcement materials for Al–Cu matrix. Powder blends consisting of 95 wt% Al and 5 wt% Cu are mixed with metal disilicides at different weight percentages (1, 2, and 5 wt%). MA for 4 h is conducted on these overall blends using a high‐energy ball mill. Microstructural and thermal properties of the as‐blended and mechanically alloyed powders are determined, and then they are compacted under 450 MPa and sintered at 550 °C for 2 h. Mechanical characterization of the composites reveals an increase in hardness and wear resistance with an increasing amount of reinforcement content. Among bulk samples, 5 wt% WSi2‐reinforced composites have the highest microhardness (165 ± 15 HV) and lowest wear rate (1.69 × 106 μm3 Nm−1) values. However, under the compression forces, the highest toughness and strength are obtained from 2 wt%‐reinforced composites.