Pressureless Sintering Research Articles

A Rapid Thermal‐Radiation‐Assisted Sintering Strategy for Nd–Fe–B‐Type Magnets

The green transition has spiked demand for high‐performance sintered Nd–Fe–B permanent magnets, necessitating advanced powder consolidation technologies to enhance production efficiency. This study explores the rapid sintering methodology for an Nd–Fe–B powder using a radiation‐assisted sintering approach. The case study material is an industrially used powder, prepared through strip‐casting, hydrogen decrepitation, and jet milling, with a mean particle size of 5.5 μm. The powder is sintered to full density in a modified spark plasma sintering furnace, achieving pressureless conditions and eliminating electrical currents in the sample to preserve grain alignment and prevent decomposition of the hard‐magnetic phase. Fully dense samples are obtained with heating rates ranging from 10 to 200 °C min−1 and up to 5 min of dwell time at 1100 °C. Rapid heating results in grain size and microstructure comparable to conventionally sintered magnets prepared from the same powder, without compromising magnetic performance after postsinter annealing at 520 °C for 120 min. This sintering method contributes to a novel strategy for optimizing magnet production by utilizing efficient thermal‐radiation heat transfer. The combination of rapid heating and pressureless sintering drastically reduces heat‐up and dwell times, providing a fundamental advantage over slow conventional sintering.

Rapid manufacturing of copper-graphene composite EDM electrode for improved machining performance

ABSTRACT Recently, graphene has emerged as reinforcing material for copper-based electric discharge machine (EDM) tools because of its superior electrical, thermal and high melting properties. 3D printing methods to fabricate electrodes are convenient and faster compared to conventional methods. Therefore, this study investigated the fabrication of copper-graphene composite electrodes for EDM using rapid tooling and pressureless microwave sintering. The influence of graphene content on the machining performance was examined, revealing enhanced electrical conductivity and machining efficiency. Electrical conductivity of the composite electrodes having 0.25 and 0.5 wt% of graphene was higher than pure copper. Additionally, upto 89% improvement in material removal rate and upto 14% reduction in electrode wear rate was obtained. Optical study showed an improvement of upto 28% in surface finish of the workpiece machined using composite electrode having 0.25 wt% graphene. A complex shaped composite tool was successfully fabricated and machined cavity was obtained.

Improved mechanical properties of 3YSZ ceramics prepared by pressure-less sintering assisted by cold sintering under mild conditions

It is an effective method to prepare ZrO2 ceramics by cold sintering (CS) and the subsequent pressure-less sintering (PLS), while the compacting problems are serious during the CS process with high pressure and high temperature, such as delamination, cracking and sticking to steel mold. To solve these problems, polyacrylic acid (PAA) is utilized as the organic binder, with the aid of which the 3 mol% yttrium-doped zirconia (3YSZ) nanopowder can be well compacted by CS under mild conditions (200 MPa, 100 °C). Compared to the conventional dry-pressed compacts, the particle agglomerations are observed in cold-sintered compacts as well as the slightly increased relative density, which lead to a significant improvement of Vickers Hardness (Hv) by 78 %. Consequently, the CS-PLS ceramics sintered at 1400 °C also exhibit a higher relative density (99.76 %) and better mechanical properties (Hv = 14.8 GPa, fracture toughness KIC = 6.49 MPa m1/2, flexural strength σ = 380.64 MPa) than the PLS ones without cold sintering, indicating that the CS-PLS process with mild conditions for CS is promising for preparing the denser and stronger YSZ ceramics.

In-situ formed TiCx reinforced Ti composites derived from polyzirconocarbosilane precursor: Synthesis, characterization and mechanical properties

Discontinuously reinforced titanium (Ti) matrix composites (DRTMCs) strengthened by in-situ TiCx particles are synthesized via pyrolysis of polyzirconocarbosilane (PZCS) and pressureless sintering. The Ti matrix reacts with the pyrolysis product of PZCS to form TiCx particles, which effectively refine the α-Ti grains and create a clean, well-bonded semi-coherent interface. A notable orientation relationship is observed between the TiCx and α-Ti phases: (111)TiCx||(101¯1)Ti and [1¯10]TiCx||[12¯10]Ti. The Ti-2PZCS composite demonstrates a significant reduction in average grain size, from 101.5 μm in pure Ti to 39.49 μm, coupled with superior comprehensive mechanical properties: an ultimate tensile strength of 710 MPa, a yield strength of 588 MPa, and an elongation of 9.5 %. The enhanced strength of Ti/PZCS composites is mainly due to grain refinement, solid solution strengthening, and load transfer mechanisms. This study introduces a novel approach for fabricating high-performance Ti composites, highlighting the potential for advanced material applications.

Synthesis and evaluation of thermal and optical properites of Fe2AlB2 phase alloy for thermal control in space applications

Ternary laminated Fe2AlB2 alloy holds its significance in displaying high strength, damage tolerance, oxidation and radiation cracking resistance. It is synthesized through a pressureless sintering route from elemental powders. Synthesis of Fe2AlB2 is studied from two different molar ratios (2/1/2 and 2/1.5/2) and three different sintering temperatures (1100°C, 1150 °C and 1200 °C). Phase analysis is carried out through an x-ray diffractometer and scanning electron microscopy (SEM). In addition, thermal decomposition, thermal conductivity and absorbance spectra are examined. Fe2AlB2 begins to decompose at 1169 °C into borides and aluminides. Thermal conductivity and diffusivity of Fe2AlB2 decreases with increase in temperature. Lower solar reflectivity and high thermal emittance of Fe2AlB2 implies the thermal control and optical behavior that can be expected for extreme space environments.

Rapid Synthesis and Sintering of La2O2S and Its Physical, Optical, and Mechanical Properties

Rare-earth oxysulfides are a class of functional ceramic materials with excellent physico-chemical properties and rich functionality. In this study, La2O2S powders were prepared from La2S3 and La2O3 powders at 1000 °C by pressureless sintering. La2O2S compacts were synthesized from La2S3 and La2O3 powders at 800–1600 °C by spark plasma sintering. The influences of sintering temperature and time on the preparation of La2O2S were studied. XRD results indicated that La2O2S ceramics were synthesized successfully and that the lattice constants of La2O2S were close to the theoretical values. SEM showed that the microstructure of La2O2S compacts was homogeneous. The specific heat of La2O2S mainly came from lattice contribution, and its Debye temperature was 237 K. The UV–visible absorption spectra showed different absorption levels in the 240–300 nm range. Raman spectroscopy revealed distinct peaks at different temperatures, indicating changes in the covalence band. The relative density of La2O2S ceramics was 92% and lower than theoretical values. Hardness of the synthesized La2O2S was greater than that of Gd2O2S ceramics.

Microstructure and mechanical property of high-density 7075 Al alloy by compression molding of POM-based feedstock

Metal injection molding of aluminium alloy (MIM-Al) attracts attention, owing to the lightweight, corrosion resistance and good thermal conductivity. However, it is hard to fabricate high-quality MIM-Al due to the hard-to-sinter powder and poor mechanical properties. Here, we report a facile compression molding process for fabricating high-density 7075Al alloy parts using polyformaldehyde (POM)-based feedstock. Pressureless sintering with a high nitrogen flow rate was adopted to promote sintering densification process. The wetting behavior, rheological properties, and morphology of the feedstock were characterized, showcasing the shear-thinning behavior and suitable viscosity for POM-PP-SA binder. Through controlling the compact pressure, mold temperature and holding time, green gear part with good shape retention and dense microstructure was achieved. Influence of process factors and sintering temperature on the microstructure and mechanical properties of 7075Al alloy are investigated. Remarkably, the aluminum alloy components sintered at 610 °C exhibited excellent performance, with a relative density reaching 97.6 % and a tensile strength of 214.8 MPa. This achievement provides a foundation for the industrial application of complex-shaped aluminum alloy components through the compression molding process.

Graphene nanoplatelet reinforced Al-based composites prepared from recycled powders via mechanical alloying and pressureless sintering

This study reports on the powder metallurgy preparation and characterization of aluminum-graphene nanoplatelet (Al-GNP) composites synthesized using recycled Al powders. Recycled Al and GNP powders (0.1–1 wt%) were mechanically alloyed (MA'd) for 4 h, followed by cold pressing (at 450 MPa) and pressureless sintering at 590 °C for 2 h. Starting powders were analyzed using an optical emission spectrometer (OES) and a Raman spectrometer. Also, MA'd powders and sintered samples were characterized using an X-ray diffractometer (XRD), a scanning electron microscope/energy dispersive spectrometer (SEM/EDS), and a differential scanning calorimeter (DSC). Particle size analyses, pycnometer, and Archimedes' densities, Vickers microhardness, dry-sliding wear, and compression tests were also conducted. The Al4C3 formation was observed in the XRD patterns of sintered compositions. The highest and lowest relative densities were measured for the 1 wt% and 0.1 wt% GNP reinforced samples as 97 % and 92 %, respectively. The highest hardness value was obtained as approximately 1.31 GPa for 1 wt% GNP reinforced. With the addition of reinforcement GNP, the wear rate developed to approximately 0.00225 mm3/Nm. The compressive strength increased from nearly 70 MPa to 162 MPa.

Characterization of Compacted and Pressureless Sintered Parts for Molybdenum Oxide Powder according to Hydrogen Reduction Temperature

Characterization of Compacted and Pressureless Sintered Parts for Molybdenum Oxide Powder according to Hydrogen Reduction Temperature

Development of a highly loaded zirconium carbide paste for material extrusion additive manufacturing

A highly loaded, aqueous-based zirconium carbide (ZrC) paste was developed for ceramic on-demand extrusion (CODE). Commercial ZrC powder was ball milled to reduce the particle size to ∼1 μm. Paste composition was determined by rheological characterization and observation of printing behavior; the final paste consisted of 46–47 vol% ZrC, 43.5 vol% distilled water, 8.1 vol% dispersant, and 1.6 vol% binder. Rheological characterization of the paste showed a yield stress of ∼8 Pa and shear thinning behavior (ΔG′/Δσ* = −55). The paste was printed using a 610 μm nozzle; a test print showed shape fidelity using the developed paste. A pressureless sintering study was performed; a sintering temperature of 2000°C with a 2-hour hold produced samples of 90 % relative density with 3.8 μm grains, which was consistent with literature. The developed paste, in combination with the pressureless densification results, adds a new, ultra-high temperature material to ceramic extrusion additive manufacturing.

Residual Stress and Warping Analysis of the Nano-Silver Pressureless Sintering Process in SiC Power Device Packaging.

Chip bonding, an essential process in power semiconductor device packaging, commonly includes welding and nano-silver sintering. Currently, most of the research on chip bonding technology focuses on the thermal stress analysis of tin-lead solder and nano-silver pressure-assisted sintering, whereas research on the thermal stress analysis of the nano-silver pressureless sintering process is more limited. In this study, the pressureless sintering process of nano-silver was studied using finite element software, with nano-silver as an interconnect material. Using the control variable method, we analyzed the influences of sintering temperature, cooling rate, solder paste thickness, and solder paste area on the residual stress and warping deformation of power devices. In addition, orthogonal experiments were designed to optimize the parameters and determine the optimal combination of the process parameters. The results showed that the maximum residual stress of the module appeared on the connection surface between the power chip and the nano-silver solder paste layer. The module warping deformation was convex warping. The residual stress of the solder layer increased with the increase in sintering temperature and cooling rate. It decreased with the increase in coating thickness. With the increase in the coating area, it showed a wave change. Each parameter influenced the stress of the solder layer in this descending order: sintering temperature, cooling rate, solder paste area, and solder paste thickness. The residual stress of the nano-silver layer was 24.83 MPa under the optimal combination of the process parameters and was reduced by 29.38% compared with the original value of 35.162 MPa.

High depolarization temperature and low dielectric loss in high-temperature fine-grain composite piezoceramics

High depolarization temperature (Td) and low dielectric loss (tanδ) were obtained in Sc2O3/BiScO3-PbTiO3-Bi(Zn1/2Hf1/2)O3 (Sc2O3/BS-PT-BZH) high-temperature fine-grained composite piezoceramics prepared by high-energy ball milling and one-step pressureless sintering. The heterogeneous interfacial polarization effect and thermal stress between the Sc2O3 semiconductor secondary phase and the BS-PT-BZH perovskite piezoelectric matrix phase contribute to the polarization orientation of the ferroelectric domain in the electric field and the maintenance of the poled ferroelectric domain state under temperature excitation, respectively. Benefiting from the above two heterogeneous interfacial effects, the Sc2O3/BS-PT-BZH fine-grained (∼0.4 μm) ceramic exhibits a high Td of 349 °C, a low tanδ of 5.1 % and a moderate piezoelectric constant d33 of 495 pC/N, which are comparable to those of many BS-PT-based coarse-grained (>2 μm) ceramics. The advent of Sc2O3/BS-PT-BZH fine-grained piezoceramic is helpful to promote the development of high-performance miniaturized high-temperature piezoelectric devices.

Influence of grain size and grain edge phase on the flexural strength of aluminum nitride ceramics with Y2O3 sintering additive

The necessity for efficient heat dissipation compels AlN packaging materials towards a thinner profile. In response to the ensuing concerns regarding the reliability of strength, the effects of the grain edge phases and grain size on flexural strength are investigated in this study. Different preparation processes, including (hot-pressed sintering, pressureless sintering, and annealing) are employed to prepare AlN ceramics with different grain edge phases and grain sizes. After classifying the morphology of the grain edge phases and fitting the Hall-Petch relationship, it is found that the grain edge phases had a more significant influence on the flexural strength. Depending on the morphology, the negative effects can be divided into three degrees. While the finer grain size proves advantageous for the flexural strength. Consequently, in the non-additive system, the flexural strength of AlN ceramic is 422 MPa, with an average grain size of 8.47 μm. For the additive-doped system, the maximum flexural strength can reach 532 MPa, which can be attributed to the limited average grain size resulting from the hot-pressed sintering process, approximately 1.32 μm. These results could potentially be employed in the microstructural design of high-strength AlN ceramics.

The Oxidation of ZrB2/MoSi2 Ceramics in Dissociated Air: The Influence of the Elaboration Technique.

In order to investigate the most extreme conditions in which materials potentially applicable in reusable thermal shields can be operated, ultra-high-temperature ZrB2 ceramics with 20 vol.% MoSi2 were prepared using two different techniques, cold isostatic pressing (CIP) and robocasting (RC, an additive manufacturing technique), followed by consolidation using pressureless spark plasma sintering (SPS). The oxidation behavior of the resulting materials was analyzed in low-pressure dissociated air at three different temperatures, namely 1800, 2000 and 2200 K. Using XRD and surface and cross-section SEM (coupled with EDS), zirconia was found to form at all three temperatures, while silica was only present at 1800 K, with gaseous SiO forming at a higher temperature. The elaboration technique influences the density of the ceramic, and less dense materials undergo deeper oxidation. This investigation suggests that 2000 K is already beyond the maximum temperature threshold at which damage to ceramics is limited by the formation of protective silica. This study confirms that the selected material is a promising candidate for thermal protection applications.

Microstructure evolution of diboride and carbide phases in platelets-toughened (Zr, Ti)C-(Ti, Zr)B2 ceramics via reaction pressureless sintering

Fully dense (Zr, Ti)C-(Ti, Zr)B2 composites were sintered via reactive pressureless sintering of ZrB2 and TiC based on the in-situ reaction and solid solution coupling effects. The effects of TiC content and sintering temperature on microstructure evolution and mechanical properties were explored. Elongated (Ti, Zr)B2 platelets were observed, stemming from the in-situ reaction between ZrB2 and TiC, and exhibited a high aspect ratio with the increase of TiC content. Besides, phase decomposition structures featuring semi-coherent interfaces were identified in the ZrB2-70mol.% TiC system sintered at 2200 °C, attributed to the chemical miscibility gap. High values of hardness (27.0 GPa), flexural strength (533 MPa), and fracture toughness (4.6 MPa m1/2) are found in ZrB2-70 mol.% TiC composite sintered at 2200 °C. The low porosity, solid solution strengthening, grain refinement caused by carbide phase decomposition, and high dislocation densities and strain concentration at the semi-coherent interfaces of carbide phase decomposed structures play an important role on hardening and strengthening. Furthermore, the presence of in-situ synthesized (Ti, Zr)B2 platelets with a high aspect ratio significantly contributes to toughening.

Synergistic effect of YAG particulate reinforcement on microstructural and thermo-mechanical properties of pressureless sintered MgAl2O4 spinel ceramic composites

Yttrium aluminum garnet (YAG) powders, synthesized through a partial wet chemical process, exhibit controlled morphology, high phase purity, and uniform homogeneity etc. This process involves the precipitation of aluminum ions onto Y2O3 particles. The transformation process of the Al-compound/Y2O3 precursor structure leads to the direct crystallization of pure YAG at 900 °C, passing through an intermediate stage where hexagonal YAH phase forms. A comprehensive study was conducted on the impact of preformed YAG as a reinforcing phase on the densification, mechanical, and thermo-mechanical properties of MgAl2O4-YAG composite, sintered under pressureless sintering in temperature range of 1500 °C–1600 °C. The inclusion of YAG particulate up to 5.0 wt% significantly enhances the densification, hardness, fracture toughness, flexural strength and high-temperature mechanical behavior of magnesium aluminate spinel (MAS) ceramics, however, increasing the YAG content beyond 5.0 wt% causes a reduction in these properties. The optimized addition of YAG particulate contributes to reducing the average grain size and refining the microstructure of the spinel ceramic matrix.

Phase evolution in reaction sintered Th-Monazite ceramic waste forms by SPS and PLS

Monazite ceramic is one of the potential waste forms to incorporate actinides in planned nuclear waste repositories. Inspired by recent progress on the low-temperature sintering and enriching radionuclides by wet-chemistry, the sintering processes of monazite ceramic with/without pre-sintering are investigated by spark plasma sintering and pressureless sintering using La0.8Sr0.1Th0.1PO4·nH2O rhabdophane as raw materials. Some specific issues such as the behavior of dissociation, structural evolution, and physicochemical properties affected by SPS and PLS are discussed in detail. The results reveal that a low-temperature appears to be a prevailing factor for SPS under 40 MPa with a suggested 1050 °C for 15 min process to prepare La0.8Sr0.1Th0.1PO4 monazite ceramic without forming Th(PO3)4 phase, and the relative density and Vickers hardness of ceramic are more than 98 % and 600 (HV10). The dissolution experiment based on the ASTM-1220 method indicates that La0.8Sr0.1Th0.1PO4 monazite ceramics is a typical example of an incongruent reaction, associated with Sr2+ is more likely to be dissolved out form monazite lattice compared with Th4+ and La3+.

Few-layered graphene/Al-Cu alloy matrix composites: Mechanical, tribological and corrosion properties

Nano-sized graphene incorporated Al-5.5 wt% Cu alloy matrix composites were produced via powder metallurgy. Nano-sized graphene powders obtained via the arc-discharge method were incorporated into Al and Cu powders in varying amounts (0–5 wt%) by applying a high-energy ball milling (HEBM) process for different durations. The consolidated composites were prepared by the succeeding uni-axial pre-compaction and pressureless sintering stages. The as-blended and mechanically alloyed (MAed) powders and bulk composites were comparatively characterized in terms of their physical, thermal, microstructural, tribological, mechanical, and corrosion properties depending on the graphene amount and the duration of mechanical alloying (MA). After 4 h of MA, the starting as-blended powders presented as coarse and discrete particles gained a refined and homogenized microstructure with more equiaxed dimensions. Bulk FLG/Al-5.5Cu (FLG in amounts of 0, 0.5, 1, 2, and 5 wt%) composites are ever-increasing hardness values with rising graphene as 109, 101, 128, 238, and 263 HV, respectively. The compressive strength improved gradually to 495 MPa using graphene up to 1 wt%, though contrary to the high hardness values, a drastic decline was observed by 5 wt% graphene incorporation. Besides, the wear resistance of composites outperformed that of the Al-5.5Cu matrix by incorporating this amount of reinforcement together without a deterioration in the corrosion resistance.

A novel plasma-facing NdB6 particulate reinforced W1Ni matrix composite: Powder metallurgical fabrication, microstructural and mechanical characterization

Tungsten (W) is one of best candidate metal for plasma-facing materials (PFM), especially due to its high melting temperature and neutron absorption capability. However, converting W into bulk PFM is hard because of its high melting point. This problem can be solved by adding metallic sintering aids with low melting points. In this study, W matrix with 1 wt% Ni aid was reinforced by adding NdB6 particles (1, 5, and 10 wt%). It can be introduced as a novel potential PFM, thanks to its low volatility and high neutron absorbability. The ceramic and composite powders produced via mechanochemical synthesis and mechanical alloying were examined in terms of composition, particle size, crystallite size, and lattice strain. Samples sintered via pressureless sintering (PS) and spark plasma sintering (SPS) were microstructurally analyzed by using an X-ray diffractometer (XRD), a scanning electron microscope (SEM) attached with an energy dispersive spectroscope (EDS), and mechanically analyzed in terms of microhardness and wear behavior. Based on the results, W2B and WB phases emerged in the SPS'ed W1Ni-5NdB6 and PS'ed./SPS'ed W1Ni-10NdB6 composites. SPS'ed W1Ni-10NdB6 composite had the highest hardness value and the lowest specific wear rate. The SPS'ed W1Ni-5NdB6 composite showed fewer surface damages and higher irradiation resistance as compared with other samples after exposure of He+ irradiation.

Pressureless sintering of HfC-HfB2-SiC-HfSi2 ceramics and their ultra high-temperature ablation resistance

HfC-HfB2-SiC-HfSi2 nanocomposites were synthesized by pressureless sintering with varying quantities of the HfSi2 additive. The morphology and densification behavior of HfC-HfB2-SiC-HfSi2 were investigated, and the results indicated that the sintering aid generated silicides to fill the grain gap. Plasma ablation tests investigated the ablation properties at 2700 °C, and the HfC-HfB2-SiC-HfSi2 nanocomposites exhibited excellent ablation resistance. The incorporation of the HfSi2 additive (4–7 wt%) not only improves the densification of the ceramic composite but also significantly promotes the mass ablation rate from -0.41mg∙cm−2s−1 to -0.048mg∙cm−2s−1. This was mainly attributed to the forming of a dense oxidation layer consisting of the Hf-Si-O phase, which acted as an excellent barrier to prevent the diffusion of O into the ceramic matrix.