Search Results (113)

Hydroxyapatite (HAp) is a key bioceramic for biomedical applications, but conventional pressureless sintering (PS) requires high temperatures that can promote phase degradation. Here, we compare PS (1100 °C/180 min) and cold sintering process (CSP) (150 °C/450 MPa/30 min) for pure HAp and an HAp composite containing 4 wt.% niobium–phosphate bioglass (BG), using a 2 M H₃PO₄ transient liquid (10 wt.%). CSP increased relative density from 73.10% to 79.92% for HAp and from 68.43% to 83.54% for HAp/BG, representing up to a 22.1% gain compared with PS. One-way ANOVA confirmed a significant effect of processing route/composition on relative density (F(3,24) = 919.69, p < 0.05), and Tukey HSD indicated that all groups differed statistically. SEM revealed a markedly more consolidated and homogeneous microstructure for CSP, particularly for HAp/BG, consistent with enhanced dissolution–reprecipitation and pore filling. XRD showed that PS at 1100 °C led to partial HAp degradation with β-TCP formation, whereas CSP preserved the HAp phase with broader peaks, smaller crystallite size, and higher specific surface area. These results demonstrate CSP as an efficient low-temperature alternative for densifying HAp-based bioceramics, with BG addition further improving consolidation. Full article

The use of monolithic alumina is limited by its intrinsic brittleness, which is commonly addressed through second-phase reinforcement. Silicon carbide (SiC) is an attractive reinforcement due to its high-temperature stability; however, its oxidation behavior strongly influences composite processing and properties. In this study, alumina/SiC composites containing 1, 5, and 10 wt.% SiC were prepared by conventional powder mixing, calcined at 800 °C for 1 h, and pressureless sintered at 1400 °C in air. Phase evolution, microstructure, densification, and mechanical properties were investigated using XRD, SEM/EDS, density–porosity measurements, and flexural testing. Air sintering led to SiC oxidation and the formation of silica-rich glassy phase and mullite, which significantly affected densification. The composite containing 1 wt.% SiC exhibited the best performance, with a flexural strength of 248.7 MPa, a Weibull modulus of 5.7, an average grain size of 1.86 µm, and a porosity of 11.08%. Higher SiC contents resulted in excessive porosity and severe degradation of mechanical properties. Full article

Compared to pure aluminum powder, aluminum alloy powders exhibit higher strength and hardness, which leads to greater difficulty in plastic deformation during cold compaction, consequently impairing green compact formation and subsequent densification. This study introduces pure aluminum powder into the raw material system based on 2024 aluminum alloy powder and SiC powder, effectively improving the powder compaction characteristics. A systematic investigation was conducted to examine the effects of sintering temperature (460–640 °C) and holding time (5–120 min) during pressureless sintering on the sintering shrinkage, relative density, mechanical properties, and microstructure of SiC_p/Al composites reinforced with 35 wt.% of 31.9 μm SiC particles. The results indicate that sintering at 600 °C for 30 min constitutes the optimal process condition, achieving effective interparticle bonding while preventing coarsening of both precipitates and pores. Subsequent hot pressing effectively enhanced the relative density and mechanical properties of the sintered preforms, achieving a maximum tensile strength of 343 MPa, which represents an improvement of over 70% compared to the sintered-only state. For the hot-pressed state, elevated levels of SiC particle content and size compromised its mechanical performance. This work demonstrates a highly operable and industrially viable processing route for manufacturing aluminum matrix composites using alloy powders. Full article

Reinforced alumina ceramics are renowned for their high hardness and strength among common oxide ceramics. However, high-temperature or high-pressure treatment is necessary for maximizing values of strength and hardness. In this paper, liquid-phase-assisted pressureless sintering of alumina reinforced with zirconia was studied. Sintering of dense ceramic bodies in relatively low temperatures (up to 1100 °C) was possible with the usage of CuO-TiO₂-Nb₂O₅-based additive, together with an intense milling process. By using the XRD method, the formation of dominant α-Al₂O₃ and m-ZrO₂ phases with small concentrations of secondary ones in experimental samples was confirmed. SEM studies showed that uniform distribution of components in the composite was achieved in samples sintered from intensively milled powders. The significant increase in the values of Vickers hardness and biaxial flexural strength (by 2.6 times) in samples from intensively milled powders at a sintering temperature of 1050 °C was explained by reduced porosity, improved grain distribution, and the formation of the t-ZrO₂ phase in the alumina-reinforced composite. The study clearly showed high potential of the proposed low-temperature sintering method for zirconia-toughened aluminum oxide, which can be used in manufacturing of advanced ceramics. Full article

Zirconia, also known as zirconium dioxide ZrO₂, is well known for its good mechanical properties, like its inertness, good wear resistance, high temperature resistance and good strength. To enhance the mechanical properties of many materials, a technique known as transformation toughening is widely used today. This research focuses on achieving an optimized composition of zirconia and alumina Al₂O₃ to achieve zirconia-toughened alumina ZTA with a maximum density and other mechanical properties using a cost-effective and time-efficient approach. Doing so will make it possible to make more and more use of this valuable ceramic. The curing of zirconia and alumina samples with 3d—printing resins in silicone dies was performed so that we could obtain the optimum ratio of the resin and ZTA powder that would produce the most desirable results and properties. For 3d printing, ZTA samples with 19% zirconia ZrO₂ were used with alumina at two different temperatures (i.e., Sample 1, consisting of three pellets weighing 5–6 g, was sintered at 1500 °C, and Sample 2, also containing three pellets weighing 5 g (approx.), was sintered at 1600 °C). The green-state preparation of these samples (Sample 1 and Sample 2) was performed using milling media of WC balls/ethanol and a milling ratio of 1:3, and a milling time of 4 h 100 rpm was used while drying at 80 °C for 5.5 h. The relative density (70%) and Vickers hardness (14–17 GPa) were obtained for Al₂O₃/ZrO₂/MgO samples. Mechanical properties like hardness and strength strongly depend on the holding time, the rate of the temperature increase while sintering and the sintering temperature itself. Full article

Complex concentrated alloys (CCAs) have attracted significant attention due to their potential to develop materials with enhanced properties, such as increased hardness and strength. These properties are strongly influenced by the chemical composition and the processing method used. Body-centred cubic (BCC) structures are known to have high hardness but low fracture toughness, whereas face-centred cubic (FCC) structures typically exhibit lower hardness but higher toughness. In this study, Al-Cr-Cu-Fe-Mn-Ni CCAs with three distinct compositions were produced using pressureless sintering. One set of samples was prepared with equiatomic composition (composition E), whereas the compositions of the other two sets were defined based on thermodynamic calculations to obtain sintered samples predominantly formed by BCC (composition B) or FCC (composition F) phases. The samples were characterized using X-ray diffraction, scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction, density measurements, hardness measurements, and uniaxial compression tests. For all compositions, good agreement was obtained between the phases predicted by thermodynamic calculations and those experimentally detected. In addition, significant differences in the mechanical properties were observed between samples with each composition. The samples with composition B exhibited the highest hardness, but almost no ductility. In contrast, samples with composition F showed the lowest yield strength and hardness, but the highest ductility. Samples with composition E had intermediate values between those of samples B and F. These differences were attributed to differences in the proportions and properties of the BCC and FCC phases in each composition and demonstrate that the mechanical properties of Al-Cr-Cu-Fe-Mn-Ni CCAs can be tailored using compositions defined based on thermodynamic calculations. Full article

In the quest to produce high-purity alumina, bottom-up engineering via architecting the interior of ceramic with bimodal structures of alumina powders in the absence of any additives has gained considerable attention owing to the simplicity offered. The present work investigated the influence of bimodal structures containing micron (~35 μm) and submicron (~600 nm) Al₂O₃ powders on the formation of dense Al₂O₃ ceramic. To this end, ball-milling was conducted to prepare the desired sizes of powders, followed by two-step sintering in a vacuum at 1450 °C and 1650 °C with 6 h and 4 h holding times, consecutively. The bimodal structures induced the formation of Al₂O₃ ceramic with nearly full densification (>99%; ρ 3.95 g/cm³). Both the coarse and fine-grained moieties synergistically balanced the densification kinetics whilst suppressing abnormal grain growth. The uniform and homogeneous grain size minimized the plasma porosity down to <6.0%, limiting the penetration of plasma during the etching process. Full article

Sintered silver, widely used in WBG electronic device packaging for its excellent electrothermal properties and high-temperature stability, faces challenges in macroscopic mechanical behavior and reliability due to porosity, especially for pressureless sintered silver. However, the intrinsic pores inside sintered material introduce uncertainties during nanoindentation tests for mechanical characterization. This study investigated the impact of pore distribution on the dislocation behavior of pressureless sintered silver during nanoindentation. Firstly, pressureless sintered silver models with 8–33% porosity were prepared and characterized through scanning electron microscope (SEM) for porosity, electron backscatter diffraction (EBSD) for the geometrically necessary dislocation (GND) density distribution, and transmission electron microscopy (TEM) for the crystal structure and microscopic strain. The EBSD results indicated that nanoindentation caused localized plastic deformation in sintered silver, closely related to its porous structure. The TEM results revealed that sintered silver undergoes dislocation slip during nanoindentation, leading to complex dislocation network formation, while the strain decreased with distance from the indentation. To further investigate the relationship of pore distribution and dislocation behavior during nanoindentation, molecular dynamics (MD) simulations were carried out. The MD results revealed that the dislocation distribution was consistent with the EBSD and TEM results. During loading, with the increased porosity from 10% to 23.7%, the total dislocation length was reduced by 63%, while it led to a 38% increase in total dislocation length with the average pore size decreased from 3.84 nm to 2.88 nm under similar porosity conditions. This study improves the understanding of the deformation mechanisms of porous sintered silver under nanoindentation and provides insight into the mechanical characterization of porous materials. Full article

Cylindrical targets have a high utilization rate, but are difficult to manufacture. A large hollow ITTO segment with thin walls was prepared by cold isostatic pressure and two-stage sintering. The fabrication process yielded a segment with an outer diameter of 153 mm, an inner diameter of 135 mm, and a length of 700 mm, indicating a length to thickness ratio of up to 78. The dense and uniform green bodies ensure the achievement of high density and uniformity of the sintered body throughout its volume. The segment exhibited a high relative density of about 99.5% and a low resistivity of below 3.4 × 10⁻⁴ Ω·cm. The density and resistivity illustrate a minimal inhomogeneity along the length of the segment. The segment exhibits a cubic bixbyite phase and is characterized by densely packed fine grains with an average size of several microns. Therefore, these results establish a substantial foundation for the large-scale production of cylindrical ITTO segments. Full article

In this study, the Gd₂Zr₂O₇-based ceramic target was densified via pressureless sintering which follows well with Kingery’s three-stage sintering theory. Sintering temperature is the key factor affecting the densification of targets. In the initial stage, when the sintering temperature is in the range of 1200–1450 °C, the porosity decreases with the density of targets slowly increasing to 64.71%. Grain boundary diffusion controls the densification process. In the middle stage, at 1450–1500 °C, the density ratio of the target rapidly rises to 77.6%. The competition between grain boundary migration rate and pore shrinkage rate leads to the maximum isolated porosity. In the final stage, when the sintering temperature is above 1500 °C, the density ratio of the target significantly increases to 97.28% at the temperature of 1600 °C. Even when the holding time is extended to 7 h at 1500 °C, the density ratio of the target only reaches 85.72%. With the increase in sintering temperature and prolongation of holding time, the fracture toughness of the ceramic targets exhibited a trend of initial increase followed by a decrease. Density ratio and grain size were identified as key factors influencing fracture toughness. When the density ratio reached approximately 80%, the fracture toughness achieved its maximum value of 2.245 MPa·m^0.5. When the sintering temperature exceeds 1450 °C, both the Young’s modulus and hardness of the targets increase rapidly, which significantly enhances their fracture toughness. However, with the increase in sintering temperature or holding time, the grain grows rapidly. This excessive grain growth reduces grain boundary. Full article

This paper investigates grain growth and densification kinetics in alumina ceramics subjected to spark plasma sintering (SPS) and conventional pressureless sintering (CS). The findings reveal that, under both sintering conditions, grain growth primarily occurs after reaching the sintering ‘freezing point’. The analysis of densification kinetics indicates that the activation energies of densification of alumina ceramics are 173.6 KJ/mol and 261.2 KJ/mol during the early stage, and 362.2 KJ/mol and 383.7 KJ/mol in the late stage for SPS and CS conditions, respectively. Therefore, a two-step sintering method (TSS) is proposed, where SPS rapidly sinters alumina powder to reach the grain growth ‘freezing point’, and then, the sintered bodies are subjected to CS to obtain dense alumina ceramics. The results show that the flexural strength of alumina prepared using this TSS can reach 489.6 MPa, about 19% improvement over those processed solely through CS. Full article

In this work, a multicomponent PZT-type material doped with manganese Mn, antimony Sb, samarium Sm, and tungsten W was fabricated using classical powder technology. Sintering of the ceramic samples was performed by the free sintering method (pressureless sintering). The influence of samarium on the properties of PZT was analyzed using a variable amount of samarium Sm³⁺ (from 0.8 to 1.2 wt.%) and tungsten W⁶⁺ (from 1.4 to 1.2 wt.%) admixture compared to the Pb(Zr_0.49Ti_0.51)_0.963Mn_0.021Sb_0.016O₃ + W⁶⁺1.8 wt.% reference composition. XRD studies have shown that PZT-type ceramic samples have a tetragonal structure with a point group of P4mm. Field emission scanning electron micrographs (FE-SEMs) showed fine and properly crystallized grains with an average grain size of 5.65–7.70 μm and clearly visible grain boundaries. The polarization–electric field (P-E) hysteresis measurement confirmed the ferroelectric nature of the ceramic materials with high P_m maximum polarization values (from 12.38 to 16.46 μC/cm²). Dielectric studies of PZT-type materials have revealed high permittivity values (from 1025 to 1365 at room temperature (RT) and from 18,468 to 25,390 at phase transition temperature T_m) with simultaneously low tanδ dielectric loss factor values (from 0.004 to 0.011 at RT) and low DC electrical conductivity, which are important parameters for microelectronic applications. The most homogeneous structure and the most favorable set of utility parameters are represented by the composition with an equal content of Sm and W admixtures, i.e., for 1.2 wt.%. Full article

Sinter-based additive manufacturing (AM) requires sintering for the densification of green bodies. Al powder is difficult to sinter due to the dense oxide film on the surface, and it is difficult to apply to sinter-based AM. Liquid phase sintering using Al-based eutectic alloy powder is promising for sintering Al powder without external pressure. In this study, Al powders with various oxide film thicknesses were sintered using Al-X eutectic alloy powders (X=Cu, Ca, and Mg) to clarify suitable alloy elements in the sintering aids for the liquid phase sintering. When an as-supplied Al powder with an oxide film thickness of approximately 2 nm (presumably amorphous Al₂O₃ film) was used, Al-Cu and Al-Ca aids promoted the densification, whereas numerous pores were observed in the sample sintered using Al-Mg aid. The pores would be formed during the cooling after sintering, along with the homogenization of Mg distribution. When Al powder with an oxide film thickness of around 4 nm was used, a high relative density of over 95% was maintained using Al-Cu aid, whereas the relative density of the sample sintered using Al-Ca aid significantly degraded, presumably due to the formation of Ca-based oxide. These results indicate that the Al-Cu eutectic alloy powder is a promising sintering aid for the liquid phase sintering of Al powder. Full article

This work investigates the fabrication of Ti6Al4V composites manufactured by powder metallurgy through pressureless infiltration. Porous Ti6Al4V alloy compacts with different particle sizes were fabricated by sintering and then, liquid Ag was infiltrated to obtain composites. Computed microtomography was used to analyze the samples before and after infiltration. Numerical flow simulations and dilatometry tests evaluated the kinetics of Ag infiltration into porous Ti6Al4V compacts. Microstructure was observed by SEM and mechanical strength was evaluated by compression tests. Results showed that the pore properties play a crucial role in the infiltration timing and the distribution of the Ag’s liquid. In particular, large pores allowed the infiltration to start a few °C degrees earlier than samples with smaller pores. Three-dimensional images after infiltration showed that most of the pores were filled and the remaining ones were isolated. The resulting microstructure was composed of Ti₂Ag, α-Ti and Ag phases, indicating that the Ag diffusion occurred. Furthermore, the mechanical strength depends on the interparticle neck sizes and the Ag improves the plastic deformation reached during compression tests. The best results were obtained for the samples with larger pore sizes because the resulting mechanical properties (E = 23 GPa and σ_y = 403 MPa) are close to that of human bones, making it the best candidate as an antibacterial material for biomedical use. Full article

The primary grain size, sintering conditions, and admixtures have a significant impact on the grain growth of alumina ceramics. Three kinds of alumina powders with varying grain sizes and zirconia nanoparticles were selected and configured into five compositions of zirconia-toughened alumina (ZTA) ceramics. As-received granules were used to sinter bulk ceramics at temperatures of 1520 °C, 1600 °C, and 1680 °C for durations of 0.5–2 h, respectively. The average grain sizes of alumina in ZTA ceramics were studied as a function of the sintering temperature, time, and particle size of raw materials. The results demonstrated that the addition of nano alumina led to a slight reduction in the grain size of alumina and a more uniform grain size distribution. The incorporation of nano zirconia (15 wt.%) resulted in the concentration of zirconia among the alumina grains, effectively inhibiting grain growth and resulting in a significant reduction in the average grain size of alumina from 11.73 to 5.63 μm after sintering at 1600 °C for 1 h. Full article