Pressureless Sintering Technique Research Articles

Composition-driven superior CMAS corrosion resistance of high-entropy rare-earth disilicates

Achieving superior calcium-magnesium-aluminosilicate (CMAS) corrosion resistance in high-entropy rare-earth disilicates (HEREDs) is crucial to their applications as environmental barrier coatings in aerospace. Stimulated by the vast compositional field of HEREDs, we achieve exceptional CMAS corrosion resistance in HEREDs at elevated temperatures via a composition-driven strategy for the first time. Specifically, the equimolar 3- to 8-cation high-entropy rare-earth disilicate (3–8HERED) samples are first successfully designed and fabricated using a high-throughput pressure-less sintering technique and then their CMAS corrosion resistance up to 1773 K is evaluated. Among all the as-fabricated samples, the as-fabricated 7HERED samples are found to exhibit the best CMAS corrosion resistance. The outstanding CMAS corrosion resistance of the as-fabricated 7HERED samples is primarily attributed to pronounced sluggish diffusion induced by configurational entropy, with the synergistic effect of their chemical activity, which facilitates the formation of dense apatite protective layers. Our work provides valuable ways to design remarkable anti-CMAS corrosion high-entropy ceramics.

Nanoindentation and isothermal compression behaviour of Al–4Cu–Ni composites

The present investigation explores the nanoindentation and isothermal compression behaviour of Al–4Cu-xNi (x = 2, 4, 6, 8 and 10 wt%) Aluminium matrix composites produced by pressureless sintering technique. The scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses of sintered samples reveal the presence of core-shell type Al3Ni and Al3Ni2 intermetallic formation at the interface between Ni particulates and the Al–Cu matrix. Nanoindentation study shows hardness of 1.55 ± 0.15, 4.65 ± 0.44, 21.02 ± 0.42, 13.17 ± 0.13, 5.53 ± 0.89, 6.28 ± 0.19, 8.39 ± 0.14 GPa corresponding to α-Al matrix, Ni, Al3Ni2, Ni and Al3Ni2 interface, Al3Ni, and their respective interface. The composite containing 10 wt % Ni possesses the highest yield strength at room temperature. At 200 °C it could retain 54 % of its room temperature strength (91 MPa). The HR-TEM analysis shows that particulate and precipitation hardening was the predominant mechanisms during the initial stage of an isothermal compression. However, as the compression progressed, the pinning effect, dynamic recovery (DRV), and work hardening mechanisms became active. According to the current study findings, interface qualities substantially impact composite performance at both room temperature and elevated temperatures. The creation of strong intermetallic compounds (Al3Ni2 and Al3Ni) due to an intensive endothermic interaction between the Al and Ni particles is linked to this effect.

Multiple relaxation behavior of Cf/mullite composites and their electromagnetic wave performance at high temperatures

Reliability and stability of electromagnetic wave absorption (EMA) materials used in high-temperature fields such as aircraft and aero-engines are of great interest. In this regard, the Cf/mullite composites with an opportune heterogeneous interface were fabricated using gel-casting and pressureless sintering techniques. The as-prepared composites exhibited excellent EMA performance with multiple relaxation behavior upon increasing testing temperature ranging from 25 °C to 800 °C (the minimum reflection loss (RLmin) of −62.67 dB with a thin thickness of 2 mm at 600 °C). The results demonstrated that the temperature-dependence complex permittivity with multiple relaxation characteristics plays a key role for the EMA performance evolution. The newly generated Cole-Cole semicircle and κ curves with an increasing number of intersections at zero values suggest that the polarization relaxation is enhanced with rising temperatures, thus improving the EMA capacity. It is believed that the Cf/mullite composite could be a new promising EMA material at high temperatures.

Potential plasma facing materials: A case study on CeB6 particulate reinforced W–1Ni matrix composites produced via different sintering techniques

Tungsten (W) is the most promising material for future plasma-facing materials. Its alloys or composites reinforced with various compounds have been studied to improve the irradiation resistance of W. This study aimed to investigate the effect of boron-containing reinforcement in the W matrix against He+ ion irradiation. Cerium hexaboride (CeB6) particulates, which have high neutron shielding properties, were incorporated into the W matrix pre-alloyed by 1 wt% Ni (W1Ni). CeB6 powders were home-made and prepared from CeO2/Mg/B2O3 powder blends via mechanochemical synthesis and purification steps. 1, 5, and 10 wt% CeB6 powders were added to pre-alloyed W1Ni by mechanical alloying, and then they were consolidated by using pressureless sintering (PS, 1400 °C, 1 h) and spark plasma sintering (SPS, 1410 °C, 1 min) techniques. CeB6 particle-reinforced W1Ni composites contained different amounts of reinforcements were prepared by two different sintering methods and were compared with respect to their compositional, microstructural, and microhardness properties and wear and irradiation behaviors. Based on the results, increasing the CeB6 reinforcement amount in the composite triggered the formation of the W2B phase, especially in the W1Ni–10CeB6 composite after both sintering methods. The mechanical and irradiation properties were enhanced more by increasing the CeB6 amount in the case of using the SPS method. When compared to other sintered samples, the SPS'ed W1Ni–10CeB6 composite has the lowest specific wear rate of ∼4 × 10−7 mm3/Nm and the maximum hardness value of ∼21 GPa. According to surface deformation, the W1Ni–5CeB6 composite exhibited comparatively higher resistance to He+ ion irradiation.

Efficient Sintering of Mo Matrix Composites—A Study of Temperature Dependences and the Use of the Sinter Additive Ni

Mo matrix composites (MMC) with Mo-9Si-8B inclusions were fabricated by pressure-less sintering (PLS) and spark plasma sintering (SPS) techniques at temperatures between 1200–1500 °C using 1 wt.% Ni sinter additive. The positive impact of the addition Ni addition on the sinterability and formation of a continuous Mo matrix of MMC with randomly distributed Mo3Si and Mo5SiB2 inclusions was determined. The Ni addition increased the shrinkage of MMC during PLS by almost a third. The continuous Mo matrix of MMC and a relative density of more than 98% was obtained after SPS at 1400–1500 °C. The composite with the maximum relative density of 98% showed a Vickers hardness of 482 ± 9 (HV20). The potential of using Ni-activated PLS and SPS to produce high-density MMC is shown.

EFFECTS OF BINARY OXIDE COATED MWCNTS TOWARD TAILORING THE MECHANICAL, THERMO-MECHANICAL, AND MICROSTRUCTURAL PROPERTIES OF PRESSURELESS SINTERED MgAl2O4 SPINEL CERAMIC COMPOSITE

This study presents a performance evaluation of MgAl-binary oxide-coated multi-walled carbon nanotubes (MWCNTs) as a reinforcement phase in a magnesium aluminate spinel MgAl2O4 ceramic composite system fabricated using a pressureless sintering technique. The physical properties, including porosity, bulk density, and flexural strength of spinel composites were evaluated in the temperature range of 1500 – 1600 ?C. The results reveal the coated MWCNTs architecture's promising influence on the composite system's property enhancement. The developed thin-film coating on the MWCNTs favors enhanced dispersibility with better matrix-reinforcement interfacial connectivity, thereby improving the mechanical and thermo-mechanical performance of the composites.

Tailoring layered Csf/SiBCN composites with pseudoplastic fracture behavior: Strengthening and toughening mechanisms

AbstractThe layering process using phenolic resin coupled with pressureless sintering technique was explored to prepare short carbon fiber‐reinforced SiBCN composites (Csf/SiBCN). The effect of phenolic resin content on the microstructure and mechanical properties of Csf/SiBCN was mainly studied. Results suggest that the addition of phenolic resin can appropriately improve density and flexural strength and reduce total pore volume of the composites because phenolic resin can partly consolidate SiBCN powders to promote shaping via low‐temperature pressureless sintering. As the phenolic resin content increases, the fracture behavior changes from delamination fracture to monolithic fracture, thereby reducing fracture displacement. The reason for this performance is that the interlayer bonding is enhanced to hinder crack propagation along the layers.

Synthesizinge a novel Zr2Al-GNS MAX phase ceramic with superior electrical properties using pressureless sintering technique.

A unique Zr2Al-GNS MAX phase ceramic supported nanographene sheet was prepared using a cost-effective pressureless sintering technique under relatively low temperature. An experimental investigation was conducted to explore the lattice parameters using different temperatures, such as 1000, 1150, and 1300 °C. To characterize the crystal structure of the MAX phase ceramic, X-ray diffraction, field emission scanning electron microscopy imaging, energy-dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HRTEM), and selected area diffraction (SAED) were utilized. The results revealed that the pressureless sintering technique was successfully utilized to synthesize the Zr2Al-GNS MAX phase ceramic under 1150 °C with a low impurity ratio of secondary phases such as Zr3AL2, Zr3AL5, and ZrC components. The high percentage of the Zr2Al-GNS MAX phase ceramic was obtained at 49.0% at 1150 °C compared with different temperatures. The BET surface area (SBET), pore volume, and pore size were also investigated. The SBET of the prepared Zr2Al-GNS MAX phase was increased to 30% using graphene nanosheet, while the porosity was highly decreased to 8% from its original value. The electrical properties were also studied in this research for potential applications, such as the absolute value of impedance (Z), absolute value of admittance (Y), induction (L), capacitance (C), resistance (R), conductance (G), susceptibility (B), and phase angle (ϴ). It was found that the capacitance and the phase angle were improved using the prepared Zr2Al-GNS MAX phase ceramic, depending on the frequencies. The results presented here may facilitate the improvements in the features of the MAX phase type of Zr2Al-GNS-enhanced one-layer nanographene sheet for electrical applications ceramic.

Thermal stability of ReB2‐type transition metal diborides

AbstractThe thermal stability of metastable ReB2‐type transition metal diborides (TMB2), which are considered as new type of superhard material, is of vital importance to obtain bulk samples. In the present work, thermal stability of four kinds of ReB2‐type TMB2 powders, ReB2, OsB2, Os1–xRexB2, and Os1‐xWxB2, were synthesized with varied transition metal (TM)‐to‐B molar ratio by mechanochemical methods and the subsequent annealing was compared. The as‐synthesized powders were then consolidated using a pressureless sintering technique. The results showed that the B content required to obtain the pure hexagonal ReB2‐type Os1–x(TM)xB2 phase varied, which indicated different thermal stabilities, such as OsB2 < Os0.1W0.1B2 < Os0.9Re0.1B2 < Os0.8W0.2B2 < ReB2 < Os0.6W0.4B2 and Os0.5W0.5B2. Among them, Os0.6W0.4B2 and Os0.5W0.5B2 were found to be relatively thermally stable and could be synthesized with a stoichiometric molar ratio of (Os + W):B = 1:2. It was also found that the thermal stability of TMB2 with a hexagonal ReB2 structure could be mainly governed by the length of lattice constant c. The results have guiding significance for the design and preparation of new type of TM borides. In addition, the hardness of TMB2 can be increased by tailoring the B content in the raw materials more precisely.

Densification kinetics and sintering behavior of UO2 and 0.5 wt.%MnO‐doped UO2

AbstractIn this work, the sintering kinetics of pure UO2 and 0.5 wt.%MnO‐doped UO2 was studied by a high‐temperature dilatometer heated up to 1500°C. In addition, the sintering behavior of pure UO2 and 0.5 wt.%MnO‐doped UO2 was studied by pressureless sintering technique. The results showed that MnO doping enhanced the grain boundary diffusion of UO2, which can effectively decrease the densification temperature and promote grain growth. The sintering temperature of UO2 was significantly reduced by about 200°C with the addition of 0.5 wt.%MnO. The microscopic morphology studies showed that there were still fine particles agglomerated, forming sintered spheres in the matrix even if no severe agglomeration and bimodal size distribution were observed in raw UO2 powder. The microstructure evolution of the sintered sphere and UO2 matrix during the densification process was studied by isothermal sintering. Finally, the present analyses indicated that the densification of UO2 matrix can be accelerated by adding MnO or increasing the sintering temperature, thus improving the densification inhomogeneity of UO2 matrix.

Sintering behaviour and properties of zirconia ceramics prepared by pressureless sintering

Improvements in the sintering process and powder quality can lead to wider application of zirconia in ceramics. In this study, the effects of different temperatures on the stability, relative content of the tetragonal phase, and composition of Al2O3–ZrO2 ceramic powders were explored using pressureless-assisted sintering. The crystallinity of the sintered Al2O3–ZrO2 samples was significantly improved. The content of the tetragonal-phase ZrO2 in sintered ceramic powders was 52.07%, 52.46%, 56.16%, 63.99%, and 64.90%, respectively, which was significantly higher than those of the raw materials. The average particle size of the sintered samples decreased from 1.07 μm to 0.17 μm with an increase in temperature, indicating that the ceramic powder particles were refined. The sample that was subjected to pressureless-assisted sintering at 1200 °C and held for 1 h exhibited the best stability and more uniform particle distribution compared to other samples. The particle size distribution data were closer to the standard line, satisfying the requirements of the normal distribution law. The results revealed that a high temperature was more favourable to the solid solution, and the formation of an Al2O3–ZrO2 solid solution can diminish the influence of the volume expansion of ceramic powders on the sample properties during sintering. Therefore, the addition of the sintering aid Al2O3 significantly promotes the densification of the powders, and the pressureless sintering technique reduces the sintering temperature of the solid solution, thus imparting a crystalline structure and excellent mechanical properties to the material.

Unveiling thereinforcement potentiality of MWCNTs architecture towards the improvement of microstructural vis-a-vis mechanical and thermo-mechanical properties of pressureless sintered MgAl2O4 spinel ceramic composite

The study presented in this work focuses on the preparation of composites made of multi-walled carbon nanotubes (MWCNTs) and magnesium aluminate spinel through a pressureless sintering technique. The effects of sintering temperature and MWCNTs content on the densification behavior and mechanical properties of the composite were investigated in detail. The results revealed that the composite exhibited improved densification behavior, enhanced mechanical performance and decent control at the microstructural refinement with increasing MWCNTs content up to 0.75 wt%. The superior mechanical properties obtained in case of sintered composite were attributed to the synergetic toughening mechanisms involving the fiber pull out, cracks bridging and cracks deflection effects of reinforcing phases, observed as a resultant effect of strong interfaces with the matrix phase. The crystalline structural arrangement obtained from the XRD and Raman spectral analysis of the composites structure also satisfies the attainment of the promising reinforcement potential of the nanotubular structure. Most importantly, the study also exemplified the subsidiary interference of a surface decorative coating on the MWCNTs structure with the MgAl-binary oxide network. The strategic grafting of the oxide protective shell and the consequent development of a robust interfacial bridging network between the matrix and reinforcement phase regulated the energy dissipation characteristic for fiber debonding. This, in turn, restricted the crack growth phenomenon during mechanical loading, and thereby ultimately contributing to the enhanced fracture toughness, reduced thermal expansion co-efficient and augmented thermo-mechanical performance of the evolved spinel-based composite structure

Green synthesis, formation mechanism and oxidation of Ti3SiC2 powder from bamboo charcoal, Ti and Si

Using low-cost and highly reactive bamboo charcoal, Ti and Si elemental powders as starting materials, Ti3SiC2 powder was synthesized via a simple and cost-efficient pressureless sintering technique in argon atmosphere. The influences of synthesis temperature, holding time and Si content on the Ti3SiC2 content of the synthesized products were investigated, and the analysis indicated that the relative content of Ti3SiC2 reached 98.9 wt% with a molar ratio of 3Ti/1.2Si/2.2C at 1400 °C for 1.5 h. The Ti3SiC2 with good crystallinity and homogeneous nanolayered structure was synthesized at lower temperatures due to the high reactivity and high specific surface area of bamboo charcoal. The non-isothermal oxidation behavior showed that Ti3SiC2 powder was stable in air below 540 °C. With the temperature increasing up to 1300 °C, continuous and dense TiO2 and SiO2 oxidation layers were formed on the surface of Ti3SiC2 particles, which conferred good oxidation resistance to Ti3SiC2 powder.

Microstructure and properties of aluminum nitride ceramics prepared by aqueous gelcasting method with nontoxic curdlan as gelling agent

Aluminum nitride ceramics were prepared by aqueous gelcasting method and pressureless sintering technique in N2 atmosphere using Y2O3 as sintering additives with nontoxic curdlan as gel system. The solidification mechanism of curdlan was studied. The effects of curdlan content and solid content on the microstructure, relative density and flexural strength of green bodies were investigated. The influences of Y2O3 content and sintering soaking time on the microstructure and properties of sintered bodies were also studied. The results show that, as the temperature increases to 80 °C, the ceramic powders solidify through three-dimensional gel networks of curdlan during gelling process. The green bodies can be successfully fabricated through aqueous gelcasting method with modified powder as original materials. Suitable curdlan content and solid content contribute to fabricating green body with uniform microstructures and high flexural strength. The relative density and flexural strength of sintered bodies enhance as the Y2O3 content and soaking time increase. The flexural strength and thermal conductivity are about 107.5∼172.3 MPa and 75.2∼112.5 W/(m·K), respectively. The sintered body with 4 wt% Y2O3 soaking for 3 h exhibits the highest thermal conductivity because of appropriate relative density, uniform microstructure and reasonable intergranular phase distribution. The mechanical property and thermal conductivity of sintered bodies can be improved by optimizing the gelcasting process parameter, Y2O3 content, and soaking time. The nontoxic gelling system will have wide application for aqueous gelcasting ceramic with complex shape.

Micromechanical properties and microstructures of AC and DC flash-sintered alumina

Alumina (α-Al2O3), one of the most widely used structural ceramics for industrial applications, requires high temperature and long sintering time to achieve a fully dense structure through conventional pressureless consolidation and sintering methods. Recently, several attempts have been made to reduce sintering time and temperature of Al2O3 through the application of electric field with limited success. In this study, alternating current (AC) and direct current (DC) flash sintering techniques were employed to obtain high-density Al2O3 within 10 min. The density of flash-sintered (FS) specimens was greater than the specimens prepared by the pressureless sintering technique (hereinafter designated as conventional sintering) at the same temperature for 10 h. Furthermore, AC technique can reduce the electric field necessary for flash sintering substantially comparing to the DC flash sintering. The microindentation measurements reveal that the AC-FS Al2O3 provides the highest fracture toughness and a high hardness among all sintered specimens. The improved mechanical properties of AC-FS Al2O3 are attributed to defects, such as dislocations as well as stacking faults observed in the FS specimens. This study provides a fresh perspective on the low-energy manufacturing of structural ceramics with improved properties.

Optical, thermal properties, and hydrolysis resistance of transparent YF3-doped CaO ceramics fabricated by vacuum sintering

Transparent calcium oxide (CaO) ceramic was successfully fabricated by pressureless vacuum sintering technique with 0.5 at% YF3 as the sintering additive. In consideration of the hydrolysis of CaO, the precursor powders were calcined at 600 °C. Within the sintering temperature range of 1150–1450 °C, the effect of YF3 additive on the phase structure, relative density, transmittance, and microstructure of CaO ceramics was studied. Benefiting from the assisted liquid-phase sintering mechanism and vacancy diffusion mechanism of YF3, full-dense (3.35 g/cm3) YF3-doped CaO transparent ceramic was fabricated at 1350 °C with high thermal conductivity of 15.42 W/(m·K). The in-line transmittance attained 47.10% at the wavelength of 1200 nm. Moreover, the hydrolysis resistance was extremely enhanced due to the pore-free structure. Compared to the CaO ceramic with low relative density of 78.66%, the weight gain of full-dense CaO ceramic greatly decreased from 22% to 1.2% within 35 days.

High temperature applications of carbon nanotubes (CNTs) [v]: thermal conductivity of CNTs reinforced silica nanocomposite

Consolidated functionalized carbon nanotubes/silica refractory ceramic nanocomposites (FCNTs/silica) were fabricated by pressureless sintering technique. Thermal conductivity of the nanocomposites with various amounts of carbon nanotubes (0, 1, and 4 wt.%) were investigated. The thermal conductivity increases with temperature, 1 wt. % FCNTs/silica nanocomposite gave the highest thermal conductivity. Therefore, it can be concluded that the carbon nanotubes (CNTs) are promising reinforcement for improving thermal conductivity of the silica refractory ceramics.

Investigation of the ion dynamics by a particular excitation due to the stoichiometric imbalances on BiFeO[formula omitted] ceramics

In this work, bismuth ferrite ceramics have been synthesized with various stoichiometric imbalances by the pressure-less sintering technique. UV–Vis spectroscopy and scanning electron microscopy show that both the band gap and the grain morphology of the ceramics are comparable to the stoichiometric imbalances. More necessarily, excess iron in the stoichiometry shows new charge carrier dynamics with the existence of completely new crystalline biferroic Bi1.2Fe0.8O3+δ ceramics. This means the iron ion valence fluctuation is well tailored by modifying iron contents. The conductivity study explains that two discrete forms of Johnscher’s power law and Super-linear power law types of conductivity mechanism can be induced by the change of stoichiometry. Obtained outcomes illustrate that stoichiometric imbalances play a major role in ion dynamics, leading to identifying a new path to engineer the properties of bismuth ferrite ceramics.

Preparation and mechanical properties of (Ti0.2Zr0.2V0.2Nb0.2Ta0.2)(C0.6N0.4)-Co high-entropy cermets

A novel (Ti 0.2 Zr 0.2 V 0.2 Nb 0.2 Ta 0.2 )(C 0.6 N 0.4 )-Co high-entropy cermets were prepared by two-steps method and the microstructure and mechanical properties were investigated. The single-phase high-entropy carbonitride (HECN) (Ti 0.2 Zr 0.2 V 0.2 Nb 0.2 Ta 0.2 )(C 0.6 N 0.4 ) powders were synthesized at 1600 °C. Subsequently, the HECN-Co cermets were successfully fabricated by the pressureless sintering technique. The results revealed that HECN powders (grain size ~0.79 μm) with uniform composition could be prepared at a lower temperature (1600 °C) by using oxide and carbon powders as raw materials. This preparation method also maintained the oxygen content and free carbon at low levels of 0.13 wt% and 0.03 wt%, respectively. The HECN-Co high-entropy cermets sintered at 1500 °C showed uniform distribution of the metal phase along the ceramic phase and good denseness (apparent porosity of only 0.08%). In addition, the samples possessed excellent mechanical properties, with hardness and fracture toughness of 15.72 GPa and 12.9 MPa·m 1/2 . Because of the unique features of high-entropy ceramics and the effective combination of ceramic and metal phases, HECN-based high-entropy cermets offer excellent overall properties. • A novel (Ti 0.2 Zr 0.2 V 0.2 Nb 0.2 Ta 0.2 )(C 0.6 N 0.4 )-Co high-entropy cermets were designed and prepared. • (Ti 0.2 Zr 0.2 V 0.2 Nb 0.2 Ta 0.2 )(C 0.6 N 0.4 ) powders were synthesized at 1600 °C with the O 0.13 wt% and C f 0.03 wt%. • (Ti 0.2 Zr 0.2 V 0.2 Nb 0.2 Ta 0.2 )(C 0.6 N 0.4 )-Co cermets were fabricated at 1500 °C with H V 15.72 GPa and K IC 12.9 MPa·m 1/2 . • The synthesis of (Ti 0.2 Zr 0.2 V 0.2 Nb 0.2 Ta 0.2 )(C 0.6 N 0.4 ) powders and its densification behavior were discussed.

A comparative study of spark plasma and conventional sintering of undoped SnO2 sputtering targets

SnO2 ceramics were fabricated by spark plasma sintering (SPS) and conventional (pressureless) sintering techniques using undoped submicron SnO2 powders. The effect of sintering temperature and dwell time on the densification behavior, phase evolution, and microstructural development of sintered ceramics were investigated. The relative density of SPSed ceramics increased when dwell time was raised from 1 to 10 min at 950 °C. However, full densification was prevented by the decomposition of SnO2 to Sn. The decomposition started after ∼10 min at 950 °C. In parallel to these observations, as sintering temperature increases, the elemental Sn in agglomerated form increases. On the other hand, the relative densities of conventionally sintered ceramics (at 1200°C-1400 °C) were relatively low (i.e., 63% relative density), and abnormal grain growth was observed due to the shift in sintering mechanisms to evaporation-condensation as a dominant mechanism. Since the undoped SnO2 ceramics, SPSed at 950 °C for 5 min under 30 MPa exhibit 93% relative density, high chemical purity, homogeneous grain size distribution, and smaller average grain size, they demonstrate great potential as sputtering targets for the production of high-quality thin film gas sensors.