Conventional Pressureless Sintering Research Articles

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

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

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

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

Sintering and toughening of self-reinforced AlN with integrated AlN whiskers via spark plasma sintering

Aluminum nitride (AlN), prized for its high thermal conductivity and electronic insulating properties, finds widespread use in electronic applications, particularly as a substrate material. However, its relatively low fracture toughness limits its application. In this study, to enhance both the thermal and mechanical properties of AlN ceramics, we thoroughly investigate the microstructure, thermal, and mechanical properties of self-reinforced AlN ceramics fabricated by spark plasma sintering, with self-reinforcement achieved through the integration of AlN whiskers within the AlN matrix. Unlike previous studies utilizing conventional pressureless sintering with limited sintering conditions, this work employs spark plasma sintering with a wide range of temperatures (1600–1900°C) and holding times (10–30 min), providing a comprehensive understanding of the sintering behavior. Microstructural features, observed through optical and scanning electron microscopy, reveal distinct toughening mechanisms via crack deflection and branching, with a maximum 20% enhancement. The self-reinforced AlN ceramics exhibit remarkable improvements in thermal conductivity, reaching up to 150Wm−1K−1, surpassing reported values for spark plasma-sintered AlN ceramics. Adding AlN whiskers enhances the thermal and mechanical properties of AlN simultaneously, particularly at 1800°C for 10 min with controlled grain size and nicely integrated AlN whiskers. This work not only contributes valuable insights into the complex relationship between processing parameters, microstructure, and resulting thermal and mechanical properties but also highlights the enhanced thermal and mechanical properties of self-reinforced AlN ceramics.

Conventional sintering of nano-crystalline Yttria-Stabilized Zirconia enables high-strength, highly translucent and opalescent dental ceramics

Development of restorative materials capable of mimicking optical and mechanical performance of natural teeth is a quest in aesthetic density. Yttria-Stabilized Zirconia (YSZ) ceramics represent one of the most popular choices for dental restorations, owing to their biocompatibility, white colour, and the possibility to use CAD-CAM technologies. In particular, YSZ doped with 3 mol. % yttria (3YSZ) is popular because it presents high strength. Nonetheless, the limited light transmission of commercially available high strength 3YSZ does not meet the requirements of highly aesthetic cases. On the other side, YSZ presenting a larger portion of yttria are more translucent but exhibit modest strength. Here, we report on fabrication of dense zirconia nanostructures in bulk form via conventional pressure-less sintering at temperatures down to 1100–1200 °C, achieving highly translucent and strong 3YSZ with significant opalescent behaviour. Both Hall−Petch and inverse Hall-Petch relationship were observed in 3YSZ samples with average grain size in the range of 250 nm and 55 nm, demonstrating the importance of grain size control to enhance both optical and mechanical properties of zirconia ceramics, simultaneously. Maximum biaxial strength of 1980 ± 260 MPa, in-line light transmission of 38% in the visible spectrum and opalescence approaching that of enamel were obtained at optimum grain size of 80 ± 5 nm. The notable optical properties are linked to the miniaturization of the residual pores and refinement of grain size towards the nanoscale while the superior mechanical strength is justified by the activation of different energy dissipation processes at nano and macroscale.

How does cold sintering affect the microstructural evolution of high-entropy pyrochlore during pressureless sintering?

The challenge of producing dense fine-grained high-entropy ceramics is hereby addressed by a combined cold and conventional pressureless sintering method for the first time. It provides nearly fully dense high-entropy pyrochlores with better microstructural homogeneity, and only half the grain size of conventional pressurelessly sintered samples with identical density. Both the finer and more homogeneous microstructures benefit better mechanical properties in hardness and reduced Young’s modulus. The kinetics and activation energies of both densification and grain growth were quantified and discussed to understand how cold sintering affects the microstructural evolution during the following pressureless sintering. More importantly, cathodoluminescence analysis suggested that cold sintering could cause more defects on the surface of the original particles, which could promote the grain boundary diffusion and have drag effects on grain boundary motion. The lessons learned here offer scientific understanding and technological guidance towards pressureless sintering of dense bulk high-entropy ceramics.

Pressureless two-step sintering of ultrafine-grained high-entropy zirconate ceramics with excellent mechanical properties

The challenge of preparing dense ultrafine-grained high-entropy ceramics through conventional pressureless sintering is hereby addressed by a simple two-step sintering method. Ultrafine-grained (Ce0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 high-entropy zirconate with 99.2 % theoretical density and 135 nm grain size has been fabricated by pressureless two-step sintering for the first time. Compared to the conventional sintering, two-step sintering provides finer grain sizes and better microstructural uniformity, which yield excellent comprehensive mechanical properties with high hardness of 16.0 GPa, large elastic modulus of 262 GPa and high fracture toughness of 2.8 MPa·m1/2. It is believe that pressureless two-step sintering can be utilized to prepare other high-entropy ceramics with fine grain size and high quality.

Ultrafast high-temperature sintering and densification of ZrC-based ceramics

Zirconium carbide (ZrC) ceramics are promising candidates for high-temperature structural components and nuclear reactors. However, their poor sinterability has limited widespread application. This study explored the sintering and densification of ZrC-based ceramics with graphite or B4C additive using ultrafast high-temperature sintering (UHS). A nearly fully dense ZrC ceramic (> 98%) could be obtained by adding 2.5 wt% B4C additive via ultrafast high-temperature sintering at 2300 °C within 3 min. Additionally, the results revealed that adding graphite into ZrC was beneficial to reduce the grain size but detrimental to densification. Compared to ZrC ceramics sintered using conventional pressureless sintering (PLS) at 2400 °C for 60 min, ZrC-based ceramics with 2.5 wt% B4C sintered via UHS at 2400 °C for 30 s exhibited higher relative densities, smaller grain sizes, and greater Vickers hardness due to faster heating rates and shorter sintering processes.

Lithium Dendrite Propagation in Ta-Doped Li7La3Zr2O12 (LLZTO): Comparison of Reactively Sintered Pyrochlore-to-Garnet vs LLZTO by Solid-State Reaction and Conventional Sintering.

Ta-doped Li7La3Zr2O12 (LLZTO) garnet is a promising Li-ion-conducting ceramic electrolyte for solid-state batteries. However, it is still challenging to use LLZTO in Li metal batteries operating at high current densities because of the tendency for Li metal to nucleate and propagate along the grain boundaries. In this study, we carry out a detailed investigation to elucidate the effect of microstructure and grain size on the electrochemical properties and short circuit behavior in LLZTO. Pellets were prepared using reactive sintering from pyrochlore precursors (a method called pyrochlore-to-garnet, P2G) and compared with LLZTO synthesized using solid-state reaction (SSR) followed by conventional pressureless sintering. Both preparation methods were controlled to keep the phase and elemental composition, ionic and electronic conductivity, relative density, and area-specific resistance of the pellets constant. Reflection electron energy loss spectroscopy and X-ray photoelectron spectroscopy confirm that both types of LLZTO have similar band gaps and chemical states. Microstructure analysis shows that the P2G method results in LLZTO with an average grain size of around 3 μm, which is much smaller than the grain sizes (as large as 20 μm) seen in SSR LLZTO. Galvanostatic Li stripping/plating and linear sweep voltammetry measurements show that P2G LLZTO can withstand higher critical current densities (up to 0.4 mA/cm2 in bidirectional cycling and >1 mA/cm2 for unidirectional) than those seen in SSR LLZTO. Post-mortem examination reveals much less Li deposition along the grain boundaries of P2G LLZTO, particularly in the bulk of the pellet, compared to SSR LLZTO after cycling. The improved cycling behavior in P2G LLZTO despite the higher grain boundary area could be from more homogeneous current density at the interfaces and different grain boundary properties arising from the liquid-phase, reactive sintering method. These results suggest that the effect of grain size on Li dendrite propagation in LLZO may be highly dependent on the synthesis and sintering method employed.

Densification and grain growth of UO2 and MnO-UO2 during pressureless sintering

Large-grain UO2 has attracted much attention due to its advantage of suppressing fission gas release by reducing the specific surface area of grain boundaries. In this study, a single grain growth additive MnO is selected to modify UO2 by conventional pressureless sintering method. The effect of MnO modification on UO2 densification and grain growth at temperatures between 1200 and 1700 °C was investigated in detail. 0.5 wt%MnO-UO2 pellets can reach nearly 95% T.D. when sintered at 1200 °C for 24 h. An average grain size greater than 300 µm can be obtained at 1700 °C for 24 h. A series of sintering experiments were designed and conducted to verify the specific mechanism of MnO on the grain growth of UO2. The classical grain growth kinetics indicates that the grain growth exponent of MnO modified UO2 in the early stage is 3, which corresponds to the lattice diffusion under pore control, while that of UO2 is 2 and conforms to the grain boundary migration control. The SEM results show that there is solute segregation of MnO at the grain boundaries, followed by nucleation of lamellar structure in the form of piles and plates on the boundaries after high temperature treatment and traces of liquid phase in some region. Diffusion sintering experiments show that the mechanism of MnO-promoted UO2 grain boundary migration is based on solid phase diffusion. The XRD results of UO2 modified with different amounts of MnO demonstrate that the growth degree of UO2 grains can be related to the variation of lattice parameters resulted from the MnO modification. This is due to the enhanced mobility of UO2 grain boundaries because of the large distortion of surrounding lattice caused by Mn modification. These results can provide new ideas for the manufacture of UO2 ceramic fuels.

Preparation of extraordinary high-density indium-tin oxide target on the basis of pressure-less sintering method

Density is the most important parameter determining the performance of indium-tin oxide (ITO) targets. However, the full densification of ITO ceramics is extremely difficult. Previously reported strategies are based on the addition of components such as sintering aids or significant process adjustments. In this study, we aimed to realise the full densification of an ITO target by the fine regulation of parameters involved in the conventional pressure-less sintering process. The phase transitions, BET surface area, and particle size changes of indium-tin co-precipitated nanoprecursors sintered at different temperatures were studied. Moreover, the effects of the ITO powder state and sintering temperature on the density, resistivity, and microstructural uniformity of the ITO target were investigated. The results showed that the existence of rhombohedral In2O3 in the ITO powder and ceramic sintering temperature are vital to the full densification of the ITO target. Density was found to dominate ITO-target resistivity when the sintering temperature was <1585 °C. In this study, an extraordinary high-density (99.81 %), low-resistivity (1.707 × 10−4 Ω cm), and uniform-microstructured ITO target is prepared under optimised operation conditions.

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.

Pressure-less spark plasma sintering of 3D-plotted titanium porous structures

Additive manufacturing of titanium porous structures by direct ink writing involves the removal of the binder needed for powder extrusion and subsequent sintering to consolidate the 3D-plotted body. In this work, pressure-less spark plasma sintering (PL-SPS) was systematically studied for fast consolidation of titanium porous structures. Furthermore, poloxamer 407 was used as the binder and the lowest temperature possible was identified for its thermal elimination. The results show for the first time that PL-SPS generated sintering conditions similar to those generated by conventional pressure-less sintering, producing hierarchical titanium porous structures with equivalent densification, shrinkage, and surface roughness, but with minimal grain growth. The thermal responses of the die and material showed efficient radiation heat transfer, allowing fast heating (100 °C/min) of one sample per run, promoting the formation of sintering necks and powder densification in 10 min, which is much faster than conventional sintering that requires at least 2 h of dwell time. However, the process operates at a sintering temperature 200–300 °C above the conventional sintering temperature, and at the expense of high consumption of electrical energy to achieve such a high heating rate. The mechanical strength of the resulting titanium structures increases with increasing strand densification at nearly constant strand separation, resulting in strong and plastic porous structures.

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.

Liquid phase assisted synthesis of (Ti,V,Nb,Ta,W)C-Ni high entropy carbide cermets by conventional pressureless sintering

Two-phase fully dense high entropy carbide (HEC) cermets were successfully prepared following the conventional cemented carbide/cermet processing route. Cold isostatically pressed equimolar TiC, VC, NbC, TaC and WC powder mixtures with 12 vol% Ni binder were pressureless sintered in the solid state and liquid phase sintering regimes respectively. A homogeneous rock-salt (space group Fm3¯m) HEC phase embedded in a nickel alloy binder was attained by liquid phase sintering at 1420°C. Thermodynamic simulations were conducted to predict the equilibrium phases and carbide/binder phase contents. The phase evolution was investigated at seven different sintering temperatures in the 900–1420°C window. Thermogravimetry and differential scanning calorimetry analysis were performed to follow up the sintering process. Electron probe microanalysis was used to map the elemental distribution as well as to quantify the constituent phase contents. Nanoindentation were performed to correlate the difference in mechanical properties of the core-rim structured HEC grains with the chemical composition. The liquid phase sintered (Ti,V,Nb,Ta,W)C - 12 vol% Ni had a Vickers hardness of 14.3 GPa and Palmqvist indentation fracture toughness of 9.2 MPa. m1/2.

Microstructure, mechanical and wear properties of mechanically alloyed and conventionally sintered Nb-W and Nb-Mo alloys

Nb-W and Nb-Mo exhibit unrestrained solubility in liquid and solid phase, leading to influence the mechanical properties of Nb. In the present work 90Nb-10 W (alloy A) and 90Nb-10Mo (alloy B) (in weight%) are synthesized by mechanical alloying for 10 h and consolidated by conventional pressureless sintering in argon atmosphere at 1450 °C with 2 h of soaking time. The effect of compaction pressure (375 MPa, 500 MPa, 625 MPa) on densification before sintering and densification, hardness after sintering of the alloys is also investigated. Plastic-assisted deformation is more in alloy B against alloy A which enhances the densification during sintering. Elemental mapping and line scanning result of sintered alloy A show the incidence of W in Nb phase, suggesting profound inter-diffusion of W/Nb. Maximum % relative sintered density of 68.89% is achieved in alloy B at 500 MPa pressure. Alloy A exhibits maximum hardness of 3.11 GPa at 625 MPa compared to 2.55 GPa hardness for alloy B compacted at identical compaction pressure. The wear resistance of alloy B is comparatively higher than alloy A, both compacted at 500 MPa pressure.

Rapid manufacturing of silica glass parts with complex structures through stereolithography and pressureless spark plasma sintering

This paper reports a method for preparing silica glass by pressureless spark plasma sintering (PL-SPS), which can rapidly manufacture silica glass parts with complex structures by coupling with stereolithography 3D printing technology. The rapid sintering process and microstructure evolution of silica glass prepared by PL-SPS were mainly investigated. The experimental results showed that the sintering temperature and dwelling time were the main factors affecting the PL-SPS of silica glass. The microstructure evolution indicated that the densification rate of the sample was very fast from 1250 °C to 1300 °C, and the interlayer defects caused by the printed layer thickness could be healed in the final stage of densification. Like conventional pressureless sintering, silica glass with a relative density of more than 99% and a visible-light transmittance of more than 90% could also be obtained through PL-SPS, but the entire working time was shortened from 22.53 h to 0.49 h.

A Novel Method for Densification of Titanium Using Hydrogenation-Induced Expansion under Constrained Conditions

Full density of sintered Ti alloys is difficult to achieve through conventional pressureless sintering techniques. This paper introduces a novel approach for densifying powder metallurgy Ti alloys through constrained hydrogenation assisted densification (CHAD). This method utilizes hydrogenation-induced volumetric expansion and constrains a sintered Ti body in a rigid mold to achieve densification. The results demonstrate that the residual porosity in powder metallurgy CP-Ti and Ti-6Al-4V alloys can be effectively reduced or removed by CHAD, leading to improved tensile properties. The described method can be used as an efficient and cost-effective technique to make Ti and Ti alloys with high density and improved mechanical properties.

Young’s modulus evolution during sintering and thermal cycling of pure tin oxide ceramics

Pure tin oxide (SnO2) ceramics is well known for its bad sinterability, more precisely for the difficulty to densify without additives by conventional pressureless sintering. This is related to the fact that the sintering mechanisms in pure tin oxide ceramics are non-densifying (surface diffusion at low temperature and evaporation-condensation at high temperature). On the other hand, for the same reason, pure tin oxide ceramics is a very unusual model system that can be used to demonstrate the effects of microstructural changes on effective properties without the otherwise dominating effect of changes in porosity. In this paper we show that pure tin oxide ceramics uniaxially pressed at 50 MPa, pre-sintered at 500 °C and re-sintered at 1000, 1200 and 1400 °C exhibit relative Young’s modulus increases of 30, 70 and 120 % while the porosity remains essentially constant at a value of 51.6 ± 0.7 %.

Asynchronous densification of zirconia ceramics formed by stereolithographic additive manufacturing

Stereolithography has been proven as a feasible approach to make crack-free ceramic macrostructure with customized designs, but the microstructure, especially pore structure remains to be tailored more precisely for better performance, where the sintering protocol and related densification characteristics could play a vital role as the slurry preparation and debinding protocol do. Herein we report a phenomenon named “asynchronous densification”, that is, the surface region of zirconia ceramics formed by stereolithographic additive manufacturing would be densified prior to the bulk at 1200°C during the conventional pressureless sintering in air. The cause of this asynchronism is unclear but supposed to be correlated with low packing density, high sintering activity, poor thermal conduction of ceramics and impurities. Early densification of the surface may have negative effects towards ceramic components with more homogeneous microstructure, suppressed pore coalescence and limited grain growth, and therefore needs to be better controlled through optimization in sintering protocol.

Self-reinforced and high-thermal conductivity silicon nitride by tailoring α-β phase ratio with pressureless multi-step sintering

A self-reinforced Si3N4 ceramic with high thermal conductivity was prepared through multi-step pressureless sintering with Y2O3 and MgO additives. The steps consisted of phase control and densification stages to provide additional crystallization sites using a partial α-β phase transformation, which also established a bimodal microstructure. The optimal temperature for the intermediate stage that exhibited the maximum flexural strength was determined by comparing the phase content and linear shrinkage. Thus, 1400 °C, which resulted in 14.4 wt% β-Si3N4, was found to be optimal. After final sintering, a flexural strength of 932 MPa and a thermal conductivity of 74 W/mK were achieved, which are 11.5% and 8.0% higher, respectively, than those obtained using conventional pressureless sintering. The rate constant and activation energy of the pressureless sintering with the Y2O3–MgO additive system were also calculated to evaluate the potential of the α-β transformation to obtain the proper quantity of β-Si3N4 seeds.