Liquid Phase Sintering Research Articles

Interface migration and grain growth in NbC-Ni cemented carbides with secondary carbide addition

Cemented carbides are widely used for cutting and drilling tools. They usually combine a WC hard carbide phase and a Co-based ductile binder. NbC-Ni materials are considered as a possible alternative, especially for wear applications. The advantageous economic situation for raw materials sourcing, their interesting mechanical properties and low density have raised a new interest for these materials. However, mechanical properties can be limited by the rapid grain growth during liquid phase sintering, as compared to WC-Co. Grain growth can be controlled by the addition of secondary carbides. In this paper, a quantitative EBSD analysis of grain growth is performed for NbC-12 vol%Ni materials sintered at 1450 °C with controlled addition of Mo2C and WC. The average grain size decreases continuously with Mo2C addition. The results are discussed based on a more detailed interface characterization and on a previous model for the cooperative migration of phase boundaries and grain boundaries.

Transient liquid phase sintering in spark plasma sintered tungsten heavy alloys with Nb and Mo additives

In the present investigation spark plasma sintered tungsten heavy alloys of composition 90W-7Ni-3Fe, 84W-7Ni-3Fe-6Mo, and 84W-7Ni-3Fe-6Nb were analyzed to determine the effect of Mo and Nb addition on microstructure and phase formation. The alloys were sintered at temperatures ranging from 1150 °C to 1275 °C and at a fixed sintering pressure of 30 MPa. Nb added alloy exhibited a maximum relative density of 99% at sintering temperature of 1200 °C. Whereas the base alloy and Mo added alloy, exhibited a maximum relative density of 97.9% and 95.9% at 1150 °C respectively. Transient liquid phase sintering was found to play a crucial role in the base alloy and Mo added alloy whereas the Nb added alloy was found to undergo persistent liquid phase sintering. Microstructure characterisation by SEM revealed formation of finer tungsten grains with average grain size ranging from 1 μm to 4.3 μm. Mo addition restricted the tungsten grain growth while Nb addition promoted the tungsten grain growth compared to the base alloy. Phase analysis by XRD revealed the formation of Ni4W intermetallic (at lower temperatures) for the base alloy, NbO2 and NbW intermetallics for Nb added alloy and MoNi rich phase for Mo added alloys. Hardness measured by Vickers hardness method revealed a declining trend in hardness with increase in sintering temperature, owing to tungsten grain growth in all the alloys and along with Kirkendall pore formation and growth for the base alloy and Mo added alloy. Nb addition resulted in maximum hardness of 678 HV1 which is highest ever reported for SPSd WHA with 90 wt% concentration of W.

Microstructure, electrical and magnetic properties of MgFe2O4 sintered with (1-x)Bi2O3.xFe2O3 additives

In this research, liquid phase sintering of MgFe2O4 with a Bi2O3-Fe2O3 additive was studied. Initially, MgFe2O4 powder was synthesized at 1150 °C, then 3 wt% additive with (1-x)Bi2O3.xFe2O3 (x=0.00, 0.04, 0.08, 0.12) was added to the powder and sintered at 1050 °C. The x=0.04 sample showed the highest density of ∼4.40 g/cm3. Cubic spinel ferrite and phases containing bismuth oxide were formed in all samples, notably, only x=0.08 and x=0.12 samples consist hematite. Uniform microstructure was achieved for x=0.00 and 0.04 samples while other samples exhibited non-uniform porous microstructures. EDS analysis revealed uniform iron distribution in the grains of x=0.04 sample, unlike the x=0.00 sample. Impedance spectroscopy indicated increased electrical resistivity in x=0.04 compared to x=0.00. Saturation magnetization peaked at 18.5 emu/g for x=0.04, then decreased, while coercive force rose from 13.2 to 30.2 Oe with increasing x. The x=0.04 additive acted as the optimal sintering aid, improving electrical and magnetic properties.

Evolution of microstructure and mechanical properties of TiB2-7.5 wt.% WC-based cermets sintered at different temperatures

The evolution of microstructure and mechanical properties of TiB2–7.5 wt% WC-based cermets sintered at different temperatures were investigated. The results showed that eutectic liquid phase appeared at around 1335 °C, indicating that solid-phase sintering occurred below that temperature, while liquid-phase sintering occurred above that temperature. TiB2, W2CoB2, WB and CoNi phases were detected in the XRD patterns of cermets sintered at different temperatures, and TEM analysis showed the presence of TiC phase. So the following chemical reaction should occur: TiB2 + WC + Co → W2CoB2 + WB + TiC. The TiB2 core - (Ti, W, Co, Ni)(B, C) rim phase were observed in the cermet sintered at 1397 °C. As the increase of sintering temperature, the solubility of WC and TiB2 in CoNi binder phase, and the number of core-rim structure hard phase particles increased. The formation of core-rim structure enhanced the interfacial bonding strength between TiB2 phase and CoNi binder, which was beneficial for improving the strength and toughness of cermets. The cermets sintered at 1465 °C had good comprehensive mechanical properties, with a relative density of 99.62%, a hardness of 17.34 ± 0.43 GPa, a transversal rupture strength of 2038 ± 53 MPa, and an indentation fracture toughness of 12.15 ± 0.32 MPa·m1/2.

Approaching crystal’s limit of thermoelectrics by nano-sintering-aid at grain boundaries

Grain boundary plays a vital role in thermoelectric transports, leading to distinct properties between single crystals and polycrystals. Manipulating the grain boundary to realize good thermoelectric properties in polycrystals similar as those of single crystals is a long-standing task, but it is quite challenging. Herein, we develop a liquid-phase sintering strategy to successfully introduce Mg2Cu nano-sintering-aid into the grain boundaries of Mg3(Bi, Sb)2-based materials. The nano-aid helps to enlarge the average grain size to 23.7 μm and effectively scatter phonons, leading to excellent electrical transports similar as those of single crystals and ultralow lattice thermal conductivity as well as exceptional thermoelectric figure of merit (1.5 at 500 K) and conversion efficiency (7.4% under temperature difference of 207 K). This work provides a simple but effective strategy for the fabrication of high-performance polycrystals for large-scale applications.

Multi-phase-field modeling of sintering applicable to solid-state and liquid-phase sintering in multiphase and multicomponent systems

A thorough understanding and control of the microstructural evolution during sintering is essential for improving the properties of various sintered materials. In this study, a new multi-phase-field (MPF) model of sintering that applies to both solid-state sintering (SSS) and liquid-phase sintering (LPS) in multiphase and multicomponent systems was developed. The MPF model incorporates the advection term to account for the rigid-body motion (RBM) of the sintered particles. Moreover, the MPF model provides thermodynamically reasonable results of phase transformation and solute diffusion when coupled with a thermodynamic database. The effect of incorporating the RBM was validated by evaluating the neck growth and densification rates obtained from the simulation results of SSS. The MPF simulation results of the initial stage of LPS demonstrated that the a) MPF model could reproduce the densification behavior, which depends on the phase transformation between the solid and liquid phases, and b) RBM is an important factor in reproducing the densification behavior. A long-term MPF simulation of the LPS also demonstrated that the microstructural evolution, including three densification-related mechanisms—contact flattening, Ostwald ripening, and solid-state bonding—could be analyzed from the initial to final stages of the LPS. This study is expected to advance the sintering process applicable to multiphase and multicomponent systems.

Microstructure and high-temperature oxidation resistance of Ti-6Al-4V alloy with in-situ SiC-SiO2 nano-composite coating by LPDS technique

A SiC-SiO2 nano-composite coating was prepared via a novel liquid-phase plasma-assisted particle deposition and sintering (LPDS) method to enhance the oxidation resistance of the Ti-6Al-4V alloy. For comparison, a conventional plasma electrolytic oxidation (PEO) ceramic coating is fabricated on the Ti-6Al-4V alloy. The microstructure and formation mechanisms of both ceramic coatings were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results indicated that the thickness of the SiC nano-composite coating (∼40 μm) increased significantly by 285 % compared to the PEO coating (∼14 μm). The microstructural evolution and isothermal oxidation performance of the PEO and SiC-SiO2 nano-composite coating were comparatively investigated at 800 °C. After 100 h, the thickness gain of the SiC-SiO2 nano-composite coating (∼14 μm) was lower than that of the PEO coating (∼26 μm). The improved oxidation performance is primarily attributed to the outermost layer containing abundant SiC nanoparticles, which transform into SiO2 during the oxidation process and effectively inhibiting the inward penetration of oxygen.

Microstructures and mechanical properties of W-Co-Al tungsten heavy alloy prepared by different formation processing

High density, high strength and high susceptibility to adiabatic shear banding (ASB) have always been pursued by tungsten heavy alloys (WHAs) for kinetic energy armor-piercing projectiles. To meet the demand, this work reported a novel tungsten heavy alloy with high-density L12 precipitation-strengthened Co9Al9W binder (WCAW). WCAWs were prepared by liquid phase sintering (LPS) and laser ultrashort-time liquid phase sintering (LULPS) respectively. Experimental results show that LULPS technique with large molten pool convection and rapid solidification can produce WCAWs with uniform W distribution, low W-W contiguity and fine W particles. Conversely, W particles in LPS WCAWs were excessively connected and segregated. The uniform distribution, low W-W contiguity and fine W particles enable LULPS WCAW to exhibit better mechanical properties. The ultimate compressive strength of the LULPS WCAWs is ∼2442 MPa, 16.4 % higher than that of the LPS WCAWs. As well, the dynamic yield strength of LULPS WCAWs is high, which is ∼2281 MPa and 37.4 % higher than that of LPS WCAWs.

Additively manufactured, long, serpentine submillimeter channels by combining binder jet printing and liquid-phase sintering

Metallic microfluidic devices made from powder-bed additive manufacturing systems have received increasing attention, but their feasible channel geometry and complexity are often limited by lack of an effective approach to removing trapped powder particles within the channels or conduits of the sintered parts. Here, we present an innovative approach to fabricating long serpentine, high-aspect-ratio submillimeter channels made of stainless steel 316L (SS) by binder jet printing (BJP) and liquid-phase sintering. We leverage the unique nature of the BJP process, that is printing and consolidation steps are decoupled, enabling us to join two or more parts during the sintering step. Instead of constructing the channel device as a single part, we print multiple parts for easy powder removal and later join them to form enclosed channels. The key innovation lies in adding sintering additives like boron nitrides (BN) to the SS stock powder—at the SS/BN interfaces, liquid phase is locally generated at temperature much lower than the SS melting temperature, facilitating the bonding of the multiple parts as well as the consolidation of parts for near-full density. We systematically vary the sintering temperature to show how it affects the joining quality and the channel shape distortion. The joining quality such as the fracture strengths of the joined samples is measured by a pull test while the shape distortion is characterized by various imaging techniques. The feasibility of the proposed approach is demonstrated by fabricating a 400-mm-long, fully enclosed serpentine channel with a rectangular cross-section of 0.5 mm in width and 1.8 mm in height. The pressure drop across this 3D-printed SS serpentine channels is measured for air flow and compared to a standard gas flow model, showing that the device is free of leakage or clogs.

Effects of sintering method and Al2O3 content on microstructure and mechanical properties of WC-10Co cemented carbide

Ultra-high toughness and ultra-high hardness WC-10Co-xAl2O3 cemented carbides were successfully prepared by vacuum liquid phase sintering (VS) and spark plasma sintering (SPS) using ultrafine WC and commercial Co powders as raw materials, respectively. The effects of nano-Al2O3 content and sintering process on the relative density, average grain size, microstructure and mechanical properties of WC-10Co alloys were investigated. The results showed that the densification degree of the samples decreased with the increase of Al2O3 content. In addition, the relative densities of the samples prepared by SPS were lower than those of the samples prepared by VS. For the samples prepared by VS, the average grain size increased first and then decreased as the Al2O3 content increased from 3 wt% to 9 wt%, and the percentage of coarse grain showed the same trend. When the addition of Al2O3 was 6 wt%, the average grain size reached the maximum value of 1.39 ± 1.03 μm. Meanwhile, the sample with 6 wt% Al2O3 had a typical inhomogeneous dual-grain structure, and the percentages of fine grain (< 1 μm) and coarse grain (> 1 μm) were about 50%, which displayed the highest fracture toughness of 18.59 MPa·m1/2. The dual-grain structure not only improved the toughness, but also compensated the loss of hardness. However, for the samples prepared by SPS, with the increase of Al2O3 content, the average grain size of WC decreased, while both hardness and toughness were continuously improved. When the Al2O3 content increased to 9 wt%, the grain size reached the minimum value of 0.25 ± 0.08 μm, and meanwhile, the hardness and toughness reached the maximum value of 19.69 GPa and 12.29 MPa·m1/2, respectively. Moreover, as the Al2O3 content increased, the transverse rupture strength (TRS) of the samples prepared by VS increased first and then decreased, and the TRS of sample with 3 wt% Al2O3 reached the maximum value of 1410 MPa. However, the TRS of the sample prepared by SPS showed a decreasing trend.

Periclase-magnesium aluminate spinel ceramics filter with excellent purification efficiency on molten steel prepared from Mg(OH)2: Effect of fused MgO content

Reticulated ceramic filters are widely used in the field of steel purification to improve the quality of steel products. In this work, periclase-magnesium aluminate spinel ceramic filters with microporous MgO coating were fabricated by using Mg(OH)2 as raw material. The effect of fused MgO content on the microstructures, mechanical properties and filtration capacity of filters on Al2O3 inclusions in molten steel was comparatively studied. The results indicate that the particle packing and liquid phase sintering cooperated to promote the formation of neck connections between microporous MgO particles, which increased the cold compressive strength and thermal shock resistance of the filters. Furthermore, the results from the immersion test in molten steel demonstrate that an appropriate amount of high-temperature liquid phase and the relatively microporous structure of filter skeleton were favorable for filters to filtrate Al2O3 inclusion in molten steel. Overall, the filter with 30 wt% fused MgO had the best comprehensive performance with a cold compressive strength of 0.68 MPa, a residual retention of cold compressive strength after three thermal shock recycles of 79.07 % and a filtration efficiency of 79.06 % in reducing the total oxygen content of the steel.

Enhancing the performance of SiC-based varistors through the use of SPS processing and fluxes

SiC varistors, which are used as surge arrestors in high power/high energy applications, often contain high levels of porosity (∼25 %) which can degrade varistor performance. Starting with SiC-clay varistors (∼43 µm grain size SiC) we employed spark plasma sintering (SPS) and flux additions (to enable liquid phase sintering) to reduce the level of porosity and understand its impact on the microstructure and the electrical properties. Sintering by SPS at 1200 °C for 5 min increased the density to 94 % theoretical, increasing energy density capability by almost a factor of 10 (to ∼1100 J cm−3), but the non-linear coefficient (α) fell by 40 % (to 3.5) and breakdown field (Eb) fell by 75 % (to ∼200 V cm−1). Varistors prepared with nepheline syenite as a flux (replacing clay) which were pressureless sintered at 1200 °C had maximum densities of 80 %. The samples in which all the clay was replaced by the flux exhibited exceptional energy density performance (over 10,000 J cm−3) but suffered reductions in other parameters. Using small grain size SiC (∼3 µm) and 40 % syenite flux yielded consistently superior properties: low power range α value 12.2, high power range α value 32.5, breakdown field ∼5880 (V cm−1) and very low leakage currents ∼10−3 (mA/cm2).

Formation of polycrystalline-Co particle-chains in Cu–Co alloy during liquid-phase sintering in a high magnetic field

Formation of polycrystalline-Co particle-chains in Cu–Co alloy during liquid-phase sintering in a high magnetic field

Effect of Ti3AlC2 addition on the densification and high- temperature erosion resistance of TiB2 composite ceramics fabricated by SPS

TiB2-Ti3AlC2 composite ceramics were fabricated by spark plasma sintering (SPS) technique. The effects of Ti3AlC2 additive on the densification, mechanical properties and resistance to high-temperature aluminum liquid erosion of composite ceramics were investigated, and the mechanisms of densification and erosion resistance were highlighted, respectively. At sintering temperature of 1800 °C, pressure of 40 MPa, and holding time of 10 min, TiB2 composite ceramic with 40 wt% Ti3AlC2 acquired 98.9 % densification, 98.2 % thermal shock resistance, and 13.71 MPa∙m1/2 fracture toughness. The in-situ decomposition of Ti3AlC2 was clearly observed during the sintering process, and the generated liquid Al induced the liquid phase sintering mechanism, promoting the densification of composite ceramics. Besides, the simultaneous in-situ generation of TiC phase strengthened the TiB2 matrix and induced a mixed toughening mechanism of transgranular fracture and intergranular fracture, resulting in superior mechanical properties and better resistance to high-temperature erosion of TiB2-Ti3AlC2 composite ceramics.

Preparation of 3D printed silicon nitride bioceramics by microwave sintering

Silicon nitride (Si3N4) is a bioceramic material with potential applications. Customization and high reliability are the foundation for the widespread application of Si3N4 bioceramics. This study constructed a new microwave heating structure and successfully prepared 3D printed dense Si3N4 materials, overcoming the adverse effects of a large amount of 3D printed organic forming agents on degreasing and sintering processes, further improving the comprehensive performance of Si3N4 materials. Compared with control materials, the 3D printed Si3N4 materials by microwave sintering have the best mechanical performance: bending strength is 928 MPa, fracture toughness is 9.61 MPa·m1/2. Meanwhile, it has the best biocompatibility and antibacterial properties, and cells exhibit the best activity on the material surface. Research has shown that the excellent mechanical performance and biological activity of materials are mainly related to the high-quality degreasing, high cleanliness sintering environment, and high-quality liquid-phase sintering of materials in microwave environments.

Alumina-based ceramic coatings obtained by the SHS process for high temperature corrosion and wear protection of steel tubulars

Ceramic coatings have much potential for steel tubulars' protection against high temperature corrosion of gases and liquids, often containing abrasive solid particles, and to enhance their integrity in severe power generation, mineral and oil & gas production environments. Combining self-propagating high-temperature synthesis (SHS) with high-speed centrifugal process, alumina-based coatings were produced onto the inner surface of tubulars for these applications. According to the developed SHS batch formulation and the designed centrifugal device and process, well-consolidated composite ceramic layers with a thickness of 1.5–3 mm have been obtained. The developed SHS process does not create hazardous gases, and it takes ∼30 s for the ceramic phase formation. A high-level compaction due to centrifugal forces and a liquid phase sintering approach provided a dense gradient ceramic structure formed by oxide grains cemented by a glassy phase and bonded with a steel substrate through a thin iron layer. The coatings were successfully produced onto the inner surface of carbon and stainless steels’ tubes of up to 12 ft-lengths and standard dimensions from 2 to 3/8″ to 7.5” OD. The coatings were tested, for the first time, in wear and high-temperature corrosive environments, e.g., in steam–H2S–CO2 gases and molten salts, with encouraging results due to high contents of Al2O3 and some other oxides and consolidated ceramic structure. They can be recommended for applications in high-temperature (900 °C and greater) corrosive environments and erosive flows.

Ultrafast pressureless sintering of high conductivity Ni-based cermets inert anode for Carbon‐free electrolytic aluminum

Cermet is a promising inert anode material to solve the problem of high carbon emission and bearing C-F harmful gas generation of primary aluminum produced from bauxite using molten electrolysis with carbon anode. However, a major challenge for traditional methods is to ensure that the cermet inert anode simultaneously has good conductivity, corrosion resistance, and strength. Herein, a new ultrafast pressureless sintering (UPS) method is developed to greatly improve the conductivity of metal ceramic inert anode materials while ensuring good corrosion resistance and strength. In a departure from conventional practice that metal phase undergoes local enrichment accompanied by irregular formation of ceramic particles at low heating rates, here UPS achieves rapid diffusion and uniform distribution of metal phase at regular ceramic particle interfaces during liquid phase sintering. The new method is applicable to Ni-based cermet and the production of Ni-based cermet inert anode with excellent conductivity in a few tens of seconds has been demonstrated, which is able to improve the quality of cermet inert anode requiring a low energy input. The conductivity of cermet produced by the applied UPS process is 15-fold higher than that of other pressureless sintering processes with low heating rates. These Ni-cermet inert anodes exhibit high corrosion resistance and density with excellent mechanical properties. This rapid sintering technique facilitates the structural design of inert anodes and develops a pathway of new systems with scalable, efficient, and high-quality sintering of composite materials.

Research on material jetting fabrication and dielectric constant calculation of LTCC based on silica-borosilicate glass

Several material systems are available for low-temperature co-fired ceramics, and to achieve more efficient designs and applications using additive manufacturing (AM) techniques such as material jetting, the predictability of the dielectric constant of the sintered structure will significantly reduce the consumption of material development. In this work, the LTCC part was fabricated by 3D material jetting using silica-borosilicate glass LTCC ink and densified at 850°C. The borosilicate glass composition was derived from nanopowders prepared by radio frequency plasma spheronisation and configured as surface-grafted glass sols. Scanning electron microscopy results showed that silica particles were embedded in the glass matrix by a liquid phase sintering mechanism accompanied by several confined pores that were difficult to remove. A dielectric constant of around 3.78 was tested from room temperature to 100°C. Molecular dynamics sintering simulation was used to create sintered structures that matched the experimental values. The dipole moment fluctuation of the silica/glass bulk was counted to obtain an accurate value of the dielectric constant, and then the Bruggeman model based on the effective medium theory predicted the dielectric constant of the sintered body, with a prediction of 3.65, which is within 4 % difference from the measured value (3.78). The dielectric constant was predicted for different ceramic/glass compositions and sintering temperatures and found that 35 wt% and 60 wt% should be the upper and lower bounds for borosilicate glass additions to satisfy a stable structure with a relatively low dielectric constant range (3.38–4.42). In conclusion, an effective molecular dynamics approach to predict the dielectric constant of sintered bodies was developed to assist material design in LTCC AM fabrication.

Silicon carbide ceramics manufactured by digital light processing and low temperature sintering

The rapid development of additive manufacturing techniques enables the preparation of SiC ceramics with complex configurations; however, the high sintering temperature and defects-control are still great challenges during processing. Thus, SiC ceramics were prepared by digital light processing followed by liquid phase sintering using Al2O3–Y2O3 additives in this study. The photopolymerization of the SiC slurry, microstructures and mechanical properties of the sintered ceramics were investigated. The Al2O3–Y2O3 additives enhanced the curing ability of the photosensitive SiC slurries by reducing the absorbance and scattering of UV light. The liquid phase accelerated the densification of the ceramics by particles rearrangement and interfacial bonding. Thus, the flexural strength of the ceramics increased from 41.0 MPa to 62.8 MPa with sintering temperature increasing.

Preparation of porous β-Si3N4/Si5AlON7 composite ceramics for digital light processing and study of mechanical and dielectric properties

Porous β-Si3N4/Si5AlON7 composite ceramics are anticipated to serve as high-temperature wave-transparent materials, yet the fabrication of high-performance porous β-Si3N4/Si5AlON7 composite ceramics remains a significant challenge. Addressing this issue, this study introduces a technique for crafting high-performance porous β-Si3N4/Si5AlON7 composite ceramics. This method involves the homogeneous blending of SiO2 and Si3N4 powders, followed by digital light processing and high-temperature liquid-phase sintering at 2000 °C. The impact of SiO2 content on various properties of the ceramic slurry and the resultant ceramics was thoroughly examined, including rheology, curing depth, microstructural morphology, phase composition, flexural strength, hardness, dielectric constant, and dielectric loss. It was observed that a SiO2 content of 30 % enables the slurry to fulfill the criteria for digital light processing, leading to the production of porous β-Si3N4/Si5AlON7 composite ceramics that exhibit outstanding mechanical and dielectric properties. Specifically, these ceramics demonstrated a flexural strength of 149.2 ± 7.2 MPa and a hardness of 11.8 ± 1.1 GPa. Moreover, at a frequency of 10 GHz, the ceramics exhibited a dielectric constant of 4.02 and a dielectric loss of 0.11, highlighting their potential as high-temperature wave-transparent materials.