Field Assisted Sintering Technique Research Articles

Fabrication of Boron Carbide Ceramics Reinforced with Silicon Carbide Fibers

In the present study, the boron carbide ceramics reinforced with silicon carbide fibers (B4C-SiCf) were fabricated by incorporating various contents of SiCf ranging from 2.5 to 10 wt.%. The B4C-SiCf powder mixtures were sintered using the Field Assisted Sintering Technique (FAST), also known as Spark Plasma Sintering (SPS), at temperatures of 1800 °C and 2000 °C for 5 min by applying a pressure of 70 MPa in argon atmosphere, while heating and cooling rates were maintained at 100 °C/min and 50 °C/min, respectively. The influence of the initial powder ratio on the sintering behavior, relative density, microstructural development, and mechanical properties of the obtained B4C-SiCf composites was investigated. The results indicated that the sintered ceramic materials contained only the initial compounds, i.e. B4C and SiC phases. Scanning Electron Microscopy (SEM) showed that the sintered composites consisted of densely compacted B4C grains. The highest relative density (> 99%) was achieved for the sample with 95 wt.%B4C and 5 wt.%SiCf, sintered at 2000 °C. Depending on the constituent content and the densification temperature the obtained composites demonstrated hardness values ranging from 29 to 43 GPa. The maximal hardness was attained in the composite with 95 wt.%B4C sintered at 2000 °C. To assess the performance of the composites under extreme conditions, the ablation resistance was evaluated using a flowing oxyacetylene torch test. The material containing 5 wt.%SiCf exhibited superior ablation resistance when compared to the other compositions. The attained results of this study showed that the SPS technique is a highly effective method for the densification of additive-free B4C-SiCf ceramic composites, showcasing promising properties for applications in extreme environments.

Microstructure and thermal property of W/Cu multilayer composites

In this study, a series of W/Cu multilayer composites were prepared by W and Cu foils bonded at temperatures ranging from 700 to 900 °C through field assisted sintering technique. The microstructure, bending property, thermal conductivity and thermal diffusivity of the W/Cu multilayer composites were investigated. The microstructure in the W layer is similar and the grains grow larger in the Cu layer with increasing bonding temperature. And the interfacial defects between the W layer and Cu layer decrease as the bonding temperature increases. Therefore, the interfacial bonding strength increases as the bonding temperature increases. The interfacial debonding disappears in the W/Cu multilayer composites bonded at 900 °C during three-point bending. Meanwhile, the W/Cu multilayer composites bonded at 900 °C show the highest thermal conductivity (234.6 W/(m⋅K). In addition, the thermal conductivity decreases with the increase of test temperature. The thermal expansion coefficient of the W/Cu multilayer composites bonded at 700 °C is highest. And the model showing the thermal expansion of the W/Cu multilayer composites has been proposed.

Effect of dopant solubility and excess doping on the superconducting properties of doped Nb3Sn prepared by field assisted sintering technique

The present work is focused on understanding the role of Ruthenium (Ru) and Yttrium (Y) on the superconducting properties of the A15 Nb3Sn intermetallic compound. Ru and Y shows different solubility limit within the Nb3Sn matrix. Ru and Y-doped Nb3Sn superconducting alloys were prepared through mechanical alloying (MA) route followed by sintering using Field Assisted Sintering Technique (FAST). Analysis by X-ray diffraction demonstrates a significant decrement in crystallite size up to 0.5 wt% for the Ru-doped Nb3Sn followed by a significant increment when doping level reaches 1 wt% Ru. This could be the combined effect of structural disorderliness and Ru dissolution within the Nb3Sn matrix. While a significant increase in the crystallite size is observed for 0.5 wt% Y-doped Nb3Sn and no further change up to 1 wt% Y. This change could be a result of limited solubility of Y within the Nb3Sn matrix. Meanwhile, excessive Y plays an important role in the formation of additional flux pinning centers. The critical current density (Jc) shows a significant increase for all the Y-doped Nb3Sn alloys by a factor of 29 % due to the formation of Y2O3 from the residual Y, which is assumed to serve as additional flux pinning center. The Y2O3 particles was also considered as a possible cause of pinning down of grain boundaries in addition to NbO leading to grain refinement through Zener pinning. Ru-doped Nb3Sn alloys also exhibit an increase in Jc compared to pristine Nb3Sn, but only in concentrations up to 0.5 wt% Ru. At higher applied field, even 1 wt% Ru shows slight increase in Jc than pristine Nb3Sn. Y-doped samples showed superior functional properties.

Effect of dopant solubility and excess doping on the superconducting properties of doped Nb3Sn prepared by field assisted sintering technique

Effect of dopant solubility and excess doping on the superconducting properties of doped Nb3Sn prepared by field assisted sintering technique

Evaluating the Impact of ZnO Doping on Electrical and Thermal Properties of Calcium-Aluminosilicate Oxynitride Glass-Ceramics

This study aimed to investigate the impact of ZnO content on the structure, thermal, and electrical properties of oxynitride glass-ceramic(s) within the Ca-Al-Si-O-N (CASON) system. The base glass had the composition of Ca7Al14Si17O52N7, with ZnO additions ranging from 3 to 15% by weight. A pristine Ca7Al14Si17O52N7 glass was successfully prepared by melt-quenching technique followed by converted into glass-ceramic by incorporating various amounts of ZnO using the field-assisted sintering technique. XRD and FESEM analysis confirmed that increasing the amount of Zn increases the crystallinity in the glass matrix. The observed crystalline phases were formed mostly from ZnO and showed higher conductivity than the remaining dielectric matrix. IR spectra confirmed the presence of bands correlated with the presence of Zn and suggested the progressive depolymerization of the silicate-aluminate network as a consequence of increasing Zn content. Density values varied between 2.75 and 2.94 g/cm3 and increased with increasing the Zn content in the glass-ceramic. The thermal expansion and thermal conductivity values increased and decreased, respectively, with the increase of Zn content in the matrix. The electrical properties of the samples were investigated using impedance spectroscopy over a wide range of frequencies (10 mHz to 1MHz) and temperatures (153 K to 623 K). The results showed that in the glass without ZnO and glass-ceramic(s) with a small addition of ZnO, the conductivity is mainly dominated by the transfer of oxygen ions and, to a small extent, by the presence of electronic conductivity. As the ZnO content increases, continuous conduction paths are formed between the ZnO crystallites, and the electrical conductivity increases rapidly and becomes dominated by electron transfer.

Exploring FAST Technique for Diffusion Bonding of Tungsten to EUROFERE97 in DEMO First Wall.

The European Fusion Reactor (DEMO, Demonstration Power Plant) relies significantly on joining technologies in its design. Current research within the EUROfusion framework focuses on developing materials for the first wall and divertor applications, emphasizing the need for suitable joining processes, particularly for tungsten. The electric field-assisted sintering technique (FAST) emerges as a promising alternative due to its high current density, enabling rapid heating and cooling rates for fast sintering or joining. In this study, FAST was employed to join tungsten and EUROFERE97 steel, the chosen materials for the first wall, using 50-µm-thick Cu foils as interlayers. Three distinct joining conditions were tested at 980 °C for 2, 5, and 9 min at 41.97 MPa to optimize joint properties and assess FAST parameters influence. Hardness measurements revealed values around 450 HV0.1 for tungsten, 100 HV0.1 for copper, and 390 HV0.1 for EUROFER97 under all joining conditions. Increasing bonding time improved joint continuity along the EUROFER97/Cu and W/Cu interfaces. Notably, the 5 min bonding time resulted in the highest shear strength, while the 9 min sample exhibited reduced strength, possibly due to Kirkendall porosity accumulation at the EUROFER97/Cu interface. This porosity facilitated crack initiation and propagation, diminishing interfacial adhesion properties.

Field-assisted sintering of high-entropy alloy-reinforced aluminium matrix composites: phase identification and microstructural properties

This study investigates the design, phase identification, and microstructural properties of high-entropy alloy (HEA)-reinforced aluminium (Al) matrix composites. Thermophysical expressions for HEAs were employed during the design phase of the HEA; both theoretical frameworks and experimental analyses were used to anticipate stable phases while a field-assisted sintering technique was employed to consolidate the samples. Calculation of phase diagram (CALPHAD) predictions for the phases present in the HEA align with valence electron concentration (VEC) calculations as both predicted the presence of BCC and FCC phases. The microhardness results reveal a substantial increase in the hardness value of the composites as compared to the pure Al, such that as low as 5 wt% HEA addition resulted in over a 100% improvement, while the densification of the composites was found to decrease with an increase in the wt% of HEA. SEM micrographs and XRD analyses show fair dispersion, bonding, and phase integration in the HEA-reinforced composites.

Plasma formation during flash sintering of boron carbide – Part I: Plasma characteristics

In so-called Field Assisted Sintering Techniques such as Spark Plasma Sintering (SPS) or Flash Sintering (FS), ceramic powder compacts are densified at high sample temperature in the presence of an electrical field. The formation of a plasma is often discussed without actual evidence for its appearance. In this work, which is the first of two parts, the mechanisms of plasma formation as well as its properties under FS conditions were investigated. Field induced breakdown of various atmospheres was conducted by a DC-field (up to 350 V/cm) at temperatures between 1000 and 1500 °C. Experiments were conducted both with and without a sample mounted between the electrodes. Alumina, boron carbide and silicon carbide in green or sintered form were used as the samples. The electrical and optical properties of the plasma were characterised, and the early stages were visualised by high-speed camera recordings. The results showed that the breakdown is caused by a flashover mechanism and the conductance of the plasma under the present conditions was of the same order as the conductivity of a boron or silicon carbide green body. Plasma formation did not lead to extensive sintering but the plasma was able to infiltrate the green body and cause transfer of matter from the interior to the surface and beyond. Based on the present results, it seems unlikely that a gas plasma is formed under the normal conditions of SPS or FS.

Flash sintering can induce an anisotropic microstructure in ceramics. A phase field modeling insight

Field-assisted sintering techniques are at the forefront of advanced energy-saving procedures to create fully dense ceramic materials. At present, there is an extraordinary interest in understanding the sintering process and effectively controlling the final specimen's microstructure. This latter aspect is critical, but unfortunately, the short sintering times and intense electric fields under non-equilibrium conditions prevent a rigorous prediction in most cases. This paper is a theoretical contribution to gain insight into an outstanding effect of applying intense electric fields: the formation of non-spherical grain structures. A phase field simulation approach has been carried out to reproduce and determine the origin of this effect. The final simulated microstructures are proved to be textured. These results are explained in the framework of a recent model and compared with experimental evidence reported in the literature.

Combined Effect of Particle Reinforcement and T6 Heat Treatment on the Compressive Deformation Behavior of an A357 Aluminum Alloy at Room Temperature and at 350 °C

Solid state sintering of cast aluminum powders by resistance heating sintering (RHS), also known as spark plasma sintering or field-assisted sintering technique, creates a very fine microstructure in the bulk material. This leads to high performance material properties with an improved strength and ductility compared to conventional production routes of the same alloys. In this study, the mechanical behavior of an RHS-sintered age-hardenable A357 (AlSi7Mg0.6) cast alloy and a SiCp/A357 aluminum matrix composite (AMC) was investigated. Aiming for high strength and good wear behavior in tribological applications, the AMC was reinforced with a high particle content (35 vol.%) of a coarse particle fraction (d50 = 21 µm). Afterwards, separated and combined effects of particle reinforcement and heat treatment were studied under compressive load both at room temperature and at 350 °C. At room temperature compression, the strengthening effect of precipitation hardening was about twice as high as that for the particle reinforcement, despite the high particle content. At elevated temperatures, the compressive deformation behavior was characterized by simultaneously occurring temperature-activated recovery, recrystallisation and precipitation processes. The occurrence and interaction of these processes was significantly affected by the initial material condition. Moreover, a rearrangement of the SiC reinforcement particles was detected after hot deformation. This rearrangement lead to a homogenized dispersion of the reinforcement phase without considerable particle fragmentation, which offers the potential for secondary thermo-mechanical processing of highly reinforced AMCs.

Laser-based resonant ultrasound spectroscopy for monitoring secondary phases in metastable Ti-alloys: From growth kinetics to high-throughput characterization

Metastable beta-Ti alloys undergo complex microstructural changes when annealed for extended times (hours, days, weeks) at temperatures below beta-transus. Contact-less RUS turns out to be a perfect tool for monitoring these processes, as well as for analyzing the properties of the alloys after the heat treatment. In the talk, we will illustrate this ability with three examples of recent advancements in this field. In particular, we will show that RUS is capable of analyzing (i) growth kinetics [1], from fast non-equilibrium phenomena to extremely slow processes; (ii) cubic symmetry conservation in single crystals with particles of various secondary phases [2]; and (iii) phase composition-elasticity relationships in series of oligocrystalline samples [3]. [1] Nejezchlebová et al. Elastic constants of β- Ti15Mo (2019) J Alloys Compounds, 792, pp. 960–967. [2] Olejňák et al. An ultrasound-based evaluation of cubic symmetry preservation and homogeneity in elastic behavior of β+ω and β+α Ti-alloys (2023) Materials and Design, 236, art. no. 112474. [3] Preisler et al., High-throughput characterization of elastic moduli of Ti-Nb-Zr-O biomedical alloys fabricated by field-assisted sintering technique (2023) J Alloys Compounds, 932, art. no. 167656.

Microstructure Characterization and Mechanical Properties of Polymer‐Derived (HfxTa1−x)C/SiC Ceramic Prepared upon Field‐Assisted Sintering Technique/Spark Plasma Sintering

The high‐temperature microstructural evolution and mechanical properties of two SiC‐based polymer‐derived ceramics with different Hf:Ta molar ratios are investigated using electron microscopy techniques and manipulated by nanoindentation. The as‐pyrolyzed ceramic powder consists of an amorphous Si(HfxTa1−x)C(N,O) structure (where x = 0.2, 0.7) with localized nanocrystalline transition metal carbides (TMCs). Subsequent application of the field‐assisted sintering technique (FAST) for high‐temperature consolidation results in a crystalline (HfxTa1−x)C/SiC ultra‐high temperature ceramic nanocomposite. The microstructure contains powder particle‐sized grains and sinter necks between the former powder particles. The powder particles consist of a β‐SiC matrix and small TMCs. Large TMCs are observed on the internal surfaces of former powder particles. This is due to the pulsed direct current and the resulting Joule heating that facilitates diffusion as well as oxygen impurities. Sinter necks of large β‐SiC grains form during the FAST process. The microstructural regions are assessed using high‐throughput nanoindentation. The hardness for SiC/(Hf0.7Ta0.3)C is measured on the formed grains and the sinter necks giving mean hardness values of about 27 and 37 GPa, respectively.

Pinning behaviour of carbon black reinforced MgB2 superconductor fabricated by field assisted sintering technique

In this study, the pinning behaviour of carbon black reinforced MgB2 bulk superconductors produced by in-situ field assisted sintering technique (FAST), also called spark plasma sintering (SPS) technique, was investigated. It is well known that the superconducting parameters of MgB2 are enhanced by chemical doping. Carbon-containing additives replace boron in the MgB2 crystal structure, creating pinning centres and increasing both the upper critical field and the critical current density. For this purpose, a series of polycrystalline samples with chemical composition MgB2-xCx (x = 0, 0.1, 0.2, 0.3 and 0.4) were prepared. The density of the samples was determined by the Archimedes principle and it was found that the relative density decreased with increasing carbon content. It is well known that carbon-containing additives suppress the transition or critical temperature (Tc) of the MgB2 superconductor. In this study, the critical temperatures of superconductors decreased with carbon black reinforcement. The critical current densities (Jc) of the samples were determined from the width of the hysteresis curves by using the Bean critical model, and it was determined that the critical current densities deteriorated with this reinforcement. This degradation in critical current densities was accompanied by a reduction in the irreversibility field (Hirr) and pinning force (Fp). It was concluded that the carbon black used as a reinforcement element did not act as a pinning centre.

Effects of Mechanical Milling and FAST Sintering on Mg Powders: Microstructural Analysis and Mechanical Properties

This study investigates the sintering mechanism of commercially pure Magnesium (Mg) using the Field Assisted Sintering Technique (FAST). FSEM and XRD analysis identified four distinct bonding and sintering mechanisms influenced by particle geometry, residual stress, and microstructure developed through mechanical milling. XRD analysis demonstrated variations in crystallite size and lattice micro-strain with increasing milling time. The combination of mechanical milling and FAST exhibited significant effects on the microstructural and mechanical properties of Mg powders, with the Mg36 sample displaying promising strength and hardness.

Nickel Coatings on Ceramic Materials Using Different Diffusion Techniques

Diffusion bonding is a process that has proven effective for the joining of metal to ceramic, but the differences in coefficient of thermal expansion still pose challenges during and after the bonding process. This work details the exploration of traditional diffusion-bonding processes using two traditional approaches, which include bonding of a 99.9+% pure Ni foil to SiC, Si3N4, and YSZ disks using (1) a hot isostatic press (HIP), with and without added weight to promote interfacial contact, and (2) field-assisted sintering (FAST). Samples were consolidated by heating to 1200 °C and held for 6 h under vacuum before cooling to room temperature during the HIP method. For the FAST technique, bonding experiments were performed at both 800 °C and 1200 °C in a vacuum environment under 10 MPa uniaxial pressure. After the Ni was bonded to the ceramics, diffusion heat treatments were carried out in the HIP. For electroless-plated samples, the heat-treatment temperature was chosen as 825 °C to avoid melting. For electroplated samples, heat treatment occurred at 925 °C or higher. Electroplated YSZ samples were heat-treated at 1150 °C as the Ni-Si eutectic is not a concern in this system. The time at temperature varied from 6 h to 48 h depending on the material combination tested. Post-heat-treatment diffusion characteristics were analyzed using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). A main cause of poor bonding performance in the HIP samples was reduced interfacial contact, while cohesive failures in the FAST samples are likely due to the formation of brittle intermetallic Ni-Si phases. Preliminary results indicate success in bonding Ni to SiC, Si3N4, and YSZ using a diffusion-enhanced approach on electroplated specimens.

Assessing wear and corrosion in oxide reinforced AlCrFeNi high entropy alloy fabricated via field assisted sintering technique

AlCrFeNi high-entropy alloy (HEA) reinforced with 4%wt TiO2 was fabricated through mechanical alloying (MA) followed by Field Assisted Sintering Technique (FAST) at 1100 °C temperature, 50 MPa pressure and 10 mins holding time, with subsequent investigation into FAST-induced powder deformation, microstructure, wear, and corrosion properties. TiO2 slowed down powder deformation during FAST of the AlCrFeNi, reducing density by 0.8%. The microstructure consisted of (Fe, Cr)-enriched BCC and (Al, Ni)-rich B2 phases, with the even distribution of a dark TiO2 phase in the AlCrFeNi HEA. The composite exhibited enhanced properties, including a high indentation resistance of 679 HV. The resistance to corrosion of the AlCrFeNi-TiO2 composite exhibited a substantial enhancement, as evidenced by a significant reduction in the corrosion current density (Icorr). Additionally, the wear tests unveiled improved tribological behavior in the composite, characterized by reduced friction coefficients (CoF) and wear rates. The wear mechanism was abrasive in the alloy and adhesive in the composite.

Processing of Long Ti-15Mo Alloy Rods by Upscaling the Field-Assisted Sintering Technique

Processing of Long Ti-15Mo Alloy Rods by Upscaling the Field-Assisted Sintering Technique

On the role of the electric field in the last-stage sintering of ceramics: A phase-field modelling approach

“Field-assisted sintering techniques” (FAST) are one of the landmarks for ceramists nowadays. From the beginning of their discovery, their physical mechanisms are a pending question. In this paper, the growth of a collective of 3D ceramic grains (with or without an electric field) has been modelled through phase-field. The model allows switching off thermal effects from pure non-thermal electrodynamical ones.The results show that strong electric fields affect the sintering process, but, contrary to usual statements, they retard grain growth. This effect weakens as temperature rises. Therefore, the origin of FAST techniques must have a thermal component A hypothesis is proposed: local non-homogeneities of transport properties at the grains are demonstrated to induce intense temperature gradients even in stationary conditions, and it can act as driving force for flash. The intrinsic cause of electric field-induced inhomogeneities must be related to local atomistic processes, which are still unknown.

Using photothermal radiometry to enable real-time sintering density measurements

Measurements of thermal diffusivity can increase speed, efficiency, and reliability of electronic field assisted sintering techniques.

Channelling electric current during the field-assisted sintering technique (FAST) to control microstructural evolution in Ti-6Al-4V

Perhaps the most defining feature of field-assisted sintering technology (FAST) is the application of an electric current, in addition to the uniaxial pressure, to create resistive heating in and around the sample region. However, with a few exceptions, most research takes this as an unchangeable part of the process. Here, this current flow has been directed to specific regions within the toolset, using boron nitride as electrically insulating material. This caused the heating to occur in differing regions within the Ti-6Al-4V sample and mould over four insulating configurations, with the shift in current density resulting in an extreme disparity in the final microstructures. The samples were imaged and analysed with deep learning in MIPAR, alongside comparisons with finite element analysis (FEA) models for 20 s and 5 min dwell times, to provide the technique with predictive capabilities for grain size and microstructure. The results gathered imply significant potential for this concept to improve the flexibility of FAST, and reduce negative effects such as undesirable temperature profiles in size scaling sintering for industry.