Spark Plasma Sintering and Densification Mechanisms of Conductive Ceramics under Coupled Thermal/Electric Fields

In spark plasma sintering (SPS), thermal and electric fields are applied simultaneously as a material is densified under pressure. The interactions between these two types of physical fields influence the densification behavior during SPS. Moreover, the uniformity and spatial distribution of these fields are also influenced by sample size. In the current investigation, the densification behavior of electrically conductive aluminum‐doped zinc oxide (AZO) ceramics is studied to provide insight into the role played by the thermal and electric fields on densification mechanisms, as a function of sample size. Our results demonstrate that field uniformity and densification behavior depend on sample size, and that ultimately, this behavior can be rationalized in terms of the electrical conductivity characteristics. Our results show that in small samples with a diameter of 20 mm, both thermal and electric fields are spatially uniform, which result in homogeneous microstructure. In large samples with a diameter of 80 mm, however, spatial variations in both thermal and electric fields lead to microstructural inhomogeneities, such as incomplete particle–particle bonding. Furthermore, as the density of the AZO sample increases, the effective electrical conductivity increases due to a decrease in void/pore volume, which changes the densification mechanisms, especially for the larger sample. Thus, for effective sintering of larger samples, a two‐stage sintering sequence is proposed, which relies on the thermal field that evolves once the effective electrical conductivity increases in the sample. We provide experimental confirmation to this suggestion on the basis of results which demonstrate that by extending the hold time from 3 to 30 min, high‐density (99.4%), homogeneous AZO ceramic samples with a diameter of 80 mm can be achieved after sintering at 1200°C.     

Numerical modeling of spark plasma sintering—Discussion on densification mechanism identification and generated porosity gradients

A multi-physical numerical model of the SPS process has been developed in order to evaluate electrical, thermal and mechanical fields undergone by the powder during sintering. The reliability of this numerical model has been validated by comparison with experimental data carried out on a submicrometric alpha alumina powder. This numerical approach is applied to achieve a critical analysis of the main methodologies used in the literature for the identification of the densification parameters, and the possible misinterpretations of involved deformation mechanisms. Moreover, as shown during sintering of alumina pellets of larger diameter (50 mm in diameter), the induced porosity gradient seems to be mainly correlated to the existence of thermal gradient within the powder, and in a lesser level to stress gradient. Consequently, the temperature gradient seems to be a crucial point to be controlled during SPS process of large size samples in order to obtain fine and homogeneous microstructure.     

Spark plasma sintering and hot pressing of ZTA-NbC materials – A comparison of mechanical and electrical properties

Zirconia toughened alumina can be made electrically conductive and thus electric discharge machinable by addition of a percolating dispersion of niobium carbide. In order to boost the productivity of the sintering process spark plasma sintering was tested at identical temperature and pressure but shorter dwell than in hot pressing. SPS sintering parameters for ZTA-NbC are developed and spark plasma sintered ceramics are compared to the hot pressed benchmark.During SPS a percolating NbC backbone of niobium carbide grains is formed which enhances electrical conductivity but impedes densification. Identical strength at however higher sintering temperature is achieved by SPS but the fracture resistance and hardness were always superior in hot pressed samples. The monoclinic content of zirconia grains in as fired SPS samples is higher despite smaller average grain size and the transformation toughening effect is less pronounced. SPS promises economic benefits due to shorter dwell and cooling cycles.     

Effects of Current Confinement on the Spark Plasma Sintering of Silicon Carbide Ceramics

Despite numerous works have recently reported the densification of silicon carbide (SiC) ceramics using the Spark Plasma Sintering (SPS) technique, the effect of the localized electric current flow near the specimen over the liquid phase sintering process of SiC ceramics and on their microstructural features has not been completely addressed. In the present work, two different SPS setups affecting current flow are selected, one based on the ordinary die/punch setup configuration, and the other employing a BN electrically insulating coating on the inner wall of the die to force the electric current to locally flow through the inner graphite foil in contact with the ceramic compact. The effective electrical resistance and the energy consumed during the SPS runs for both setups, as well as the sintering behavior, microstructure, and mechanical properties of the SPSed materials are analyzed. The BN die coating considerably increases the effective resistance of the system, decreases the power consumption, and accelerates the SiC densification. Besides, ceramic specimens experience significantly higher real temperatures than the set values and, accordingly, coarser microstructures and tougher materials than those for the ordinary setting are produced. The thermoelectrical properties of SiC materials are proposed as fundamental in their SPS process, especially when electrical current is forced through the inner part of the SPS setting around the specimen.     

Rapid Grain Growth in 3Y‐TZP Nanoceramics by Pressure‐Assisted and Pressure‐Less SPS

Pressure‐less spark plasma sintering (SPS) is a new approach during which rapid densification of ceramic nanopowder green bodies is accompanied by rapid grain growth. Although the origin of this phenomenon has not yet been fully understood significant, difference in grain growth between pressure‐less and pressure‐assisted SPS was expected. In this work 3Y‐TZP nanopowder with average particle size of 12 nm was consolidated using two‐step approach: (1) at an intermediate temperature (600°C to 1000°C) SPS warm pressing followed by (2) high temperature (1400°C to 1600°C) pressure‐less SPS. The standard one step pressure‐assisted SPS experiments were quoted as references. Rapid grain growth was observed during both pressure‐less and standard SPS. The samples prepared by both approaches at the same sintering temperature (1400°C–1600°C) achieved identical grain size and grain size distribution, if large pores were eliminated in early stage by SPS warm pressing. The electric current, electromagnetic field, and mechanical pressure is proven to have a negligible direct influence on grain growth in 3Y‐TZP ceramics at temperatures above 1000°C under standard SPS conditions.     

A Screening Design Approach for the Understanding of Spark Plasma Sintering Parameters: A Case of Translucent Polycrystalline Undoped Alumina

An experimental screening design was used to evaluate the effects of spark plasma sintering (SPS) parameters such as heating rate, sintering temperature, dwell duration, and green-shaping processing on the relative density, grain size, and the optical properties of polycrystalline alumina (PCA). It is shown that heating rate and sintering temperature are the most critical factors for the densification of PCA during SPS. Green-shaping processing could prevent grain growth at low SPS sintering temperatures. No predominant SPS parameters are observed on the optical properties. Hence, the optical properties of PCA are controlled by microstructural evolution during the SPS process.     

Spark plasma sintering microscopic mechanisms of metallic systems: Experiments and simulations

The microscopic densification mechanisms of metallic systems (TiAl, Ag-Zn) by spark plasma sintering (SPS) have been studied by simulations and experiments. Finite element simulations showed that, despite very high current densities at the necks between metallic powder particles (≈5 × 10⁴ A/cm²), only very limited Joule overheating can be expected at these locations (<1°C), because of very fast heat diffusion. The microscopic plasticity mechanisms under these high electric currents have been studied by transmission electron microscopy. For this purpose, thin foils have been extracted by focused ion beam at the necks between TiAl powder particles. This is the first time, to the best of our knowledge, that microscopic plasticity mechanisms at the necks between powder particles are investigated by TEM during densification of a metallic powder. Dislocation glide and climb mechanisms were identified, followed by recovery and recrystallization. The elementary mechanism kinetically controlling these phenomena is proposed to be bulk diffusion of Al, which activation energy (360 kJ/mol) is close to the activation energy measured for densification (308 ± 20 kJ/mol). Comparisons of densification kinetics by SPS (≈60-110 A/cm²) and by hot pressing (0 A/cm²) showed no influence of current on these mechanisms. Finally, reaction experiments in the Ag-Zn system did not show any influence of very high currents (>1000 A/cm²) on diffusion kinetics. Consequently, densification by SPS occurs by classical mechanisms not affected by the current.     

Direct Characterizing of Densification Mechanisms during Spark Plasma Sintering

Spark plasma sintering (SPS) is a convenient and rapid means of producing dense ceramic compacts. However, the mechanisms responsible for rapid densification have not been identified satisfactorily, with different studies using an indirect approach yielding varied values for the densification parameters. This study involved SPS in high purity nanocrystalline alumina with temperatures ranging from 1173 to 1423 K and stresses from 25 to 100 MPa. A direct approach, with analyses at a constant density, revealed a stress exponent of ~1 and an inverse grain size dependence of ~3, consistent with Coble creep process. Whereas the direct approach gives a stress exponent of ~1, the indirect approach used previously gives stress exponents ranging from ~2.2 to 3.5 with the same data, thereby revealing potentially spurious values of the densification parameters from conventional indirect approaches to characterizing densification. The rapid densification during SPS is related to the finer grain sizes retained with the rapid heating rates and the imposed stress that enhances the driving force for densification.     

Flash sintering of Al2O3–ZrO2 ceramics under alternating current electric field

In this study, the influence of alternating current (AC) electric field on flash sintering and microstructural evolution of alumina–zirconia (Al₂O₃–ZrO₂) composite was systematically investigated at furnace temperature of 800 °C. Compared with direct current (DC) electric field, AC electric field not only promoted densification and grain growth of Al₂O₃–ZrO₂ composite, but also improved the uniformity of microstructure of ceramics. Grain size of AC flash-sintered samples was found to be inversely related to electric field, and positive correlation was observed with current density limit. Dense Al₂O₃–ZrO₂ composite ceramic was fabricated via AC flash sintering under 60 mA mm^?2 at low furnace temperature within 120 s, and as-sintered samples exhibited relatively good mechanical properties. The mechanism involving synergistic effect of Joule heating and defects generation under the influence of electric field was proposed to explain rapid densification during AC flash sintering. These results indicate the feasibility of preparation of dense composite ceramic with homogeneous microstructure via AC flash sintering.     

Modelling of elimination of strength-limiting defects by pressure-assisted sintering at low stress levels

The structural reliability of sintered products depends on large defects introduced during powder processing, which cannot be removed by pressureless sintering. Here, we present a model how a large single ellipsoidal void is deformed, and finally disappears by pressure-assisted sintering. Taya-Seidel’s model is applied to predict the shrinkage of a large void in a compressible linear viscous material by using bulk viscosity, shear viscosity, and sintering stress that are determined experimentally for sintering of alumina powder at low stress levels. The application of mechanical stress promotes the densification rate. Its effect is maximum for hot isostatic pressing (HIP) and minimum for sinter forging. The effect is intermediate for hot pressing (HP) and spark plasma sintering (SPS), because the hydrostatic component of stress varies with densification. While a crack-like defect can be removed during densification, a spherical void must be eliminated by shear deformation in the final stage during dwell time.     

Microstructure homogeneity control in spark plasma sintering of Al2O3 ceramics

Homogeneous microstructure control in the SPS (spark plasma sintering) sintered big size Al₂O₃ ceramic was realized by the synergy effect of grain boundary tailoring and proper pressure profile design. Two-step pressure profile itself did not show any efficient densification enhancement if no grain boundary modifier MgO added. The two-step pressure profile can effectively reduce average grain size and grain size difference over the sintered specimen, while MgO doping can reduce the average grain size in the whole sintered samples. Finally, a general strategy to overcome the intrinsic temperature gradient in SPS is suggested.     

Review of flash sintering: materials,mechanisms and modelling

Flash sintering (FS) is an energy efficient sintering technique involving electrical Joule heating, which allows very rapid densification (<60?s) of particulate materials. Since the first publication on flash-sintered zirconia (3YSZ) in 2010, it has been intensively researched and applied to a wide range of materials. Going back more than a century ago, we have found a close similarity between FS of oxides and Nernst glowers developed in 1897. This review provides a comprehensive overview of FS and is based on a literature survey consisting of 88 papers and seven patents. It correlates processing parameters (i.e. electric field magnitude, current density, waveforms (AC, DC) and frequency, furnace temperature, electrode materials/configuration, externally applied pressure and sintering atmosphere) with microstructures and densification mechanisms. Theorised mechanisms driving the rapid densification are substantiated by modelling work, advanced in situ analysis techniques and by established theories applied to electric current assisted/activated sintering techniques. The possibility of applying FS to a wider range of materials and its implementation in industrial scale processes are discussed.     

Kinetic Analysis of Solution-Precipitation During Liquid-Phase Sintering of Alumina

Densification controlled by solution-precipitation during liquid-phase sintering was analyzed for the aluminamagnesium aluminosilicate glass system. As a model system for liquid-phase sintering, narrowly sized alumina powders and up to 20 vol% magnesium aluminosilicate glass samples were isothermally sintered at 1550° to 1650°C. Densification rate increases with increasing liquid content and sintering temperature but decreases with increasing density. For samples with >15% grain growth, the densification rate during the solution-precipitation stage of sintering was proportional to (particle size)⁻² and thus interface reaction-controlled. Activation energies ranged from 270 to 500 kJ/mol over the relative density range of 66% to 96%, respectively. The low activation energy is attributed to densification by particle rearrangement, whereas the higher activation energy is due to densification controlled by interface-reaction-controlled solution-precipitation. Intermediate activation energies are attributed to simultaneous densification by the two mechanisms.     

Pulse Electric Current Sintering of Al2O3/3 vol% ZrO2 with Constrained Grains and High Strength

The pulse electric current sintering technique (PECS) was demonstrated to be effective in rapid densification of fine-grained Al₂O₃/3Y-ZrO₂ using available commercial powders. The composites attained full densification (>99% of TD) at 1450°C in less than 5 min. The composites sintered at a high heating rate had a fine microstructure. The incorporation of 3 vol% 3Y-ZrO₂ substantially increased the average fracture strength and the toughness of alumina to as high as 827 MPa and 6.1 MPa·m^1/2, respectively. A variation in the heating rate during the PECS process influenced grain size, microstructure, and strength, though there was little or no variation in the fracture toughness.     

Densification of alumina by SPS and HP: A comparative study

In this study, the densification of alumina by spark plasma sintering (SPS) was investigated and compared to conventional hot pressing. It was shown that SPS is very effective in the sintering of alumina leading to higher densities and allows to work at lower temperatures and with shorter sintering cycles. The effect of the heating rate is dependent on the heating mode (SPS or HP). The identification of active sintering mechanisms was attempted by an isothermal and an anisothermal methods, showing that other mechanisms probably related to electrical effects enhance the densification. We suggest the higher contribution of surface diffusion mainly during the initial stage of sintering and an influence of the presence of impurities segregated at the grain boundaries. They could create conductive layers and also introduce ions with a lower valence than Al³⁺; defects are created in the surface layers and the diffusion of the species is increased.     

Oxidation effects on spark plasma sintering of molybdenum nanopowders

Surface-oxidized molybdenum nanopowders are compacted by spark plasma sintering (SPS). The oxide impurity behavior is analyzed under various sintering temperatures. The densification mechanism of the nanopowders with a melted oxide phase is identified in situ by regression analysis of the experimental data on the temperature-dependent porosity change and on the SPS multistep pressure dilatometry. To increase the density of the compacted pellets, the nanopowders with the oxide phase are consolidated by SPS using the two in situ oxide removal methods: carbothermic reduction and particle surface cleaning by the electric current flow through the powders. The advantages and disadvantages of these methods in terms of the density, grain size, and mechanical properties of the final products are discussed.     

Transparent polycrystalline alumina using spark plasma sintering: Effect of Mg,Y and La doping

Transparent polycrystalline alumina (PCA) is a promising replacement for sapphire. Its optical properties however are highly dependent on the grain size and residual porosity which need to be controlled for real inline transmittances (RIT), that are high enough for possible applications.To achieve high RITs, doping as well as pressure assisted sintering is often used. In this study spark plasma sintering (SPS) and doping are investigated. A systematic experimental design is used to study the influence of Mg, Y and La single or co-doping (75–450 ppm) as well as the SPS sintering pressure and temperature on the RIT and grain size of PCA.Using optimized sintering parameters, RITs of >50% were attained in the visible wavelength (640 nm) for 0.8 mm thick samples for almost all doping strategies. The best RIT of 57% was for triple-doped samples at a total dopant level of 450 ppm. These results are significantly better than previously published SPS studies and illustrate that SPS sintered alumina can attain high and reproducible optical transmittances under various doping and sintering conditions.     

Spark plasma sintering of UO2 nanopowders: Pressure,heating rate and current effects

We analysed with different methods the densification of UO₂ nanopowders in SPS under constant heating rate (CHR) and isothermal sintering conditions. The apparent activation energy of densification in SPS (75 kJ/mol with CHR method) is significantly smaller than in conventional sintering. It is shown that this is likely not an effect of the applied current. We also observed a threshold stress at 64 MPa for the transition from pressure-insensitive sintering (stress exponent n≈0) to pressure-assisted sintering, suggesting that the contribution of the capillary stresses in such nanopowders is comparable with the typical stress applied in SPS.     

Densification of complex shape ceramics parts by SPS

Spark Plasma Sintering (SPS) equipments, with higher productivity and hybrid heating modes, allow reducing thermal gradients in large samples and broaden their application potential. Furthermore, strong shape limitations remain: only simple shapes can be obtained because of the use of uniaxial pressure.The aim of this paper is to propose a SPS densification method for complex shape ceramic or powder metallurgy parts, without any modification of the tools and equipment. The samples to be sintered are placed in a powder bed in the classical die used in SPS. Different powder beds have been tested.A numerical model was developed in order to allow a rapid simulation of the force field around a part surrounded by a powder bed in a SPS tool. The optimization of the operating conditions was carried out, first, on small simple-shape samples, then, on complex geometry parts which were densified to demonstrate the feasibility of the proposed sintering process.     

Effects of heating rate on microstructure and transparency of spark-plasma-sintered alumina

Commercial alumina powder was densified by spark plasma sintering (SPS) at 1150 °C. During SPS processing, the effects of the heating rate were examined on microstructure and transparency. With decreasing heating rate, the grain size and the residual porosity decreased, while the transparency increased. At a heating rate of 2 °C/min, the grain size was 0.29 μm, and the in-line transmission was 46% for a wavelength of 640 nm. The mechanisms for the fine microstructure and low porosity at slow heating, which are conflicting with some existing results, were explained by considering the role of defect concentration and grain-boundary diffusion during densification.