Spark Plasma Sintering Cycle Research Articles

Production of complex microchanneled parts of ZrB2-MoSi2 ultra-high temperature ceramics by freeze casting and pressureless spark plasma sintering

A novel approach for manufacturing complex parts of ultra-high-temperature ceramics (UHTCs) is proposed, combining freeze casting with pressureless spark plasma sintering (SPS). With the aid of MoSi2 sintering additive (15 vol% or more), this combination enables the production of fully dense ZrB2-based UHTC parts at 1900 °C. Producing complex-shaped samples including an intricate network of internal microchannels seems feasible, by using additive manufacturing techniques in the production of templates for the external silicone mould and the internal microchannel network to be used during the freeze casting process. Although defects are prone to occur either during freeze drying, due to thermal stresses arising from the expansion mismatch between the internal resin template and the ceramic preform, or during the resin burn-out or SPS cycles, the resulting samples exhibited a fully dense microstructure and similar hardness (17 ± 1 GPa) as the bulk material. Thus, the proposed approach shows promise in the production of arbitrarily complex-shaped UHTC parts that may find application in numerous industrial sectors including aerospace, aviation, and energy generation and storage.

Spark plasma sintering of hard wear‐resistant graphene nanoplatelet–reinforced TiB2 + SiC composites from TiC–B4C–SiC

AbstractGraphene nanoplatelet (GNP)–reinforced TiB2 + SiC composites were fabricated by reactive spark plasma sintering (SPS) from a TiC–B4C–SiC powder mixture with equal‐volume percentages, optimizing their SPS temperature and evaluating their unlubricated sliding wear against diamond. First, it is shown that during the heating ramp of the SPS cycle, TiC and B4C react according to the chemical reaction 2TiC + B4C → 2TiB2 + 3C, and that the thus‐formed C is graphenized as GNPs, leading to composites with microstructures consisting of a ceramic matrix of fine TiB2 and SiC grains with abundant randomly oriented GNPs at grain boundaries. It is also shown that this reactive SPS is optimal at 2000°C (under 75 MPa pressure and 5 min soaking), resulting in a very hard (∼28.5–29.9 GPa) and very tough (∼6.7(3) MPa m1/2) composite. And second, it is shown that these two properties and its proneness to develop an oxide tribolayer make this composite very resistant to unlubricated sliding wear against diamond (∼2.8(1)·108 (N m)/mm3), undergoing only very mild two‐body abrasion. Finally, opportunities for the fabrication of toughened and very hard ceramic composites for contact‐mechanical and tribological applications are discussed.

Fabricating exotic boron carbide ceramics by shaker milling and reactive spark plasma sintering of B and C: Application to the elusive B6C

A route based on the shaker milling of B and C powders followed by reactive spark plasma sintering (SPS) of the resulting B+C powder mixture is presented for the fabrication of exotic boron carbide monolithic ceramics with controlled stoichiometry, and applied to the elusive B6C ceramics. Firstly, it is shown that shaker milling for only 1 h refines down to the nanoscale, mechanically activates, and intimately mixes the B and C powders, making the resulting B+C powder mixture far more sinterable. Secondly, it is shown that B and C react during the heating ramp of the SPS cycles, resulting in a fully dense superhard (∼30 GPa) boron carbide monolith with the desired stoichiometry (B6C in this case) and fine twinned-grain microstructure at an SPS temperature (1550 °C in this case) at which typical commercial B4C powders barely densify. This low-temperature densification is possible mainly due to the enhanced solid-state sinterability promoted by both shaker milling and structural ordering, with the exothermic heat released during the formation of the boron carbide contributing little. And thirdly, it is shown that there is an optimal SPS temperature above which the hardness of these exotic boron carbide monoliths decreases because there is less twinning hardening.

Spark-plasma sintering of boron carbide–silicon carbide composites at 1400 °C from B4C+Si: Densification and sintering/reaction mechanisms

The feasibility of fabricating novel boron carbide–silicon carbide composites by spark-plasma sintering (SPS) of B4C+Si powder mixtures at only 1400 °C was investigated. First, it is shown that B4C can be fully densified at 1400 °C if ~20 vol% Si aids are used, leading to bi-particulate composites constituted by boron carbide (major phase) and SiC (minor phase). The formation of these composites is due to the fact that Si acts as a reactive sintering additive during SPS. Lower and higher proportions of Si aids are not optimal, the former leading to porous bi-particulate composites and the latter to dense triplex-particulate composites with some residual free Si. Importantly, it is also shown that these novel boron carbide–SiC composites are fine-grained, nearly-ultrahard, moderately tough, and more affordable to fabricate, a combination that makes them very appealing for many engineering applications. Second, it is demonstrated that during the heating ramp of the SPS cycles a eutectic melt is formed that promotes full low-temperature densification by transient liquid-phase sintering if sufficient Si aids are used. Otherwise, a subsequent stage of solid-state sintering is required at higher temperatures once the eutectic liquid has been consumed in the in-situ formation of SiC. And third, it is demonstrated that during SPS the original B4C undergoes a gradual isostructural crystallographic transition towards a Si-doped carbon-deficient boron carbide that is more relevant with increasing proportion of Si aids, and it is identified that the carbon source for the formation of SiC is almost exclusively the carbon exsoluted from the B4C crystals themselves during their isostructural transition. Finally, implications of interest for the ceramic and hard-material communities are discussed.

Proportional Integral Derivative Control in Spark Plasma Sintering Simulations.

The prediction of microstructure evolution and densification behavior during the spark plasma sintering (SPS) process largely depends on accurate temperature regulation. A loop feedback control algorithm called proportional integral derivative (PID) control is a practical simulation tool, but its coefficients are often determined by an inefficient “trial and error” method. This paper is devoted to proposing a numerical method based on the principles of variable coefficients to construct an optimal linear PID controller in SPS electro-thermal simulations. Different types of temperature profiles were applied to evaluate the feasibility of the proposed method. Simulation results showed that, for temperature profiles conventionally used in SPS cycles, the PID output keeps pace with the desired profile. Characterized by an imperfect time delay and overshoot/undershoot, the constructed PID controller needs further advancement to provide a more satisfactory temperature regulation for non-continuous temperature profiles. The first step towards a numerical rule for the optimal PID controller design was undertaken in this work. It is expected to provide a valuable reference for the advanced electro-thermal modeling of SPS.

Morphology and luminescence of MgAl2O4 ceramics obtained via spark plasma sintering

Spark plasma sintering cycles were applied to a commercial magnesium aluminate spinel powder to evaluate its suitability for producing small-grained optical ceramics. Translucent stoichiometric samples with a crystalline grain size as small as ∼0.5 μm were obtained. Structural, microstructural (XRD and SEM), and correlated spectroscopic (transmission and cathodoluminescence) analyses were used to further investigate the defect structure and inversion degree of the material. We argue that the transparency is closely related to F and F+ centers, which can be distinguished in the absorption and luminescence spectra of the stoichiometric compositions.

Role of LiF additive on spark plasma sintered transparent YAG ceramics

Abstract It is well known that the presence of LiF as sintering aid to process transparent polycrystalline Y 3 Al 5 O 12 (YAG) ceramics by the Spark Plasma Sintering (SPS) technique induces three positive effects: a better particle rearrangement, a higher densification rate and the elimination of carbon contamination leading to the improvement of the material optical properties. However, the reaction mechanisms responsible of these effects still need to be clarified. Two YAG powders presenting different characteristics were selected to highlight the possible interaction between impurities and the sintering aid. In order to facilitate the determination of the LiF action during processing and reveal the involved phenomena, an excessive quantity of 5 wt% LiF was added. Samples were sintered at different temperatures between 800 °C and 1450 °C under 50 MPa. XRD, SEM and EDX techniques were used to characterise the microstructural development of ceramics and to establish the mode of action of LiF. The results highlighted that the role of LiF is mainly dependent on the purity and the phase composition of starting powders and that the SPS cycle must be adapted to the sintering aid characteristic temperatures.

In-situ synthesis and densification of HfB2 ceramics by the spark plasma sintering technique

Hafnium diboride (HfB2) ceramics were in-situ synthesized and densified by the spark plasma sintering (SPS) method using HfO2 and amorphous boron (B) as starting powders. Both synthesis and densification processes were succesfully accomplished in a single SPS cycle with one/two step heating schedules, which were designed by considering thermodynamic calculations made by Factsage software. In two step heating schedule, soaking at 1000°C, which was supposed to be the synthesis temperature of HfB2 particles, caused a creep like behaviour in final ceramic microstructures. A single step synthesis/densification schedule at 2050°C with a 30min hold time under 60MPa uniaxial pressure leads to obtain monolithic HfB2 ceramics up to 94% of it's theoretical density. Considering the literature, low hardness values (max. 12GPa) were achieved, which were directly attributed to the low bonding between HfB2 grains in terms of the residual stresses occurred during the synthesis and cooling steps. Samples produced by applying one step heating schedule showed transgranural fracture behaviour with a, fracture toughness of 3.12MPam1/2. The fracture toughness of the samples produced by applying two step heating schedule was higher (5,06MPam1/2) and the fracture mode changed from transgranular to mixed mode.

Finite-element modeling of the electro-thermal contacts in the spark plasma sintering process

Spark plasma sintering (SPS) is a breakthrough process for powder consolidation assisted by pulsed current and uniaxial pressure. In order to model the temperature variations of the tools during a SPS cycle, the Graphite-Papyex-Graphite contact phenomena are studied experimentally and modeled by finite element calculations. Compared to conducting materials, the thermo graphic image of an insulating sample (alumina) shows strongly localized heating along the Papyex implying contact effects are predominant. The aim of this modeling study is to determine the main contact phenomena due to Papyex. It is based on numerous experimental data and studies the case of alumina sintering. Finally the contact model is confronted to experimental thermal images.

Mechanical and Tribological Performance of Graphite/Silicon Nitride Composites: A Comparison between Pressureless and Spark Plasma Sinter Processing

Graphite particle (GP)-reinforced silicon nitride (Si3N4) composites were fabricated using pressureless sintering (PLS) and spark plasma sintering (SPS) in the presence of Y2O3-AlN-SiO2 ternary system. Different densification behaviors of the specimens fabricated by PLS and SPS were observed. While increasing GP content drastically reduced matrix densification during PLS at 2023 K (1750 °C) with 2 hours dwell, SPS at 1923 K (1650 °C) for only 10 minute under 50 MPa pressure resulted in much dense composites even up to 3.5 wt pct GP loading. Mechanical and tribological characterizations revealed that SPS-ed composites can offer improved performance compared to pure Si3N4. SPS-ed 1.5 wt pct GP/Si3N4 composite offered the highest resistance to wear up to 20 N normal load. Wear rate (W R) of that composite reduced by ~24 pct than that obtained for pure Si3N4 (W R ≈ 1.14 × 10−3 mm3/N m). Furthermore, W R of SPS-ed 2.5 wt pct GP/Si3N4 composite at F N = 10 to 20 N was found to be only ~1/8th of W R values of PLS-ed 2.5 wt pct GP/Si3N4 composite (10N W R ≈ 6.61 × 10−3 mm3/Nm; 20N W R ≈ 9.45 × 10−3 mm3/Nm). Present study indicated promising opportunity of SPS for fabricating improved Si3N4 composites through GP addition. The reinforcing particles not only rendered its self-lubrication effect to SPS consolidated composites but also significantly promoted the rate of matrix densification during SPS cycle by the virtue of its high thermal and electrical conducting nature.

Dog-bone copper specimens prepared by one-step spark plasma sintering

Copper dog-bone specimens are prepared by one-step spark plasma sintering (SPS). For the same SPS cycle, the influence of the nature of the die (graphite or WC–Co) on the microstructure, microhardness, and tensile strength is investigated. All samples exhibit a high Vickers microhardness and high ultimate tensile strength. A numerical electro-thermal model is developed, based on experimental data inputs such as simultaneous temperature and electrical measurements at several key locations in the SPS stack, to evaluate the temperature and current distributions for both dies. Microstructural characterizations show that samples prepared using the WC–Co die exhibit a larger grain size, pointing out that it reached a higher temperature during the SPS cycle. This is confirmed by numerical simulations demonstrating that with the WC–Co die, the experimental sample temperature at the beginning of the dwell is higher than the experimental control temperature measured at the outer surface of the die. This difference is mostly ascribed to a high vertical thermal contact resistance and a higher current density flowing through the WC–Co punch/die interface. Indeed, simulations show that current density is maximal just outside the copper sample when using the WC–Co die, whereas by contrast, with the graphite die, current density tends to flow through the copper sample. These results are guidelines for the direct, one-step, preparation of complex-shaped samples by SPS which avoids waste and minimizes machining.

Pulse analysis and electric contact measurements in spark plasma sintering

In order to model the current density distribution and the temperature changes of the tools used during a spark-plasma-sintering (SPS) cycle, the variation of the power delivered by an SPS machine and the graphite-Papyex®-graphite electrical contacts were studied experimentally. The electric device was also characterized; in particular current pulse characteristics and their behavior with time were studied in various conditions of temperature, pulses sequences, materials and total electric power dissipated. It is well known that the performance of an electric contact is dependent on the applied pressure and the temperature. First, by varying the pressure during the SPS cycle the effect of the electric contacts is clearly seen. Secondly, in order to determine the behavior of such contacts experimentally over a pressure range of 10–50MPa and temperatures of 50–800°C, a Dœhlert experimental design was used.

Densification of B4C nanopowder with nanograin retention by spark-plasma sintering

Densification of a commercially available B4C nanopowder by spark-plasma sintering (SPS) with nanograin retention was investigated, with emphasis on elucidating the potential of both single-step and two-step SPS cycles as well as the effect of pre-sintering annealing treatments of the nanopowders. It was found that single-step SPS does not allow a suitable processing window to obtain near-fully-dense B4C nanostructured ceramics from the as-received nanopowder, the cause being attributable to boria impurities producing coarsening. Pre-sintering annealing treatments of the nanopowder eliminate boria impurities thus facilitating the densification by single-step SPS, but at the expense of grain growth. Two-step SPS overcomes these drawbacks of the single-step approach (both without and with pre-sintering annealing treatments), enabling the desired densification of the as-received B4C nanopowder with nanograin retention (but nonetheless with a certain grain size heterogeneity). It was also found that the resulting ceramics are both superhard and toughened, highly desirable attributes in contact-mechanical and tribological applications.

Densification kinetics, phase assemblage and hardness of spark plasma sintered Cu–10 wt% TiB2 and Cu–10 wt% TiB2–10 wt% Pb composites

Abstract

Improving the mechanical reliability of cryomilled Al–Mg alloy using a two-stage spark plasma sintering cycle

Dense bimodal-grained aluminum magnesium alloys were produced by spark plasma sintering of cryomilled powders. The use of a two-stage sintering cycle did not induce grain growth or influence the hardness and flexural strength, but doubled the fracture strength distribution statistic (Weibull modulus) from 13 to 25. The increased duration of the second hold (from 5 to 20 min) marginally increased the Weibull Modulus, from 23 to 25.

Spark plasma sintering and hot compression behaviour of AZ91 Mg alloy

In the present investigation, the microstructural characterisation of the AZ91 Mg alloy produced by spark plasma sintering (SPS), as well as the evaluation of its hot compression behaviour, has been performed. Based on the differential scanning calorimetry analyses of the starting powders, three SPS cycles are investigated, using temperatures of 400 and 450°C, and at 450°C with previous solubilisation soaking at 420°C. Despite different microstructural and hardness characteristics, the three alloys display similar hot compression behaviour. At 200°C, the formation of an unstable crack, which propagates at 45° with respect to the loading axis, is observed after the occurrence of the peak load. At higher testing temperatures, after reaching the peak stress, the flow stress decreases slowly with increased strain of ∼0·51. Such behaviour corresponds with the observation of an accelerated cracking due to the propagation of decohesions at the interparticle regions. Ultimately, SPS allowed for attainment of high relative density; however, the sintering degree of the materials was quite low.

Identification of microstructural mechanisms during densification of a TiAl alloy by spark plasma sintering

This work aims at identifying, by coupled scanning and transmission electron microscopy (SEM and TEM) observations, the densification mechanisms occurring when an atomized Ti–47Al–1W–1Re–0.2Si powder is densified by spark plasma sintering (SPS). For this purpose, interruptions of the SPS cycle have been performed to follow the evolution of the microstructure step by step. The powder particles exhibit a classical dendritic microstructure containing a large amount of out-of-equilibrium α phase. During heating-up, the microstructure undergoes successive transformations. At T = 525–875 °C the α phase transforms into γ. The γ phase formed is supersaturated in W and Re. It de-saturates for T above 875 °C by discontinuous precipitation of W and Re-rich B2 phase. Densification takes place for T between 900 °C and 1150 °C by plastic deformation of the powder particles. TEM observations show that the repartition of the plastic deformation is correlated to the dendritic microstructure, and that dynamic recrystallization mechanisms occur. Microstructural phenomena directly resulting from the high currents involved in the SPS process have not been observed.

Transmission electron microscopy characterization of spark plasma sintered ZrB 2 ceramic

Microstructures of ZrB 2 ceramics consolidated by hot-pressing and spark plasma sintering were investigated by transmission electron microscopy (TEM), combining energy dispersive X-ray spectroscopy (EDX). The microstructures of both ceramics were compared. Amount of impurities was lower for ZrB 2 consolidated by spark plasma sintering than for hot-pressed ZrB 2. In particular, oxygen impurity was not detected even at the grain-boundaries in ZrB 2 consolidated by spark plasma sintering. The cleaning effect generated on the powder surfaces during spark plasma sintering cycle was displayed. In addition, dislocations were present only in the spark plasma sintered ZrB 2 ceramic, as a result of localized high stresses.

Reducing effect of contaminants in solid oxide fuel cell electrolyte by spark plasma sintering

Spark plasma sintering (SPS) technology was adopted to reduce the detrimental effect of SiO2 and Cr2O3 contaminants on 8 mol-% yttria stabilised zirconia (YSZ) electrolyte. Contamination of SiO2 not only significantly reduces the ionic conductivity of the YSZ electrolyte, especially grain boundary conductivity, it also retards the densification process during sintering. Also, the addition of 0.5 wt-%Cr2O3, whilst marginally reducing grain boundary conductivity, can promote the densification process regardless of sintering technique adopted, be it conventional electrical resistance heating or SPS. The effects of SPS sintering conditions, such as sintering temperature, isothermal dwell time, applied uniaxial pressure and number of repeated SPS cycles on the relative density and ionic conductivity of the contaminated YSZ electrolytes were investigated in detail. The results showed that SPS sintering could alleviate the harmful effect of contaminants on grain boundary conductivity, with minimal effect on the grain interior conductivity. It can also sinter the YSZ electrolyte to near full density (about 98%) in around 3 min.

Spark plasma sintering of Sm 2O 3-doped aluminum nitride

The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm 2O 3) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm 2O 3 as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm 2O 3 as sintering additive, with a thermal conductivity value about 118 W m −1 K −1) for the 3 wt.% Sm 2O 3-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm 2O 3 greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm 2O 3 dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts.