Spark Plasma Sintering Route Research Articles

A review on mechanical alloying and spark plasma sintering of refractory high-entropy alloys: Challenges, microstructures, and mechanical behavior

Refractory high-entropy alloys (RHEAs) are promising candidates for those applications requiring of strong materials at high temperatures with elevated thermal stability and excellent oxidation, irradiation, and corrosion resistance. Particularly, RHEAs synthesized using mechanical alloying (MA) followed by spark plasma sintering (SPS) has proven to be a successful path to produce stronger alloys than those produced by casting techniques. This superior behavior, at both room and high temperature, can be attributed to the microstructural features resultant from this powder metallurgy route, that include the presence of homogeneously distributed non-metallic particles, fine- and ultrafine-grained microstructures, and higher content of interstitial solutes. Nevertheless, the powder metallurgy fabrication relies over a complex balance of several operational variables, and the process is no exempt of certain challenges, such as contamination or the presence of pores in the bulk parts. This review aims to cover all the peculiarities of the MA + SPS route, the resultant microstructures, their mechanical properties, and the strengthening and deformation mechanisms behind their superior performance, as well as a brief description of their oxidation resistance.

Aluminum-based ceramic/metal composites with tailored thermal expansion fabricated by spark plasma sintering.

We have devised a moderate temperature spark plasma sintering route for preparing aluminum matrix composites which possess tailored coefficients of thermal expansion (CTEs) in combination with tunable electrical and thermal conductivities. Due to its isotropic negative thermal expansion over a wide temperature range, cubic-phase ZrW2-xMoxO8 (x = 0.0, 1.0) is an ideal secondary phase for metal matrix composites with suitable CTEs. In this study, high-density ZrW2O8/Al and ZrWMoO8/Al composites containing 30-70 vol% Al were fabricated using spark plasma sintering. X-ray diffraction analysis indicated that the composites were composed of a thermally-stable cubic phase at temperatures as high as 873 K for ZrW2O8 and 773 K for ZrWMoO8, without any orthorhombic high-pressure phase derived from the large thermal mismatch between the ceramic and metal during sintering. The thermal expansion curves of the ZrW2-xMoxO8/Al composites were consistent with the predictions made using the Rule-of-Mixtures. The CTEs could be controlled from negative to positive and even close to zero by simply varying the volume fraction of aluminum. Similarly, the thermal and electrical conductivity of the ZrW2-xMoxO8/Al composites increases with increasing Al content, which is thought to be mainly related to the contribution of the free electron conduction path of Al in the composites.

Fabrication of High-Entropy Alloys Using a Combination of Detonation Spraying and Spark Plasma Sintering: A Case Study Using the Al-Fe-Co-Ni-Cu System

The use of pre-alloyed powders as high-entropy alloy (HEA) coating precursors ensures a predetermined (unaltered) elemental composition of the coating with regard to the feedstock powder. At the same time, it is interesting to tackle a more challenging task: to form alloy coatings from powder blends (not previously alloyed). The powder-blend-based route of coating formation eliminates the need to use atomization or ball milling equipment for powder preparation and allows for the introduction of additives into the material in a flexible manner. In this work, for the first time, a HEA was obtained using detonation spraying (DS) followed by spark plasma sintering (SPS). A powder mixture with a nominal composition of 10Al-22.5Fe-22.5Co-22.5Ni-22.5Cu (at.%) was detonation-sprayed to form a multicomponent metallic coating on a steel substrate. The elemental composition of the deposited layer was (9 ± 1)Al-(10 ± 1)Fe-(20 ± 1)Co-(34 ± 1)Ni-(27 ± 1)Cu (at.%), which is different from that of the feedstock powder because of the differences in the deposition efficiencies of the metals during DS. Despite the compositional deviations, the deposited layer was still suitable as a precursor for a HEA with a configurational entropy of ~1.5R, where R is the universal gas constant. The subsequent SPS treatment of the substrate/coating assembly was carried out at 800–1000 °C at a uniaxial pressure of 40 MPa. The SPS treatment of the deposited layer at 1000 °C for 20 min was sufficient to produce an alloy with a single-phase face-centered cubic structure and a porosity of <1%. Interestingly, the hardness values of the as-sprayed and SPS-treated coatings were close to each other (~320 HV0.3). The hardness of the coatings measured in two perpendicular directions did not differ significantly. The features of the DS–SPS route of the formation of HEA coatings and its potential applications are discussed.

Evaluation of three-component boron-free glass fiber/TiO2/polyimide nanocomposite properties prepared by 3D turbula dispersion and spark plasma sintering method

Interest has grown in recent time on the development of three-component polymer nanocomposite with enhanced properties. However, the study focuses on the effect of TiO2 nanofiller on the mechanical, tribological, dielectric, and corrosion resistance properties of boron-free glass fiber (BGF) reinforced polyimide (PI) composite produced with spark plasma sintering route. Microstructure of the samples was examined using scanning electron microscopy. Mechanical, tribological, dielectric, thermal, and corrosion behaviour of the samples were characterized by nanoindenter, ball-on-disc, LCR meter, TGA, and potentiodynamic polarization test, respectively. The SEM results show that the introduction of the nanoparticles into the BGF/PI composite aids in more uniform distribution of the BGF particles within the PI matrix, and thus good interfacial interaction of the glass fiber and the PI matrix structure. Comparing the BGF/PI composite sample with BGF/TiO2/PI nanocomposite, it was observed that the nanocomposite depicted better hardness, elastic modulus, low friction coefficient and wear rate, dielectric, and enhanced corrosion properties. With TiO2 additions, hardness and modulus of the PI composite was improved by 56.6% and 10.1%, respectively. In addition, PI composites filled with TiO2 depicted a 0.07 coefficient of friction and wear rate of about 67.7% in reduction than that of neat PI and 9.0% in reduction than that of the pristine BGF/PI composites. Improved interactions between the reinforcements and the host matrix are suggested as the possible mechanism resulting to the desirable properties of the three-component PI nanocomposite recorded. Finally, the developed nanocomposite is suggested to be favourable for automobile, aerospace, and microelectronic device applications.

Magnesium-based composite by nano-nucleation of β-Mg17Al12 using spark plasma sintering route for advanced structural application

The influence of nano AlN on the microstructural evolution, mechanical (nanohardness, elastic modulus, microhardness, tensile, and yield strength), and tribological performance were examined. The outcome of the research suggested that the AlN inclusion at varying fraction have a direct connection to the enhancement of the investigated properties and a maximum result was achieved by the MgAZ91D-12 wt%AlN nanocomposite. The scanning electron microscope shows the nucleation and dissolution of the dominant β-Mg17Al12 found in the monolithic MgAZ91D following the addition of nano-AlN. Maximum microhardness of 84.0 ± 0.21 HV, nanohardness of 2.2 ± 0.037 GPa, the elastic modulus of 58.333 ± 0.26 GPa, elastic recovery (We/Wt) of 0.228, elastic strain to failure (H/Er) of 0.040, yield pressure (H3/Er2) of 0.0035 GPa, and the lowest plasticity index (Wp/Wt) of 0.7362 were achieved by MgAZ91D-12 wt%AlN. The wear rate of the sintered composite increases with the increase in the load applied but decreases with an increase in the percentage weight fraction of the nano AlN. The coefficient of friction result is observed to be high in unreinforced alloy and decreases as a function of the nano inclusion and the applied load.

Biotribological behaviour, biodegradability and osteocompatibility of Mg-3Zn/HA composite based intramedullary inserts in avian model

Bioactivity, structural integrity and tribological behaviour of biodegradable orthopaedic fracture fixing accessories considerably impact their actual performance in the body environment. Immune system in the living body quickly responds to the wear debris as foreign material and begins a complex inflammatory response. Magnesium (Mg) based biodegradable implants are widely studied for temporary orthopaedic applications, due to their similar elastic modulus and density to natural bones. However, Mg is highly vulnerable to corrosion and tribological damage in actual service conditions. To address these challenges via a combined approach, the Mg-3 wt% Zinc (Zn)/x hydroxyapatite (HA, x = 0, 5 and 15 wt%) based composites (fabricated via spark plasma sintering route) are evaluated in terms of biotribocorrosion and in-vivo biodegradation and osteocompatibility behaviour in an avian model. The addition of 15 wt% HA in the Mg-3Zn matrix has significantly enhanced the wear and corrosion resistance in the physiological environment. X-ray radiograph analysis of the Mg-HA-based intramedullary inserts implanted in the humerus bone of birds showed consistent progression of degradation and positive tissue response up to 18 weeks. The 15 wt% HA reinforced composites have shown better bone regeneration properties than other inserts. This study provides new insights into developing next-generation Mg-HA-based biodegradable composites for temporary orthopaedic implants, with excellent biotribocorrosion behaviour.

Development of functionally graded ZrB2–B4C composites for lightweight ultrahigh-temperature aerospace applications

In the present work, a lightweight three-layer ZrB2–B4C functionally graded composite material has been developed by spark plasma sintering route. The functionally graded material (FGM) is free from interlayer defects and displays a smooth transition between the individual layers. The composition of each layer was designed to reduce the overall density without sacrificing the functionality of the material. The density of the FGM is almost 40% lower than monolithic ZrB2 and 24% lower than ZrB2–30B4C rendering it potentially very attractive for high-temperature aerospace applications. A detailed structural characterization of the FGM was carried out to evaluate the elemental distribution in the graded composite, as well as determine the spatial distribution of the crystalline phases. Vickers hardness was measured within each layer and in the interlayer regions to further evaluate the gradient structure and interlayer transitions. Longitudinal elastic constants of the FGM along the thickness and across the layers measured using ultrasound phase spectroscopy showed that despite the gradient structure, the FGM can be treated as a quasi-isotropic solid.

High virucidal potential of novel ceramic-metal composites fabricated via hybrid selective laser melting and spark plasma sintering routes.

In this work, we combine selective laser melting (SLM) and spark plasma sintering (SPS) to fabricate new materials with high virucidal potential. Various bioactive disc-shaped ceramics, metal alloys, and composites were fabricated and tested against bacteriophage Phi6—a model system for RNA-enveloped viruses. We prepared silver-doped titanium dioxide (TiO2 + 2.5‒10% Ag), copper-doped titanium dioxide (TiO2 + 2.5‒10% Cu), Cu2NiSiCr, and Cu15Ni8Sn composite materials (metal lattices filled with ceramics). The virucidal tests of the ceramic and metal powders were performed in buffered suspensions, while the surfaces of the discs were tested by swabbing. The results show that the virus titer on the TiO2 + 10% Ag ceramic and CuNi2SiCr metal discs decreased by 4 logs after 15 min of exposure to the surfaces compared to the control ceramic and steel discs. We show that SLM 3D printed pre-alloyed CuNi2SiCr filled with bioactive TiO2 + 10% Ag nanopowders and sintered by the SPS process combines the simplicity of printing with the strength and virucidal properties of Ag and Cu materials. The proposed new virucidal materials were also used for the fabrication of prototype elevator buttons.

Comparative study of reactive and nonreactive spark plasma sintering routes for the production of TaB2‐TaC composites

AbstractThe influence of two different spark plasma sintering (SPS)‐based processing routes (i.e., reactive SPS [RSPS] and nonreactive SPS) on the properties of TaB2‐TaC composites was investigated. Ta2O5 and B4C powders were used as starting materials in the RSPS method, and synthesis and densification of TaB2‐TaC composites were accomplished in a facile single step. The effect of sintering temperature and time on the microstructure and densification of the in‐situ RSPS were investigated. The obtained results were compared with nonreactive spark plasma sintered TaB2‐TaC composites. The highest densification (∼99.5%) was achieved for the TaB2‐TaC composite with 6.64 vol.% TaC after reactive sintering at 1550°C under 40 MPa with a 5 min holding time. Although lower SPS temperature was used in the RSPS method, better densification and higher Vickers hardness were obtained compared to the nonreactive SPS. While platelet‐shaped TaC formation was observed in both processes, the average grain size was smaller in the RSPS method. On the other hand, no significant difference was detected in the oxidation behavior of the composites produced by RSPS and nonreactive SPS.

Microstructure and properties of FeSi6,5 soft magnetic materials prepared by spark plasma sintering

In this paper, FeSi6,5 (6.5 wt.% Si) soft magnetic materials have been prepared Via a Spark Plasma Sintering (SPS) technique at difference sintering temperatures in the range of 1150 to 1300 °c for 15 min and a heating rate of 100 °c/min. The results show that the density of sintered samples increased with the rising sintering temperature resulted in the enhancement of Vickers hardness and magnetic properities otsintered samples. Hovvever, the highest magnetic saturation (Ms) of 209.6 emu/g and lowest coercivity (Hc) of 1.85 Oe were obtained for the SPSed sample at 1250 °c. In contrast, the sample sintered in vacuum at 1300 °c for 1h with heating rate of 20 °c/min shows the lower magnetic properties due to the lower density and high amount of pores in the structure. The results of research show that the spark plasma sintering route has a high potential of fast sintering Fe-Si soft magnetic materials and for application.

Spark plasma sinterability of TiC ceramics with different nitride additives

This research aimed to assess the effect of various nitride additives, i.e., AlN, hexagonal BN, Si3N4, TiN, and ZrN, on the sinterability, microstructure, and mechanical-physical characteristics of TiC-based materials. All ceramics were sintered at the same sintering conditions using the spark plasma sintering (SPS) process. A relative density over 100% was secured by introducing 5 wt% AlN to the TiC matrix. By contrast, the Si3N4 additive had the most detrimental influence on the sinterability of TiC, resulting in a relative density of around 90.4%. Both ZrN and Si3N4 additives were fully consumed over the SPS route, contributing to the in-situ formation of (ZrC + TiN), and SiC ingredients, respectively. Additionally, the finest microstructure was obtained by using BN as an additive. The best flexural strength (688 MPa) was secured for the TiC-AlN sample, while the lowest (219 MPa) was related to the TiC-Si3N4 composite. Considering the hardness values, the highest number (3128 HV0.1 kg) was associated with the monolithic sample, followed by the TiC-AlN ceramic, at 3050 HV0.1 kg, while the lowest hardness of 2649 HV0.1 kg was attained for the TiC-ZrN composite. Ultimately, both BN and AlN additives enhanced the thermal conductivity of TiC, reaching values of 18.6 and 21.9 W/mK, respectively.

Synergistic effects of Si3N4 and CNT on densification and properties of TiC ceramics

A novel TiC-based material was fabricated by introducing Si3N4 and carbon nanotubes (CNTs) additives to enhance the thermal conductivity, flexural strength, and hardness of TiC. Spark plasma sintering route was used to produce four different specimens of TiC, TiC-5 wt% CNTs, TiC-5 wt% Si3N4, and TiC-5 wt% CNTs-5 wt% Si3N4. Though the individual impact of both additives on the relative density of TiC was detrimental, the co-addition of them resulted in a near fully dense composite. According to the results, some carbon left the crystalline structure of TiC, deposited at the grain boundaries as graphite flakes. However, the presence of CNTs in the microstructure stopped the formation of such flakes. The incorporated Si3N4 was completely used up over the SPS process, assisting in the in-situ generation of SiC. Additionally, the co-addition of Si3N4 and CNTs led to a decline in the mean grain size of the TiC matrix by almost 30%. Apart from this effect, the presence of both these additives provided a wider range of particle size and morphology, leading to a denser microstructure. However, the role of the added CNTs in promoting some chemical reactions in lower temperatures in the ternary system should not be overlooked. Finally, the ceramic co-reinforced with both additives presented the highest values of thermal conductivity (19.2 W/mK), hardness (3213 HV0.1 kg), and flexural strength (530 MPa).

Polarization induced dielectric and electrical response of electrovector hydroxyapatite and ferroelectric sodium potassium niobate ceramics

The ability of hydroxyapatite (HA) to convert readily into an electret under low polarizing fields (⩽1 kV cm−1) makes it as an excellent substitute for electro-active prosthetic implants. In the present study, compacts of electrovector HA and ferroelectric Na0.5K0.5NbO3 (NKN) were developed via the spark plasma sintering route. The compacts were then polarized at various polarizing fields of 30, 50, and 90 kV cm−1, respectively, at 150 °C, for 1 h. Thereafter, samples were depolarized at heating rates of 1, 5, and 10 °C min−1, respectively. The thermally stimulated depolarization current (TSDC) measurement reveals that the charge density in HA increases from ∼0.2 nC cm−2 to ∼2 µC cm−2 with an increase in the polarizing field from 30 to 90 kV cm−1. The dielectric (ϵ), as well as the ac conductivity behaviour of unpolarized and polarized (90 kV cm−1) HA, are compared. The activation energy is measured to be in the range of 0.6–0.7 eV, which is mainly associated with proton (H+) migration. For NKN, TSDC spectra reveal a slight shift in the phase transition temperatures (TO−T, TC) towards higher temperatures, with an increase in the polarizing field (30–90 kV cm−1) as well as the heating rate. The dielectric and ac conductivity measurements suggest that the polarization of NKN slightly decreases TO−T and TC, as compared to its unpolarized counterpart. The conductivity in NKN is mainly attributed to the migration of oxygen vacancies and n-type charge carriers. Furthermore, x-ray photoelectron spectroscopy analyses suggest that the polarization of HA at such a high field (90 kV cm−1) does not alter its surface chemistry.

Characterization of ZrB2–TiC composites reinforced with short carbon fibers

Solid solution formation, phase evolution, microstructure development, and mechanical behavior of ZrB2–TiC composites with/without carbon fiber additive were investigated. Fully dense composites were manufactured by spark plasma sintering route under 40 MPa load at 1900 °C for 420 s. FESEM and XRD studies were carried out to unveil the densification and toughening mechanisms in the sintered specimens. Results showed that the addition of only 1 wt% short carbon fibers increased the hardness from 21.8 GPa to 23.5 GPa. Moreover, a 3 % enhancement in the indentation fracture toughness was also observed due to the incorporation of short carbon fibers. The in-situ formation of ZrC and TiB2 phases were identified and related to a chemical reaction between the ZrB2 and TiC constituents. The solid solution development between the ZrB2 matrix and the in-situ formed TiB2 was also proved by the microstructural observation and the shifted peaks of ZrB2 in the XRD patterns.

Spark plasma sintering plus heat-treatment of Ta-doped Li7La3Zr2O12 solid electrolyte and its ionic conductivity

Ta-doped Li7La3Zr2O12 (Ta-LLZO) solid electrolyte ceramics were synthesized by solid-state sintering, spark plasma sintering (SPS) and SPS plus heat-treatment (two-step method) using LiOH·H2O, La2O3, ZrO2 and Ta2O5 as the raw materials. The Ta-LLZO samples prepared from these three routes are all in cubic phase structure. However, the solid-state method results in Ta-LLZO samples with different particle sizes and porous morphology, while the samples prepared by SPS route contain fewer internal voids and smaller particle size. The two-step method not only retains the high compactness of SPS samples, but also the increment of grain size. The SEM results demonstrate that Ta-LLZO sintered by the two-step method shows optimal electrical properties. And the total ionic conductivity is 4.6 × 10−3 S cm−1 at 150 °C and the activation energy is calculated to be 0.38 eV.

Structural and electrical properties of BaZr0.7Ce0.2Y0.1O3−δ proton conducting ceramic fabricated by spark plasma sintering

Spark plasma sintering (SPS) is an attractive method to obtain fully densified polycrystalline ceramics at rapid sintering rates. Cerium- and yttrium-doped barium zirconate, BaZr0.7Ce0.2Y0.1O3−δ, was prepared by a SPS route, resulting in high values of density of above 98%. Investigations of microstructural and crystallographic properties with determination of grain sizes were performed by means of Scanning Electron Microscopy, X-ray Diffraction and Static Light Scattering and discussed in correlation to bulk and grain boundary conductivities obtained by AC impedance spectroscopy. Samples prepared by SPS were generally found to consist of small grains that exhibit high bulk conductivities, whereas their grain boundary conductivities were comparably small, possibly also dominated by the presence of minor phases.

Influence of mechanically activated annealing on phase evolution in Al0.3CoCrFeNi high-entropy alloy

In the present work, the concept of mechanically activated annealing (MAA) has been applied to produce nanocrystalline Al0.3CoCrFeNi high-entropy alloys (HEAs) with reduced contamination levels. Phase evolution during conventional mechanical alloying (MA), MAA and subsequent consolidation by spark plasma sintering (SPS) have been studied in detail. Complete alloying is obtained after 15 h of MA, while milling time of 5 h and annealing at 1100 °C for 1 h have been used to achieve alloy formation during MAA. Both the MA–SPS and MAA–SPS routes have shown major FCC phase. The contamination of WC observed during MA was successfully eliminated during MAA, while the volume fraction of Cr7C3 reduced from 20% during MA–SPS to 10% after MAA–SPS. This method can serve as an effective way to produce nanostructured HEAs with minimum contamination.

Synthesis, microstructure and mechanical properties of a MoAlB ceramic prepared by spark plasma sintering from elemental powders

In this study, the high-pure MoAlB ceramic was successfully synthesized by the spark plasma sintering (SPS) method from elemental powders. The reaction process and intermediate products were determined by differential scanning calorimetry and X-ray diffraction analysis. It was established that the synthesis temperature could be as low as 800 °C via SPS route. Herein, the lattice constants of the synthesized MoAlB ceramic are confirmed to be a = 3.203 Å, b = 13.976 Å and c = 3.10 Å. It also can be established that the synthesized MoAlB ceramic by SPS method shows better enhanced comprehensive mechanical properties compared with the reported results. The Vickers hardness, compressive strength, flexural strength and fracture toughness are measured as 12.3 GPa, 1.67 GPa, 476.6 MPa and 6.5 MPa m1/2, respectively. No obvious delaminating deformation can be found on the fractured surfaces of MoAlB ceramic, despite the fact that the MoAlB ceramic has laminate structure. The transgranular and intergranular fracture play key roles during the compression and bending tests.

Sintering kinects of (K0.48 Na0.52)NbO3 lead-free piezoelectric ceramics densified by spark plasma sintering: Construction of master sintering curve

In this work, (K0.48Na0.52)NbO3 (KNN) ceramics were prepared by using Spark Plasma Sintering (SPS) method applying different heating rates. The microstructural evolution of the ceramics was analyzed and the results were correlated with possible sintering mechanisms. The analysis revealed homogeneous microstructures with nanometric grains. It was noticed that with increasing the heating rate the grain size decreases and the bimodal grain growth is suppressed. The Master Sintering Curve was constructed and an apparent activation energy of 660 kJ/mol was determined. From this result, it was possible to predict the possible densification mechanisms present in the KNN by SPS route.

Spark Plasma Consolidation and Wear Behaviour of Nanoceramic addition into Titanium Alloy

Abstract Ti–6Al–4V–TiN–TiCN powders were processed using spark plasma sintering route. The powder consolidation was carried out at the temperatures of 1000 °C and a constant pressure of 50 MPa with a holding time of 10 mins. Results indicate that with the sintering temperature of 1000 °C, the microstructure shows some level of rapid growth and provided the optimal combination of a dense microstructure. The fractography of the sintered samples at sintering temperature and holding time was observed to investigate the sintering mechanism as well. Wear behaviour was studied and worn surface of the composite showed improved abrasive wear resistance with non-continuous grooves as compared to the sintered Ti−6Al−4V without TiN/TiCN addition.