Pulse Electric Current Sintering Technique Research Articles

Densification, microstructure, and nanomechanical evaluation of pulsed electric sintered zirconia-silicon nitride reinforced Ti-6Al-4 V alloy

In this study, the influence of silicon nitride (Si3N4) and zirconia (ZrO2) ceramics was examined on the titanium alloy using the pulsed electric current sintering technique to investigate the microstructural behavior, densification, and nanomechanical properties of these composites. Si3N4 and ZrO2 were dispersed in Ti-6Al-4 V at a functional pressure of 50 MPa, a sintering temperature of 1200 °C, a heating rate of 100 °C/min, and a holding time of 10 min. The ternary composite samples were prepared viz, Cs1 (Ti6Al4V-3 vol.% Si3N4-15 vol.% ZrO2), Cs2 (Ti6Al4V-3 vol.% Si3N4-10 vol.% ZrO2), and Cs3 (Ti6Al4V-3 vol.% Si3N4-5 vol.% ZrO2). The bulk morphology of the composites was studied using scanning electron microscopy (SEM) equipped with EDS, and the phase contents were identified with an X-ray diffractometer (XRD). The relative density results for the tri-composites showed that the Cs1 sample recorded the highest at 99.94%, producing a fully dense sintered composite. However, there was a drop in the relative density of composites Cs2 and Cs3, recording 97.73% and 97.11%, respectively, comparable to the unreinforced Ti-6Al-4 V alloy with 98.65%. The nanoindentation examination conducted for the trio-composites showed linear mechanical responses/improvement, with Vickers hardness, from 589.31 to 865.70 MPa; nano hardness, from 6.466 to 9.441 GPa, and elastic modulus, from 113.52 to 185.95 GPa.

High-Temperature Oxidation Behavior of Ni/Al<sub>2</sub>O<sub>3</sub> Composites with Various Ni Content

Al2O3 composites dispersed with 1, 5, 10, and 15 vol% nickel particles were prepared using a pulsed electric current sintering technique. High-temperature oxidation was conducted at temperatures ranging from 1200°C to 1350°C for 6–24 h in air. After oxidation, the sample surface mainly consisted of NiAl2O4 oxide. The growth of the oxidized zone in all sample groups followed a parabolic pattern. The influence of the Ni content on the high-temperature oxidation behavior of Ni/Al2O3 is discussed. A kinetic model was proposed to explain the growth of the oxidized zone. The diffusion coefficient of oxygen ions through the grain boundaries of the Al2O3 matrix can be obtained using the proposed model, which is in agreement with those in the literature.

Nanostructure and nanoindentation study of pulse electric-current sintered TiB2–SiC–Cf composite

A carbon-fiber (Cf) doped TiB2–SiC composite was prepared and investigated to determine its densification behavior, micro/nanostructural properties, and mechanical characteristics. TiB2–25 vol% SiC–2 wt% Cf was prepared at 40 MPa and 1800 °C for 7 min using the pulsed electric-current sintering technique, and a relative density of 98.5% was realized. The as-sintered composite was characterized using advanced techniques, e.g., X-ray diffractometry, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, field-emission electron probe micro-analysis, and nanoindentation. The Cf additive could remove the surface oxide layers from the TiB2 and SiC domains, thus transforming them into TiB2 and SiC. According to micro/nanostructural studies, Cf could not retain its initial structure and was eventually converted into graphite nanosheets. In addition, the prepared composite was examined using the nanoindentation technique, and the following results were obtained for the calculated hardness, elastic modulus, and stiffness values: TiB2 > SiC > TiB2/SiC interface.

Insight into tribological and corrosion behaviour of binderless TiCxNy ceramic composites processed via pulsed electric current sintering technique

The study presents the corrosion behaviour and wear response of pulsed electric current sintered binderless TiC 50 N 50 , TiC 70 N 30 , and TiC 90 N 10 based ceramic composites, consolidated by spark plasma sintering (SPS), and the relative densities were evaluated using the Archimedes principle. The microstructural evolutions of the sintered samples were examined through various microscopy techniques, and their susceptibility to corrosion in aggressive chloride environment was assessed using open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. The microstructural examination of the specimens showed the presence of different phases within the titanium carbonitride (TiCN) cermets. The wear resistance evaluated using the frictional coefficient (COF) and calculated wear rate showed that the specimens exhibited an improved resistance. The specimens further showed enhanced resistance to corrosion in the test electrolyte, as the TiC 50 N 50 cermet displayed enhanced resistance to the aggressive chloride ions in comparison to the other specimens.

Crack-healing performance and oxidation behavior of SiC dispersed yttrium silicate composites

ABSTRACT Yttrium silicate-based composites dispersed with silicon carbide particles have been proposed as a novel material concept of self-healing environmental barrier coatings applied for Si-based ceramics and their composites. Crack-healing effectiveness as well as high-temperature oxidation behavior of the yttrium silicate composites was investigated as functions of oxidation time and temperatures. Dense samples of the composites dispersed with 5 vol% SiC particles were fabricated by using the pulsed electric current sintering technique. Thermal oxidation for self-healing was conducted at temperatures ranging from 1000°C to 1300°C for from 1 to 24 h in air. High-temperature oxidation experiments were carried out at temperatures ranging from 1200°C to 1400°C for from 1 to 60 h in air. As a result, the surface cracks with approximately 200 µm in length introduced on the sample surface were disappeared completely after heat treatment at 1300°C for 1 h in air. Crack-healing performance of SiC/Y2SiO5 is better than that of SiC/Y2Si2O7. The crack-healing performance of SiC/Y2SiO5-Y2Si2O7 is in the middle of that of SiC/Y2SiO5 and SiC/Y2Si2O7. The mechanism of crack healing was identified as the consequence of SiC oxidation into SiO2, which accompanies a volume expansion, and outward diffusion of Y3+ cations caused the formation of Y2Si2O7 outer layer. Oxidation of SiC particles within the matrix developed an oxidized zone. Growth of the oxidized zone obeyed the parabolic law, which meant diffusion process in the oxidized zone is the rate-controlling process.

Structural and Thermoelectric Properties of Solid\u2013Liquid In4Se3-In Composite

The aim of our work was to investigate thermoelectric properties of a composite of solid In4Se3 and solid or liquid indium. Polycrystalline In4Se3-In composites were prepared by a direct reaction of elements, powdering of products and sintering powders by pulsed electric current sintering technique. Microstructural and structural properties of obtained composites were analyzed using SEM + EDX and XRD techniques. Electrical transport properties and thermal conductivity were measured over a temperature range of 323 K ≤ T ≤ 673 K. Results show that the electrical conductivity of composite increases about four times in comparison with that of pristine In4Se3. The thermal conductivity decreases in a systematic way with the increase of In content and reaches a value of about 0.44 W m−1 K−1. As a result, the addition of indium enhances the thermoelectric figure of merit ZT from 0.8 to 1.2 at 673 K. However, we found that the melting of indium at about 430 K has no significant influence on thermoelectric properties of composites. We assume that the improvement of electrical properties is mainly due to the formation of point defects in In4Se3 phase and metallic properties of the In phase. To analyze formation energies of possible defects in In4Se3 structure, DFT calculations within the molecular cluster model were carried out. It was found that the In interstitial atoms are energetically more favorable than the formation of Se vacancy in In4Se3 structure.

Feasibility of a high stable PbTe:In semiconductor for thermoelectric energy applications

High-efficiency thermoelectric conversion is achieved by using materials with a maximum figure of merit Z = S2σ/κ, where S is the Seebeck coefficient, and σ and κ are the electrical conductivity and thermal conductivity, respectively, over a wide temperature range. Lead telluride alloys were some of the first materials investigated and commercialized for generators; however, their full potential for thermoelectrics has only recently been revealed to be greater than commonly believed. The maximal value of Z, as a function of electron density, is attained only for a specific location of the Fermi level EF relative to the conduction band edge EC. A systematic study of structural, microstructural, and thermoelectric properties of bulk PbTe doped with indium is presented. Samples were prepared by the pulsed electric current sintering technique. The high dimensionless figure of merit ZT ≈ 0.8 over 200–500 °C temperature range for PbTe doped with 0.05–0.1 at. % of In was obtained. Moreover, ZT is practically the same for Pb0.9995In0.0005Te and Pb0.99In0.01Te compounds at high temperature. Therefore, indium dopant in PbTe stabilizes the optimal location of the Fermi level. The effect of the negative process of indium diffusion into the matrix during the long service time of the TE generator could be avoided by doping heavily with indium the hot side of n-type functionally graded PbTe:In leg.

Sustainable thermoelectric materials fabricated by using Cu2Sn1-xZnxS3 nanoparticles as building blocks

Uniform Cu2Sn1-xZnxS3 (x = 0–0.2) nanoparticles (NPs) with a characteristic size of about 40 nm were chemically synthesized. The primary crystal phase of the NPs was wurtzite (WZ) with a mean crystalline size of about 20 nm. The NPs were sintered to form nanostructured pellets with different compositions preserving the composition and grain size of the original NPs by the pulse electric current sintering technique. The pellets had a zinc blende (ZB) structure with a residual WZ phase, and the mean crystalline size was found to remain virtually unchanged for all pellets. Among all samples, the pellets of Cu2Sn0.95Zn0.05S3 and Cu2Sn0.85Zn0.15S3 exhibited the highest ZT value (0.37 at 670 K) which is 10 times higher than that of a non-nanostructured Cu2SnS3 bulk crystal thanks to effective phonon scattering by nanograins, the phase-pure ZB crystal structure, and the increase in hole carrier density by Zn doping.

Self-Healing Behavior of Y<sub>2</sub>SiO<sub>5</sub> Toughened by SiC Particles

Crack-healing effectiveness of Y2SiO5-based composites dispersed with 5 vol% SiC particles was investigated. Dense specimens of 5 vol% SiC/Y2SiO5 were fabricated by pulsed electric current sintering technique. Surface cracks were introduced by using a Vickers indenter to be a crack length of approximately 200 μm. Self-healing performance of SiC/Y2SiO5 composites via high-temperature oxidation in air was studied as a function of heat treatment temperature. As a result, surface cracks disappeared perfectly after heat treatment at 1300°C for 6 h in air.

Influences of Al2O3 grain size on high-temperature oxidation of nano-Ni/Al2O3 composites

Two 5 vol% Ni/Al2O3 composites with the difference in Al2O3 grain size were fabricated by pulsed electric current sintering technique to investigate the influence of Al2O3 grain size on oxidation behavior of the composites. Average Al2O3 grain sizes of two fabricated composites were 1.1 μm and 0.5 μm after sintering. Oxidation tests were conducted at temperatures ranging from 1100 to 1350 °C for 1–48 h in air. A thin NiAl2O4 layer was observed in exposed surface of samples after oxidation. An oxidized zone that consisted of Al2O3 matrix and NiAl2O4 grains was defined. Growth of the oxidized zone obeyed the parabolic law. Influences of Al2O3 grain size on high-temperature oxidation of the composites were discussed.

Pulsed Electric Current Sintering of the Al<sub>2</sub>O<sub>3</sub> - 15wt% ZrO<sub>2</sub> Nanocomposites with 3wt% of Different Solid Lubricants

High temperature low friction materials are sought for use in engines in order to reduce energy consumption of the machines. Due to the high service temperatures solid lubricating materials are necessary. This study is designed to find the optimal processing conditions for preparing these materials by pulsed electric current sintering. In this study, the Al2O3- 15wt% ZrO2(AZ) nanocomposite was modified with 3 wt% of self-lubricating component (CaF2, BaF2, MoS2, WS2, h-BN, or graphite). After the preparation of the alumina-zirconia powder mixture solid lubricant powder was added. Powders were then mixed in ethanol for 24 h, dried in a rotary evaporator, and in oven at 80°C for 24 h. The particle size distribution of the powders was established with the laser method. Powders were compacted by using pulsed electric current sintering technique at 1300 °C with 50 MPa for 5 min in vacuum. The structure of the materials was studied with XRD and SEM. Density of the compacts was measured with the Archimedes method and their hardness was evaluated by applying HV1 hardness with the instrumented indentation techniques. Their mechanical behavior was further studied with the instrumented scratch testing.

Dielectric breakdown and thermal conductivity of textured alumina from platelets

A textured alumina was fabricated from anisotropic α-alumina particles (platelets) by heating at 1650°C for 15 min under applied pressure of 60 MPa using a PECS (Pulse Electric Current Sintering) technique. Subsequently, the thermal conductivity, mechanical properties and dielectric breakdown of the textured alumina were evaluated, are compared to those of conventional polycrystalline alumina and single crystal alumina. The results showed that the thermal conductivity of the textured alumina could be to exhibit almost the same dielectric breakdown as the conventional alumina, that is, the dielectric breakdown was 15 kV/mm and thermal conductivity was 36 W/mK, when the alumina was cut on the vertical plane in the load direction while sintering.

Abnormal morphology of nanocrystalline Mn–Zn ferrite sintered by pulse electric current sintering

Nanocrystalline manganese–zinc (Mn–Zn) ferrite powders prepared by the sol–gel auto-combustion method are sintered to form bulk ferrite by pulse electric current sintering technique. The sample phase, before sintering and after sintering, is carried out by X-ray diffraction (XRD). The morphology of the sample is observed by scanning electron microscopy (SEM). The results indicate that the bulk ferrite obtained has a pure spinel structure. With special graphite die, a special morphology is observed, which is explained by pressure, temperature and induced electromagnetic field.

Evaluation of Nano-Sized Nickel Dispersed Alumina Materials

In order to develop new ceramics for a power electric device, alumina/nickel (Ni) nanocomposites were fabricated using the soaking method and the pulse electric current sintering technique. The volume fractions of Ni were 0.5, 1, and 5% in γ–alumina powder. 10mass% α–alumina powder was added as a sintering seed. SEM and TEM observations revealed that the nanocomposites exhibited an intra-granular fracture mode, and agglomerated Ni particles existed mostly along alumina grain boundaries especially for 5vol% Ni. The experimental results showed that the strength and fracture toughness values of the nanocomposites were higher than those of monolithic alumina, and the thermal conductivity decreased slightly with increase in Ni content. Moderate improvements in the strength and fracture toughness seemed to result from the fact that the nano-sized Ni particles dispersed within alumina grains creating dislocations around the Ni particles, which reduced sintering residual stresses in alumina grains and expanded the frontal process zone size. It was concluded that nanocomposites with a lower Ni content had better mechanical, electrical, and thermo-mechanical properties.

Nanocomposites – Toughened Ceramics

In order to improve fracture toughness of ceramics, an intrinsically small frontal process zone (FPZ) size must be expanded. An intra-type nano-structure, where nano-particles are embedded within matrix grains, yields dislocations around the dispersed particles due to residual stresses. These dislocations become sessile dislocations at room temperature, operate as origins of small stress concentration in the matrix, and create nano-cracks in the FPZ. To produce the intra-type nano-structure, we developed a soaking method. This method makes it possible to produce nano-sized metallic particles dispersed within ceramic powders. In this research, alumina-nickel nanocomposite powder was obtained using the soaking method. The powder mixed with α-alumina as a seed was sintered using a pulse electric current sintering technique. The sintered nanocomposites are then annealed to disperse dislocations around the nanoparticles into alumina grains. Results showed that the maximum fracture toughness was 7.6 MPam1/2, which was two times higher than that of alumina.

Effect of α-Alumina as a Seed in Alumina/Silicon Carbide Composites

Intra-type structure of ceramic matrix composites (CMCs) can improve the mechanical properties of ceramic materials. In this work, we used γ-alumina as a matrix including nano-pore for fabricating alumina/silicon carbide composites using pressure-less sintering and pulse electric current sintering (PECS) techniques. We added α-alumina as a seed in order to improve densification of this commercially available γ-alumina. The mixture was sintered from 1250 to 1450 by pressure-less sintering and PECS techniques. Densification of the specimen sintered by pressure-less sintering is very difficult and bulk density of the specimen sintered at 1450 showed lower than 3.0 g/cc. In the case of the PECS technique, the density was improved to 3.8 g/cc in seed added specimen sintered at 1450. Maximum Vickers hardness and maximum fracture strength were obtained 17 GPa and 500 MPa for the seed added specimens sintered at 1450, respectively.

Enhance the heating rate of the PECS process by inserting a conducting body into BN powder compact

Pulsed electric current sintering (PECS), or the so-called spark plasma sintering (SPS), is a novel sintering process with the characteristic of short firing cycle. The process can be applied to densify metallic or ceramic bodies within a matter of minutes instead of several hours. The process is similar to the conventional hot pressing in the way that the powder compact is heated with the application of an external load. Nevertheless, instead of using an external heating source, a pulsed electric current is passed directly through the electrically conducting, such as graphite, mold. Tokita et al. claimed that the pulsed current generates spark discharge and even plasma between conducting particles [1]. They suggested that the generation of spark discharge or plasma removes the adsorbed species from the surfaces of particles during the initial stage of PECS. The grain boundary diffusion is thus enhanced in the later stage of the process. However, Wang et al. [2] claimed that the pulsed electric current sintering on alumina is similar to that of hot-pressing. The pulsed electric current heated the graphite mold first by Jouel heat, then the heat transferred inside into the powder compact. As the heating rate is fast, a temperature gradient exists. A dense outer region is thus observed within an alumina specimen after PECS [2]. Munir et al. suggested that the presence of pulsed electric field contributes to the fast densification [3]. Shen et al. suggested that the discharge can still take place between the non-conducting particles [4]. Many attempts have been made to explore the sintering mechanism of PECS. Though systematic studies have been conducted [1–5], the mechanisms for the densification and grain growth are far from clear. Nevertheless, most previous studies agreed upon the significance of PECS technique to the industrial production. In the present study, the presence of spark discharge between conducting particles is confirmed by inserting a conducting body into nonconducting particles.

Fabrication of Alumina-Based Toughened Nanocomposites

Highly toughened nickel dispersed alumina-based nanocomposites were fabricated using our developed soaking method, based on our previously proposed toughening mechanism of nanocomposites. Commercially available γ-alumina agglomerate with high porosity was used as a starting material. Nickel nitrate solution was penetrated into the nano-pores of the γ-alumina agglomerate using this new technique. The alumina-nickel composite powder with α-alumina seeds was then sintered using a pulse electric current sintering technique. To disperse dislocations generated around the nano-particles into alumina grains, the sintered materials were annealed. The results showed that the maximum fracture toughness was 7.6 MPam1/2, which was about two times higher than that of dense alumina.

Sintering and Mechanical Properties of AlZrC2

AlZrC 2 powder prepared by solid-state reaction was sintered using a pulse electric current sintering technique in a temperature range from 1700 to 2000°C under the pressure of 40 MPa in vacuum for 5 min. The density and mechanical properties of the sintered AlZrC 2 monolith were estimated. As heating temperature increased, the density of the AlZrC2 monolith increased and the mechanical properties were improved. The bulk density of the AlZrC 2 monolith sintered at 2000°C was 5.12 g/cm 3 . The maximum values of the fracture strength and Vickers' hardness were 380 MPa and 11.1 GPa, respectively.

In situ sintering bonding of oxide dispersion strengthened superalloys using pulsed electric current sintering technique

The in situ sintering bonding of oxide dispersion strengthened superalloys MA956 and MA754 was carried out using a pulsed electric current sintering (PECS) technique. Insert layers consisting of MA956 and MA754 powders were positioned between the respective bonding alloys, and PECS bonding was then carried out at 1023–1415 K for 0–64.8 ks applying a bonding pressure of 40–70 MPa. The number of pores between particles in the bond layer diminished with holding time. The densification behaviour of the bond layer during the PECS bonding process could be expressed by a kinetic equation with the sequential rate determining steps of plastic flow (creep) and volume diffusion. Transmission electron microscopy observations revealed that oxide strengthening particles of Y2O3 were dispersed finely and uniformly not only inside the powder particles but also at the prior particle boundaries. The creep rupture lives of MA956 and MA754 joints bonded at 1323 K for 54.0 ks and 1373 K for 64.8 ks were about 70% of that of the base metal in the longitudinal direction. The heat conduction analysis suggested that the rapid densification in PECS bonding could be attributed to the promotion of sintering by the local Joule heat generation (local increase in temperature) at the necked region. It was deduced that the superior mechanical properties of PECS bonded joints resulted from the fine and uniform dispersion of strengthening oxides in the bond layer.