Pulsed Electric-current Pressure Sintering Research Articles

Fabrication of dense ZrB2/B4C composites using pulsed electric current pressure sintering and evaluation of their high-temperature bending strength

ZrB2/x·vol%B4C (x = 30–90) composites were fabricated from ZrB2 and amorphous B/C powders using pulsed electric current pressure sintering (PECPS) from 1600 °C to 1900 °C for 6.0 × 102 s (10 min) under 50 MPa in a vacuum, accompanied by self-propagating high-temperature synthesis (SHS). Since the B4C phase was formed at 1600 °C, the relative density (Dr) was evaluated; the composites sintered at 1900 °C attained the highest Dr. Their Dr values increased gradually from 99.35% to 99.99% with increasing B4C contents up to 60 vol% and showed a constant value above 60 vol%. At room temperature, the mechanical properties of Vickers hardness (Hv), fracture toughness (KIC) and three-point bending strength (σb) were measured. Hv exhibited a monotonous increase from 20.3 to 32.7 GPa. On the other hand, KIC and σb revealed the same behavior for each of the compositions; both exhibited the highest values, i.e., 10.2 MPa m1/2 for KIC and 870 MPa for σb, in the 60 vol%B4C sample, and then the KIC decreased gradually to 9.73 MPa m1/2, and σb dropped suddenly from 850 MPa (70 vol%) to 340 MPa (80 vol%) and stayed as low σb in the 90 vol% B4C sample. Next, the high-temperature σb values of the composites (40–70 vol%) were measured in Ar. The composites (40–60 vol%) revealed high σb (≥640 MPa) from R.T.~1600 °C; the maximum value of 803.5 MPa was observed for the 60 vol%B4C composites at 1600 °C, and then the σb of all composites dropped to around 340 MPa at 1800 °C. From their stress-strain curves, elastic and plastic deformations were observed at 1600 °C and 1800 °C, respectively.

Simultaneous Synthesis and Densification of Carbon Nano-Materials Dispersed Boron Carbide Composites Using Pulsed Electric-Current Pressure Sintering (PECPS)

Dense [boron carbide (B4C)]/[carbon nanomaterials] composites were synthesized and sintered simultaneously using pulsed electric-current pressure sintering (PECPS) at 2173 K for 6.0×102 s (10 min) under 50 MPa in a vacuum. The starting powders were amorphous B and C nanopowders and nanocarbons. The latter were acid-treated carbon nanofiber CNF and carbon nanotube CNT. The sintered composites were evaluated from the viewpoints of mechanical properties at high temperatures up to ~ 2023 K in inert atmosphere. Thus fabricated composites with 10vol%CNF maintained high bending strength σb around 750 MPa even at 1973 K; this temperature is 100 K higher than that of conventional B4C/CNF composites, and furthermore 600 MPa at 2023 K. These high σb at elevated temperatures might be explained by both the low content of catalytic Fe particles and the rough surface of CNF after the acid-treatment. On the other hand, B4C/CNT composites displayed 770 MPa at 1723 K. The stress-strain curves demonstrate that B4C/CNF composite deformed elastically until 1273 K and plastically up to 1973 K, however, the B4C/CNT composites displayed elastic deformation up to around 1873 K.

TiC/TiCN-based hard material with high oxidation resistance at high temperature

TiC/TiCN-based cermet is a candidate material for alternatives to WC-Co used in tools and dies. Typically, Ni and Co are used as metal binders of TiC/TiCN-based cermets. Being types of rare metals, it is therefore desired that these metal binders be replaced with the ubiquitous materials. Iron aluminide (FeAl) intermetallic compound is a candidate material for an alternative rare metal binder made from ubiquitous materials. FeAl is well known as an extremely corrosion-resistant material under oxidizing atmospheres, sulphidizing atmospheres, and in molten salts. Although TIC/TiCN-FeAl is a combination of intermetallic compound and carbide with very high hardness, it is brittle. TiC/TiCN-FeAl was processed through wet milling and pulsed electric current pressure sintering to improve mechanical properties, thus improving transverse rupture strength while keeping hardness intact. In this work, we attempted to improve the oxidation resistance performance of TiC/TiCN-FeAl at high temperature by changing raw materials.

Fabrication of dense B4C/CNF composites having extraordinary high strength and toughness at elevated temperatures

Fully dense B4C/CNF composites have been fabricated from the mixture of amorphous fine powders of boron and carbon, and carbon nano-fiber (CNF) using pulsed electric-current pressure sintering (PECPS) at 2173K (1900°C) for 10min under 30MPa in a vacuum. They showed a moderate mechanical properties at room temperature: bending strength σb more than 450MPa, Vickers hardness Hv of 29–36GPa, and fracture toughness KIC ranging from 4.1 to 5.7MPam1/2. However, they revealed extraordinary high strength more than 600MPa at 1673–1873K (1400–1600°C) in an inert gas atmosphere, especially, σb of 830MPa at 1773K (1500°C) (12.5vol% CNF added composite) and σb of 800MPa at 1873K (1600°C) (10.0vol% CNF added composite). In addition, these B4C/CNF composites also showed much improved high-temperature fracture toughness by a factor of around 20, in comparison with monolithic B4C ceramics at 1773K (1500°C). These extraordinary both high strength and toughness might be explained in terms of the suppression of grain growth by CNF at elevated temperatures, an inverse strength temperature-dependence for carbides, and anchor effect induced by CNF.

ZrO<sub>2</sub>(Y<sub>2</sub>O<sub>3</sub>)-Al<sub>2</sub>O<sub>3 </sub>系固溶体粉体を用いて作製したジルコニア系セラミックスにおける機械的特性のAl<sub>2</sub>O<sub>3 </sub>組成依存性

ABSTRACTZrO2 composite ceramics containing 10~30 mol% Al2O3 and 1.5 mol% Y2O3, i.e., 90~70 mol% ZrO2(1.5 mol% Y2O3)-10~30 mol% Al2O3 ceramics, have been fabricated by sintering cubic ZrO2 solid solution (ss) nano-powders prepared via the neutralization co-precipitation method. The sintering was performed using a pulsed electric-current pressure sintering (PECPS) at 1598 K for 10 min under 50~60 MPa in Ar. Dense tetragonal-ZrO2 (ss) ceramics containing α-Al2O3 were composed of 277-130 nm grains. FE-SEM observation revealed that their microstructures, that is the grain sizes of both t-ZrO2 (ss) and α-Al2O3, and the distribution of α-Al2O3 particles, were much influenced by the content of α-Al2O3. The best mechanical properties of extreme high bending strength (σb ≥ 1 GPa) and high fracture toughness (KIC ≥ 15 MPa • m1/2) simultaneously were obtained at the composition of 25 mol% Al2O3 with a high relative density 99.9 %, as the same composition reported for the HIP sintered ZrO2(2~3 mol% Y2O3)-Al2O3 composite ceramics.

スピネル型(Fe, Mn)Al<sub>2</sub>O<sub>4 </sub>系フェライトのパルス通電加圧焼結と物性評価

スピネル型(Fe, Mn)Al<sub>2</sub>O<sub>4 </sub>系フェライトのパルス通電加圧焼結と物性評価

Fabrication of novel ZrO2(Y2O3)–Al2O3 ceramics having high strength and toughness utilising pulsed electric current pressure sintering (PECPS)

Pulsed electric current pressure sintering (PECPS) makes it possible to produce novel monolithic and composite materials with well controlled microstructure or nanostructure, which has not been realised before. ZrO2 based ceramics containing 25 mol.-%Al2O3 and 0·90–1·125 mol.-%Y2O3, i.e. ZrO2(1·2–1·5 mol.-%Y2O3)–25 mol.-%Al2O3, revealing high bending strength σb≧1 GPa and high fracture toughness KIC ≧15·5 MPa m1/2 simultaneously, have been fabricated by PECPS of cubic ZrO2 solid solution (ss) powders containing both Al2O3 and Y2O3 prepared via a sol–gel process. Tetragonal ZrO2 (t-ZrO2) phase composite ceramics sintered at 1573 to 1623 K (1300–1350°C) for 10 min under 60 MPa in Ar were composed of ∼φ200 nm grains with high relative density ≧97·5%. Transmission electron microscopy observation cleared that these mechanical properties might originate from homogeneous distribution of α-Al2O3 particles in the dense t-ZrO2 matrix; the precipitation of α-Al2O3 could be achieved by adopting the ss powders and PECPS.

中和共沈法で調製したZrO2(Y2O3)–Al2O3 系固溶体粉体のパルス通電加圧焼結法による 高強度・強靭性セラミックスの作製

ZrO2 composite ceramics containing 25mol%Al2O3 and a small amount of Y2O3, i.e., 75mol%ZrO2 (0.5～2.0mol%Y2O3: 0.5～2.0Y)–25mol%Al2O3 ceramics, have been fabricated by sintering cubic ZrO2 solid solution (ss) nano-powder prepared via the neutralization coprecipitation method. Their sintering was performed using a pulsed electric-current pressure sintering (PECPS) at 1300～1350˚C for 10 min under 60 MPa in Ar. Dense tetragonal-ZrO2 (ss) ceramics containing α–Al2O3 were composed of 135 − 150 nm grains with a high relative density ≥99.9%. They showed extreme high bending strength (σb≥1 GPa) and high fracture toughness (KIC≥15 MPa • m1/2) simultaneously. FE-SEM observation revealed that these mechanical properties were originated from homogeneous distribution of α–Al2O3 particles around the t–ZrO2 matrix; the precipitation of α–Al2O3 could be achieved by adopting both the (ss) powders containing α–Al2O3 and PECPS.

Fabrication of Full‐Density Mg‐Ferrite/Fe–Ni Permalloy Nanocomposites with a High‐Saturation Magnetization Density of 1 T

Dense nanocomposites consisting of spinel Mn‐doped Mg‐ferrite and permalloy, (Mg[Fe1 − xMnx]2O4(0 ≤ x ≤ 0.4)/53Fe–47Ni), have been fabricated from powder mixtures of MgO/α‐Fe2O3/MnO and N2‐atomized metal particles utilizing high‐pressure cold isostatic press (CIP), pulsed electric current pressure sintering (PECPS), and capsule‐free hot isostatic pressing (HIP). Metal particles were coated with the mixed oxide powders homogeneously under high shear compression in Ar. The coated powders were densified into relative densities of 83–85% by high pressure CIPing (1 GPa). They were then presintered at 823 K for 3 min under uniaxial pressure of 100 MPa in Ar using a PECPS. The pre‐sintered bodies had relative densities of 93–95% composed of spinel ferrite and permalloy; the mixed metal oxides were changed into magnesium ferrites by heating briefly at low temperature. Then, HIP (1123 K/6 h/196 MPa‐Ar) was adopted to densify the pre‐sintered bodies into a nearly full density; furthermore, a sintering atmosphere control was performed using ferrite muffles to maintain both phases. Sintered materials consisting of granular metal particles (φ 8.5 μm) isolated and surrounded by a thin ferrite discontinuous layer (0.2–1.3 μm) with a relative density ≥99.5% revealed both a saturation magnetization density ≥1 T and moderate permeability values of 30–35 at 1 MHz.

Improvement of CNFs/SiC nanocomposite properties obtained from different routes and consolidated by pulsed electric-current pressure sintering

The influence of the preparation route and composition on carbon nanofibers–silicon carbide (CNFs/SiC) nanocomposites' properties was studied. Nanopowders were mixed by ultrasonic dispersion or high attrition milling and the consolidation was done by pulsed electric-current pressure sintering technique. The relative density and fracture strength of high-energy attrition milled CNFs/SiC nanocomposites gradually increased with the increase of sintering temperature, from 1400 to 1800°C and holding time 1 to 30min. A chemical surface coating of CNFs with alumina precursor is proposed as a very effective way for improving the interaction between CNFs and SiC. An increase of 54% in fracture strength was achieved on the nanocomposites when the surface coating was used. As a consequence of the stronger interaction between the components, which is achieved through the use of suitable processing route and sintering parameters, and the role of nano-alumina as sintering aid improved mechanical properties was achieved.

Fabrication of Full-Density Mg-Ferrite/Fe-Ni Permalloy Nanocomposites with a High-Saturation Magnetization Density of 1 T

Dense nanocomposites consisting of spinel Mn-doped Mg-ferrite and permalloy, (Mg[Fe1 − xMnx]2O4(0 ≤ x ≤ 0.4)/53Fe–47Ni), have been fabricated from powder mixtures of MgO/α-Fe2O3/MnO and N2-atomized metal particles utilizing high-pressure cold isostatic press (CIP), pulsed electric current pressure sintering (PECPS), and capsule-free hot isostatic pressing (HIP). Metal particles were coated with the mixed oxide powders homogeneously under high shear compression in Ar. The coated powders were densified into relative densities of 83–85% by high pressure CIPing (1 GPa). They were then presintered at 823 K for 3 min under uniaxial pressure of 100 MPa in Ar using a PECPS. The pre-sintered bodies had relative densities of 93–95% composed of spinel ferrite and permalloy; the mixed metal oxides were changed into magnesium ferrites by heating briefly at low temperature. Then, HIP (1123 K/6 h/196 MPa-Ar) was adopted to densify the pre-sintered bodies into a nearly full density; furthermore, a sintering atmosphere control was performed using ferrite muffles to maintain both phases. Sintered materials consisting of granular metal particles (φ 8.5 μm) isolated and surrounded by a thin ferrite discontinuous layer (0.2–1.3 μm) with a relative density ≥99.5% revealed both a saturation magnetization density ≥1 T and moderate permeability values of 30–35 at 1 MHz.

Fabrication of ZrO2 Solid Solution Ceramics Containing Al2O3 Having High Bending Strength (σb≥1 GPa) and High Fracture Toughness (KIC≥20 MPa·m1/2) Simultaneously by Pulsed Electric-current Pressure Sintering (PECPS)

ZrO2 solid solution (ss) ceramics containing 25mol%Al2O3 and 1.125mol%Y2O3, i.e., 75mol%ZrO2(1.5mol%Y2O3:1.5Y)-25mol%Al2O3 ceramics, have been fabricated by sintering cubic ZrO2 (ss) nanoparticles prepared via a sol-gel process with a pulsed electric-current pressure sintering (PECPS) at 1100°∼1350°C for 10 min under 60 MPa in Ar. Dense tetragonal-ZrO2 (ss) ceramics with a small amount of α-Al2O3, sintered at 1300° to 1350°C, were composed of 0.2-0.3 μm grains with a high relative density of ≥97.5 %. They showed extreme high bending strength (σb≥1 GPa) and high fracture toughness (KIC≥20 MPa·m1/2) simultaneously. High-resolution TEM observation revealed that these mechanical properties were originated from homogeneous distribution of α-Al2O3 particles in the nanometer-grain matrix of t-ZrO2; the precipitation of α-Al2O3 could be achieved by adopting the solid solution powders containing α-Al2O3 and PECPS.

Fabrication of Mg Ferrite/permalloy Magnetic Nano-composites

Dense {Mg(Fe1-xMnx)2O4/53Fe-47Ni permalloy (0≤x≤0.4)} magnetic nano-composites consisting of spinel and alloy phases have been fabricated from N2-atomized metal particles and fine-powder mixtures of MgO/α-Fe2O3/MnO using a combination of pulsed electric current pressure sintering (PECPS) and hot isostatic pressing (HIP). Fe-Ni permalloy particles were coated with the mixed metal oxide powders homogeneously by passing these mixtures through a narrow gap (1 mm) under high shear compression in Ar. Mixed powder compacts of permalloy with the fine metal oxides were densified into the green bodies having relative densities of 83-85 % under isostatic high pressure of 1 GPa at room temperature. Then green powder compacts were pre-sintered at 823 K for 3 min under a uniaxial pressure of 100 MPa in Ar using a PECPS. The pre-sintered bodies composed of spinel ferrite and permalloy revealed the relative densities of 94-96 %; the mixed metal oxides changed into magnesium spinel ferrites Mg(Fe1-xMnx)2O4 during a short heating time. Then hot isostatic pressing (HIP: (1123 K/6 h/200 MPa-Ar)) was adopted to sinter the pre-sintered bodies into a near full density without an encapsulation, however, a sintering atmosphere control was performed using a ferrite muffle in order to maintain the both phase. Sintered nano-composites with the relative density more than 99.5 % have a saturation magnetization density ≥1 T and moderate permeability values of 30-35 at 10 MHz, which values have been expected from a material design.

The Study on Carbon Nanofiber (CNF)‐Dispersed B4C Composites

Carbon nanofiber (CNF)‐dispersed B4C composites have been synthesized and consolidated directly from mixtures of elemental raw powders by pulsed electric current pressure sintering (1800°C/10 min/30 MPa). A 15 vol% CNF/B4C composite with ∼99% of dense homogeneous microstructures (∼0.40 μm grains) revealed excellent mechanical properties at room temperature and high temperatures: a high bending strength (σb) of ∼710 MPa, a Vickers hardness (Hv) of ∼36 GPa, a fracture toughness (KIC) of ∼7.9 MPa m1/2, and high‐temperature σb of 590 MPa at 1600°C in N2. Interfaces between the CNF and the B4C matrix were investigated using high‐resolution transmission electron microscopy, EDS, and electron energy‐loss spectroscopy.

パルス通電加圧法を用いたカーボンナノファイバー分散／Ｂ４Ｃナノコンポジットの合成同時焼結と機械的特性

Carbon nanofiber (CNF)-dispersed B4C nanocomposites containing 2.5 vol% Al2O3 as a sintering aid were fabricated directly from mixtures of amorphous B and C, CNF, and Al2O3 powders by pulsed electric-current pressure sintering (PECPS) at 1800°C for 10 min under 30 MPa in a vacuum. Dispersion of CNF into the B4C matrix up to ∼15 vol% introduced the enhancement of densification. A 15 vol% CNF/B4C nanocomposite revealed excellent mechanical properties: a bending strength (σb) of ∼710 MPa, a Vickers hardness (Hv) of ∼36 GPa, and a fracture toughness (KIC) of ∼7.6 MPa·m1/2, in comparison with those (σb∼530 MPa, Hv∼29 GPa, and KIC∼4.3 MPa·m1/2) of monolithic B4C.Interfaces between CNF and B4C matrix were investigated using high-resolution TEM and Eels. Thin Al-B-C-O layer with a several nm order thickness was observed along with CNF, and Eels showed that the densification and mechanical properties of CNF/B4C nanocomposites might be related to this thin binding layer.

Simultaneous Synthesis and Consolidation of W-Added ZrB<sub>2</sub> by Pulsed Electric Current Pressure Sintering and their Mechanical Properties

Dense zirconium boride (ZrB2)-based materials with and without tungsten (W) have been fabricated directly from mixtures of constituent elemental powders by pulsed electric current pressure sintering (PECPS) at 1800°C for 10 min under 30 MPa in a vacuum. Formation processes of monolithic, W-doped ZrB2 solid solutions (Zr1-xWx)B2 (0<x≤0.12), and composites consisting of ZrB2(ss) and WB2 were investigated. Their mechanical properties of Vickers hardness (Hv), fracture toughness (KIC), and bending strength (σb) at room temperature were evaluated. Solid solution and composite materials gave higher Hv (~20.7 GPa), KIC (∼4.4 MPam1/2), and σb (~600 and 690 MPa for the former and the latter, respectively) than those (14.1 GPa, 3.21 MPam1/2, and ~500 MPa) of the monolithic ZrB2 fabricated under the same conditions. Furthermore, the latter two materials exhibited excellent high-temperature σb values (~550-600 MPa) up to 1600°C in N2, in comparison with that (~320 MPa) of monolithic ZrB2 materials.

Fabrication of carbon nanofiber(CNF)-dispersed Al2O3 composites by pulsed electric-current pressure sintering and their mechanical and electrical properties

Dense Al2O3-based composites (≥99.0% of theoretical) dispersed with carbon nanofibers (CNFs) were fabricated using the pulsed electric-current pressure sintering (PECPS) for 5 min at 1300°C and 30 MPa in a vacuum. The dispersion of CNFs into the matrix depended much on the particle size of the starting Al2O3 powders. Mechanical properties of the composites were evaluated in relation with their microstructures; high values of three-point bending strength σb (∼800 MPa) and fracture toughness K IC (∼5 MPa·m1/2) were attained at the composition of CNF/Al2O3 = 5:95 vol%, which σb and K IC values were ∼25% and ∼5%, respectively, higher than those of monolithic Al2O3. This might be due to the small Al2O3 grains (1.6 μm) of dense sintered compacts compared with that (4.4 μm) for the pure Al2O3 ceramics, resulting from the suppression of grain growth during sintering induced by uniformly dispersed CNFs in the matrix. Electrical resistivity of CNF/Al2O3 composites decreased rapidly from >1015 to ∼2.1 × 10−2 Ωm (5vol%CNF addition), suggesting the machinability of Al2O3-based composites by electrical discharge machining.

Mechanical properties of simultaneously synthesized and consolidated carbon nanofiber (CNF)-dispersed SiC composites by pulsed electric-current pressure sintering

Carbon nanofiber (CNF) dispersed β-SiC composites with the addition of 0.2 mass% boron and 2.0 mass% carbon as sintering aids have been synthesized and consolidated simultaneously from mixtures of Si, amorphous C and B powders and CNF by pulsed electric-current pressure sintering (PECPS). Synthesis and consolidation process, which were observed from their expansion and shrinkage curves during PECPS, have been examined using X-ray diffraction and scanning electron microscopy for the powder compacts. CNF/SiC composites sintered at 1800 °C for 10 min under 40 MPa in a vacuum have ∼96.0% of theoretical density and homogeneous structures consisting of ∼4.0 μm grains. A 10 vol% CNF/SiC composite exhibited excellent mechanical properties: a bending strength of ∼720 MPa, a Vickers hardness of ∼26.0 GPa, and a fracture toughness of ∼5.5 MPa m 1/2. High-temperature bending strength of ∼890 MPa at 1200 °C in air was attained with the same nanocomposites.

Simultaneous synthesis and consolidation of chromium carbides (Cr 3C 2, Cr 7C 3 and Cr 23C 6) by pulsed electric-current pressure sintering

Chromium carbides (Cr 3C 2, Cr 7C 3 and Cr 23C 6) have been synthesized and consolidated simultaneously from mixtures of Cr and amorphous carbon powders by pulsed electric-current pressure sintering (PECPS). Dense ceramics thus obtained were composed of chromium carbides with a small amount of Cr 2O 3, which originates from a trace amount of oxygen adsorbed on the as-received starting Cr powder. Synthesis and consolidation processes, which were observed from their shrinkage curves during PECPS, have been examined using X-ray diffraction (XRD) and scanning electron microscopy for the powder compacts. Cr 3C 2 ceramics sintered at 1300 °C for 10 min under 30 MPa have a 98.9% of theoretical density and fine structures with a 3.6 μm grain size. They exhibit excellent mechanical properties: a bending strength σ b of 690 MPa, a Vickers hardness H v of 18.9 GPa and a fracture toughness K IC of 7.1 MPa m 1/2.