High-pressure Sintering Research Articles

Effect of N, N′-m-phenylene bismaleimide on mechanical performance of waste rubber powder sintered by high-pressure high-temperature method

Waste rubber with three-dimensional structure is insoluble and non-melting, which creates recycling problem. The technique called “high-pressure high-temperature sintering” (HPHTS) can convert vulcanized rubber powder into usable rubber products. However, reversion occurs when vulcanizates are exposed to a prolonged treatment at high temperature. This reversion results in the decrease of mechanical properties of sintered waste rubber powder (WRP) samples. To address this problem, a method of enhancing the mechanical properties of sintered WRP by incorporating different content of maleic anhydride (MA) and N, N′-m-phenylene bismaleimide (BMI) is discussed in this paper. The results show that BMI and MA can slow down the formation of the conjugated double bonds. Through Diels–Alder reaction, BMI inhibits conjugated double bonds formation and improves the adhesion between particles and particles, which is beneficial to improvement of final properties. Therefore, BMI improves the mechanical properties of sintered WRP greatly.

Stress-strain relationship of translucent nanocrystalline Gadolinium Zirconate ceramic with grain size below 10 nm using nanoindentation

Translucent nanocrystalline Gadolinium Zirconate (Gd2Zr2O7) ceramic with grain size below 10 nm was successfully fabricated using high pressure sintering technology at 5 GPa and low temperature of 550 °C. The bilinear compressive stress-strain relationship of this sample was firstly extracted from nanoindentation procedure based on the partial unloading method. It indicated that the plastic deformation did not originate from dislocation sliding and was controlled by the grain boundary sliding. Moreover, the compressive yield strength, σy=7.8 GPa of the translucent nanocrystalline Gd2Zr2O7 ceramic was also determined by the intersection of the theoretical elastic line and the indentation compressive stress-strain line.

Hierarchical nanostructured W-Cu composite with outstanding hardness and wear resistance

Hierarchical nanostructured W-Cu composite with an average W size below 200 nm and nanocrystalline structure inside the W phase was obtained by refining the inner structure of the initial ultrafine powders combined with high-pressure spark plasma sintering. It revealed that an atomic scale combination can be formed at both the W grain boundaries and W/Cu interfaces. Accordingly, the nanostructured W-Cu composite exhibits twofold hardness and greatly improved wear resistance with satisfactory electrical conductivity, as compared to those of their fine grain structured counterparts. The upgraded wear resistance is attributed to the restricted micro-plowing and the mechanically mixed layer, induced by a refined microstructure, intrinsic high hardness, and the composition modulation on the wear surface.

Thermoelectric performance of (GeTe)1−x(Sb2Te3)x fabricated by high pressure sintering method

(GeTe)1−x(Sb2Te3)x is one of good middle temperature thermoelectric materials. Here, the effects of x values and heat treatment (673 K 24 h) on the thermoelectric property of (GeTe)1−x(Sb2Te3)x (x = 0, 0.02, 0.04, 0.06) prepared by solid-state synthesis (873 K) and high pressure sintering (6 GPa) method were studied, the composition, microstructure, and the thermoelectric properties in temperature range of 300–731 K were investigated. The results indicate that high pressure sintering (HPS) is beneficial to the dissolution of Sb2Te3 in GeTe and the acquisition of nanocrystalline (GeTe)1−x(Sb2Te3)x. Soluble Sb2Te3 can greatly change the crystal structure of GeTe, and results in the high pressure phase and different thermoelectric property in (GeTe)1−x(Sb2Te3)x (x ≠ 0). Heat treatment does not change the high pressure phase of (GeTe)1−x(Sb2Te3)x (x ≠ 0), meanwhile it increases the crystal grain size of (GeTe)1−x(Sb2Te3)x (x ≠ 0). Samples of x = 0.04 and x = 0.06 show the maximum Seebeck coefficient value at 724 K, and (GeTe)0.98(Sb2Te3)0.02 shows the highest power factor (PF = 3.70 μW/mmK2 at 724 K). The largest value of ZT = 0.56 at 724 K is obtained for (GeTe)0.94(Sb2Te3)0.06.

Design of Mg-6wt%Al alloy with high toughness and corrosion resistance prepared by mechanical alloying and spark plasma sintering

Microstructural optimization was performed on Mg alloy produced by high-energy milling and spark plasma sintering (SPS) through systematic investigation of mechanical properties and corrosion resistance. The SPS Mg alloys degassed under low vacuum condition led to continuous and thick magnesium oxide stringer consisted of MgO crystals at particle boundary even at high sintering pressure of 130Mpa. Simultaneously, large and irregularly shaped oxide particles at grain boundary and grain inside was identified from TEM observation. Meanwhile, for fully degassed at high vacuum and SPS Mg samples, particle boundaries were discontinuous and the cuboidal MgO particles were uniformly distributed within the grain. Sufficient degassing prior to sintering is critical for forming strong interfaces by facilitating the easy breakup of the oxide film during SPS because Mg powders is highly reactive and form easily Mg oxide on their surfaces, which makes sintering rather difficult. The width of particle boundary in the SPS samples from current experiment appears to be critically depended on degassing vacuum and sintering condition. The removal of continuous or thick oxide stringers with attainment of theoretical sintering density may be a critical parameter for ensuring better mechanical and electrochemical performance in Mg alloys fabricated by high-energy milling SPS process. A fine grained Mg alloy with high toughness and corrosion resistance is obtained by using this degassing process prior to SPS.

Pressure induced structure distortion in ferroelectrics with high Curie point and enhanced piezoelectric properties

High-temperature ferroelectric materials perform piezoelectric properties at elevated temperature, and they could detect strains, vibrations, and noise under severe thermal conditions. Therefore, these materials could be widely applied in material processing, automotive, aerospace, and power industries. As the promising materials, perovskite-like layered (PL) structure ferroelectrics, like La2Ti2O7, were widely investigated due to their high Curie points (Tc > 1300°C). However, the low piezoelectric constants restricted the applications. Here, the high pressure treatment was surprisingly found to be an effective method to improve the piezoelectric/ferroelectric performances on bulk La2Ti2O7 ceramics. The high-pressure-treated La2Ti2O7 shows an enhanced piezoelectric constant by 74% compared to un-treated sample, which is the highest d33 achieved in PL-structure ferroelectrics with Tc > 1400°C. This is because the high-pressure sintering distorts the structures, increasing the spontaneous polarizations (Ps) and piezoelectric behaviors. These results offer a method to fabricate the bulk ceramics with strain/stress effect, and might be instructive for the materials design in the future.

Synthesis and characterisation of Φ62 mm polycrystalline diamond compact

To solve the problem of uniform high-quality sintering of large-diameter polycrystalline diamond compact (PDC) cutting tool material on a cubic press, the design of a high-temperature, high-pressure (HTHP) sintering cavity assembly structure of a cubic press is optimised. PDC cutting tool materials with Φ62 mm were successfully sintered at a pressure of 5.5–5.8 GPa and a temperature of 1550–1650 °C. The microstructure and phase composition of PDC samples were detected by scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The microhardness, flexural strength, interfacial shear strength, wear resistance, and heat resistance of the samples were tested by a Vickers hardness tester, universal testing machine, and an abrasion ratio tester, and their uniformity was analysed. The test results show that the microhardness of Φ62 mm PDC is 86.25 GPa, which is 13.34% higher than that of the conventional PDC cutting tool material on the market; the bending strength is 1398.6 MPa; the interfacial shear strength is 2690.4 MPa; and the wear ratio is 29.8 × 104 with only an 8.09% decrease from the peripheral point to the central point. The wear resistance is basically uniform in the radial direction, indicating that the HTHP sintering process and the structure of the sintering cell assembly are reasonable, and the temperature and pressure field distributions inside the high-pressure sintering cavity are uniform, thereby realising uniform, dense sintering of the Φ62 mm PDC. When the PDC sample was calcined at 750 °C for 2 min, the wear resistance was 29.8 × 104, which is 10.59% lower than the wear resistance at room temperature, and the sample had good heat resistance.

Self-bonding mechanism of pure polycrystalline cubic boron nitride under high pressure and temperature

Cubic boron nitride (cBN) powders with different grain size were used as a starting material, and the pure polycrystalline cubic boron nitride (PcBN) samples were sintered under the same conditions at 7GPa, 1700 °C, 270 s. Abrasion resistance and compressive strength of sintered pure PcBN samples were tested, and the microstructure was observed and analyzed by SEM, HRTEM, XRD and Raman spectrum. The results show that the particle size of cBN has not only a great influence on microstructure and property of the sintered pure PcBN un-der the same conditions but also influence on the bonding mechanism of the cBN particles. The inner stress of the pure PcBN sintered with the cBN particle size of 10 μm during the high pressure sintering process is about 199–222% higher than that of the sample with the cBN size of 1 μm. The wear ratio and the compressive strength of PcBN samples sintered with 10 μm cBN was increased by 47.7% and 39.7% than that of the sintered sample with 1 μm cBN respectively. The coarser-grained PcBN contains serious deformation and crushing, which can be attributed to the twin boundary and the stacking faults in the samples. As a consequence, the abrasion ratio and compressive strength of the pure PcBN are dramatically increased. It is assumed that self-bonding mechanism generated from plasticity deformation, which is a main bonding mechanism for the high pressure sintering of pure cubic boron nitride.

Crystal Structures of Al2Cu Revisited: Understanding Existing Phases and Exploring Other Potential Phases

When processing single crystal X-ray diffraction datasets for twins of Al2Cu sample synthesized by the high-pressure sintering (HPS) method, we have clarified why the crystal structure of Al2Cu was incorrectly solved about a century ago. The structural relationships between all existing Al2Cu phases, including the Owen-, θ-, θ’-, and Ω-Al2Cu phases, were investigated and established based on a proposed pseudo Al2Cu phase. Two potential phases have been built up by adjusting the packing sequences of A/B layers of Al atoms that were inherent in all existing Al2Cu phases. The mechanical, thermal, and dynamical stability of two such novel phases and their electronic properties were investigated by first-principles calculations.

Dense amorphous Si2BC1-4N monoliths resistant to high-temperature oxidation for hypersonic vehicle

Dense amorphous Si2BC1-4N monoliths were subjected to aggressive environments at high-temperatures to explore the oxide scale microstructure and growth kinetics. The effects of high-pressure sintering and crystallization on oxidation resistance were studied in detail. Dense amorphous Si2BC1-4N monoliths oxidize according to a parabolic rate law or linear equation, depend on the carbon content. Oxidation first attacks at the outermost surfaces of amorphous phases, subsequently proceeds at the interfaces of the crystallized nuclei and neighboring amorphous phases. High-temperature oxidation induces nucleation and thermo-oxidative coupling promotes the nucleation and growth of the nanocrystals.

Microstructure of novel SiCp/Al-Si-Mg composites fabricated by high pressure sintering

Three kinds of SiCp/Al-Si-Mg composites, containing 20%, 40% and 60% volume fraction of silicon carbide particles (SiCp) were respectively sintered at high cubic anvil pressure, using SiC powders prepackaged by aluminum. Then microstructures of the composites were investigated, and the influences of pressure, sintering temperature and volume fraction of SiCp on the microstructures were analyzed. Results show that the composites own homogeneous microstructures, high density and few porosities. In the composites, SiCp and Al matrix were bounded to each other tightly and few agglomeration of SiCp or aluminum was observed. With the volume fraction of SiCp increased, the pinning effect of the reinforcement on the Al matrix is enhanced. With the sintering temperature and pressure increased, densities of the samples are also increased. Energy Dispersive Spectrometer (EDS) and X-ray diffraction (XRD) results show that there are transition layers and Al4C3 phases formed at the interface between SiCp and Al matrix when fabricated under pressure of 3.0GPa and temperature of 620 °C.

Recent Progress of Iron-Based Superconducting Round Wires

Recent progress of round wires using iron-based superconductors is reviewed. Both transport and magnetic Jc have been enhanced by several techniques such as purification of polycrystalline powders, high-pressure sintering (HIP), and control of drawing and sintering conditions. The present record of transport Jc of round wires is realized when the wire with 122-type compounds is processed at 175 MPa with a value at 4.2 K under 100 kOe being 38 kA/cm2 using (Ba,K)Fe2As2. We also discuss HIP round wires of (Sr,K)Fe2As2 and recently fabricated CaKFe4As4. Details of the optimization of round wires to achieve large Jc values are described.

Preparation, structure and flame retardancy of poly(butylene terephthalate) with gradient intumescent flame retardant distribution

In this manuscript, the content-gradient distributions of an intumescent flame retardant (IFR) island-shaped structure in poly(butylene terephthalate) (PBT) were prepared using compression sintering processing technology inspired by powder metallurgy, to achieve efficient flame retardancy and mechanical properties with low flame retardant dosage. The PBT/IFR master-batch was first prepared by extrusion and then pulverized into powder. At low temperature (20 °C below the melting temperature) and high pressure (≥ 50 MPa) sintering processing conditions, two PBT/IFR powders with different levels of IFR content were fabricated into restricted island-like structure composites. The island with a higher IFR content exhibited flame retardancy, while the island with lower IFR content maintained the mechanics of the matrix. Compared with the uniform dispersion PBT/20 wt% IFR composite, the combustion of the PBT/IFR (25 wt%:15 wt% = 1:1) gradient content composite improved from V-1 to V-0 using a UL-94 test, while the tensile strength increased by 10%. The performance of the flame retardant was improved by periodically changing its distribution in the polymer matrix via restricted island-like structures fabricated using compression sintering processing technology inspired by powder metallurgy.

“Frozen” pressure effect in GGAG:Ce3+ white light emitting nanoceramics

Nanocrystalline powders of Ce3+ doped Gd3Ga3Al2O12 (GGAG) with the size of tens nanometers were prepared using modified Pechini method. Low Temperature High Pressure sintering method was used for the fabrication of the translucent nanoceramics using as starting materials nanocrystalline GGAG:Ce3+ powders. For growth of GGAG:Ce single crystal the Czochralski method was utilized. XRD measurement was used in order to characterize the grain size of the GGAG:Ce3+ nanostructures. Excitation and emission and luminescence decays measurements were done to observe impact of the grain size and applied pressure on the spectroscopic properties of crystallites and ceramics. It was observed that for the smallest grains both spectra are significantly changed. The spectroscopic properties of the nanopowders were compared to the GGAG:Ce3+ ceramics made from them as well as to the single crystal. It was found that is possible to modify permanently the crystal field in the nanocrystals by applying the pressure during sintering.

Zirconium Phase Transformation under Static High Pressure and ω-Zr Phase Stability at High Temperatures.

High-purity Zr has been observed to undergo a phase transformation from the α-phase to the hexagonal ω-phase under high pressure generated either statically or by shock loading. The transition pressure from α-Zr to ω-Zr at 300 K is 2.10 GPa. The main aim of this research was to determine the conditions of α-Zr in ω-Zr transformation and the state of stresses after the high-pressure pressing and sintering of zirconium powders. Commercially acquired zirconium powders of 99.9% and 98.8% purity were used in this study. Qualitative and quantitative phase analysis of the materials was carried out using X-ray diffraction. The materials were statically pressed and sintered using a Bridgman-type toroidal apparatus at under 4.0 and 7.8 GPa. After pressing, the transformation proceeded for the zirconium powder containing 98.8% purity (with hydrides admixture) but did not occur for the high-purity zirconium powders with 99.9% purity. The zirconium powders were sintered using the HPHT (High Pressure—High Temperature) method at temperatures of 1273 K and 1473 K. The transformation proceeded for both powders. The highest contribution of the ω-Zr phase was obtained in the zirconium (98.8% purity with the hydrides contents) sintered for 1 min at a temperature of 1473 K and a pressure of 7.8. The ω-phase content was 87 wt.%. The stress measurement was performed for the pressed and sintered materials using the sin2ψ X-ray diffraction method. The higher sintering temperature resulted in a decrease of the residual stresses in the ω-Zr phase for the sintered zirconium. The higher levels of stress limited the transformation of the α-Zr phase into the ω-Zr phase. Investigated materials characterized by higher compressive macrostresses were also typical of the greater stability of the ω-Zr phase at high temperatures.

Effect of Y2O3 additive on properties of cBN composites with Si3N4–Al2O3–Al as binder by high temperature and high pressure sintering

Abstract In this study, cBN composite was prepared by high temperature high pressure (HTHP) sintering using Si3N4, Al2O3 and Al as main binders. Effects of Y2O3 addition on thermal and mechanical properties of cBN composites were systematically evaluated. The results show that a eutectic melt Y–Si–Al–O–N was formed in composites with the addition of Y2O3 under HPHT sintering process, and the presence and quantity of eutectic liquid accelerated the densification of composites. Mechanical properties of cBN composites by incorporating Y2O3 fillers had an obvious improvement. The composite incorporating 2 wt% Y2O3 at 1550 °C presented outstanding relative density of 99.38%, high bending strength of 610.5 MPa, superior hardness of 49.2 GPa and oxidation temperature of 1215.9 °C, while 5 wt% Y2O3 doped composite exhibited the max fracture toughness of 7.2 MPa⋅m1/2. Overall, cBN composites look promising for future use in cutting inserts due to their improved mechanical properties and thermal performances.

Sintering of translucent and single-phase nanostructured scandia-stabilized zirconia

Fully-dense and single-phase nanostructured scandia-stabilized zirconia specimens were produced by high-pressure spark plasma sintering technique. Nanocrystalline powders were prepared by the coprecipitation method. Green pellets were sintered at temperatures varying from 700 to 900 °C and pressures from 1.4 to 2 GPa, resulting in dense microstructures with single-phase fluorite-type cubic structure within a wide range of Sc2O3 content (6–15 mol%). The average grain size of sintered specimens ranged from 8 to 20 nm. Transmittance spectra confirm translucence in sintered specimens, which is consistent with full density. The results reported here reveal that the polymorphism challenge in the zirconia-scandia system can be successfully suppressed by this consolidation technique, which allows for controlling the grain size of bulk specimens.

Estudio de la descomposición de fases durante el sinterizado por plasma de compósitos Al-5%Ni<sub>3</sub>Al

La síntesis de compósitos base Al reforzados con partículas intermetálicas nanocristalinas de Ni3Al se llevó a cabo mediante la técnica de Spark plasma Sintering (SPS). La mezcla de polvos Al-5%Ni3Al fue sinterizada por SPS a dos diferentes condiciones: (i) 450 °C, 47 MPa y 180 s (4M47) y (ii) 520 °C, 16 MPa y 600 s (5M16). Las partículas de Ni3Al fueron obtenidas mediante el proceso de aleado mecánico después de 1260 ks de molienda. Los compósitos muestran una parcial descomposición en la intercara Ni3Al-Al, formando las fases Al3Ni y Ni2Al3. El análisis por MET confirman las características nanométricas del Ni3Al en los compositos Al-5%Ni3Al. Los compósitos sinterizados 4M47 y 5M16 presentaron 95% y 87% de la densidad teórica y valores de microdureza Vickers de 62,3±7 y 55,3±5, respectivamente. Los ensayos de nanoindentación muestran que las propiedades mecánicas de los intermetálicos Al3Ni, Al3Ni2 y Ni3Al, son mayores en condiciones de sinterizado a alta presión.

Assessment of the performance of TiB2 nanoparticles doped cBN-TiN-Al-Co composites by high temperature high pressure sintering

In this study, cBN composites incorporating TiN–Al–Co were synthesized via high temperature and high pressure (HPHT) methods. TiB2 nanoparticles were employed to optimize cBN cutting tools capacity for precise cutting. The effects of TiB2 nanoparticles on the preparation and properties of cBN composites were comprehensively investigated. The results show that the presence of TiB2 nanoparticles uniformly distributed in the interface not only promotes the formation of AlCo but also inhibits crack propagation, which contributed to the enhanced mechanical properties. With the appropriate addition of TiB2 nanoparticles, cBN composites obtained the compact and fine microstructure and presented better mechanical properties. Consequently, cBN composites incorporating 6 wt% TiB2 nanoparticles exhibited excellent relative density of 99.81%, bending strength of 731 MPa, hardness of 49.5 GPa, fracture toughness of 7.4 MPa⋅m1/2, and abrasive ratio of 2.1. Such composites with outstanding mechanical properties might be promising for cutting tools.

Fabrication of composites with excellent mechanical properties based on cubic boron nitride reinforced with carbon nanotubes

Abstract Composites consisting of cubic boron nitride (cBN) as a matrix and carbon nanotubes (CNTs) as reinforcing additives were fabricated by high-temperature and high-pressure sintering (HTHP). Microstructures, mechanical properties, fracture modes and toughening mechanisms of these composites were investigated. Composites exhibited excellent bending strength, wear resistance, and fracture toughness. Fracture toughness of composites reached 7.02 MPa·m1/2. Comparing to pure cBN matrix, bending strength improved from 475.27 to 600.15 MPa, and wear resistance increased by 43.23%. Such improvements of mechanical properties were mainly attributed to pullout and bridging reinforcements by CNTs. CNTs incorporation also changed fracture mode from inter-to trans-granular.