Hot-press Sintering Process Research Articles

Fabrication and ferroelectric/dielectric properties of La-doped PMN-PT ceramics with high optical transmittance

Relaxor ferroelectric 0.75(Pb1–3x/2Lax)(Mg1/3Nb2/3)O3-0.25(Pb1–3x/2Lax)TiO3 (La3+:PMN-PT x/75/25, where x=2.8, 3.0, 3.5, and 4.0mol% of La3+) transparent ceramics were fabricated by the combination of oxygen atmosphere pressureless sintering and hot-pressing sintering process. The optical transmittances of above four ceramics are higher than 60% at the wavelength of 500–900nm. La3+:PMN-PT 3.0/75/25 exhibits the highest transparency around 70% at 900nm which is very close to the theoretical transmittance 71%. Each of the four ceramics exhibits the pure perovskite phases. They show fully dense microstructures and their relative densities are higher than 99.8%. The ferroelectric and dielectric measurements indicate that these four ceramics exhibit relaxation characteristics. With increasing La3+ content, (200) peak in XRD patterns shifts to higher angles and the average grain size increases, while the temperature Tεmax corresponding to the maximum εr, the remanent polarizations Pr and coercive fields Ec decrease gradually.

Hot-press sintering Sr2Nb2O7 ceramics and their electrical properties

Sr2Nb2O7 ceramics with high densities were synthesized by the hot-press sintering process, and the effects of sintering temperature and holding time of phase composition and microstructure were studied. The dielectric, piezoelectric, and ferroelectric properties of Sr2Nb2O7 ceramics were also investigated. A relative density of 98% was obtained when the Sr2Nb2O7 ceramics were prepared by hot-pressing (30 MPa) at temperature as low as 1200 °C for 2 h. The grain size of Sr2Nb2O7 increases gradually with the variation of temperature from 1150 to 1200 °C. The grains are plate-like and the length of grain size is 15 µm when the temperature increased to 1200 °C. In addition, the grain boundary became unclear at 1300 °C for 2 h. The dielectric constant and dielectric loss of the ceramics sintered at 1200 °C for 2 h were 57 and 1.6% at 10 kHz, respectively. The piezoelectric constant reached 2.1 pC/N. The remanent polarization and the coercive field were 0.24 µC/cm2 and 15.1 kV/cm, respectively.

Low temperature sintering of Ag nanoparticles/graphene composites for paper based writing electronics

With the great demand in the applications of flexible electronics, the methods leading to improvements in the electrical and mechanical performance have been widely investigated. In this work, we firstly prepared a hybrid composite ink using Ag nanoparticles and graphene. Then, a hot-press sintering process was deployed to obtain the desired electrical tracks which could be applied in flexible electronics. We have systematically investigated the effects of sintering time, pressure and temperature, as well as the different percentage of weight (wt%) of graphene for the electrical and mechanical performance of sintered electrical tracks. We achieved reasonably low electrical resistivity at low sintering temperature (120 °C). Specifically, the resistivity reaches 6.19 × 10−8 Ω · m which is just 3.87 times higher than the value of bulk silver. Additionally, the prepared hybrid composite ink obtained better electrical reliability against bending test comparing with Ag nanoparticle ink. Finally, the optimal wt% of graphene and potential effect to the electrical and mechanical performance were also investigated.

FABRICATION AND WEAR BEHAVIOR ANALYSIS ON AlCrFeNi HIGH ENTROPY ALLOY COATING UNDER DRY SLIDING AND OIL LUBRICATION TEST CONDITIONS

In this paper, AlCrFeNi high entropy alloy coating was fabricated on the surface of Q235 steel using hot pressing sintering process. The coating has the controlled thickness size and excellent mechanical properties. Scanning electron microscopy (SEM), XRD and hardness testing method were used to study the morphology, phase structure and hardness of high entropy alloys coating. The lattice distortion plays a significant role in increasing the hardness. Coating formation mechanism caused by the element diffusion under the hot pressing effect is also discussed in the paper. Simultaneously, the dry sliding and oil lubrication wear tests, wear morphology observation and wear mechanism discussion were completed. As the result shows, AlCrFeNi high entropy alloys coating exhibits superior wear resistance either at dry sliding or oil lubrication tests owing to its hard high entropy solid solution structure.

Self-Sharpening Failure Characteristic of a Si<sub>3</sub>N<sub>4</sub> Ceramic Tool in High Speed Cutting of Inconel 718

A Si3N4 ceramic tool material with high mechanical properties was fabricated by hot-pressing sintering process. The high speed machining of Inconel 718 tests were carried out with round ceramic inserts. The failure surface and microstructure were analyzed by scanning electron microscopy (SEM) to reveal the ceramic tool failure mechanisms. The results showed that the main failure mechanisms of the Si3N4 ceramic tool were flaking, micro-chipping, abrasive wear and adhesive wear in the turning process. On the other hand, chipping, flaking and adhesive wear were the main failure reasons in the milling process. Meanwhile, some small flaking along the cutting edge and step-shaped flaking on the rake face closed to the cutting edge were found on the failure surfaces, which was a typical self-sharpening failure characteristic of the ceramic tool in the high-speed cutting process. This tool failure evolution characteristic of the ceramic tool can be attributed to its higher flexural strength and fracture toughness, which was beneficial to improve the tool life and was constrained by cutting conditions.

SiC and ZrN nano-particulate reinforced AlON composites: Preparation, mechanical properties and toughening mechanisms

Due to excellent chemical stability, high rigidity, superior corrosion and wear resistance, aluminum oxynitride (AlON) has been considered as one of most promising candidate ceramic materials in high-performance structural, advanced abrasives and refractory fields. However, it usually exhibited relatively low flexural strength and poor fracture toughness. The study is aimed to develop silicon carbide (SiC) and zirconium nitride (ZrN) nano-particulate reinforced AlON composites with improved mechanical properties and fracture resistance via a hot-press sintering process. It was found that the addition of ZrO2 nanoparticles would be transformed into ZrN during sintering. Due to the pinning effect of SiC and ZrN nano-particles positioned at grain boundaries of micro-sized AlON particles, the presence of SiC and ZrN nano-particles resulted in the reduction of both porosity and grain size, and a change of fracture mode from intergranular cracking in AlON to intragranular cracking in composites. With presence of 8wt% SiC and 5.2wt% ZrN nano-particles, the relative density, microhardness, Young’s modulus, flexural strength and fracture toughness increased. Different toughening mechanisms including crack bridging, crack branching and crack deflection were observed, thus effectively increasing the crack propagation resistance and leading to a considerable improvement in flexural strength and fracture toughness.

Barium hexaferrite thick films prepared by screen printing for microwave applications

In this work, hexagonal barium ferrite thick films with a thickness of about 90 μm were prepared on alumina substrates by screen printing method. Microstructure and magnetic properties of the thick films have been studied. The results show that organic vehicles have a great influence on the microstructure of the ferrite thick films. It is found that the density and surface flatness of the barium ferrite thick films can be improved greatly by hot-press sintering process. The barium ferrite films prepared by hot-press sintering show a clear magnetic anisotropy. Along the easy axis, the squareness ratio of the hexagonal barium ferrite thick films is about 0·75 with a coercive field of 3·4 kOe. A prototype of microwave circulator with the prepared BaM thick films was fabricated.

Structure and Properties of W-Cu/AlN Composites Prepared via a Hot Press-Sintering Method

Abstract W-30wt% Cu nano-crystalline powder was prepared by high-energy ball-milling, and then different mass fractions of AlN nanoparticles were added to the powder via the ball-milling. Finally W-30Cu/x% AlN composites were obtained by hot press-sintering process. The microstructures, the physical and mechanical properties of as-sintered composite samples were studied. The results indicate that the W-30Cu/x% AlN composites possess denser and more uniform microstructure. Moreover, the degree of refinement of W particles and the hardness of the composites increase with the addition of AlN, but the bending strength decreases with increasing of AlN nanoparticles due to the nanoparticles excessive distribution on the grain boundaries of the matrix. It is beneficial to increase thermal conductivity of W-Cu composites, when the content of AlN nanoparticles is ≤1%. Resistivity of the composites increases with increasing the amount of AlN nanoparticles, but the conductivity of the composites declines.

A comparative study on wear properties of nanostructured Al and Al/Al2O3 nanocomposite prepared by microwave-assisted hot press sintering and conventional hot pressing

In this study, wear behavior of nanostructured Al and Al/2O3 nanocomposite samples prepared by an advanced technique called microwave- assisted hot press sintering process was investigated. The specimens were characterized by XRD analysis, pin-on-disk wear test and scanning electron microscopy observations. Compared to nanostructured Al, nanocomposite containing 4 wt-%Al2O3 showed a higher hardness of 175 HV and a lower wear rate of 7 × 10-3 mg m-1. Also, wear properties of hot-pressed samples were compared with those of nanocomposite conventionally prepared by Tavoosi et al. method. The results showed that contribution of ceramic particles in enhancing the wear resistance of nanocomposite can be eliminated by increased porosity content. Higher sintering temperature due to rapid heating rate of microwave-assisted hot press sintering method led to more densification and improvement in wear resistance in comparison with conventional method.

Effect of Co Content on the Densification, Microstructure and Mechanical Properties of cBN-WC/Co Composites

Hot pressing sintering process for cemented carbide preparation was investigated with cBN-WC/Co composite powders with different Co content from 4wt% to 10wt% at temperatures between 1300°C and 1400°C. The results showed that the Co content had remarkable influence on the densification and mechanical properties of cBN-WC/Co composites. Near-full densification can be obtained when cBN-WC/8wt%Co powders were sintered at temperatures at 1350°C, and pressures at 20MPa, combining an excellent Vickers hardness of 17.57GPa with an acceptable flexure strength of 601.58MPa.

Hot-pressing of monazite Ce0.5Pr0.5PO4 ceramic and its chemical durability

Pr-doped monazite-(Ce) ceramic with the composition of Ce0.5Pr0.5PO4 was successfully fabricated utilizing uniaxial hot-press sintering process. The effects of sintering temperature and holding time on bulk density and microstructure of ceramic were investigated, and sintering parameters were optimized. The chemical durability of ceramic was examined through MCC-1 test. The relatively optimal sintering temperature and holding time at 30 MPa were 1150 °C and 2 h, respectively. The hot-pressed ceramic has a smaller fine-grained, closer packed structure than the pressureless sintered ceramic. The element mapping images of Ce, Pr, P and O show that all the elements are almost distributed homogeneously in the monazite ceramics. Furthermore, the normalized dissolution rates of Ce and Pr decrease with increasing time and remain almost unchanged after 28 days. The 42 days normalized dissolution rates of RL(Ce) and RL(Pr) were equal to 5.7 × 10−6 and 5.3 × 10−5 g m−2 d−1, respectively.

Comparison of microstructure and chemical durability of Ce0.9Gd0.1PO4 ceramics prepared by hot-press and pressureless sintering

Gd-doped monazite-(Ce) ceramics (Ce0.9Gd0.1PO4) have been fabricated by hot-press sintering (HPS) and pressureless sintering (PLS). The influence of sintering temperature and holding time on the bulk density and microstructure was investigated. The results show all of HPS samples are denser than the PLS samples, even if the optimal PLS temperature is as high as 1400°C and the HPS temperature is only 1150°C. Low-temperature densification of the HPS ceramics should be attributed to particle rearrangement under pressure. In addition, chemical durability testing was carried out using the standard MCC-1 static leach test method. The results indicate that the normalized elemental leach rates of HPS samples are higher than those of PLS sintered specimens before 21 days, and remained almost unchanged (RL (Ce)≤6.0×10−6gm−2d−1, RL (Gd)≤1.8×10−5gm−2d−1) in both HPS and PLS ceramics after 28 days. The low-temperature HPS process to prepare monazite-based ceramic is a promising method for immobilizing trivalent minor actinide.

Influences of milling on the dielectric and microwave absorption properties of Ti3SiC2/cordierite composite ceramics

Cordierite ceramics doped with Ti3SiC2 powders milled for different time are fabricated by hot pressed sintering process. For the Ti3SiC2 powders, the particle size is decreased and the morphology is changed from lamellar to flaky with the increasing milling time. The finer Ti3SiC2 particles are detrimental to the densification process but favorable to the improvement of flexural strength of the full dense ceramics. Dielectric and microwave absorption properties of the ceramics are investigated in the frequency range of 8.2–12.4GHz. As the milling time increases, both the real and imaginary part of the complex permittivity are enhanced which is attributed to the decrease of particle size and formation of flaky particles. The ceramic doped with Ti3SiC2 powders milled for 10h presents the optimal microwave absorption property. The effective absorption bandwidth (RL<−10dB) with a minimal RL value of −16.4dB is obtained in 9.7–12.4GHz for the ceramic in 1.6mm thickness.

Study on Preparation and Physical Mechanical Properties of Si3N4Composite Ceramics

In this paper, by using the hot-pressed sintered technology, pure Si 3 N 4 ceramics and Si 3 N 4 composite ceramics containing different volume fraction(hBN) are prepared with the sintered catalyst of Al 2 O 3 and Y 2 O 3 (total 10vol%) by HIGH MULTI 5000 multi-functional high temperature sintering furnace. And the phase compositions, microstructure and mechanical properties of specimen were studied. The results show that, based on the hot pressing sintering process, the α-Si 3 N 4 is all changed into β-Si 3 N 4 . Addition of hBN into Si 3 N 4 inhibits the growth of the crystal grain and makes the ceramic materials generate hBN segregation zone in internal materials, which causes the addition of hBN that cannot improve the physical mechanical properties effectively just because the serious stress concentrates and fatigue break down. In addition, compared with other sintering process, the physical and mechanical properties of Si 3 N 4 prepared by hot-pressing sintering are better than other methods. Improvement of sintering temperature and time can refine the crystal grain and be a helpful densification. The changed β-Si 3 N 4 grain completely forms an intertwined structure and increases the physical and mechanical properties of Si 3 N 4 . Meanwhile, Y 2 O 3 and Al 2 O 3 additives are melted and filled into Si 3 N 4 particles at the high temperature, leading to a significant drop of the porosity and the increase of density.

Magnesia tuned multi-walled carbon nanotubes–reinforced alumina nanocomposites

Magnesia tuned alumina ceramic nanocomposites, reinforced with multi-walled carbon nanotubes, were condensed using pressureless and hot-press sintering processes. Densification, microstructure and mechanical properties of the produced nanocomposites were meticulously investigated. Electron microscopy studies revealed the homogenous carbon nanotube dispersion within the alumina matrix and confirmed the retention of carbon nanotubes' distinctive tubular morphology and nanoscale features during the extreme mixing/sintering processes. Pressureless sintered nanocomposites showed meagre mechanical responses due to the poorly-integrated microstructures with a slight improvement upon magnesia addition. Conversely, both the magnesia addition and application of hot-press sintering technique resulted in the nanocomposite formation with near-theoretical densities (~99%), well-integrated microstructures and superior mechanical properties. Hot-press sintered nanocomposites incorporating 300 and 600ppm magnesia exhibited an increase in hardness (10 and 11%), flexural strength (5 and 10%) and fracture toughness (15 and 20%) with respect to similar magnesia-free samples. Compared to monolithic alumina, a decent rise in fracture toughness (37%), flexural strength (22%) and hardness (20%) was observed in the hot-press sintered nanocomposites tuned with merely 600ppm magnesia. Mechanically superior hot-press sintered magnesia tailored nanocomposites are attractive for several load-bearing structural applications.

Structural, thermal, dielectric and ferroelectric properties of Na0.5K0.5NbO3 and Na0.5K0.5NbO3+0.5mol%MnO2 ceramics

Lead-free Na0.5K0.5NbO3 and Na0.5K0.5NbO3+0.5mol%MnO2 ceramics were fabricated by solid state hot pressing sintering process. The phase structure and thermal, dielectric and ferroelectric properties of these ceramics were investigated. X-ray measurements showed that all samples have a single perovskite structure and possess orthorhombic–tetragonal and tetragonal–cubic phase transitions. Sequence of phase transitions was confirmed by differential scanning calorimetry (DSC) and dielectric measurements. Obtained results showed that optimal temperature sintering of Na0.5K0.5 NbO3+0.5mol%MnO2 ceramics is 1100 °C. The results show that investigated ceramics are one of the promising materials as electronic ceramics.

Enhanced mechanical, dielectric and microwave absorption properties of cordierite based ceramics by adding Ti3SiC2 powders

Cordierite ceramics doped with different contents of Ti3SiC2 are successfully fabricated by hot-pressed sintering process. The influences of Ti3SiC2 addition on the properties of the ceramics are investigated. No chemical reactions occur between the Ti3SiC2 and cordierite phases during the sintering process. The dispersed Ti3SiC2 powders inhibit the densification process of the ceramics and improve the sintering temperature. The ceramic sintered at higher temperature has larger complex permittivity and dielectric loss value when the Ti3SiC2 content is fixed, which is attributed to its higher density. As for the dense ceramics, the flexural strength and complex permittivity are both enhanced with the increasing Ti3SiC2 content. The permittivity of the ceramic doped with 20wt% Ti3SiC2 is observed to increase dramatically because of the surge of electric conductivity. The ceramic doped with 15wt% Ti3SiC2 presents the optimal microwave absorption property. The effective absorption bandwidth of 9.7–12.4GHz is obtained for the ceramic in 1.6mm thickness, and the minimum absorption peak is −16.4dB at the matching frequency 11.1GHz.

Study on the mechanical properties, microstructure and corrosion behaviors of nano-WC–Co–Ni–Fe hard materials through HIP and hot-press sintering processes

Abstract This study aims to explore a series of Hot Isostatic Pressing (HIP) and hot-press sintering processes in order to examine the effects on the mechanical properties, microstructures and corrosion behaviors of micro- and nano-WC materials. The experimental results showed that the transverse rupture strength (TRS) values of micro- and nano-WC increased to 1627.3 and 1842.7 MPa after 1250 °C, 125 MPa, 100 min HIP treatments, respectively. Meanwhile, the porosity decreased slightly. The corrosion test results also showed that HIP-treated micro- and nano-WC effectively improved corrosion resistance in a 3.5 wt% NaCl solution. In addition, the lowest porosity (0.21%), highest hardness (91.7 HRA) and highest K IC (14.7 MPa√m) values appeared in nano-WC after 1250 °C, 15 MPa, 1 h hot-press sintering. Moreover, the hot-press sintering procedure significantly inhibited the grain growth of the tungsten carbide materials.

Effect of La content on dielectric, ferroelectric and electro-optic properties of Pb(Mg1/3Nb2/3)O3–PbTiO3 transparent ceramics

La-doped Pb(Mg1/3Nb2/3)O3–PbTiO3 (PLMNT) transparent ceramics were prepared by a hot-press sintering process, and their phase structure, dielectric, ferroelectric, optical and electro-optic properties were investigated systemically. All the samples are in pure perovskite phase according to the X-ray diffraction analysis. With the increase of La content, the value of the maximum dielectric constant decreases, and the corresponding peak temperature Tεmax shifts to lower value. In the meanwhile, the P–E hysteresis loops become slimmer, and the saturation polarization (Ps), coercive electric-field (Ec), and remanent polarization (Pr) significantly decrease. All the PLMNT samples exhibit good optical transparency. At room temperature, the quadratic electro-optic coefficient decreases obviously while the La content increasing.

Effects of sintering processes on the mechanical properties and microstructure of Ti(C,N)-based cermet cutting tool materials

In this study, two types of Ti(C0.7,N0.3)-based cermet cutting tool materials (Ti(C,N)–Mo–Ni–Co, named as TMNC, and Ti(C,N)–WC–Mo–Ni–Co–TaC–HfC, named as TWMNCTH) were fabricated by the hot pressed sintering process at different temperatures (from 1380°C to 1500°C) for different holding times (from 30min to 60min) in a vacuum atmosphere and at a compressive stress of 32MPa. The polished surface and the fracture surface of the two types of cermets were observed by a scanning electron microscope (BSE/SEM) and energy dispersive spectrometry (EDS), and the relationships among sintering processes, mechanical properties and microstructure were discussed. The experimental results showed that the sintering temperature and holding time both had a great influence on the flexural strength and a small effect on the hardness and the fracture toughness of the two types of cermets. The two cermets both had the optimal comprehensive mechanical properties when they were sintered at 1400°C for 30min. The sintering temperature and holding time also had a great influence on the microstructure of the two cermets, and the grain sizes increased when the sintering temperature varied from 1400°C to 1500°C and the holding time varied from 30min to 60min. The properties and microstructure of the two cermets were also compared. The results indicated that the cermet TWMNCTH had a lower flexural strength, a similar value of fracture toughness, a higher hardness and a thicker rim in the microstructure.