A comparison of sintering techniques using different particle sized β-SiAlON powders

In the present study sintering behavior and mechanical properties of β-SiAlON ceramics were investigated using different sintering techniques (gas pressured sintering (GPS), pressureless sintering and spark plasma sintering (SPS)) and different particle sized powders (with D_BET 216 and 130 nm). After sintering of the microstructure and phase characterization were carried out using a scanning electron microscope (SEM) and the X-ray diffraction (XRD) method, respectively. All the samples, prepared using fine powder, were sintered at lower temperatures than samples prepared by conventional powder, by two sintering techniques (GPS and pressureless). Additionally, the results showed that cooling rates had an important effect on the formation and the amount of intermediate phase in the sample. As a result, it was shown that the particle size of starting raw materials, the amount of additive, the sintering temperature and the technique had a significant effect on the microstructure and mechanical properties of the SiAlON samples.     

Preparation and indirect selective laser sintering of alumina/PA microspheres

Indirect selective laser sintering (SLS) is a promising additive manufacturing technique to produce ceramic parts with complex shapes in a two-step process. In the first step, the polymer phase in a deposited polymer/alumina composite microsphere layer is locally molten by a scanning laser beam, resulting in local ceramic particle bonding. In the second step, the binder is removed from the green parts by slowly heating and subsequently furnace sintered to increase the density. In this work, polyamide 12 and submicrometer sized alumina were used. Homogeneous spherical composite powders in the form of microspheres were prepared by a novel phase inversion technique. The composite powder showed good flowability and formability. Differential scanning calorimetry (DSC) was used to determine the thermal properties and laser processing window of the composite powder. The effect of the laser beam scanning parameters such as laser power, scan speed and scan spacing on the fabrication of green parts was assessed. Green parts were subsequently debinded and furnace sintered to produce crack-free alumina components. The sintered density of the parts however was limited to only 50% of the theoretical density since the intersphere space formed during microsphere deposition and SLS remained after sintering.     

聚碳酸酯粉末的选择性激光烧结成型

以聚碳酸酯(PC)粉末为烧结材料,研究了激光功率等工艺参数对PC烧结件的微观形态、密度和力学性能的影响规律。结果表明,PC烧结件的密度、拉伸强度及拉伸弹性模量、冲击强度均随激光功率增加而增大;但过高的激光功率会导致激光扫描区域的粉末过热,使PC烧结件产生颜色变黄、轮廓不清晰等缺陷。     

Effect of high-speed sintering on the microstructure,mechanical properties and ageing resistance of stereolithographic additive-manufactured zirconia

Digital light processing (DLP) demonstrates significant application potential in the fine printing of dental zirconia. However, its complicated print process and post-treatments, such as sintering, are time consuming and sensitive to technical details. Therefore, the feasibility of using a high-speed sintering (HS) method for the fabrication of DLP-based zirconia was investigated in this study. Zirconia samples fabricated using DLP and conventional subtractive manufacturing techniques were all sintered following the different protocols: HS and conventional sintering (CS). Then, the density, Vickers hardness, fracture toughness, surface micro-topography, phase assemblage, and ageing resistance were assessed. The results showed that samples fabricated using the HS technique presented moderate Vickers hardness, fracture toughness, and ageing sensitivity in comparison with the other groups of specimens; moreover, they exhibited moderate initial cubic phase content and average grain size. Conversely, specimens sintered using the CS protocol with a peak temperature of 1580 °C showed high ageing sensitivity and unbalanced mechanical properties. The DLP- and SM-fabricated specimens showed similar trends for the studied properties. Overall, sintering parameters can significantly affect the macro- and micro-properties of DLP specimens, and the proposed HS method showed potential for producing DLP-based zirconia that is acceptable for clinical applications.     

激光烧结制备塑料功能件

采用选择性激光烧结技术(SLS)直接制备高强度塑料功能件，考察了激光功率、扫描速度、单层层厚等工艺参数及无机填料对成型件强度的影响。在激光功率10W、扫描速度1500mm／s、单层层厚0．15mm的较佳烧结工艺参数下，制备了拉伸强度、弯曲强度、冲击强度分别达到44MPa、50．8MPa、37．214／m^2的SLS成型件。随着填料用量的增加，烧结件的拉伸强度略有增加，冲击强度下降，在填料用量为40％(质量含量，下同)时弯曲强度达到最大值。     

Sintering behavior and mechanical properties of spark plasma sintering SiO2-MgO ceramics

SiO₂-MgO ceramics containing different weight fractions (0, 0.5, 1, 2, and 4 wt%) of SiO₂ powder were prepared by mixing nano MgO powder, and the powder mixtures were densified by spark plasma sintering (SPS). The effect of SiO₂ addition and SPS method on the sintering behavior, microstructure and mechanical properties were investigated. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The highest relative density, flexural strength and hardness of 2 wt% SiO₂-MgO ceramics reached 99.98%, 253.99 ± 7.47 MPa and 7.56 ± 0.21 GPa when sintered at 1400 °C by SPS, respectively. The observed improvement in the sintering behavior and mechanical properties are mainly attributed to grain boundary "strengthening" and intragranular "weakening" of the MgO matrix. Furthermore, the spark plasma sintering temperature could be decreased by more than 100 °C as compared with the HP method, SPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.     

Microstructural evolution and sintering properties of palygorskite nanofibers

In this study, the morphological evolution and sintering properties of the palygorskite nanofibers were studied along with the increase of temperature, using raw palygorskite as materials. The palygorskite powder was calcined at different temperatures in the range of 100°C-1200°C, and the microstructural evolution of the palygorskite nanofibers was investigated by thermogravimetric and differential thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscope (HRTEM). Furthermore, the palygorskite powder was shaped to bars by dry pressing and sintered from 700°C to 1200°C. The properties of the sintered palygorskite were characterized by bending strength, mercury intrusion porosimeter (MIP), and stepwise isothermal dilatometry (SID). The results showed that the morphology of palygorskite nanofibers maintained unchanged till 1000°C. The palygorskite nanofibers molted to bind each other and formed a solid interwoven network structure at 1100°C. Correspondingly, it was shown from the sharply decrease of the sintered palygorskite porosity from 45.46% at 1000°C to 1.82% at 1100°C that the dense sintering of palygorskite started at 1100°C. With the sintering proceeding, some closed micropores fused each other to form bigger opening pores, resulting in a slight increase of porosity at 1200°C. However, the pore size distribution got more uniform and the density of the sintered body increased. So the bending strength of the sintered body reached the maximum of 176.67 Mpa and finally the main crystalline phases of the sintered sample changed to quartz, enstatite, and kyanite. The sintering activation energy of the palygorskite was measured by means of SID with a value of 906.46 kJ·mol⁻¹.     

Initial sintering mechanism and additive effect in zirconia ceramics

Y₂O₃-stabilized ZrO₂ (YSZ) ceramics have been used for various engineering applications because of their excellent mechanical properties or oxide-ion conductivity applicable to solid-electrolyte. The performance of YSZ depends on the sintered texture that is directly determined by sinterability of raw powders. A new kinetic analysis method of diffusion mechanism based on an initial sintering theory (grain-boundary or volume diffusion) is theoretically derived, the initial sintering mechanism of hydrolytic YSZ powder is experimentally determined, and the effects of powder characteristics on sinterability are discussed. Furthermore, the additive-enhanced sintering is proposed. A small amount of Al₂O₃ significantly enhances the densification. Using the additive effect, the low-temperature degradation that is the fault of zirconia ceramics can be improved by decreasing the sintering temperature.     

Effect of sintering temperature on the morphology,crystallinity and mechanical properties of carbonated hydroxyapatite (CHA)

Effect of sintering temperature on the physical and mechanical properties of synthesized B-type carbonated hydroxyapatite (CHA) over a range of temperature in CO₂ atmosphere has been investigated. The B-type CHA in nano size was synthesized at room temperature by using a direct pouring wet chemical precipitation method. The synthesized CHA powders were subsequently consolidated by sintering treatment from 800 to 1100 °C. The sintered CHA samples were evaluated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometry, X-ray fluorescence (XRF), carbon-hydrogen-nitrogen-sulfur-oxygen (CHNS/O) elemental analyzer, Field emission scanning electron microscopy (FESEM), and Vicker's indentation technique. The results obtained from XRD and FESEM indicated that the synthesized B-type CHA powders were nanometer in size. The crystallinity and crystallite size of the sintered CHA samples were increased due to increasing sintering temperature. The heat treatment between 800 °C and 1000 °C has resulted in coarsening and increased hardness of the sintered CHA samples. However, these properties began to deteriorate when sintering beyond 1100 °C due the formation of calcium oxide.     

Selective laser flash sintering of 8-YSZ

Flash sintering of ceramics is characterized by rapid sintering during simultaneous application of electric field and heat. Previous studies of flash sintering have been conducted in furnace environments, where sample temperatures are approximately uniform. In this work, we use highly dynamic heating from a scanning laser to initiate flash sintering while simultaneously applying a DC electric field. Onset of flash sintering is determined by a measurable increase in current through the sample. Our results show that stage I and stage II flash sintering can be initiated by laser heating. At low-to-moderate combinations of laser energies and applied electric fields, measured current rises slightly when the laser is scanned completely across the specimen from the positive to the negative electrodes. Microstructures for these samples show that powder consolidation is minimal in this regime (stage I flash sintering), and thus the observed current is likely due to onset of neck growth between powder particles rather than densification. At higher laser energies and fields, current rises steeply and microstructures show significant consolidation (stage II flash sintering). The demonstration that flash sintering occurs when ceramic is heated by laser-scanning supports future utilization of selective laser flash sintering as an additive manufacturing process.     

Liquid phase assisted high pressure sintering of dense TiC nanoceramics

To address the difficulty of achieving high density while effectively suppressing grain growth in the fabricating of nanoceramics, this paper demonstrates a novel integrated approach, consisting of a solid-liquid reaction, high pressure and low temperature sintering, to prepare dense nanocrystalline TiC ceramics. Using Si as the additive, a low-viscosity Si liquid phase is formed under the sintering condition of 1200?°C/4.5?GPa. It is shown that both of the sintering aid and high pressure are crucial in achieving high density nanostructured TiC ceramics and controlling their microstructure and thus their mechanical properties. The pure TiC can be sintered to reach 95.3% of its theory density and, with the assistance of liquid Si additive, it can be sintered to full densification without grain growth by high pressure technique.     

Microstructure evolution during spark plasma sintering of FJS-1 lunar soil simulant

A spark plasma sintering (SPS) process has been explored to densify FJS-lunar soil simulants for structural applications in space explorations. The effect of SPS conditions, such as temperature and pressure, on the densification behavior, phase transformation, microstructural evolution, and mechanical properties of FJS-1 have been examined by conducting the X-ray diffraction analysis, electron microscopy imaging, and nano/micro indentation testing. Test analysis results were also compared to results from the FJS-1 powder and sintered samples without pressure. The FJS-1 powder was composed of sodian anorthite, augite, pigeonite, and iron titanium oxide. When FJS-lunar soil simulants were sintered without pressure, the main phase evolved from sodian anorthite to the intermediate sodian anorthite, jadeite and glass, and iron titanium oxide at 1000°C, which were further transformed into filiform and feather-shaped augite and schorlomite at 1100°C. Most densification processes in pressureless sintering occurred at 1050°C-1100°C. During the SPS process, the main phases were sodian anorthite, pigeonite, and iron titanium oxide at 900°C. These phases were transformed to sodian anorthite, glass, and feather-shaped augite at 1000°C and 1050°C, with the nucleation of dendritic schorlomite at 1050°C. Significant densification by SPS can be observed as low as 900°C, which indicates that the application of pressure can substantially lower the sintering temperature. The SPSed samples showed higher Vickers microhardness than the pressureless sintered samples. The mechanical properties of the local phases were represented by the contour maps of elastic modulus and nanohardness. Multiscale mechanical test results along with the microstructural characteristics further imply that the SPS can be considered a promising in-situ resource utilization (ISRU) method to densify lunar soils.     

Direct selective laser sintering of hexagonal barium titanate ceramics

A direct selective laser sintering (SLS) process was combined with a laser preheating procedure to decrease the temperature gradient and thermal stress, which was demonstrated as a promising approach for additive manufacturing of BaTiO₃ ceramics. The phase compositions in BaTiO₃ ceramics fabricated by SLS were investigated by X-ray and neutron diffractions. The surface morphologies and cross-section microstructures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A dense hexagonal h-BaTiO₃ layer was formed on the surface and extended to a depth of 500 μm, with a relative density higher than 97% and absence of pores or microcracks. SLS resulted in the formation of the high-temperature phase, h-BaTiO₃, which was retained at room temperature possibly due to the high cooling rate. The grain boundaries of SLSed h-BaTiO₃ ceramics consist of a Ti-rich secondary phase. Compared with that of the pressureless sintered t-BaTiO₃ ceramics, the Vickers hardness of SLSed h-BaTiO₃ is 70% higher.     

Effects of powder preparation and sintering temperature on consolidation of ultrafine WC-8Co tool material produced by spark plasma sintering

In this study, ultrafine tool materials were produced by spark plasma sintering using three sets of WC-8Co nanopowders mixed by different methods. Effects of powder preparation method and sintering temperature on the consolidation of WC-8Co cemented carbides were investigated. At sintering temperature of 1250 °C, cemented carbide sintered from the powder mixed by ultrasonic vibration method exhibited homogeneous microstructure, high relative density (99.1%), small average grain size (280 nm), and excellent mechanical properties (H_V: 18.8 GPa, K_IC: 11.4 MPa⋅m^1/2). However, cemented carbide sintered from heavily ball-milled powder (ball milling for 24 h) showed increased grain coalescence and microdefects as well as lower relative density of 94.6%. Moreover, its hardness decreased to 17.7 GPa due to the decrease in relative density. Furthermore, straight cracks along grain boundary became dominant, causing fracture toughness to decrease to 10.5 MPa⋅m^1/2. Additionally, high sintering temperature caused grain coarsening, which was detrimental to mechanical properties of cemented carbides.     

Sintering behaviour and properties of zirconia ceramics prepared by pressureless sintering

Improvements in the sintering process and powder quality can lead to wider application of zirconia in ceramics. In this study, the effects of different temperatures on the stability, relative content of the tetragonal phase, and composition of Al₂O₃–ZrO₂ ceramic powders were explored using pressureless-assisted sintering. The crystallinity of the sintered Al₂O₃–ZrO₂ samples was significantly improved. The content of the tetragonal-phase ZrO₂ in sintered ceramic powders was 52.07%, 52.46%, 56.16%, 63.99%, and 64.90%, respectively, which was significantly higher than those of the raw materials. The average particle size of the sintered samples decreased from 1.07 μm to 0.17 μm with an increase in temperature, indicating that the ceramic powder particles were refined. The sample that was subjected to pressureless-assisted sintering at 1200 °C and held for 1 h exhibited the best stability and more uniform particle distribution compared to other samples. The particle size distribution data were closer to the standard line, satisfying the requirements of the normal distribution law. The results revealed that a high temperature was more favourable to the solid solution, and the formation of an Al₂O₃–ZrO₂ solid solution can diminish the influence of the volume expansion of ceramic powders on the sample properties during sintering. Therefore, the addition of the sintering aid Al₂O₃ significantly promotes the densification of the powders, and the pressureless sintering technique reduces the sintering temperature of the solid solution, thus imparting a crystalline structure and excellent mechanical properties to the material.     

Influence of oxygen content on structure and properties of pressurelessly sintered aluminum-nitride- and zirconium-boride-doped silicon-carbide ceramics

SiC is a widely used material. Understanding how oxygen content affects the SiC structure and properties is crucial. In this paper, heat treatment was used to prepare SiC powder samples with different oxygen contents, which were doped with AlN and ZrB₂ and were densified by pressureless sintering at 2050 °C. The effect of oxygen content on the sintered SiC structure was determined by X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The results indicated that the oxygen content influenced the SiC phase composition, grain boundaries, and densification. Additionally, the interaction between oxygen defects and AlN played an important role in sintering. The nanoindentation, alternating-current impedance, and thermal conductivity of the densified SiC specimens were also evaluated to elucidate the influence of the oxygen content on the densified-SiC functional properties. The results revealed that the oxygen content affected all the measured mechanical, electrical, and thermal properties. Furthermore, surface oxygen impurities suggested that oxygen content had similar critical effects on both the densified SiC structure and properties.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Selective laser sintering of polyamide 12/potassium titanium whisker composites

The excellent performance of potassium titanium whiskers (PTWs) reinforced plastics has been recognized; however, because of their large length‐to‐diameter ratio, they have not been applied in selective laser sintering (SLS). This article reports a new method for preparing polyamide 12 (PA12)/PTWs composite (PPC) powders for applications in SLS that uses a dissolution–precipitation process. The characteristics of the powders were evaluated. The results indicated that when the PTWs content of the composites was low (<10 wt %), the shape of the powder became more regular, and the particle diameter distribution became narrower. The crystallinity of PPC was 13 wt % higher than that of PA12. The sintering characteristics and mechanical properties of PA12 powder, glass‐filled PA12 (GF–PA), and PPCs were compared. The results showed that the sintering characteristics of PPCs (10 or 20 wt % PTWs) were as good as those of PA12. The mechanical properties were greatly improved by PTWs. The maximum tensile strength, bending strength, and bending modulus of the composites containing 20 wt % PTW were 68.3 MPa, 110.9 MPa, and 2.83 GPa, respectively, and were much higher than those of PA12 and GF–PA. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The effect of sintering parameters on spark plasma sintering of B4C

The effects of sintering temperature, heating rate, and holding time on the density and hardness of the spark plasma sintered B₄C were investigated. Experimental results are compared with the predictions from computational thermodynamics. It is explained how the choice of sintering parameters can affect the mechanical properties of the sintered samples. The fundamental mechanisms of how the sintering parameters affect the properties of the sintered B₄C are discussed with the sintering experiments and the predictions from the CALPHAD (Calculation of Phase Diagrams) approach. The effect of the number of graphite foil layers to pack the powder was also investigated. It is proposed that increasing the number of graphite foil layers may increase the driving force for the C-B₂O₃ reaction to proceed. Higher density and hardness is thus achieved with the removal of free carbon and B₂O₃ from the sample.     

Effect of sintering atmosphere on microstructures and properties of synthesized microporous MgO refractory raw material

Effects of sintering atmosphere on the microstructure and strength of magnesite were investigated using magnesite powder as raw material through X-ray diffraction, scanning electron microscopy, mercury porosimetry measurement, and so on. The results showed that the sintering atmosphere strongly affected the sintering behavior of magnesite. The specimens sintered in the reducing atmosphere had more and finer micro-sized pores inside the MgO particles compared with that in the oxidizing atmosphere at the same sintering temperature. Besides, MgO refractory raw material containing porous MgO microparticles with core–shell structure was obtained through the carbothermal reduction of MgO microparticles and subsequent oxidation of Mg vapor at the surface of MgO particles at 1500°C in the reducing atmosphere. At the reducing atmosphere and 1500°C, the microporous MgO refractory raw material with the core–shell structure of external dense and internal porous had an apparent porosity of 22.1% and a compressive strength of 51.6 MPa.