Relation between microstructure,properties and spark plasma sintering (SPS) parameters of pure ultrafine WC powder

A combined experimental/numerical methodology is developed to fully consolidate pure ultrafine WC powder under a current-control mode. Three applied currents, 1900, 2100 and 2700 A, and a constant pressure of 20 MPa were employed as process conditions. The developed spark plasma sintering (SPS) finite-element model includes a moving-mesh technique to account for the contact resistance change due to sintering shrinkage and punch sliding. The effects of the heating rate on the microstructure and hardness were investigated in detail along the sample radius from both experimental and modeling points of view. The maximum hardness (2700 HV10) was achieved for a current of 1900 A at the core sample, while the maximum densification was achieved for 2100 and 2700 A. A direct relationship between the compact microstructure and both the sintering temperature and the heating rate was established.     

Low-temperature microwave sintering of TiN–SiC nanocomposites

Densification kinetics study during microwave sintering of titanium nitride-based nanocomposite has been conducted. A series of TiN–SiC compositions with 1, 3, 5 wt% of silicon carbide were microwave sintered at relatively low sintering temperatures (900–1,300 °C) for 0–30 min. The SiC content influenced on heating uniformity and final density and grain-size achieved. Densification process during microwave sintering obeyed the mechanism of grain-boundary diffusion with activation energy of 235 kJ mol⁻¹. Microwave sintering resulted in fine microstructure (~300 nm) and hence high values of micro hardness (~20 GPa).     

On sinterability of Cu-coated W nanocomposite powder prepared by a hydrogen reduction of a high-energy ball-milled WO3-CuO mixture

Cu-coated W nanocomposite powder was prepared by a combination of high-energy ball-milling of a WO₃ and CuO mixture in a bead mill and its two-stage reduction in a H₂ atmosphere with a slow heating rate of 2 °C/min. STEM-EDS and HR-TEM analyses revealed that the microstructure of the reduced W–Cu nanocomposite powder was characterized by ~50-nm W particles surrounded by a Cu nanolayer. Unlike conventional W–Cu powder, this powder has excellent sinterability. Its solid-phase sintering temperature was significantly enhanced, and this led to a reduction in the sintering temperature by 100 °C from the 1,200 °C required for conventional nanocomposite powder. In order to clarify this enhanced sintering behavior of Cu-coated W–Cu nanocomposite powder, the sintering behavior during the heating stage was analyzed by dilatometry. The maximum peak in the shrinkage rate was attained at 1,073 °C, indicating that the solid-phase sintering was the dominant sintering mechanism. FE-SEM and TEM characterizations were also made for the W–Cu specimen after isothermal sintering in a H₂ atmosphere. On the basis of the dilatometric analysis and microstructural observation, the possible mechanism for the enhanced sintering of Cu-coated W composite powder in the solid phase was attributed to the coupling effect of solid-state sintering of nanosized W particle packing and Cu spreading showing liquid-like behavior. Homogeneous and fully densified W–20 wt% Cu alloy with ~180 nm W grain size and a high hardness of 498 Hv was obtained after sintering at 1,100 °C.     

Densification,microstructure and mechanical properties of multicomponent (TiZrHfNbTaMo)C ceramic prepared by pressureless sintering

A multicomponent (TiZrHfNbTaMo)C ceramic has been fabricated by pressureless sintering at temperatures from 2100 ℃ to 2500 ℃,using an equimolar multicomponent carbide powder synthesized by carbothermal reduction as the starting material.Influence of sintering temperature on densification,microstructure and mechanical properties of the ceramics was investigated.The relative density increases with increasing sintering temperature,and a nearly fully dense sample is achieved by pressureless sintering at 2500 ℃.Average grain size increases from 3.7 to 15.2 μm with increasing sintering temperature from 2300 to 2500 ℃.The (TiZrHfNbTaMo)C ceramic sintered at 2400 ℃ exhibits a single phase fcc structure with homogeneous chemical composition,an average grain size of 7.0 μm and a relative density of 96.5％,while its measured hardness is 33.2 GPa at 100 mN and 23.2 GPa at 9.8 N.     

升温速率对氧化铝粉末烧结行为的影响

采用不同的升温速率,在膨胀计中对脱脂后的氧化铝粉末射成形坯件进行一系列的烧结试验.结果表明,烧结致密化过程主要发生在升温阶段,快速升温有利于致密化的进行和抑止晶粒长大,但由于烧结时间较短和烧结炉最高温度的限制,产品的最终致密化程度不高.在低温时快速升温,高温时缓慢加热,可以获得较好的致密化效果和微观结构.试验和分析结果将为建立非等温烧结模型和烧结工艺参数的优化方法提供依据.     

Pore evolution and microstructure with heating profile in BaTiO3-based Ni-MLCCs with Y5V specification

The effects of the heating profile in the burnout and sintering processes, especially the heating rate and holding time, on the pore evolution and microstructure in multilayer ceramic capacitors (MLCCs) showing Y5V characteristics, were investigated in order to optimize the fabrication process. The heating rate was controlled as 1, 3, and 5 °C/min in both burnout and sintering processes or the sintering process was carried out with and without the holding time of 3 h at the final sintering temperature of 1200 °C. The pore size distribution and cumulative pore surface area became broad and small, respectively, as the heating rate and sintering temperature were increased. The microstructure revealed that the MLCCs were effectively densified in the slow heating rate of 1 °C/min in both processes with the holding time of 3 h. The heating rate in the burnout process predominantly affected the pore evolution and microstructure more so than that in the sintering process, showing the effects of the holding time on removing the residual pores and on developing the final microstructure.     

Fabrication of nano-grained Ti–Nb–Zr biomaterials using spark plasma sintering

Nanostructured near-β Ti–20Nb–13Zr at % alloy with non-toxic elements and enhanced mechanical properties has been synthesized by spark plasma sintering (SPS) of nanocrystalline powders obtained by mechanical alloying. The consolidated bulk product was characterized by density measurements and Vickers hardness (HV), and X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) combined with energy-dispersive spectroscopy (EDX), and transmission electron microscopy (TEM) for structural details. The temperature during spark plasma sintering was varied between 800 and 1200 °C, while the heating rate and holding time of 100°K/min and 10 min were maintained constant in all the experiments. The effect of SPS temperature on the densification, microstructure, and HV was discussed. The results show that a nearly full density structure was obtained after SPS at 1200 °C. The microstructure of the obtained alloy is a duplex structure with the α-Ti (hcp) region having an average size of 70–140 nm, surrounding the β-Ti (bcc) matrix. The obtained alloy was chemically homogenized with a micro hardness value, HV of 660. The developed nanostructured Ti–20Nb–13Zr alloy is suggested for biomedical use as in implant material in dental and orthopedic applications.     

Bulk nanocrystalline Al85Ni10La5 alloy fabricated by spark plasma sintering of atomized amorphous powders

Amorphous Al₈₅Ni₁₀La₅ powders were consolidated to cylindrical samples by spark plasma sintering (SPS), and their microstructures and mechanical properties were investigated. When the powders were consolidated below the crystallization temperature, an amorphous phase was retained in the consolidated sample. Sintering above the crystallization temperature caused full crystallization. The Vickers hardness of the amorphous-containing sample was about 350 HV in the as-sintered state and increased up to 450 HV by a subsequent heat treatment just below the crystallization temperature. The highest hardness was achieved in a nanocrystalline microstructure. Compression tests revealed the brittle nature of the consolidated samples although the fracture and yield strength was higher than 1 GPa. The brittleness is due to the low relative density of the amorphous-containing samples and the presence of a large amount of intermetallic compounds in the fully crystallized sample.     

Moving finite-element mesh model for aiding spark plasma sintering in current control mode of pure ultrafine WC powder

The intricate bulk and contact multiphysics of spark plasma sintering (SPS) together with the involved non-linear materials’ response make the process optimization very difficult both experimentally and computationally. The present work proposes an integrated experimental/numerical methodology, which simultaneously permits the developed SPS model to be reliably tested against experiments and to self-consistently estimate the overall set of unknown SPS contact resistances. Unique features of the proposed methodology are: (a) simulations and experiments are conducted in current control mode (SPS-CCm); (b) the SPS model couples electrothermal and displacement fields; (c) the contact multiphysics at the sliding punch/die interface is modeled during powder sintering using a moving mesh/moving boundary technique; (d) calibration and validation procedures employ both graphite compact and conductive WC powder samples. The unknown contact resistances are estimated iteratively by minimizing the deviation between predictions and on-line measurements (i.e., voltage, die surface temperature, and punch displacement) for three imposed currents (i.e., 1,900, 2,100, 2,700 A) and 20 MPa applied pressure. An excellent agreement is found between model predictions and measurements. The results show that the SPS bulk and contact multiphysics can be accurately reproduced during densification of ultrafine binderless WC powder. The results can be used to benchmark contact resistances in SPS systems applicable to graphite and conductive (WC) powder samples. The SPS bulk and contact multiphysics phenomena arising during sintering of ultrafine binderless WC powders are finally discussed. A direct correlation between sintering microstructure, sintering temperature, and heating rate is established. The developed self-consistent SPS model can be effective used as an aiding tool to design optimum SPS experiments, predict sintering microstructure, or benchmark SPS system hardware or performances.     

Sintering/shrinkage kinetics of metal injection molded copper brown body

The sintering shrinkage kinetics of metal injection molded copper brown parts was investigated at three different heating rates up to 1050 °C using a high temperature pushrod dilatometer. Non-isothermal and isothermal measurements were simultaneously used in the multivariate non-linear regression analysis to determine the kinetic reaction parameters. A four-step kinetic model was established using “Jander” diffusion equation. Using the rate control sintering, the temperature-time profile for the predefined shrinkage was achieved with respect to time saving. The temperature-time profile was confirmed by performing an experimental run in the production furnace. The metallographic analysis of the microstructure and determination of the sintered density revealed the validation of calculated temperature-time profile.     

Preparation of β-FeSi2 thermoelectric material by laser sintering

Laser sintering has been applied for preparing β-FeSi₂ based thermoelectric alloy for the first time. Effects of laser sintering time on alloying, phase transformation and microstructure of FeSi₂ were investigated by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). Effects of annealing temperature and time on phase transformation were also studied through Seebeck coefficients. The results show that for 90 s laser-sintered samples, it takes only 15 h to obtain β phase under Ar atmosphere followed by an annealing at 1073 K. These samples exhibit homogeneous microstructure with average grain size of less than 5 μm. A maximum Seebeck coefficient at room temperature could reach 115 μV/K. It indicates that laser sintering could be an alternate faster preparation method to generate high quality β-FeSi₂ thermoelectric material with little contamination due to its advantages of rapid heating rate, high cooling rate and rapid solidification.     

On-line ultrasonic velocity measurements for characterisation of microstructural evaluation during thermal aging of β-quenched zircaloy-2

Ultrasonic non-destructive evaluation (NDE) technique has been used for characterisation of evolution of microstructure in β-quenched and thermally aged zircaloy-2 specimens. On-line ultrasonic velocity measurements have been made in β-quenched state of zircaloy-2 (A specimen) during heating at different heating rates up to 573 K (B specimen), 603 K (C specimen) and 623 K (D specimen) with holding time periods of 5 h for specimens B and C, and 2 h for specimen D, at the corresponding maximum temperature, by employing a specially designed experimental set-up. The observed change in velocity at room temperature (298 K) before and after ageing for specimens B and D is 0.52% and 0.48%, respectively, and this reveals that intermetallic precipitates are formed during the aging treatment. Ultrasonic measurements are correlated with the hardness, density and microstructural changes.     

Field assisted sintering of SiC using extreme heating rates

Several challenges still exist in the fabrication of silicon carbide (SiC) ceramic components associated with using multiple step processes and sintering at high temperature which result in long lead times. This challenge is being addressed by exploring an emerging sintering technology called field assisted sintering technology (FAST), also known as spark plasma sintering. The objective of this study is to sinter SiC to near theoretical density using the FAST technique and study the effect of using extreme heating rates during sintering. All samples were sintered under identical temperature and pressure of 2000 °C and 45 MPa. Resulting samples were characterized for density, microhardness, grain size, and microstructure evolution. The results showed that density decreased slightly and grain size increased as heating rate was varied from 50 to 400 °C/min.     

电流直加热动态热压烧结制备SiCp/Fe复合材料

用电流直加热动态热压烧结法制备SiCp/Fe复合材料,研究了工艺参数对其性能的影响.结果表明,材料的性能随着热压压力、加载电压、烧结时间和间歇电流循环次数的增大而明显提高.该工艺热效率高,升温速率快,制备时间短,放电点的弥散分布,能实现均匀加热并使颗粒表面活化,使材料组织细小均匀.用此工艺可快速制备出均质、高致密和高质量的SiCp/Fe复合材料,其最好性能为致密度99.9%,布氏硬度416HB,抗拉强度838 MPa.     

The effect of sintering time on the densification and mechanical properties of a mechanically alloyed Cr–Mn–N stainless steel

Sintering is an essential stage in powder metallurgy, which affects the final microstructure and performance of the part. This study is concerned with the sintering and mechanical behaviors of Fe–18Cr–8Mn–0.9N stainless steel prepared from mechanically alloyed amorphous/nanocrystalline powders. The contribution of sintering time to the densification at 1100 °C is considered and a sluggish densification is found for the alloy. Furthermore, the correlation between the microstructure and mechanical properties of the fabricated porous parts is studied. It is found that the yield stress is affected by both porosity and the material’s intrinsic yield strength. Nonetheless, the effect of porosity on the overall hardness typically prevails over the effect of matrix hardness. Interestingly, even after sintering at 1100 °C for up to 20 h, the nanometric structure of the material is retained.     

Studies on sintering behaviour of Al2O3–ZrO2 oxide composites processed by extended arc thermal plasma and conventional heating

Al₂O₃–ZrO₂ composites were sintered using low cost extended arc thermal plasma reactor and conventional heating. Composites prepared in a wide range of composition were studied in terms of their density, shrinkage, hardness, structure, microstructure and dielectric response. Experimental parameter such as sintering time, sintering temperature and plasma power were optimized to achieve higher sintered end product. Highly dense sintered products were obtained by plasma heating route within short sintering time compared with conventional sintered method. Interesting development pertaining to structure and phase evolution, structure and dielectric response are analyzed. It is found that compositional variation in this composite produces structural phase separation at different sintering conditions, which is more in plasma heating product than conventional heated product. Plasma sintered product always shows less dielectric constant as compared to conventional sintered sample.     

Rapid densification and reaction sequences in self-reinforced Y-α-SiAlON ceramics with barium aluminosilicate as an additive

Y-α-SiAlON (Y_1/3Si₁₀Al₂ON₁₅) ceramics with 5 wt.%BaAl₂Si₂O₈ (BAS) as an additive were synthesized by spark plasma sintering (SPS). The kinetic of densification, phase transformation sequences and grain growth during sintering process were investigated. Full densification could be achieved by 1600 °C without holding and using a heating rate of 100 °C min⁻¹, but the transformation from α-Si₃N₄ to α-SiAlON is not completed simultaneously with the densification process. The equilibrium phase assemblage could be reached after SPS at 1800 °C for 5 min and the resultant material possesses self-reinforced microstructure with high hardness of 19.2 GPa and fracture toughness of 6.8 MPa m^1/2. The complete crystallization of BAS is beneficial to the high temperature mechanical properties. The obtained could maintain the room strength up to 1300 °C.     

The influence of sintering temperature on the properties of compacted bovine hydroxyapatite

On the sintering characteristic of hydroxyapatite (HA), the resulting microstructure and properties are influenced not only by the characteristic and impurities of materials but also are found to be dependent on the thermal history during the fabrication process. This research is concerned with the effect of sintering temperature on the relative density, hardness, and phase purity after sintering process. Bovine HA (BHA) powder obtained from heated local cortical bovine bone at 900 °C for 2 h was uniaxially pressed at 156 MPa into green bodies using a 20 mm cylindrical dies. The compacted green body was pressurelessly sintered in air atmosphere at temperatures ranging from 1000 to 1400 °C, at a furnace ramp rate of 5 °C/min and dwell time of 2 h. The BHA starting powder was characterized using XRD and FTIR. SEM was also used for observing the microstructures of the starting material. The sintered BHA specimens were analyzed using Archimedes method for measuring density; XRD for phase stability; and Vickers method for hardness measurement. The analysis results show that the starting BHA powder and the sintered BHA specimens contain HA. The intensity of the three main peaks of HA decreases with increasing sintering temperature which may be due to decomposition of HA at high temperature. The density and hardness of BHA increases with increasing sintering temperature based on the results obtained.     

Microstructure characteristic, mechanical properties and sintering mechanism of nanocrystalline copper obtained by SPS process

Nano-sized copper powder with an average size of 50 nm fabricated by chemical reduction method of hydrazine hydrate was consolidated using spark plasma sintering (SPS) method. The relationship between the sintering temperature and relative density of the nanocrystalline bulk copper was studied, the microstructure and the mechanical properties were examined, and the sintering mechanism was discussed. It was concluded that the nanocrystalline copper with a relative density greater than 99% and the yield strength of nearly 650 MPa could be fabricated by SPS process with the holding pressure of 600 MPa, sintering temperature of 350 °C, holding time of 5 min, and heating rate of 100 °C/min. Both refinement of the grains and formation of the extensive nanoscale twins in the NC bulk copper are the main factors to strengthen the metal.     

The influence of high-temperature sintering on microstructure and mechanical properties of free-standing APS CeO<Subscript>2</Subscript>–Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript> coatings

In this study, ceria–yttria co-stabilized zirconia (CYSZ) free-standing coatings, deposited by air plasma spraying (APS), were isothermally annealed at 1315 °C in order to explore the effect of sintering on the microstructure and the mechanical properties (i.e., hardness and Young’s modulus). To this aim, coating microstructure, before and after heat treatment, was analyzed using scanning electron microscopy, and image analysis was carried out in order to estimate porosity fraction. Moreover, Vickers microindentation and depth-sensing nanoindentation tests were performed in order to study the evolution of hardness and Young’s modulus as a function of annealing time. The results showed that thermal aging of CYSZ coatings leads to noticeable microstructural modifications. Indeed, the healing of finer pores, interlamellar, and intralamellar microcracks was observed. In particular, the porosity fraction decreased from ~10 to ~5% after 50 h at 1315 °C. However, the X-ray diffraction analyses revealed that high phase stability was achieved, as no phase decomposition occurred after thermal aging. In turn, both the hardness and Young’s modulus increased, in particular, the increase in stiffness (with respect to “as produced” samples) was equal to ~25%, whereas the hardness increased to up to ~60%.