Sintering Cycle Research Articles

Microstructural development during liquid phase sintering of Fe and Fe–Mo alloys containing elemental boron additions

The sintering behaviour of Fe and Fe–Mo prealloyed powder compacts containing from 0·5 to 3·5 wt-%Mo and fixed boron additions has been studied with special emphasis on the microstructural development, the formation of the liquid phase and the liquid phase sintering mechanisms involved during the densification process. The basic phenomena involving the formation of a liquid phase and the temperature at which the liquid is generated is strongly influenced by the Mo/B ratio in the initial powder mixture. The effect produced by Mo and its concentration, both, on the final microstructure and on the behaviour of boron prior to, during and after the formation of the liquid phase, was studied under both the optical and the scanning electron microscope. For this purpose interrupted sintering experiments followed by water quenching from specific temperatures and times within the sintering cycle have been carried out. The study shows that the formation of a liquid phase is preceded by noticeable enhancement of solid state sintering at intermediate temperatures. This is accompanied by boron diffusion into the metallic particles, generating inter- and intragranular precipitates in amounts dependent on the Mo concentration. At a later stage boron is found to be preferentially located at the boundaries as the formation of a continuous Fe/Mo/B liquid phase with excellent wetting characteristics proceeds thus producing densification by pore filling and shape accommodation. Final densities up to 7·82 g cm−3 were obtained for these alloys.

Effect of silicon, vanadium and nickel on microstructure of liquid phase sintered M3/2 grade high speed steel

Liquid phase sintering of M3/2 grade high speed steel (HSS) was carried out at 1270°C in high vacuum reaching near full density starting from loose packed powder. Focus is placed on the study of the effects of the addition of Si, Ni and V as elemental powder and cooling rates on the as sintered microstructure, the main objective being improving M6C characteristics and control of pearlite appearance. Slow cooling from the sintering temperature and Si addition in wt% resulted in a completely fine pearlitic matrix with less elongated and more uniformly distributed M6C precipitates. Adding V or Ni in wt-% quantities decreased the amount of pearlite owing to MC formation and delayed pearlite formation. The study involved the use of thermodynamic modelling and sintering cycle optimisation as well as the evaluation of sintered material by means of optical and scanning electron microscopy, X-ray diffraction and hardness testing.

Low temperature sintering of 8YSZ electrolyte film for intermediate temperature solid oxide fuel cells

At 200 °C, 8 mol% yttria doped zirconia (8YSZ) nanocrystals were synthesized through a hydrothermal process for 24 h. Characterizations revealed that the as-synthesized 8YSZ powder was weakly agglomerated and had a surface area of 121 m 2/g and an average grain size of ∼9 nm. Sintering behaviors of the 8YSZ powder were studied via isothermal and non-isothermal experiments. It demonstrated that the powder has high sintering activity, where 8YSZ green films prepared via dip coating using the slurry directly formed by the hydrothermal process can be fully densified at 1000 °C. The thickness of the sintered film can be controlled through adjusting the 8YSZ concentration in the slurry and repeating the dipping–coating–sintering cycle. The superior sinterability achieved in the present study was mainly attributed to the high green density and homogeneity of the green bodies with narrow-distributed interparticle pore structure. The low sintering temperature is of great practical importance for improving film quality and facilitating co-sintering of 8YSZ films together with other SOFC components.

Control of Microstructures in α-SiAlON Ceramics

The effects of sintering cycles and doping elements on the microstructures of Ln-α-sialon were studied. The results showed that microstructures with an elongated α-sialon morphology could be obtained through high-temperature post-heat treatment (1800–1900°C) or by prolonging soaking times during sintering. Different rare-earth elements had a profound effect on the microstructure of the resulting α-sialon. The Ln-α-sialon doped with low-Z-value elements could easily develop elongated grains with higher aspect ratio.

The use of nepheline-syenite in a body mix for porcelain stoneware tiles

In the study, the possibility to use nepheline-syenite, as fluxing agent, in a body mix used for porcelain stoneware tile, was determined. Starting from the reference mix composition, different amounts, 5.0, 10.0 and 15.6 wt.%, of sodium feldspar were replaced with the same amounts of nepheline-syenite. Different sintering cycles were used to understand the role of the nepheline-syenite in the development of the final microstructure. The presence of nepheline-syenite strongly favours the sintering behaviour, by reducing the sintering time necessary to reach the water absorption values, requested by the International Standard. Furthermore, the microstructure results more homogeneous and the mechanical characteristics are increased. The better mechanical performances can also be attributed to the different mineralogical compositions, in particular with lower amount of glassy phase, higher flexural strength values are obtained.

Spark plasma sintering of Sm 2O 3-doped aluminum nitride

The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm 2O 3) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm 2O 3 as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm 2O 3 as sintering additive, with a thermal conductivity value about 118 W m −1 K −1) for the 3 wt.% Sm 2O 3-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm 2O 3 greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm 2O 3 dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts.

Liquid generation during sintering of Fe–3.5%Mo powder compacts with elemental boron additions

The mechanisms for liquid generation and the microstructural development during sintering of a Fe–3.5Mo–0.3B alloy were studied. Interrupted sintering experiments followed by water quenching from specific temperatures within the sintering cycle have been carried out. The influence of Mo, both, on the final microstructure and on the behaviour of boron prior to, during and after the formation of the liquid phase, was studied through observation of the quenched samples under LOM and SEM. The study shows that prior to the formation of the liquid phase, boron diffuses into the metallic particles forming inter and intragranular precipitates of the (Fe,Mo) 2B type. At higher temperatures a continuous Fe/Mo/B liquid phase, with excellent wetting characteristics, is formed thus leading to near fully dense materials. The generation of the liquid is based on a eutectic reaction involving the mixed (Fe,Mo) 2B borides previously formed. The development of the microstructure after liquid formation is described.

Effects of nano‐Al 2 O 3 particles on the superconductivity of Pb‐doped BSCCO

Nanometer Al2O3 particles were added to (Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) precursor powders during the final sintering cycle of a multi_step preparation process. The influence of Al2O3 on the melting temperature, microstructure and transport property of ceramics was studied by means of DTA, XRD, SEM/EDX, electrical and magnetic measurements. We report that the addition of a small amount of Al2O3 (0.2 wt%) increased the pellet's critical current density Jc at 77 K by ∼35% and improved Jc behaviour in applied magnetic field (parallel or perpendicular to the sample surface). The results indicate that the introduction of a proper amount of nano-Al2O3 particles during the final processing of BPSCCO samples can effectively improve the flux pinning ability and has a little detrimental effect on the (Bi,Pb)-2223 formation process. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Addition of nanometer Al 2O 3 during the final processing of (Bi,Pb)-2223 superconductors

Nanometer Al 2O 3 particles were added to (Bi,Pb) 2Sr 2Ca 2Cu 3O x (Bi-2223) precursor powders during the final sintering cycle of a multi-step preparation process. The influence of Al 2O 3 on the melting temperature, high- T c (Bi-2223) phase formation, microstructure and transport properties of ceramics was studied by means of DTA, XRD, SEM/EDX, electrical and magnetic measurements. We report that the addition of a small amount of Al 2O 3 (0.2 wt.%) increased the pellet's critical current density, J c at 77 K by ∼30% and improved J c behaviour in applied magnetic field either parallel or perpendicular to the sample wide surface (SWS). The results indicate that the introduction of a proper amount of nano-Al 2O 3 particles during the final processing of BPSCCO samples can effectively improve the flux pinning ability and has a little detrimental effect on the (Bi,Pb)-2223 formation process.

M 6C formation during sintering of cemented carbides containing (Ti,W)C

The temporary formation of M 6C during sintering of cemented carbides containing the cubic carbide (Ti,W)C is shown in samples exposed to interrupted sintering cycles. The M 6C, revealed by magnetic-saturation measurements, X-ray diffraction and microscopy, is formed during the initial stage of sintering, but is dissolved as sintering proceeds. The overall carbon content does not change much between the sintering temperatures. It is believed that the initially nearly stoichiometric (Ti,W)C releases carbon due to its equilibrium carbon content decreasing with increasing temperature, which makes dissolution possible. The temporary M 6C would not normally be detected, since sample characterisation is in practice employed only after sintering.

Thermogelation of Al2O3/Y-TZP films produced by electrophoretic co-deposition

There is a growing interest to develop reliable, economic and environmentally-friendly methods for manufacturing thin and thick coatings and layered systems. Electrophoretic deposition has demonstrated to be a suitable technique for manufacturing laminates and coatings. For thick films, the adhesion to the substrate and the shape retention become important problems that lead to low density, cracking, and heterogeneous surfaces. To overcome these problems, a new idea is proposed consisting in the formation of a thick deposit by EPD in water and its immediate consolidation by thermogelation of polysaccharides, which is introduced in the suspension and forms a gel structure on cooling during withdrawal of the substrate from the suspension bath. In this work the parameters involved in the electrophoretic codeposition of two phase suspensions (Al2O3/Y-TZP) containing carrageenan, on non-porous graphite substrates are studied. Dense self-supported films with thickness between 10 and 500 μm are obtained after a thermal treatment in which the graphite substrate burns out and the subsequent sintering cycle. The performance of gel-deposition is studied as a function of the concentration of carrageenan, the solids loading and the electrical conditions.

Porous Hydroxyapatite and Glass Reinforced Hydroxyapatite for Controlled Release of Sodium Ampicillin

Porous Hydroxyapatite and Glass Reinforced Hydroxyapatite for Controlled Release of Sodium Ampicillin

Determination of the optimal process parameters in metal injection molding from experiments and numerical modeling

The determination of optimal process parameters to produce parts by metal injection molding without defects and with required mechanical properties is discussed in the paper, based on experiments and numerical modeling. The experiments are carried out with a multi-cavity mould specially designed and equipped to measure and record different parameters during the injection stage. The debinding and sintering cycles are optimized to get the components free of defects, too. Based on modeling techniques using a biphasic flow formulation and a newly developed explicit algorithm, numerical simulations are realized to predict the segregation effects in injection. This novel algorithm in fractional steps and the related finite element software lead efficiently to accurate correlations between experiments and simulations.

Compact shape as a relevant parameter for sintering ZnO–Bi 2O 3 based varistors

Ceramic varistors based on the ZnO–Bi 2O 3–Sb 2O 3 system, are usually sintered at 1150–1200 °C in the presence of a Bi-rich liquid phase that forms at 700–800 °C. Bismuth is critical for the formation of electrically active grain boundaries, and the Bi-rich liquid phase plays a major role in the grain growth kinetics. Therefore, uncontrolled bismuth oxide vaporization during sintering can heavily deteriorate the electrical response of the varistor, and the area–volume ratio of the green compact becomes a relevant parameter, along with the sintering temperature and soaking time. In the present work, we have studied the microstructure and the current–voltage characteristics of ceramic varistors with different area–volume ratios and sintering cycles. We find that, depending on the sintering cycle, there exists a value for the area–volume ratio that leads to a strong deterioration of the varistor electrical properties.

Following the progress of sintering in steel powder compacts through original experiments

In order to evaluate the progress of sintering in powder steel compacts, two experiments were performed. First, the electrical resistivity of a die pressed compact was continuously measured during a sintering cycle up to 1130°C inside an adapted dilatometer. The main changes occur at intermediate temperatures, since the resistivity falls by a factor of about 100 between room temperature and 800°C and is almost constant afterwards. In the second set of experiments, partially sintered specimens are submitted to a tensile test until failure and their fracture surfaces are analysed in terms of welded area fraction of interparticle contacts. The yield and ultimate strengths, significantly increase between 600 and 1000°C. These results prove that interparticle joining operates early in the sintering cycle, above 1000°C. They also show that the strengthening of the material is not closely correlated with electrical resistivity changes during heating; interparticle contacts are able to conduct electricity before they are strong enough to support tensile stresses.

Effect of Li addition on the properties of Bi-based superconductors

The effect of Li addition on the formation and properties of Pb containing Bi-based superconductors has been studied. Three successive thermal sintering cycles have been applied to the oxides adjusted for the nominal composition, Bi/sub 1.6/Pb/sub 0.4/Sr/sub 2/CaCu/sub 2/O/sub 8+d/. Eight different samples having Li/sub 2/CO/sub 3/ from 0.0 up to 3.0 weight% have been synthesized. X-ray diffraction patterns, resistivity-temperature behaviors and differential thermal analysis of the samples have been examined. Li/sub 2/CO/sub 3/ addition at 650 /spl deg/C decreases the fraction of (Bi,Pb)-2212 and increases that of Ca/sub 2/PbO/sub 4/. The normal state resistances of the samples increase, the onset T/sub c/ values decrease and the lattice parameters almost remain same with Li/sub 2/CO/sub 3/ addition. Li/sub 2/CO/sub 3/ addition up to 3.0 wt% decreases the melting temperature of (Bi,Pb)-2212 phase by about 50/spl deg/C inducing a liquid phase at about 750 /spl deg/C. Our observations suggest that the high T/sub c/ phase, (Bi,Pb)-2223 may be formed at about 750-800 /spl deg/C using the precursors, (Bi,Pb)-2212 and Ca/sub 2/PbO/sub 4/ doped with Li/sub 2/CO/sub 3/.

Densification of plasma sprayed YSZ electrolytes by spark plasma sintering (SPS)

Abstract Solid oxide fuel cells (SOFC) are promising candidates for alternative power generation systems due to their high-energy conversion efficiencies, and low emissions of environmentally hazardous by-products. Plasma spray (PS) is an effective, and relatively inexpensive process for fabricating high performance yttria stabilized zirconia (YSZ) electrolyte for SOFC. Yet, because of the numerous inter-granular defects introduced to the electrolyte by the plasma spray process, the electrolyte is not gas tight and consequently, the energy efficiency of the cell is severely curtailed. In order to improve the performance of the SOFC, spark plasma sintering (SPS) is introduced as a post-spray treatment to enhance the density of the PS YSZ electrolyte rapidly, and effectively. In this study, spark plasma sintering (SPS) was performed at 1200, 1400 and 1500 °C. Each sintering cycle had a holding time of 3 min. Single and multiple SPS cycles (3 min at preset temperature per cycle) were used to treat the plasma sprayed yttria stabilized zirconia (PS YSZ) electrolytes. The microstructure of as-received and SPS treated electrolytes as examined by scanning electron microscopy (SEM) demonstrated a microstructure transition above 1200 °C, where the typical plasma sprayed lamella structure transformed to a granular-type structure. The porosity of as-received and SPS post-treated electrolytes, which were determined by a mercury intrusion porosimeter (MIP) revealed a significant reduction in pores at 1500 °C. Average pore size reduced from 0.2 to 0.08 μm. The ionic conductivity of the electrolytes is evaluated by AC impedance spectroscopy to characterize the effect of SPS on enhancing the ionic conductivity of the electrolytes.

Liquid phase sintering of P/M aluminium alloys: effect of processing conditions

Four commercial aluminium alloy premixed powders (Ecka Alumix 123, 321, 13 and 431) have been sintered in a broad range of experimental conditions in order to establish the optimum processing sintering cycle. Compacts were pressed at 400 MPa reaching a green density (wax weight excluded) of ∼92.5–95%TD, depending on the powder mixture. Sintering under nitrogen was carried out within a temperature range between 550 and 650 °C during 20 min. After determination of the optimum sintering temperature, additional experiments were performed between 0 and 120 min. The heating rate was varied between 5 and 15 °C/min. The influence of sintering atmosphere was also investigated repeating some of these cycles in hydrogen and vacuum. The effect of the sintering conditions (temperature, time and atmosphere) was evaluated by measuring density and hardness. After sintering, some samples were heat-treated using T4 and T6 treatments. SEM was used for the microstructural characterisation of the resulting materials.

Ceramic products obtained from rock wastes

Residue powders resulting from cutting and machining operations in slate quarries were processed by a press and sintering route to manufacture ceramic products such as flooring and roofing tiles. The first part of this work consisted of a laboratory study carried out by uniaxially pressing the powders to shape and sintering in air using a muffle type furnace. A plan of sintering experiments was followed in order to evaluate the effect of some processing parameters: particle size distribution of the rock waste powders, compacting pressure and sintering cycle (heating and cooling rates, sintering temperature). The sintering tests have shown that the density of the sintered specimens increased gradually up to 1150–1170 °C and then started decreasing. The second stage of this study was carried out at an industrial scale. Since particle size of the raw material was found to be an important parameter, atomisation was used to homogenise the particle dimensions and ensure proper feeding of the pressing systems. Moreover, atomisation is the best way to treat very fine wastes, both for economical and technical reasons. Tiles with the dimensions 200 mm×100 mm×7 mm were then uniaxially pressed at 40 MPa and subsequently sintered at temperatures close to 1150 °C. The properties of the products obtained are suitable for flooring tiles.

A phenomenological constitutive model for the sintering of alumina powder

This paper develops a phenomenological model for the densification and viscous behavior of alumina powder during sintering. The free sintering strain rate part and the viscoplastic strain rate part of the constitutive model were determined from experimental data by using the experimental method proposed for WC–Co mixture by Bouvard and co-workers. From densification data under free sintering by using reference and stairway heating cycles with samples of three different green densities, a simple densification equation was identified. This equation expresses the densification rate as a function of temperature, relative density, and green density. The predictive capability of the identified densification equation was discussed by investigating the densification of alumina powder during various sintering cycles and with different green densities. For the viscoplastic strain rate part of the constitutive model, the axial viscosity and the viscous Poisson's ratio of an alumina powder compact were determined by using an intermittent loading method.