Solution-reprecipitation Process Research Articles

Effect of sintering additive composition on microstructure, thermal and mechanical property of multi-step spark plasma sintered silicon nitride

Dense silicon nitride composites were fabricated by multi-step spark plasma sintering (SPS). As the induced phase, 6 wt% Mo was added and related with the unadulterated β-Si3N4 seeds produced by the in-situ reaction during the sintering process. Si3N4 samples sintered with 7 mol% CeO2 and Yb2O3 by multi-step SPS exhibited a higher mechanical property, namely, a bending strength of 981 MPa and a fracture toughness of 8.85 MPa m1/2, while the thermal diffusivity was lower than that for Si3N4 samples sintered with 7 mol% CeO2 (27.77 mm2/s). The result was attributed primarily to the fully phase transformation and the abnormal growth of large β-Si3N4 grains, resulted from a longer time of solution-reprecipitation process during SPS.

Transition-layer of core–rim structures and β→α transformation in SiC ceramics

Similar to Si3N4 ceramics, β→α phase transformation in SiC ceramics plays a key role in tailoring the microstructures thus optimizing related properties. SiC microstructures are dominated with the core–rim structures by AlN-solution, and by EBSD analysis, α-lamellae were revealed as stacking-faults (SF) and twin-boundaries (TB) in β-grains, co-existing with the core–rim structures as α/β→α’/β’ transformation by sintering. The structural transformation can proceed much further by gas-pressure-sintering than hot-pressing with only RE2O3 agents, while the latter retain a high-density of SF/TB in the metastable β-SiC grains. By high-angle secondary-electron imaging, nanoscale transition-layer (TL) was observed as an inter-phase to fully separate the core and rim, which is created by a transitory equilibrium in the solution–reprecipitation process. The enrichment of AlN or RE in TL demonstrates their segregation to core surface until reaching the super-saturation and before the growth of rims. With higher AlN or RE solution and after sintering, a shear stress can develop from TL contour to drive the expansion of SF/TB in Martensitic transition, especially under an external isotropic pressure. The combinations of β→α transformation, core–rim structures and viscous liquid-phase enable the comprehensive assessment of sintering–microstructure–property–performance relationship of SiC ceramics, as demonstrated for their creep behaviors and fracture toughness.

Low‐temperature Pr3Si2C2‐assisted liquid‐phase sintering of SiC with improved thermal conductivity

AbstractA novel Pr3Si2C2 additive was uniformly coated on SiC particles using a molten‐salt method to fabricate a high‐density SiC ceramics via liquid‐phase spark plasma sintering at a relatively low temperature (1400°C). According to the calculated Pr–Si–C‐phase diagram, the liquid phase was formed at ∼1217°C, which effectively improved the sintering rate of SiC by the solution–reprecipitation process. When the sintering temperature increased from 1400 to 1600°C, the thermal conductivity of SiC increased from 84 to 126 W/(m K), as a consequence of the grain growth. However, an increasing amount of the sintering additive increased the interfacial thermal resistance, resulting in a decrease of thermal conductivity of the materials. The highest thermal conductivity of 141 W/(m K) was obtained for the material having the largest SiC grains and an optimized amount of the additive at the grain boundaries and triple junctions. The proposed Pr3Si2C2‐assisted liquid‐phase sintering of SiC can be potentially used for the fabrication of SiC‐based ceramic composites, where a low sintering temperature would inhibit the grain growth of SiC fibers.

An in situ and ex situ study of the microstructural evolution of a novel lithium silicate glass-ceramic during crystallization firing

ObjectiveTo elucidate the compositional and microstructural developments of a novel lithium silicate glass-ceramic during its crystallization cycle. MethodsBlocks of a lithium silicate glass-ceramic (Obsidian®, Glidewell Laboratories) were cut into 1mm thick plates and polished to 1μm finish. Some of them were crystallized prior to polishing. Firstly, ex situ compositional and microstructural characterizations of both the pre- and post-crystallized samples were performed by wavelength dispersive X-ray fluorescence, field-emission scanning electron microscopy, and X-ray diffractometry. Secondly, the pre-crystallized samples were subjected to in situ compositional and microstructural characterizations under non-isothermal heating by simultaneous thermogravimetry/differential scanning calorimetry, X-ray thermo-diffractometry, and field-emission scanning electron thermo-microscopy. ResultsThe microstructure of pre-crystallized Obsidian® consists of an abundant population of perlitic-like/dendritic lithium silicate (Li2SiO3) nanocrystals in a glass matrix. Upon heating, the residual glassy matrix does not crystallize into any form of SiO2; elemental oxides do not precipitate unless over-heated above 820°C; and the Li2SiO3 nanocrystals do not react with the glassy matrix to form typical lithium disilicate (Li2Si2O5) crystals. Nonetheless, the Li2SiO3 nanocrystals grow and spheroidize through the solution-reprecipitation process in the softened glass, and new lithium orthophosphate (Li3PO4) nanocrystals precipitate from the glass matrix. SignificanceThe identification of compositional and microstructural developments of Obsidian® indicates that, by controlling the firing conditions, it is possible to tailor its microstructure, which in turn could affect its mechanical and optical properties, and ultimately its clinical performance.

Kinetic processes in the high-temperature pressure-infiltration of Al into Al2O3

We explore the influence (i) of the interaction between aluminium and alumina, and (ii) of sodium impurities present in Bayer alumina, on the pressure infiltration of alumina particle preforms with molten aluminium. At 1000°C or above, although the aluminium/alumina system is non-reactive, capillarity-driven solution-reprecipitation processes cause the liquid-solid interface to become mobile. Data show that this can result in infiltration kinetics that resemble those observed with reaction-driven pressure infiltration, namely a continuously increasing melt saturation under fixed infiltration pressure. Resulting isobaric saturation velocities are measured at 1000°C, 1050°C and 1100°C. The role of alumina particle shape and of Na-containing inclusions is investigated. It is found that the main factors affecting the rate of high-temperature isobaric infiltration in this system is the particle geometry. Measured steady infiltration rates give an activation volume on the order of ≈ 200 nm3 and an activation energy in the range of 300-500 kJmol, suggesting that isobaric infiltration kinetics are governed by diffusion through the solid alumina. Sodium impurities of Bayer alumina are present within β″-Al2O3. They do not influence steady pressure infiltration but ease initial melt penetration into the preform, possibly because evaporated Na2O alters the oxide skin layer that lines the surface of molten aluminium.

Cathodoluminescence characteristics of a novel core‒rim structure in Bi-doped (Sr,Ba)TiO3 ceramics

A novel core‒rim structure, core with double layered rim, was observed in Bi-doped Sr0.8Ba0.2TiO3 ceramics. Energy dispersive spectra revealed not only a small amount of solutes in the core, but also a significantly different solution levels in the rim. The core‒rim structure was formed from solution‒reprecipitation process. The larger diffusivities of Ba2+ and Bi3+ in the sintering liquid and constitutional supercooling led to their segregation at core‒rim interface. Point defects characteristics of the core‒rim structure were investigated by Cathodeluminescence (CL) spectroscopy. CL peaks at 2.9 eV, 2.4 eV and 2.0 eV influenced by VO··, donor doping and VSr´´ were discussed in detail.

Core‒Rim Structures and Hierarchical Phase Relationship for Hot-Pressed ZrB <sub>2</sub>‒SiC‒MC (M=Nb, Hf, Ta, W) Ceramics

Core‒rim structure emerges as a common microstructural feature of hot-pressed ZrB2‒SiC‒MC (M=Nb, Hf, Ta and W) ceramics. A combination of X-ray diffraction and microscopic analyses reveal that it originates from the re-distribution of transition-metals into their bi-solubility in the primary phase, hence turning the core‒rim structures into an intra-phase relationship. Solution‒reprecipitation process rationalizes their formation via a transient liquid-phase of Zr‒M‒B‒C‒(O). It dictates the special transformation process by enabling an exchange of metal with cores to reach the lower solubility and a transition to grow rims into higher solubility, driven by an over-saturation of boron in the liquid. A third and even higher solubility in the minor ZrC phase finalizes the liquid-phase sintering process, leading to hierarchical phase relationship. We propose further a scheme of g-point to syndicate the bi-solubility as solidus and liquidus limits of metal in ZrB2 phase, hence to schematize the coordinated evolution of multiphase microstructures. The core/rim boundaries can strengthen the ceramics by storing and re-distributing strain energy within the grains, which leaves the microstructure‒property relationship to renew and further optimize for UHTC.

Intergranular Oxynitride to Regulate Solution–Reprecipitation Process in Gas–Pressure‐Sintered SiC Ceramics with AlN–Y2O3 Additives

Core−rim structures are found as a general microstructural feature in SiC ceramics gas–pressure sintered with AlN−Y2O3 additives in different ratio, combined with a range of α/β‐SiC seeding to control the phase transformation. Using analytical electron microscopy, the authors investigate the re‐distribution of additives to detect uniform level of AlN solution into SiC rims, independent to the seeding ratio, and also not proportional to additives. The behavior of AlN solution reveals the presence of sintering melt based on nitro‐carbide to start the reprecipitation of SiC rims, while the cores are remnants from starting powders to control the phase transformation as well as the grain growth behavior. Among various intergranular phases, Y10Al2‐zSi3+zO18+zN4‐z is observed as common hence the secondary phase, while its Al level changes in an opposite trend to AlN solution in SiC rims. This oxynitride phase precipitates from the Y‐rich oxynitride melt, which extends and regulates the solution−reprecipitation process and develops into the Y−Si−O−(C) melt to create a series of residual intergranular oxides. Post‐annealing can fully devitrify the viscous intergranular glass to reach complete crystallization of ceramic body, which leads to improved creep resistance at high temperature for this SiC ceramic system.

Non-isothermal sintering of powdered vitrified composites. A kinetic model

This report sets out the results obtained on studying the sintering process of glass–zircon composites, analysing the microstructural changes that developed on modifying zircon content. The sintering of composites with moderate zircon contents only developed via particle rearrangement by viscous flow. In contrast, at high zircon contents, the zircon solution–reprecipitation process was also required. A kinetic model was developed and validated that describes the effect of the heating rate and zircon volume fraction on the composite degree of non-isothermal sintering progress associated with particle rearrangement by viscous flow.

Core–shell structure from the solution–reprecipitation process in hot-pressed AlN-doped SiC ceramics

A core–shell structure was observed in a series of 6H–SiC ceramics doped with 5–15 mol.% of AlN and hot-pressed at 2050 °C. Analytical electron microscopy revealed not only a significant AlN solution in the shell areas but also a detectable amount of solutes in the core regions. This core–shell structure was formed via the solution–reprecipitation process, which is promoted by the addition of 0.5 wt.% Y 2O 3 and the residual oxides. The low and equilibrated AlN solution in the core was due to the diffusion from the liquid before the shell started to grow. At high doping levels, AlN-based 2H-phase started to reprecipitate while the AlN solution in the shell of 6H–SiC grains was depressed. Silica in the residual oxides was depleted by a gasifying reaction, leaving the residual oxides to form Y 3Al 5O 12 phase in small triple pockets and an alumina-based film at the grain boundaries.

Grain growth during the early stage of sintering of nanosized WC–Co powder

Rapid grain growth during the early stage of sintering has been found in many nano material systems including cemented tungsten carbide WC–Co. To date, however, there have been few reported studies in the literature that deal directly with the kinetics or the mechanisms of this part of grain growth. In this work, the grain growth of nanosized WC during the early stages of sintering was studied as a function of temperature and time. The effects of other influencing factors, such as the initial grain size, cobalt content, and the grain growth inhibitor VC, were investigated. The kinetics of the grain growth process was analyzed and the evolution of the morphology of WC grains during heating-up was studied using high resolution scanning electron microscopy. The results showed that the grain growth process consists of an initial stage rapid growth process which typically takes place during heat-up and the normal grain growth during isothermal holding. The initial rapid grain growth is at least partially attributed to the process of coalescence of grains via elimination common grain boundary. The preferred orientation between WC grains within the aggregates is considered a favorable condition for coalescence of grains, hence rapid grain growth. The solution–reprecipitation process is considered a mechanism of coalescence.

Sintering Behavior of Si<sub>3</sub>N<sub>4</sub>-TaC Based Composites

Silicon nitride was the first nitride developed for engineering applications. The excellent combination of thermomechanical properties makes silicon nitride a good candidate for applications where high hardness and mechanical properties are fundamental. However, the low fracture toughness of this material limits its use as structural material. The improve of mechanical properties of silicon nitride comes from many factors, like refined microstructure by restraining grain growth, localized stress, crack tip bridging, etc. Within these factors, microstructure formation of the silicon nitride is critically important for the final properties. The design of silicon nitride based composite materials is of particular interest because of their improved high temperature strength and fracture toughness. In this work, Si3N4-TaC particulate composite was investigated. For this study was prepared a basis composition (CB) with 90%wt a-Si3N4, 6%wt and 4%wt Y2O3 and Al2O3, respectively. TaC (20%vol) was added into CB and after mixture, in high-energy milling, the powder was compacted into pellets. The kinetics of sintering was studied by means of dilatometry. The shrinkage rate versus time and temperature curves exhibit two well-defined peaks. The first peak refers to the particle rearrangement process and the second, more pronounced, to solutionreprecipitation process. It is quite clear that the presence of TaC particles has small influence on sintering kinetics of silicon nitride. It was observed the complete a®b-Si3N4 phase transformation. The microstructure shows good homogeneity both in regard of grain size and secondary phase distribution.

Liquid phase sintering of tough coated hard particles

Tough coated hard particles (TCHP) are a new microstructure opportunity for designing hard, wear resistant materials at the particle level. As opposed to coating whole devices, TCHP particles are coated at the micrometer level. During wear, these particles constantly present fresh, wear–resistance microstructures that provide a service lifetime not possible with coated devices. Further, fracture toughness is maximized by the uniform tough ligament spacing between hard particles. Such functionality has applications in abrasive wear, wire drawing, metal forming, rolling contacts, and other applications requiring performance over long service times. For the TCHP concept to succeed, new protocols are required for densification during liquid phase sintering. To avoid grain coarsening, dissolution of the coating phase must be minimized while at the same time densification is induced. This unique challenge reduces the amount of densification possible via solution-reprecipitation processes. Thus, special coatings and composition combinations may be required to control densification while preserving the desired coating–core microstructure in the densified product.

Lateral grain growth during reactive isothermal solidification in the Ni-Sn system

In this research lateral growth of Ni3Sn4 grains in the Ni-Sn system during isothermal solidification was investigated. Experiments involved multi-layered samples heat-treated for different durations at temperatures in the range 235–600∘C. During solidification Ni3Sn4 was the only intermediate phase found to grow as a layer at the solid/liquid interface. SEM and optical microscopy cross-sectional views of samples heat-treated at 500∘C and at 600∘C reveal that lateral grain growth occurs, the mean lateral grain size being proportional to √t(t-the duration of the heat treatment). This process was found to be connected with solution reprecipitation process between adjacent grains in order to equate the local curvature of the solid/liquid interface at groove zone. Rotation of the grain boundary groove due to this mass transfer induces a curvature of the grain boundary, which is the controlling mechanism for its migration. The process was numerically formulated, which provides quantitative fitting to the observed kinetics. The formulation includes a contribution due to the good wetting of the grain boundaries by the liquid phase supporting the observed enhancement in growth rate.

Gas pressure sintering of SiC sintered with rare-earth-(III)-oxides and their mechanical properties

The sintering behaviour of LPS-SiC and the influence of the size of the rare-earth cations on the secondary phase characteristics were investigated with different rare-earth oxide additions. In all cases, the most important sintering mechanism was found to be the solution-reprecipitation process. This fact was corroborated by TEM and EDS analyses. Post-sintering annealing resulted in devitrification of the secondary phases coupled with anisotropic grain growth due to the phase transformation from β-SiC to α-SiC. Improved fracture toughness of the annealed materials was attributed to crack deflection by the elongated grains. SEM microstructural analyses were performed in order to elucidate structure–property relationships. A comparative study in reference to the additive system Y 2O 3–AlN demonstrates significantly improved high-temperature properties of the Lu 2O 3-containing SiC ceramics.

A Consideration on TiC-Core/(Ti,Mo)C-Rim Structure of TiC-Mo2C-Ni Cermet in Relation to Hypothesis "Exhaustion of Diffusion-Contributable Atomic Vacancies in Core/Rim Structure"

TiC-core/(Ti, Mo)C-rim structure in TiC-Mo2C-Ni cermet where the equilibrium phase of carbide is only (Ti, Mo)C is known to be not generated by solid-diffusion process, but by solution-reprecipitation process in the case of usual sintering temperatures (1528-1723 K). On the other hand, in our recent study on the microstructure of Fe-66.7at%Si peritectoid alloy where the equilibrium phase is FeSi2, it was found that FeSi-core in FeSi-core/FeSi2-rim structure hardly disappeared by solid-diffusion process in the compact inside, but relatively easily disappeared and/or shrunk near the compact surface to the depth of FeSi-core size (about 15μm) by heating at high temperature. The phenomenon in the compact inside could be explained by our newly proposed hypothesis Exhaustion of diffusion-contributable atomic vacancies in core/rim structure.In this study, it was investigated in relation to the above hypothesis whether TiC-core in the compact inside disappears or not due to solid-diffusion process by heating at 2073-2473 K which are extremely higher than the usual sintering temperature. As the result, it was judged that TiC-core in the compact inside really disappeared by solid-diffusion process at 2273-2473 K, differing from FeSi-core in the compact inside. Based on these results on Fe-66.7 at%Si alloy and the cermet, it was generally concluded that our new hypothesis can be applied to the alloy where the rim phase is a stoichiometric compound unable to dissolve the core elements and also the diffusion from the core to the rim is not thermodynamically allowed like in Fe-66.7 at% Si alloy, but not to the alloy where the rim is a solid solution able to dissolve the core elements and thus the diffusion from the core to the rim is thermodynamically allowed like in the cermet.

Magnetic properties of liquid-phase-assisted sintered MnZn ferrites

MnZn ferrites were sintered in the presence of a Bi2O3-SiO2 - rich liquid phase. The microstructure of MnZn-ferrite samples that contained various amounts of liquid phase during sintering was investigated. The results revealed that microstructure development and final magnetic permeability depend essentially on the amount of liquid phase present during sintering. The solution-reprecipitation (S-R) process in MnZn ferrites starts when a continuous liquid-phase film is formed during grain growth. The status of the microstructure developed during solid-state sintering prior to the formation of the critical liquid-phase film is essential for the final microstructure developed during liquid-phase-assisted sintering.

The Use of MgO as a densification aid for α-SiC

The suitability of MgO as a densification aid for α-SiC has been investigated. Samples of SiC containing additions of MgO, both alone and in combination with A1 2O 3 and Y 2O 3, have been hot pressed at temperatures between 1500 and 1900°C and pressureless sintered at temperatures up to 2000°C. The MgO reacts with surface SiO 2 from the SiC grains to form a liquid phase which promotes densification by particle rearrangement and solution-reprecipitation processes. With combined additions of MgO, Al 2O 3 and Y 2O 3, a eutectic in the system MgO –Al 2O 3 –Y 2O 3 allows extensive liquid formation at ∼1700°C independent of the SiO 2 content of the SiC powder, enabling efficient densification by hot-pressing. Volatilisation due to reactions between the MgO and the SiC or with the furnace environment, however, oppose densification by pressureless sintering, and must be compensated for by the use of Mg-containing powder beds.

Development of microstructure of fired Ecuadorean clay

The development of the fired microstructure of a successful commercial Ecuadorian clay mineral with the composition 30% potassium feldspar, 30% kaolin, and 30% quartz, has been followed. A technique of etching and coating has been developed which allows direct comparison of optical and SEM images without intermediate preparation. In this way individual phases were identified and local reactions of these phases at different firing temperatures and soak times were revealed and have been related to existing phase diagram data. The presence of the feldspar is seen to promote early densification through a solution–reprecipitation process. The resulting potassium rich liquid is able to penetrate the large (500 μm) quartz particles which are fragmented, partially dissolved, and incorporated as finer particles into the surrounding material through a boundary layer of secondary mullite. The fragmentation of these large, potentially weakening quartz particles contributes crucially to the good mechanical properties of the fired ceramic and has considerable commercial implications.

Experimental and Computational Study of Grain Growth in AlN Based Ceramics

As a model experiment, the grain growth of aluminum nitride (AlN) was studied at different fractions of yttrium aluminate (7-31%) to compare to the results of computer simulation using Monte Carlo method (MC simulation) of grain growth in presence of a liquid phase. In the model experiment, the time dependence of mean grain size for sintered AlN ceramics was measured from scanning electron photomicrographs. At 2123K, yttrium aluminate appears to be a liquid. The liquid fraction remained relatively constant over the time intervals studied. The cube of mean grain size was observed to be proportional to heat treatment time. The mean AlN grain size decreased as the yttrium aluminate fraction increased. Results of these observations indicated that the solution-reprecipitation process is the dominant growth mechanism in this system and that the rate of grain growth is controlled by AlN diffusion in the liquid phase. On the other hand, microstructures obtained by MC simulation were isotropic and the dependence of grain size in simulated microstructures on fraction of liquid phase and Monte Carlo steps showed similar ones in the model experiment. As a result, it was concluded that MC simulation could express isotropic grain growth and the mechanism that was observed in AlN grain growth.