Forming Of Ceramics Research Articles

Novel <i>operando</i> observation technique based on combined optical coherence tomography–thermogravimetry for monitoring internal structural changes of drying ceramic slurries

In the wet forming of ceramics, it is critical to control the cracking and deformation of the ceramic slurry during the drying process. This necessitates a comprehensive understanding of the changes in the internal structure of the slurry during the drying process using a high-resolution technique that enables direct and high speed observation. In this study, a combined system of optical coherence tomography (OCT) and thermogravimetry (TG) was developed for the operando observation of the evolution of the internal structure of a drying SiO2 slurry. In the early stages of drying, the OCT signal intensity changed significantly owing to the Brownian motion of the SiO2 particles in the slurry. The slurry was found to shrink as it dried and then lose its fluidity, and the ceramic particles were found to agglomerate from the surface. In the final stages of drying, localized dry regions developed in the vicinity of the slurry surface and expanded with drying. Crack formation was also observed in a part of the localized dry regions. Therefore, crack formation was attributed to inhomogeneous drying-induced shrinkage and stress generated in the drying object during the progression of localized drying. The novel operando observation technique using combined OCT-TG is successfully demonstrated to be an effective approach for understanding the internal structural changes in opaque slurries during their drying.

Indirect forming of alumina-based ceramics by selective laser sintering combined with sol infiltration process and performance study

Indirect forming of alumina-based ceramics by selective laser sintering combined with sol infiltration process and performance study

Thermomechanical modelling of ceramic pressing and subsequent sintering

An elastic-visco-plastic thermomechanical model for the simulation of cold forming and subsequent sintering of ceramic powders is introduced and based on micromechanical modelling of the compaction process of granulates. Micromechanics leads to an upper-bound estimate of the compaction curve of a granular material, which compares well with other models and finite element simulations. The parameters of the thermomechanical model are determined on the basis of available data and dilatometer experiments. Finally, after computer implementation, validation of the model is performed against experiments developed on specially designed ceramic pieces, characterized by zones of different density. The mechanical model is found to accurately describe forming and sintering of stoneware ceramics and can therefore be used to analyze and optimize industrial processes involving compaction of powders and subsequent firing of the greens.

Superplastic Forging for Sialon-based Nanocomposite at Ultralow Temperature in the Electric Field

The utralow-temperature superplastic forging for sialon-based nanocomposites is reported for the first time. Sialon-based nanocomposites, with an average grain size smaller than 50 nm and 98.5% relative density, were prepared with nano-sized row powders by the spark plasma sintering (SPS) technique at a record ultralow sintering temperature of 1150 °C. An excellent gear is forged at the ultralow deformation temperature of 1200 °C with nanosized grains without any cracking. The maximum strain rate achieved is over 10−1 s−1, and a compression strain is more than 0.9. The practical application for superplastic forming of nitrogen ceramics is much more difficult than that for oxide ceramics because of the high deformation temperature and low strain rates. The present findings present a bright prospect for the near-net-shape superplastic forming of nitrogen ceramics.

From dry pressing to plastic forming of ceramics: Assessing the workability window

The shaping water content in clay-based ceramic building materials has conflicting implications in shaping and drying, decisively contributing to the cost-effectiveness of the industrial process. This work addresses the long-standing difficulties related to the quantification of the workability window for plastic forming. From uniaxial compression stress-strain curves, the yield stresses of mixtures of a red-firing clay and a ground basalt rock with different moisture contents were obtained. Combination of those results with the wet and dried bodies densities, showed that ceramic powders can be shaped only while in the funicular state: for this clay, dry pressing is effective above the pendular-to-funicular transition (∼5.7 wt% moisture) and the workability window for plastic forming begins at the maximum dried body density, which signals the transition from open-to-closed gas pores in the funicular state (∼15 wt% moisture), and extends to the funicular-to-capillary transition (∼18 wt% moisture). Drying did not alter the moist particle structure, which enables the expeditious determination of the workability window from dried body density and initial moisture content.

Gelcasting of through-pore hydroxyapatite ceramics

Although the near-net-shape forming ability of gelcasting has been well demonstrated, current attention mainly was focused on manufacturing compact ceramics. In this work we demonstrated that the combination of the gelcasting technique and the ceramic sintering process could prepare the near-net-shape through-pore hydroxyapatite (HAP) ceramics, in which the pore diameter at the range of < 2 μm could be adjusted by changing the content of HAP in green bodies and with the aid of alcohol immersion. Experiments demonstrated that the gels in green bodies and alcohol immersion played important roles in improving the strength of green bodies and decreasing the shrinkage and deformation of green bodies, and thus improving the through-pore and near-net-shape forming of HAP ceramics. Such method reported here possibly extends the gelcasting technique to manufacture other through-pore ceramics for precise filtration, protein separation, or high performance liquid chromatography.

Innovative application of PVA hydrogel for the forming of porous Si3N4 ceramics via freeze-thaw technique

The generation of PVA hydrogel through freeze-thaw technique has been utilized innovatively to complete the forming process of porous Si3N4 ceramics in the present work. In comparison with gelcasting method, green body with comparable strength has been achieved by this novel forming process, which is owing to the strong function of PVA gel caused by the freeze-thaw cycles. Herein, Si3N4 hollow spheres are employed as pore-forming agent in order to achieve closed pores. The morphology of green body illustrates that pores originate from both hollow spheres and the space occupied by ice crystals, and the latter gives rise to the increase of porosity around 10%. Studies on the mechanical, dielectric and thermal properties of porous Si3N4 ceramics dependent on freeze-thaw cycle and freezing time have been performed. It is found that porous ceramics treated for freeze-thaw 2 cycles with freezing time of 24 h present excellent properties, which are promising to be applied as radome materials.

Biopolymer‐Based Gel Casting of Ferroelectric Ceramics

Ferroelectric ceramics have a wide range of industrial applications. While components can be formed through simple ‘press and sinter’ approaches, there is an increasing need for more flexible processing methods. Suspension based forming promotes greater homogeneity, through elimination of processing defects, and allows the formation of more complex shaped components. In the present work, the application of aqueous gel casting technologies is reviewed, and in particular the use of natural biopolymers as gelation aids for such approaches. Recent studies of the application of such technology to complex shape forming of BaTiO3 ferroelectric ceramics are examined. Emphasis is placed upon a simple subtractive rapid prototyping method, utilizing aqueous gel casting in combination with green machining.

Experimental investigation of the elastoplastic response of aluminum silicate spray dried powder during cold compaction

Mechanical experiments have been designed and performed to investigate the elasto-plastic behaviour of green bodies formed from an aluminum silicate spray dried powder used for tiles production. Experiments have been executed on samples obtained from cold compaction into a cylindrical mould and include: uniaxial strain, equi-biaxial flexure and high-pressure triaxial compression/extension tests. Two types of powders have been used to realize the green body samples, differing in the values of water content, which have been taken equal to those usually employed in the industrial forming of traditional ceramics. Yielding of the green body during compaction has been characterized in terms of yield surface shape, failure envelope, and evolution of cohesion and void ratio with the forming pressure, confirming the validity of previously proposed constitutive models for dense materials obtained through cold compaction of granulates.

Room-temperature gelcasting of alumina with a water-soluble copolymer

Abstract

Shape forming of melt grown eutectic composition of Al2O3–YAG–ZrO2 ceramic

A method referred to as ‘low pressure solidification’ (LPS) was first suggested for the shape forming of melt growth eutectic ceramics and practically implemented for Al2O3–YAG–ZrO2 composite. The effects of applied pressure on the microstructure and fracture toughness of the eutectic ceramic were investigated. It was found that the defects in the intercolony region and on the surface reduced dramatically. The morphology of the Al2O3 phase changed from irregular shape into rod one with an increase in pressure. In addition, the fracture toughness increased from 3·60 to 3·99 MPa m1/2. The results show that the LPS method is promising for the manufacture of large and complex eutectic oxide ceramic components with desired microstructure and good fracture toughness.

Rheological behaviour of NiO/YSZ slurries for drying-free casting

We have recently developed a new drying-free casting method for shape forming of ceramics, in which a polymerizable solvent such as furfuryl alcohol (FA) is used to disperse the ceramic powders, and then polymerized into a poly (furfuryl alcohol) (PFA) binder during the casting process. Compared with conventional casting processes, this method offers more flexibility in controlling the microstructure of ceramics and eliminates defects that are likely to arise from the drying step. Controlling the rheological behaviour of ceramic slurries is an important step in the casting process to achieve improved microstructural properties. In this study, the viscosity of slurries comprising NiO/YSZ (yttria stabilized zirconia) powders and FA as solvent was investigated over a wide range of shear rates (1s−1–1000s−1) and at different volumetric solid concentrations (from 20% to 70%), while the effects of dispersant were quantified by adding different amounts of PVP (polyvinyl pyrrolidone) between 1wt.% and 4wt.%. The minimum viscosity of NiO/YSZ/FA slurries could be achieved with 2wt.% dispersant. The experimental data relating the changes in viscosity to the volumetric solid concentration were modeled using different viscosity models, with Liu's equation showing the best fit for NiO/YSZ/FA slurries with and without dispersant. The slurries showed a highly pseudoplastic behaviour without dispersant, while adding PVP as dispersant effectively modified the rheological behaviour toward Newtonian fluids.

Rapid Forming of Hydroxyapatite-Silica Ceramics

This paper proposes a solid freefrom fabrication (SFF) technology for fabricating hydroxyapatite(HA)-silica ceramics, which can generate porous three-dimensional physical objects. The HA powder and the silica are mixed with water into slurries form as raw materials. The slurries are paved by a scraper to from a thin layer which is selective scanned by a laser beam according to the cross-section of a 3D model. The HA particles are embeded in the sintered silica matrix to form green parts via a suitable range of process parameters. The benefits of this process are: bio-ceramic parts can be built by lower laser energy and faster fabricating speed. Following a subsequence heat treatment process has been developed to optimize the crystallization process and to increase the strength of the sintered parts. The parts were analyzed by an Atomic Force Microscope (AFM) to determine the surface roughness. The results obtained indicate that the proposed process was possible to generate multilayer, overhanging, and porous structure with brittle property but sufficient integrity for handling prior to post-processing. It was possible to produce the porous structure from the proposed hydroxyapatite-silica ceramics, which had a greater potential for possible bone scaffolds fabrication.

Shape forming of ceramics with controllable microstructure by drying-free colloidal casting

A new drying-free colloidal casting method was developed for shape forming of ceramics with controllable microstructure. A polymerizable solvent such as furfuryl alcohol (FA) was employed to disperse ceramic particles to form a colloidal ceramic suspension with good fluidity. After casting, the solvent was polymerized into a polymer, resulting in solidification of the colloidal suspension. For the purpose of demonstrating this casting method, three types of ceramic powders (NiO/YSZ, Al2O3 and YSZ) were used to fabricate a NiO/YSZ ceramic tube, an Al2O3 ceramic part with a hexagonal screw nut shape, and a YSZ thin film. Our study shows that the microstructure (pore size and porosity) of ceramics can be readily tuned by adjusting the amount of FA, and the drying-free casting method offers better controllability over the shape forming of a wide variety of ceramic materials.

Stability of concentrated suspensions of Al 2O 3–SiO 2 measured by multiple light scattering

The stability of ceramic suspensions can be predicted by means of zeta potential and particle size measurements but a major problem is that most existing techniques make use of diluted suspensions so that the concentrated suspensions frequently used in shape forming of ceramics are out of range. The propagation of light through a concentrated suspension can be used to characterise its colloidal stability. This work aims to study the variations of the structure of concentrated ceramic suspensions maintained at rest by multiple light scattering (MLS). This technique allows us to obtain information about the agglomeration processes that take place during ageing, like particle migration and particle aggregation. The results are compared with those obtained using conventional techniques such as the predictive tests (e.g. zeta potential and rheological behaviour of fresh suspensions) and ageing studies measured by rheometry. For such purpose, aqueous suspensions of alumina and alumina/silica (used to produce mullite by reaction sintering) have been studied.

Novel binder-free forming of Al 2O 3 ceramics by microwave-assisted hydration reaction

A novel binder-free forming of ceramics via microwave irradiation is developed. The irradiation of microwave to an alumina green body enhances the hydration reaction strongly between water and particle surfaces, creating surface aluminum trihydroxides structure adjacent to particles that bind them together tightly. This process makes it possible to manufacture mechanically strong green bodies with excellent shape retention ability without the use of organic binders.

Colloidal In-Situ Consolidation Forming of Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Ceramics Using Modified Starch

The forming of Al2O3 ceramics was carried out by the colloidal in-situ consolidation using modified starch in this work. The effects of the modified starch content in the alumina suspensions on their rheological behavior, consolidation, microstructure and properties of the green bodies were investigated. The different shape and high relative density of ceramic green bodies were achieved by this method. The results indicated that the alumina suspensions with modified starch were easier to cast due to their low viscosity and high fluidity. After drying, the linear shrinkage of the green bodies was low, ranging between about 1.5% and 2.2% and their bulk density was higher than 57% of the theoretical density. The maximum bending strength of the dried green bodies reached to 8.2 MPa and the microstructure of the green bodies was uniform with a narrow unimodal distribution of pore size. It is concluded that the colloidal in-situ consolidation casting using modified starch can perform near-net shaping of ceramic components with high-performance and it is a potential forming technique for ceramics.

Drying-induced forming of alumina ceramics using high solid-loaded slurries with addition of glycerol

A new drying-induced forming (DIF) of ceramics has been developed using high solid-loaded alumina slurries with addition of glycerol, in which the slurry was simply dried in an open mold at the ambient condition. The drying-induced forming approach is facilitated by the high stability of concentrated slurries during drying where glycerol was added to maximize the critical solid loading and play simultaneously as a drying control agent. The drying and rheological behaviors of slurries were examined with respect to the glycerol addition. A slurry of 60 vol% solid loading with 20 wt% glycerol addition was chosen for the drying induced forming test. The green samples exhibited no bending or cracks, and a small degree of shrinkage anisotropy during sintering. The sintered samples showed a high relative density of 99.2% and an average strength of 420 MPa. We demonstrate here a possibility of producing near-net-shaped and bulk ceramic components in a simple scheme by using the drying-induced approach.

New applications of alginates in the shape forming of ceramics

New applications of alginates in the shape forming of ceramics

Fast Uniform Mixing &amp; Controllable Colloidal Forming of Ceramics

A process of fast uniform mixing &amp; controllable colloidal forming of ceramics was invented to thoroughly avoid the crucial effect of temperature on colloidal forming of ceramics. The ceramic suspension was divided into two components A and B. The monomer was added into A, and the initiator was added into B. Under this condition, there were no reactions in separate A or B because of segregation of the monomer and the initiator. Both of the suspensions could keep good fluidity until they were mixed quickly and uniformly. The mixture would fast solidify under a determined pressure. The highly automatic equipment also had been developed for this process.