Layerwise Slurry Deposition Research Articles

Towards a debinding-free additive manufacturing of ceramics: A development perspective of water-based LSD and LIS technologies

Ceramic additive manufacturing (AM) requires a complex process chain with various post-processing steps that require expensive machines and special expertise. The key to further market penetration is AM that makes it possible to integrate into an already established ceramic process chain. Most successful AM technologies for ceramics are, however, based on processes that initially have been developed for polymeric materials. For ceramics AM, polymers or precursors are loaded with ceramic particles. This strategy facilitates the entry into AM, however the introduction of organic additives into the ceramic process chain represents a considerable technological challenge to ultimately obtain a ceramic component after additive shaping. In the present communication, two technologies based on ceramic suspensions will be introduced, the “layerwise slurry deposition” (LSD) and “laser induced slip casting” (LIS) technology. Both technologies take advantage of the high packing densities reached by conventional slip casting and moreover enable the processing of fines, even nanoparticles.

Ink development for the additive manufacturing of strong green parts by layerwise slurry deposition (LSD‐print)

AbstractObtaining dense fine ceramics by the binder jetting additive manufacturing process is challenging. A slurry‐based binder jetting process, such as the layerwise slurry deposition (LSD‐print) process, can enable the printing of dense ceramic parts. This work describes a procedure to develop and qualify a suitable ink to manufacture silicon carbide green parts by LSD‐print. Not only the printability but also the compatibility of the ink with the powder bed and the effect of the binding agent on the properties of the green parts are considered. Both aspects are important to obtain high green strength, which is necessary for printing large or thin‐walled parts. Characterization methods, such as rheological and surface tension measurements, are applied to optimize three selected inks. The interplay between ink and powder bed is tested by contact angle measurements and by comparing the biaxial strength of cast and additively manufactured specimens. Out of the three binding agents tested, a polyethyleneimine and a phenolic resin have a high potential for their use in the LSD‐print of silicon carbide green bodies, whereas a polyacrylate binding agent did not show the required properties.

Influence of the dispersant on the parts quality in slurry‐based binder jetting of SiC ceramics

AbstractBinder jetting is establishing more and more in the ceramic industry to produce large complex shaped parts. A parameter with a great impact on the quality of the parts is the binder–powder interaction. The use of ceramic slurries as feedstock for this process, such as in the layerwise slurry deposition–print technology, allows a great flexibility in the composition. Such slurries are typically composed of ceramic powder, water, and small amounts of various additives. The understanding of the effect of these components on the printing quality is thus essential for the feedstock development. Four models were developed regarding the impact of additives, such as dispersants on printing. These models were confirmed or rebutted by experiments performed for an SiC slurry system with two different concentrations of a dispersant and a commercial phenolic resin used as a binder. It is shown that for this system the influence of the dispersant on the curing behavior and the clogging of the pores by dispersant can be neglected. The redispersion of the dispersant after the curing of the resin has no or only a minor effect. However, the wetting behavior determined by the surface energies of the system seem to be most crucial. In case the surface energy of the slurry additive is significantly lower than the surface energy of the binder, the strength of the green parts and the printing quality will be low. This was shown by inverse gas chromatography, contact angle measurement, rheological characterization, and mechanical tests with casted samples.

Manufacturing of ceramic components with internal channels by a novel additive/subtractive hybridization process

A new approach for fabrication of ceramic components with inner channels is proposed, as a result of the combination of two additive and one subtractive manufacturing processes. In this project, porcelain parts are manufactured by the Layerwise Slurry Deposition (LSD) process, meanwhile end milling and Direct Ink Writing (DIW) are applied to create channels on the surface of the deposited ceramic. Unique to the LSD process is the formation of a freestanding powder bed with a mechanical strength comparable to conventional slip casted ceramic green bodies. Combining these three processes allows the manufacturing of ceramic objects containing an internal path of ink, which in this case was a graphite-based ink that can be further eliminated by heat treatment to obtain a porcelain object embedded with channels. The results show the capabilities of this method and its potential to fabricate not only parts with inner channels, but also multi-material and multi-functional components (such as integrated electronic circuits).

Sintering of ceramics for clay in situ resource utilization on Mars

The sintering of wet processed Mars global simulant green bodies is explored. Green bodies shaped using slip casting, throwing on a potter’s wheel and additive manufacturing, including material extrusion (robocasting) and layerwise slurry deposition (LSD) are sintered in terrestrial and simulated Mars atmosphere. A sintering schedule is developed using hot stage microscopy, water absorption, sintering shrinkage and sintering mass loss. Sintered parts are characterized in respect to their density, porosity, phase composition, microstructure and mechanical properties. Densification behavior for different green bodies was generally similar, enabling the fabrication of larger green bodies (tiles, cups, bowls) and parts with fines details (test cubes and cuneiform tables) with low water absorption. Sintered LSD discs had a bending strength between terracotta and typical porcelains with 57.5/53.3 ​MPa in terrestrial/simulated Mars atmosphere. Clay ISRU for sintered ceramics can be considered an eminently favorable construction technology for soft and hard ISRU on Mars.

Clay in situ resource utilization with Mars global simulant slurries for additive manufacturing and traditional shaping of unfired green bodies

The wet processing of regolith simulant for clay in situ resource utilization (ISRU) on Mars is presented. The two raw materials from the Mars global simulant family, one without clay (MGS-1) and one with clay - sodium montmorillonite smectite - (MGS-1C) were milled and mixed to produce a simulant with small particle size and reduced clay content (MGS-1C/8). All three simulants and the pure clay raw material were extensively characterized using XRF, synchrotron XRD, gas adsorption and gas pycnometry methods. In a straightforward processing approach, MGS-1C/8 was mixed with water and different dispersant approaches were investigated, all of which gave stable slurries. Particle size distribution, rheology, ion concentration, pH and electrical conductivity of these slurries were characterized. The slurry systems can easily be adapted to fit all typical ceramic shaping routes and here parts of varying complexity from slip casting, throwing on a potter's wheel and additive manufacturing, including material extrusion (robocasting) and binder jetting (powder bed 3D printing) were produced. The unique properties of the sodium montmorillonite clay, which is readily accessible in conjunction with magnesium sulfate on the Martian surface, acted as a natural nanosized binder and produced high strength green bodies (unfired ceramic body) with compressive strength from 3.3 to 7.5 MPa. The most elaborate additive manufacturing technique layerwise slurry deposition (LSD) produced water-resistant green bodies with a compressive strength of 30.8 ± 2.5 MPa by employing a polymeric binder, which is similar or higher than the strength of standard concrete. The unfired green bodies show sufficient strength to be used for remote habitat building on Mars using additive manufacturing without humans being present.

Additive manufacturing of SiSiC by layerwise slurry deposition and binder jetting (LSD-print)

The current work presents for the first time results on the Additive Manufacturing of SiSiC complex parts based on the Layerwise Slurry Deposition (LSD) process. This technology allows to deposit highly packed powder layers by spreading a ceramic slurry and drying. The capillary forces acting during the process are responsible for the dense powder packing and the good joining between layers. The LSD process can be combined with binder jetting to print 2D cross-sections of an object in each successive layer, thus forming a 3D part. This process is named LSD-print.By LSD-print and silicon infiltration, SiSiC parts with complex geometries and features down to 1 mm and an aspect ratio up to 4:1 could be demonstrated.The density and morphology were investigated for a large number of samples. Furthermore, the density and the mechanical properties, measured by ball-on-three-balls method, were in all three building directions close to isostatic pressed references.

3D printing of porcelain by layerwise slurry deposition

The Layerwise Slurry Deposition is a technology for the deposition of highly packed powder layers. A powder bed is achieved by depositing and drying layers of a ceramic suspension by means of a doctor blade. This deposition technique was combined with the binder jetting technology to develop a novel Additive Manufacturing technology, named LSD-print. The LSD-print was applied to a porcelain ceramic. It is shown that it was possible to produce parts with high definition, good surface finish and at the same time having physical and mechanical properties close to those of traditionally processed porcelain, e.g. by slip casting.This technology shows high future potential for being integrated alongside traditional production of porcelain, as it is easily scalable to large areas while maintaining a good definition. Both the Layerwise Slurry Deposition method and the binder jetting technologies are readily scalable to areas as large as >1 m2.

Rheological Characterization of Aqueous ZrO<sub>2</sub>-Suspension for Additive Manufacturing

Water-based suspensions from commercial Tetragonal stabilized zirconia (3Y-TZP) were produced and characterized with different contents of solids and two different dispersants. According to zeta potential measurements, 3Y-TZP particles showed basic surface characteristics and IEP of around 9 when in aqueous media. The critical volume fraction of solids was about 79.6 wt%, which hindered the processing of more concentrated slurries. Rheological measurements confirmed that well dispersed slurries could be obtained with solid content as high as 79.6 wt.%. The results showed that Triton X-114 was an effective dispersant for preparing well stabilized 3Y-TZP suspensions for the layer-wise slurry deposition process.

Cover Image, Volume 98, Issue 7

Espresso cups made at BAM (Federal Institute for Materials Research and Testing, Berlin, Germany) by Selective Laser Sintering (SLS) in combination with Layerwise Slurry Deposition (LSD) of a commercial silicate ceramic slurry provided by Duravit AG, Germany. See feature article, “Additive Manufacturing of Ceramics: issues, potentialities and opportunities,” by Jens Günster. image

Wollastonite-containing bioceramic coatings on alumina substrates: Design considerations and mechanical modelling

In this work we produced wollastonite-containing glass-ceramic coatings on alumina substrates by airbrush spraying of glass-based aqueous suspensions followed by sintering. Investigations by scanning electron microscopy revealed that layer-wise slurry deposition is a suitable technique to manufacture homogeneous and continuous bioceramic coatings without flaws or cracks at the coating/substrate interface. Coating thickness can be designed by controlling the number of spraying cycles. The bonding strength of the coating (about 22MPa) was found adequate for biomedical applications. A mechanical model based on the innovative concepts of quantized fracture mechanics was developed to predict the bonding strength and estimate the fracture toughness of the coating on the basis of experimental data from tensile tests. The approach proposed in this work can contribute to optimize the design and manufacturing of bioceramic coatings with the aim of improving their mechanical properties and suitability for clinical applications.

Feasibility of glass–ceramic coatings on alumina prosthetic implants by airbrush spraying method

This work explores the feasibility of glass-derived coatings (SiO2–CaO–Na2O–Al2O3 system) on bioceramic devices of complex shape by a layer-wise slurry deposition using an airbrush spray gun. Specifically, glass–ceramic coatings with thickness in the 50–500μm range were prepared on alumina curved substrates by airbrush spraying of glass-based aqueous suspensions followed by sintering. Investigations by scanning electron microscopy and micro-computed tomography revealed that, under appropriate optimization of slurry composition and spraying cycles, this technique is suitable to manufacture homogeneous and continuous coatings on model curved ceramic surfaces. It was observed that neither cracking nor delamination occurred at the interface between coating and substrate. Ad-hoc tensile tests were carried out by properly adapting the relevant ASTM standard to the specific case (curved geometry); the coating adhesion strength was found adequate (about 25MPa) for biomedical applications. A simple model describing the relationship between coating thickness and number of spraying cycles was also developed and proposed as a useful tool to improve the design and manufacturing of bioceramic coatings.

Laser Compensation for Ceramics Accuracy Improvement of Selective Laser Sintering

This research developed a feedback control system of laser compensation for the rapid prototyping (RP) machine using layer-wise slurry deposition and selective laser sintering (SLS). The slurry was prepared by silica power and silica sol with 60 and 40 wt.% with suitable rheological properties for 0.1 mm layer deposition. Four ceramics for comparison of the formability of fabricated ceramic green parts with/without the feedback control system of laser energy density for models were designed With this laser feedback control, batter quality ceramic green parts can be manufactured and the rapid prototyping machine with steady laser energy radiated on slurry layer was achieved. Experimental results validate the well performance of the measuring laser power and feedback control system.

Slurry‐Based Additive Manufacturing of Ceramics

Most additive manufacturing (AM) techniques have in common that material is spread out as thin layers of a dried powder/granulate by a roller or a shaker system. These layers are mostly characterized by a low packing rate. On the other hand, appreciable densities can be reached by the use of ceramic slurries. In this context, the layer‐wise slurry deposition (LSD) has been developed. Specific features of the LSD process are reflected on the basis of already existing additive manufacturing technologies. The microstructure of laser‐sintered bodies will be discussed, and strategies for an improved microstructure during sintering will be introduced.

Selective laser densification of lithium aluminosilicate glass ceramic tapes

Tapes, cast by blade deposition of a lithium aluminosilicate glass slurry, were sintered using a YAG-fiber laser, with the aim of finding suitable parameters for an additive manufacturing process based on layer-wise slurry deposition and selective laser densification. The influence of the laser parameters (output power and scan velocity) on the sintering was evaluated, by scanning electron microscopy and by X-ray diffraction, on the basis of the quality of the processed layer. Well densified samples could be obtained only in a small window of values for the output power and the scan velocity. The measurement of the width of a set of single scanned lines allowed also to estimate the minimum resolution of the system along the layer plane.

Rapid prototyping of porcelain products by layer‐wise slurry deposition (LSD) and direct laser sintering

PurposeThe purpose of this paper is to study the rapid prototyping of porcelain products by using layer‐wise slurry deposition (LSD), in order to reduce the time to market of new or customized porcelain products or artworks.Design/methodology/approachThe properties such as phase composition, microstructure, shrinkage, density, and mechanical strength, of laser sintered (LS) and biscuit fired (BF) samples, before and after post sintering in a furnace, were studied and compared with each other.FindingsThe laser sintered sample was comparable with the biscuit fired sample in porosity, but had just half the mechanical strength of the latter due to the layer‐wise fabrication process. The feasibility of rapid prototyping of porcelain products was validated by the successful fabrication of some porcelain samples, which showed that the relatively low mechanical strength of the laser sintered sample was still high enough for the following handling processes, such as surface glazing and glost firing.Originality/valueThe paper demonstrates the possibility of rapid prototyping of porcelain components and the models produced by using LSD process.

Feasibility study on rapid prototyping of porcelain products

In order to reduce the time to market of new or customized porcelain products or artworks, rapid prototyping using layer-wise slurry deposition (LSD) was studied in the present research. The properties such as phase composition, microstructure, shrinkage, density, and mechanical strength, of laser sintered (LS) and biscuit fired (BF) samples before and after post sintering in a furnace were studied and compared with each other. The laser sintered sample was comparable with the biscuit fired one in porosity, but just had a half mechanical strength of the latter. The feasibility of rapid prototyping of porcelain products was validated by the successful fabrication of two models (lavatory and double-heart) which showed that the relatively low mechanical strength of laser sintered sample was still high enough for the following handling processes.

Microstructural evolution during direct laser sintering in the Al2O3–SiO2 system

The microstructural evolution during direct laser sintering of LSD (Layerwise Slurry Deposition)—samples in the Al2O3–SiO2 system has been investigated. Slurries with a water content of 34 wt.% and a SiO2/Al2O3—ratio of about 3:1 have been used to manufacture layers which—after consecutive drying—have been sintered and laminated by laser treatment. Densified samples can be obtained with laser irradiances from 190 to 270 kW/cm2 and scan velocities between 35 and 65 mm/s. Elemental mappings of the layers’ cross sections suggest an inhomogeneous phase distribution in the laser sintered LSD samples with a slight alumina concentration gradient. A lower degree of particle melting in the bottom region of the layers is plausible due to attenuation of the laser beam intensity. SEM and HRTEM micrographs show that after a few seconds of laser treatment relictic starting phase, crystalline alumina plus amorphous silica, occur together with needle like mullite, the latter formed within an amorphous aluminosilicate phase. The resulting phase assemblage reflects the non-equilibrium conditions which can be expected for short time laser treatments. Mullite nucleation within the bulk of the liquid phase rather than in the vicinity of the parent alumina phase suggests that dissolution of alumina is the rate controlling step. Subsequent thermal post treatment in air in a conventional sintering furnace causes an increase of density to about 96% and leads to additional phase reactions. Amorphous silica transforms into cristobalite and the amount of alumina is reduced by additional mullite formation. By both coalescence of individual crystals and grain growth the morphology of the newly formed mullite changes during post heat treatment.