Figure 1. Schematic illustration of the additive manufacturing process based on laser sintering with Layerwise Slurry Deposition (LSD).
In the slip casting process, the slip is brought into contact with a porous body, the casting mold. When a liquid is wetting a porous body, capillary forces draw the liquid phase of the suspension into the porous mold. The particles start to form a powder compact, the cast, at the molds surface. The cast formation kinetics obeys a square-root-of-time law by depositing individual particles from the suspension to the surface of the mold. In case of the LSD process the mold is formed by the previously deposited and dried layers [6,7]. Slip casting results in the formation of powder compacts with densities potentially exceeding 60% TD. Furthermore, due to the free settling of the powder particles from the suspension, no obvious interface is formed between individual layers. The formation of a cast starts as soon as the slurry is in contact with previously deposited and dry layers. Hence, the speed of the doctor blade relative to the dry layers must be high enough to prevent collision of the cast with the doctor blade. On the other hand, when the speed of the doctor blade is too high, layer deposition is not uniform, because shear stresses within the suspension will result in an inhomogeneous deposition. For commercial silicate ceramic slurries, a speed of the doctor blade of 50 mm/s is typically chosen. For technical ceramics, the cast grows significantly faster and the speed of the doctor blade must be increased to 100 mm/s or higher. In conventional powder-based 3DP, the part built is easily released from the powder bed, but this is not the case for the slurry based process. After deposition of all layers required for building up the desired geometry, the printed body is embedded in the powder bed which now is a block of densely packed particles. For the release of the part, the powder compact must be dissolved by a solvent. In case water based slurries are used for the layer deposition, water can act as a solvent for the powder bed. Figure 2 shows an example for the release of two espresso cups fabricated by the SLS process in combination with Layerwise Slurry deposition.
Figure 2. Release of two espresso cups made of porcelain by employing the Layerwise Slurry Deposition (LSD) technology. a) powder bed, formed by 132 layers with a thickness of 200 µm, containing the two laser-sintered cups; b) cups partially released from the powder bed.
The use of a ceramic suspension for layer deposition in additive manufacturing is an upcoming technology, and only few results are available. The advantage of slurry based layer deposition is its potential to produce powder compacts with high powder packing densities, typically exceeding 55% TD. For the manufacture of a powder compact with properties comparable to those produced by classical powder processing, S-3DP has the highest potential. By the use of small amounts of organic or inorganic binders provided by a printing head, the particles in the densely packed powder bed are locally glued together, and compacts generated by this technology don’t require any additional treatment prior to sintering. Thus, we can forecast that S-3DP will be more and more used for the printing of ceramic powder compact. The laser treatment in S-SLS, however, will affect the microstructure of the powder compact typically in a direction which is not favorable for the manufacture of advanced ceramic.
Another interesting aspect from the LSD technology is related to the isotropy of the green body. Bodies generated by LSD show, previous to the laser sintering, no obvious texturing or anisotropy originated from the layer deposition process , which directly implies the absence of anisotropy after sintering. This, however, is valid for more or less spherical shaped particle, only. It is a well now fact that particles with shapes deviating significantly from spherical, e.g. whiskers, show a preferential orientation after deposition to thin layers.
The properties of the powder compact regarding green density and mechanical strength are comparable to powder compacts formed by conventional slip casting. This, in turn, gives rise to a unique feature of the LSD process: layers can be stacked without any supporting structure. A first layer is spread directly on the supporting platform followed by the successive addition of layers piling up a free standing block of material in which the three dimensional structure, generated by the laser sintering, is embedded (Figure 2a). Furthermore, due to the mechanical strength of the powder compact, a supporting structures typically required for fixation of the sintered bodies in a loose powder bed are not needed, regardless of the complexity of the part to be manufactured.
The layer spreading process works with a doctor blade which shows a dual function: first, it is responsible to feed the material, i.e. the ceramic slurry, into the build platform as well as to spread out the slurry into a uniform layer with constant thickness. The slurry is fed from a reservoir by a rotary positive-displacement pump, through a gap in the oblong, hollow doctor blade. Spreading is achieved by moving the blade at a constant distance parallel to the supporting platform surface. The layer thickness is defined as the gap between blade and platform. Some excess material is required during the spreading process allowing to run the process without precise control of the feed material volume. Each previously deposited layers acts as a support for the current deposition process. After drying, the layer information will be scribed by a laser into the topmost layer.
In case of processes using loose powders the laser sintered body is easily released from the powder bed, but not in the LSD process. For this task, the powder compact with embedded laser sintered body is placed in a pool of water. Dissolution of the powder compact is the most critical process step within the LSD process chain, because swelling of the powder compact by solvent ingress precedes its dissolution. Fortunately this swelling does not affect the entire volume of the compact simultaneously. Swelling and dissolution starts from the surface of the green body in direct contact with the solvent. The laser sintered body acts as barrier for the solvent and, thus, preventing the green body from swelling. Even though the sintered body is not completely dense it acts as an effective diffusion barrier for the solvent. Volumes of the powder compact completely encapsulated by sintered material cannot be dissolved and separated from the sintered body.
In practice, for the release of the sintered body it is helpful supporting local removal of material by spray rinsing. The intensity of the spray influences the surface quality of the sintered body. While no or a mild spray leaves material on the bodies surface, smoothening surface artifacts, an intense spray leaves only the sintered material with the body and artifacts, such as steps from the individual layers, come to the fore.
In Figure 3 a sequence of photographs taken during the release of a sanitary seat (WC) prototype (reduced in size, 80 mm height) made by the LSD process is shown. The model is formed by a subsequent stacking and laser treatment of 800 layers, in a thickness of 100 µm each. The part was generated by using conventional porcelain slurry in a fully automatic LSD machine (LSD 100, jointly development by CIC – Ceramic Institute Clausthal GmbH, Germany and T&T -Tools and Technologies GmbH, Germany). The machine operates with a kinematic resolution of 1 µm/step for vertical (lower the part for each layer) and horizontal (realize the slurry deposition) motions. The slurry is supplied by a dispenser pump (Dispenser 3NDP8, Netzsch, Germany) and deposited via a doctor blade on a heated (up to 200 °C) tile. A 100 W IPG single mode YAG-fiber laser system (wave length 1.064 µm, Burbach, Germany) and a galvano-scanner (Hurryscan, Scanlab AG, Germany) have been applied for the local sinter of the ceramic layer. The focused laser beam has a spot size of 50 µm .
Figure 3. Release steps after laser sintering of a sanitary seat prototype made of porcelain by employing SLS in combination with Layerwise Slurry Deposition (LSD).
According to Figure 4 the laser generates a hot spot on the powder bed surface with a partial dissipation of the radiation energy into heat. From the hot spot, the heat is transferred via heat conduction throughout the material. Hence, the surface near region of the irradiated area reaches the highest temperatures and densification of the powder by sintering or due to the formation of a liquid phase is highest. Deeper layers are revealing partially sintered powder or not affected material.
Fig. 4. Schematic illustration showing a laser beam interacting with a ceramic powder bed.
Figure 5 reveals a typical microstructure of a laser sintered porcelain body. Further details on the microstructure can be found elsewhere . A layered structure is clearly noticeable. Each layer reflects a graded structure with three distinct regions: heat affected or sintered, presintered and unsintered zone. The laser beam radiation has been applied from the top. Applying the discussed model for the dissipation of laser energy into heat, it is noticeable that just very little control on the state of sintering over the cross section of a single layer can be achieved. At the layer surface the direct laser irradiation results in temperatures typically exceeding conventional sintering temperatures, which results, in case of porcelain, in the formation of a liquid phase. Often this dense phase is revealing cracks as a result of the extreme temperature gradients induced by the local laser treatment. In deeper layers, however, sintering is not uniform and the powder is hardly densified.
Figure 5. Microstructure of a laser sintered porcelain body. The laser beam radiation has been applied from the top.
In order to obtain more uniformly sintered layers and, thus, a uniform microstructure, it is crucial to introduce the laser energy more uniformly into a layer. One option is to cast thinner layers, which is not always target leading, especially when parts large in size should be generated. Another possibility would be to tune the optical properties of the powder in use. In case the powder compact is semitransparent for the applied laser light, the energy of the light would be dissipated into heat over the entire cross section of a layer. Thus, independently from the location of a volume element, i.e., near to the surface or deeper into the layer, the laser energy would be directly adsorbed. This certainly allows a faster and more controlled heating of a powder layer over its entire cross section, but it also imposes the risk that deeper layers do receive some energy when sintering the topmost layer. The latter can be avoided by concentrating the intensity of the laser light into the topmost layer by applying appropriate optics.
Dense ceramics are mainly semitransparent in the near infrared-optical spectral range. For oxide ceramics, light absorption in the wavelength range between 1 µm and 5 µm is relatively low . Towards longer wavelengths, a strong increase in the absorption can be observed. In order to understand the optical properties of porous bodies, such as powder compacts, a precise knowledge of the complex refractive index m = n + i k of the material is required. The optical properties of porous media are significantly influenced by scattering processes . In the case of processing ceramic powders, scattering takes place at the air-filled pores within the powder compact. The absorption otherwise occurs in the ceramic material itself. The real part of the complex refractive index, n, causes a high reflection at the interface between air and ceramic material. For pore sizes significantly larger than the wavelength of the interacting light this effect governs the light scattering. For pore sizes in the range of the wavelength of the interacting light, light interaction is described by MIE theory. For pore sizes about 10 times smaller than the wavelength the porous medium appears as a continuum to the light . On the other hand, a large imaginary part, k, results in a high absorption coefficient. These theoretical considerations are particularly of interest when taking into consideration the wavelength of the two most common technical laser types, whose wavelengths are of approximately 10 µm for CO2 laser systems, and 1 µm for most technical relevant solid state lasers. The imaginary part of the refractive index, k, of ceramic material is generally high for a wavelength of circa 10 µm. On the other hand, around 1 µm dense ceramics are completely or at least to some extent, transparent. With a real part n is significantly higher than air, light scattering at air filled pores is strong. As a consequence, for particle sizes in the range of the wavelength, almost all intensity of the impinging light is reflected by a powder compact. Therefore the introduction of energy by laser light into a ceramic powder compact is difficult. Attempts to anneal a powder compact of amorphous SiO2 particles (quartz glass), in a mean particle size of 15 µm, by a diode laser emitting light at a wavelength of approximately 1 µm have been done. Even though quartz glass is highly transparent at this wavelength, all light has been reflected and an annealing of the powder compact was not possible. This effect can be understood on the basis of light scattering theory, in particular Mie theory , , and is a quite common phenomenon: While non porous water ice is transparent in the visible spectrum, snow appears white as a consequence of light scattering and reflection.
Considering the above discussion, it must be concluded that the processing of powder compacts made of particles significantly smaller in size than the wavelength of the interacting light would provide the best approach for the insertion of the laser energy uniformly into a volume of powder compact: In case particle sizes are significantly smaller than the wavelength of the interacting light, pores are too small for an effective light scattering and the optical properties of the compact appears uniformly as a combination of the optical properties of the air filled pores and the ceramic material used. Hence, by employing lasers emitting light with a wavelength around 1 µm, the light can penetrate into the powder compact, being dissipated into heat, in a dependence on the respective absorption coefficient of the material used. It is noteworthy that this approach is not possible for metal powders, as k is infinity for the discussed wavelengths.
Figure 6. Differential reflectivity of SiO2 powder compacts in dependence of the particle sizes.
Figure 6 demonstrates how the particle size of a powder compact influences its optical properties. In a preliminary study, powder compacts have been prepared by using amorphous SiO2 (quartz glass) powders with two different particle size distributions, i.e., with a d50 of 10 µm and 300 nm respectively. A more comprehensive study of the particle size dependent optical properties of ceramic powder compacts will be published elsewhere. In brief: The optical properties of the powder compacts have been measured by employing a Lambda 900 UV/Visible/NIR spectrophotometer from Perkin Elmer. Clearly seen is a change in the relative reflected intensity. While the powder compact made from a SiO2 powder with a d50 of 10 µm reflects almost all intensity over the entire spectral range measured, the powder compact made from the finer powder clearly shows a reflectivity progressively decreasing from 1 µm to 2.5 µm wavelengths. According to the low k value of SiO2 in this wavelength range, a significant portion of the impinging light is transmitting the compact (data are not shown here). Thus, by using even finer SiO2 powders it will be possible obtaining an appreciable transitivity of SiO2 powder compacts at the technically relevant laser wavelength of 1 µm. Once introduction of laser light into the powder compact is facilitated, a local annealing in the volume of the compact is possible. The absorbance of the powder compact can be tuned by adding the right portion of highly absorbing material to the initial powder and the intensity distribution of the laser light can be controlled by the use of appropriate optical elements. This approach offers a general strategy for controlling the laser absorption within the volume of powder compacts and, thus, improving the selective laser sintering process in a way, that local sintering is directly stimulated by laser light absorption within the volume of the compact, see also . As a result a better control of the sintering process and a faster and more uniform sintering will be achieved.
1 E.M. Sachs, J.S. Haggerty, M.J. Cima, and P.A. Williams, Inventors; Massaschussets Institute of Technology, Assignee. Three-dimensional printing techniques. US Patent 5,204,055; April 20; US Patent 5,204,055, 1993.
2 E.M. Sachs, M.J. Cima, M.A. Caradonna, J. Grau, J.G. Serdy, P.C. Saxton, S.A. Uhland, and J. Moon, Inventors; Massachusetts Institute Of Technology,Assignee. Jetting layers of powder and the formation of fine powder beds thereby. US Patent 6,596,224; July 22; 2003.
3 H.C. Yen, “Experimental studying on development of slurry-layer casting system for additive manufacturing of ceramics,” Int. J. Adv. Manuf. Technol., 1–11 (2014).
4 H.-H. Tang and H.-C. Yen, “Slurry-based additive manufacturing of ceramic parts by selective laser burn-out,” J. Eur. Ceram. Soc., 35  981–987 (2015).
5 T. Mühler, C.M. Gomes, J. Heinrich, and J. Günster, “Slurry-Based Additive Manufacturing of Ceramics,” Int. J. Appl. Ceram. Technol., 12  18–25 (2015).
6 X. Tian, T. Muhler, C. Gomes, J. Gunster, and J.G. Heinrich, “Feasibility study on rapid prototyping of porcelain products,” J. Ceram. Sci. Technol., 2  217–225 (2011).
7 A. Zocca, P. Lima, J. Günster, "LSD-based 3D printing of Alumina Ceramics," J. Ceram. Sci. Tech. 8  141-148 (2017).
8 G. C. Wei, “Transparent ceramic lamp envelope materials,” J. Phys. D., 38 3057–3065 (2005).
9 C. F. Bohren, “Multiple Scattering of light and some of its observables consequences,” Am. J. Phys., 55  524-533 (1987).
10 J. A. Adam, “The mathematical physics of rainbows and glories,” Physics Reports, 356 229–365 (2002).
11 Thomas Mühler, Jürgen G. Heinrich, Gundula Helsch, Dongxu Yao, Stephan Gräf, Frank A. Müller, Jens Günster, “Strategies for the Selective Volume Sintering of Ceramics,” J. Mater. Res. 29  (2014) 2095-2099.
Made by: BAM
Material: Porcelain VC
Postprocessing: as sintered
Made by: BAM
Material: Porcelain VC
Postprocessing: glazed and sintered
Made by: BAM
Material: Porcelain VC
Postprocessing: glazed and sintered