Ceramic Additive Manufacturing via Lithography
Although it is relatively a new method, lithography-based ceramic manufacturing is a strong candidate for the production of complex and design-flexible ceramic parts.
Additive manufacturing (AM) is expected to replace conventional methods for the integration of smart manufacturing and information technologies.1 Commercial ceramic manufacturing methods based on polymeric master parts are inadequate for applications for which complexity, flexibility and geometric precision are of importance. This limitation leads to the need for AM of advanced ceramic components.
The main AM methods reported in literature for direct and indirect ceramic manufacturing include: material extrusion, stereolithography, binder jetting, ink jet printing, powder bed fusion and sheet lamination.2 Among all ceramic AM approaches, slurry-based methods were reported to be able to provide high-density ceramics with high reliability.
Lithography-based ceramic manufacturing (LCM) is a relatively new method that works according to the photopolymerization principle.3,4 Photopolymerization enables strong polymer generation from a liquid polymer bath that is suitable for additive manufacturing. In this method, visible LED light (typically a blue light source with a wavelength of 465 nm) is used to cure the photopolymer-based ceramic slurry layer by layer to build up high-resolution, high-performance, and complex green ceramic parts5,6 with smooth as-sintered surfaces and densities better than 99.6%2,7 (see Figure 1).
LCM technology was first developed at TU Wien.3 In LCM, the blue visible light is projected by a dynamic mirror to the bottom side of the vat. This light technically cures only the exposed area. Upon curing one layer, the part moves upward and the blue light cures the next layer where new slurry is added4 (see Figure 2).
Similar to other AM methods, LCM involves pre-processing, processing and post-processing stages. The pre-processing stage consists of the selection of a suitable powder and binder, as well as the preparation of an efficient slurry. CAD analysis of the ceramic part and its form optimization should be performed by taking fracture mechanics into account. As the manufacturing process is finished, the green body is cleaned, handled and tested to ensure that it satisfies minimum requirements.
Following the green body production, the polymer binder is evaporated through a debinding process, and the ceramic part is sintered. Surface machining (grinding and polishing) is performed in order to provide dimensional tolerances and improve reliability. Mechanical testing is applied to ensure reliability. The industrialization of the LCM process can be completed if the structure-process-property relations are obtained in detail for different material-process combinations.
Developments in LCM
Typical commercial ceramic materials that have been mostly used in LCM include high-purity alumina (Al2O3)5,8,9 and yttria-stabilized zirconia (ZrO2).10-13 In addition, 45S5 bioglass, silicon carbide, silicon nitride and tricalcium phosphate have also been used in a few studies.14,15
Recent studies reported that alumina parts with a theoretical density of > 99.3% and bending strength of 430 MPa can be produced by LCM.9 The theoretical density and bending strength of yttria-stabilized zirconia manufactured via LCM were > 99.6% and 650-800 MPa, respectively. The hardness and fracture toughness of ceramic parts manufactured by LCM are independent of the layer architecture.11 The fracture origins of ceramic parts produced by LCM can be classified into five main groups: pores, agglomerates, cleaning defects, edge damage and machining damage.11
The first attempt to use LCM to produce ceramic parts containing controlled surface topology with high aspect ratio, speed and microscale details was successful.8 Detailed and fine alumina heat exchangers with a wall thickness of 100 µm and channel cross-section of 0.9 x 0.9 mm2 were produced with a high surface area-to-volume ratio.6 However, it was reported that holes with diameters smaller than 200 µm may tend to clogging where the non-cured suspension remains in this area. A future promising application of the LCM method was reported by Lantada et al. for the manufacture of high-precision auxetic metamaterials and devices using ceramic materials without the need for supporting structures.15
Design and Geometric Challenges
Compared to other materials, the design of advanced ceramic components requires more attention due to their brittle nature and low resistance to mechanical and thermal shock.16 Thus, simulations of heat transfer, fluid transfer and mechanical strength are key points of reliable design in the pre-processing stage of LCM.
In all additive manufacturing methods, the CAD and geometry optimization processes are crucial. Since LCM involves limitations in terms of wall thicknesses and hole radius values, there is a need for the development of CAD tools that include simulation tools and provide easy re-design according to simulation results.
One of the most important geometric challenges of LCM is the production of layers that combine both thin and thick walls. Each wall requires different types of light density and exposure times. In addition, the introduction of holes with very small diameters (< 200 µm) may be problematic since the inside of the holes cannot be cleaned easily without damaging the part. Inappropriate geometric design may cause crack formation in the part and unexpected mechanical performance. Therefore, either the technology should be improved so that different regions in one layer can be processed with different parameters, or the existence of thin and thick walls in one layer should be avoided.
Another geometric challenge is the limitation of ceramic part size (115 x 64 x 150 mm) that can be produced by commercially available LCM systems. In the future, the available manufacturing spaces of LCM machines should be increased.
Cleaning and Handling
Major challenges during the cleaning process of LCM involve the removal of the non-cured suspension and the handling of complex components.6 Cleaning and handling defects occur frequently on the surfaces where high tensile stresses may occur.11 Such defects tend to be the fracture origin of ceramic parts.
In addition, the cleaning of green bodies with solvent in an ultrasonic bath may cause bubbles due to the cavitation effect.8 As a result, the cleaning and handling processes should be reconsidered and improved not only in order to avoid swelling and defect introduction during cleaning, but also to optimize time, cost, and quality issues.
If support structures are used in LCM, the material chosen should be similar to that used for the ceramic component. Removing the support structures may result in problems for fine structures.6 Thus, the use of support structures may be minimized by changing the orientation of the component in regard to the building platform, or changing the component design to realize functional geometries that work as a supporting structure as well.
Debinding and Sintering
LCM technology still requires debinding and sintering like conventional ceramic manufacturing methods. Shrinkage occurring during sintering is typically 20%, and this behavior has to be considered during the part’s design stage. In addition, the debinding process should be performed carefully because of the high risk of defect formation. The occurrence of such defects (e.g., porosity, purity, microdefects and interfacial defects) will still require extra design and manufacturing efforts to ensure reliability. Thus, the effect of debinding/sintering processes on mechanical reliability should be investigated in detail.
Refining the LCM Method
Although it is relatively a new method, LCM is a strong candidate for the production of high-quality, complex and design-flexible ceramic parts in a cost- and time-effective manner. Producing solutions to major problems and minimizing the limitations described here will enable the use of LCM for the manufacture of advanced ceramic components with enhanced mechanical characteristics and new functional properties.
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