A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 3 2 4 a liquid state using one or more lasers to selectively cure to a predetermined thickness and harden the material into shape layer upon layer”[5]. The simple schematics of the SL process are shown in Fig. 2a[8,10]. SL can be visualized as a green body forming process where pre-forms require pyrolysis to form dense bodies. For fabricating ceramic structures, the ceramic powder content of the photopolymerizable slurry should be > 50 vol%[10]. Some of the critical design challenges of this process are: (a) maintaining adequate solid content and uniformly distributing ceramic particles, with particle size determining the layer thickness, as an unstable slurry can lead to inhomogeneous parts; (b) light scattering by ceramic particles that can affect the curing area; and (c) viscosity engineering of the slurry, which can resist perturbations during the deposition process (typically each layer has a thickness of 100 µm) and can be influenced by a shear stress differential in different regions[8,10]. Once the process is optimized, SL can be used to fabricate polymer-matrix composites[11] and ceramics, such as 99.3% dense alumina (Al2O3) [12]. This process can be accomplished using commercial printers,* including the Lithoz CeraFab 7500 (Lithoz GmbH, Vienna), 3DCeram FCP (3DCeram, Limoge), and CPT6060 (DDM Systems, Atlanta)[10]. The process can be adopted for manufacturing composites by using tabletop SL printers from Formlabs[11] and other commercially available printers[10]. SL can be further fine-tuned by processes, including by digital light processing (DLP) and two photon polymerization (TPP). In DLP, the light source passes through an engineered mask like a digital micromirror device (DMD), which bestows further control over theSLprocesswhere thex-y pattern is governed by pixel size, and the z-direction is dependent on the thickness of individual layers[8,13]. In TPP, an ultrashort pulsed beam is used to initiate simultaneous absorption of two photons in the photosensitive chromophore that initiates the chain reaction polymerization reaction[14]. This process allows a resolution of 100 nm, which can be adapted for complex designs[14]. Derby[15,16] classified inkjet printing (IJP) into three categories: continuous inkjet printing (CIJ); drop-on-demand inkjet printing (DOD); and electrostatic inkjet printing (EIJ). CIJ typically utilizes a drop size of 100 µm, drop generation between 20 kHz and 60 kHz, and drop velocity > 10 ms−1, whereas DOD uses a drop size 20 mm–50 µm, drop generation 1 kHz–20 kHz, and a drop velocity that can be controlled depending on use case. Both CIF and DOD have been studied for printing ceramics. In both cases, creating a ceramic suspension with a desired solid content is a critical and necessary step. In the CIJ process, drops are created by Rayleigh instability and consequently placed by charging. The drops have inherent problems such as accuracy of placement, wastage of ink during recirculation, and limited applicability of fluids due to dispersion issues (Fig. 2b). Comparatively, DOD has more control on the drop content and placement by controlling the pressure pulse using piezoelectric or heat sources (Fig. 2c). Piezoelectric actuation, which creates drops by mechanical actuation, is preferred for DOD, as the use of a heat source puts constraints on the volatility of the fluid medium in the inkjet. The efficacy of printing kinetics by DOD is dependent on the droplet size and drying kinetics. For example, drying of isolated drops can lead to inhomogeneous drying (e.g., coffee stain effect). Readers are encouraged to consult references 15 and 16 for detailed information about this process. Ceramic robocasting is a promising subset of AM processes where tailored Fig. 1 — Classification of different types of AM processes. * Specific vendor and manufacturer names are explicitly mentioned only to accurately describe the hardware used in this study. The use of vendor and manufacturer names does not imply an endorsement by the authors or the U.S. Government nor does it imply that the specified equipment is the best available.
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