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 5 ceramic suspension with engineered rheological behavior is extruded via a small orifice. The process is controlled with computer aided design, which helps in forming 3D materials. This process is not a single step process, as the 3D printed product needs an intermediate step to remove binder and other volatiles before the final sintering step. This process is relatively low cost, but some of the challenges include fabrication of a dispersed ceramic slurry with high loading, resulting in the need for special care in selection of organic additives, dispersant, plasticizer, and binder, as well as limitations with hygroscopic materials and stair-step finishing due to layer-by-layer manufacturing[17]. AMBasedonPowder Technology. Sach et al.[18] pioneered the binder jetting technique where initially a layer of powder bed is sprayed with binder then subsequent layers are built by repeating this layer-by-layer process. This method needs minimal feedstock preparation as powders alone can be used for this process (Fig. 3a). The five key factors of binder jet printing ceramics are powder selection, binders, printing parameters, equipment, and post-treatment process. Some of the critical challenges in this process are (a) milling of ceramic powders, (b) low binder strength, (c) and powder layer consistency. In addition, this process requires binder removal and sintering for densification after the printing process where green body strength, binder removal, etc. constitute some of the critical challenges during the post-processing step[19]. Deckard and Beaman pioneered selective laser sintering (SLS)[20]. During the SLS process, a laser with the desired power is used to sinter a bed of powders. Thereafter, the process is repeated to sinter the material in a layer-by-layer sequence (Fig. 3b). There has been limited success in directly sintering ceramics using this method due to the long sintering times typically required for densification due to a combination of factors, including solid state sintering process limitations, thermal shock resistance of ceramics, and thermal stresses, which can lead to cracking and high porosity[8,21]. This process is referred to as direct-SLS (d-SLS)[21]. A compromise is to bond ceramic particles with low melting inorganic or polymeric binders, which is also referred to as indirect-SLS (shown in Figs. 3a and d)[21], to overcome some of the issues with d-SLS. Meiner et al.[22] pioneered the selective laser melting (SLM) process in 2001. In this method, the bed of powder is melted layer-by-layer to build the 3D component by using a high-power laser. This promising process can be potentially used to manufacture high performance ceramic components with complicated designs in a single step[8]. Like d-SLS, SLM also needs to overcome limitations, such as poor thermal shock resistance of ceramics, residual stresses, and thermal gradients, during production. Gahler et al.[23] utilized a 100-µm beam by using aRofinSinar (Hamburg, Germany) SC 10 carbon dioxide (CO2) laser tube (100 W) and a galvano-scanner (type: hurrySCAN, Scanlab AG, Puchheim, Germany) to print the Al2O3-silica (SiO2) system. Gahler et al. [23] used Fig. 2 — (a) Schematics of SL[8,9], (b) IJP process[16], and (c) DOD inkjet printer with either thermal (left side) or piezoelectric actuation[16,17]. (a) (b) (c) Fig.3 — Schematics of (a) binder jetting of ceramics[19], (b) powder- based laser assistedmanufacturing approach which will result in (c) partially sinteredmicrostructure formed by direct-SLS, (d) ceramics bonded with inorganic or organic binder by indirect-SLS, and (e) melted structure during SL [8,21,24]. (a) (b) (c) (e) (d)
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