# Hybridizing Lithography-Based Ceramic Additive Manufacturing with Two-Photon-Polymerization
## 3 Results and Discussion
### Hybrid Printing  
2 PP structures were printed on a UV-cured substrate, with the substrate and the printed structures being from the same material. To create the UV-cured substrate, just the  
Figure 2: Transmission through a 45 wt% YZT slurry as a function of the wavelength of the incident light and as a function of the penetration depth, that is, the sample thickness. Also shown, the YSZ PEG-DA resin with 70 wt% solid loading. 2 PP-photoinitiator (BEA) was exchanged by DMPA, which is sensitive to UV-light. For more details see also reference [5]. The procedure to combine UV-substrates with 2 PP was as follows (Figure 3a): First, a drop of UV-curable resin was placed in between two glass plates and illuminated (crosslinked) with UV light from a conventional UV lamp. Exposure times are typically 10-30 s. Then, the 2 PP curable resin was placed on top of the solid substrate and is crosslinked with the 2 PP-Printer. Both the substrate and 2 PP-structure were thermally treated to remove the organic binder and to sinter the ceramic particles. Figure 3a shows such a substrate before sintering and Figure 3b after sintering. The area of the substrate generated according to the routes described in Figure 1a,b is relatively large compared to the size of 2 PP-printed features, which allows for the printing of a variety of structures on one substrate. Figure 3c gives an overview of different 2 PP-printed parts (i.e., micro-buses and lattice cube structures) on the UV-cured and sintered substrate. There is a row of micro-buses and two other rows of lattice cubes printed in different sizes. A FIB-cut (Figure 3d) through one bus shows the precision of the 2 PP-printing process. The bus is hollow, its roof only sustained by the window pillars. The roof sagged slightly, because it is widely overhanging, but nevertheless the freestanding structure is strong enough to self-support. This example is not chosen to show the maximum accuracy of the process, which can be appreciated in reference [5], but it is a reminder of the freedom in design by employing AM technologies. In a further printing job, the procedure is changed slightly as depicted in Figure 1b to combine 2 PP with LCM. The UV curable resin is printed with an LCM machine to generate a structured substrate. Compared to the UV cure substrate procedure in Figure 1a, an additional washing step is required here to remove uncured resin from the printed LCM structure. Afterwards, the 2 PP curable resin is added on top of it or in voids and  
Figure 3: UV-cured substrate from 70 wt% yttria stabilized zirconia resin: (**a**) before; and (**b**) after sintering at 800 \({}^{\circ}\)C; (**c**) overview of 2 PP-printed parts from the same resin on one substrate after sintering to 800 \({}^{\circ}\)C; and (**d**) FIB-cut of one bus plus detailed front view. 2 PP printing can take place. The LCM- printed substrates with added 2 PP-structures are sintered together. One example is an LCM-printed clock hand, shown in Figure 4a, taped onto an objective slide, with tapes to generate space for the 2 PP printing and with a small cover glass. This hybrid structure was sintered (Figure 4b). The arrows in Figure 4a indicate printed gearwheels on top of the clock-hand (Figure 4c). A comparison between non-sintered and sintered state shows a linear shrinkage of ~38%. Only one gearwheel was left after washing, which indicates a rather loose connection between the LCM print and 2 PP sample. It is important to check the washing conditions after LCM printing, which might passivate the surface and therefore hinder a good connection to the 2 PP print. The SEM-images also reveal a distortion of the gearwheel (Figure 4c). The smaller structure follows the inhomogeneous shrinkage of the supporting substrate, in this case the clock hand. The 2 PP structure was printed with an air-objective, which provided a wider field of view (FOV). Single lines are visible in Figure 4c indicating too little overlap between the individual lines, which results in under-curing and also destabilizes the structures.