## 3 Results and Discussion
### Three-Dimensional Printing and Sintering  
Since this work assumes sintering of ceramics in a muffle furnace without a protective atmosphere--that is, graphene oxide will be removed along with a polymer binder in order to reduce the porosity of the future product--a filament with the lowest GO content (0.10% GO/Al\({}_{2}\)O\({}_{3}\) filament) was selected for further study. To work out the printing parameters, elementary shapes in the form of discs were printed from the selected filament. It was noted that shrinkage and warping of the material are significantly reduced by 3D printing using filaments with GO compared to polyamide and 0%GO/Al\({}_{2}\)O\({}_{3}\) filaments. In study [43], it was noted that shrinkage of the material can be avoided in two ways: by reducing the temperature difference between the print temperature and the environment and/or by reducing the coefficient of thermal expansion of the printed material. The authors of study [44] proved in their work that the addition of GO to polymer composites significantly reduces the value of their coefficient of thermal expansion. Apparently, in our work, we observe the described effect: graphene oxide evenly distributed at a volume helps reduce the coefficient of thermal expansion and thereby reduces shrinkage and warping of the material during 3D printing using the FFF method. This effect has a positive effect on the geometry and density of the printed green body and, ultimately, on the sintered ceramic product. Working out the printing modes made it possible to select a nozzle temperature of 260 \({}^{\circ}\)C, nozzle diameter of 0.8 mm, layer height of 0.4 mm, print speed of 20 mm/s, table temperature of 100 \({}^{\circ}\)C and 100% filling. The nozzle diameter was intentionally chosen to be increased to prevent it from clogging with ceramic particles. The filling type of 100% was selected to obtain a dense product. Table heating was used to reduce shrinkage and warping of the product; blowing was not used for the same reason. Furthermore, according to the selected modes, products with a more complex shape were printed in the form of gears, which were subsequently subjected to burning of a polymer binder and graphene oxide and further sintering. Figure 8 shows a photograph of the printed gears (on the right) and sintered gears (on the left). The photograph demonstrates that the printed gears retained their shape after the sintering process but decreased in size. Figure 7: Optical images of tensile fracture cross-sections of 0% GO/Al\({}_{2}\)O\({}_{3}\) (**A**) and 0.10% GO/Al\({}_{2}\)O\({}_{3}\) (**B**) filaments. In order to confirm the sintering of aluminum oxide and the removal of polymer binder and graphene oxide, an X-ray phase analysis of the initial alumina powder and sintered sample was performed (Figure 9). All the peaks were identified as belonging to the \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) phase (ICDD 00-046-1212). The sharp peaks on the sintered sample indicate a high degree of crystallinity compared with the initial powder. The results of the analysis indicate that during debinding and sintering, the organic components were successfully removed, and the introduction of GO into the ceramic polymer composition had no effect on the phase structure of the sintered sample. Figure 10 shows the microstructure of the sintered ceramic sample at various magnifications and EDS maps of the distribution of elements. Figure 8: Sintered and printed gears from 0.10% GO/Al\({}_{2}\)O\({}_{3}\) filament. Figure 9: XRD analysis of the sintered sample (black line) and the raw alumina powder (red line). The resulting microstructure turned out to be similar to the microstructures of sintered ceramic samples made using this technology [6; 45; 46]. Small irregular pores are visible over the entire surface of the sample, formed as a result of the removal of the polymer binder and graphene oxide.