## 3 Results and Discussion
### Mechanical Testing Results  
In addition to the uniform distribution of the filler in the polymer mass and its low fluidity, there are two other important qualities that make 3D printing of the filament possible. Firstly, it has a relatively high Young's modulus, which prevents the filament from crumpling inside the extruder and allows the creation of strong pressure in the nozzle. Secondly, it has a high tensile elongation value, which allows proper winding of the filaments onto the spool without the risk of damaging them during printing [6]. In order to evaluate these factors, tensile tests were performed on all obtained filaments. The values of Young's moduli and ultimate tensile strength and elongation presented in this study are qualitative, as the tests were conducted on filament samples rather than on the samples specified in the standard. The aim was to determine the effect of the addition of graphene oxide on the elastic properties of filled filaments and compare them with each other. Figure 6A illustrates the representative curves of stress dependence on deformation of filled filament samples. The values of Young's moduli, ultimate tensile strength and elongation are shown in Figure 6B-D.  
Figure 6: Representative tensile stress–strain curves (**A**), ultimate tensile strength (**B**), elongation (**C**) and Young’s moduli (**D**) of all filaments. In the results obtained (Figure 6A), it is clearly seen that, depending on the content of GO, the shape of the deformation curve changes. It has already been noted in studies by other authors [20; 27; 41; 42] that the addition of graphene and its derivatives usually increases the strength of the filled polymer; at the same time, its plasticity is reduced, thus resulting in an overall reduction in material toughness. However, in the case of filaments filled with ceramic particles, the curves of the samples (Figure 6A) behaved a little differently. It is noted that samples of filaments with 0 and 0.50% graphene oxide content have a brittle fracture characteristic inherent in solid materials, while samples with 0.10 and 0.25% graphene oxide content show a viscous fracture inherent in plastic materials. The elongation values (Figure 6B) obtained for the 0.10 and 0.25% content samples were 16.4 \(\pm\) 2.8 and 11.1 \(\pm\) 1.6%, while for samples with 0 and 0.50% GO, the values were 3.9 \(\pm\) 0.9% and 3.7 \(\pm\) 0.8%, respectively. The value of the ultimate tensile strength of the filaments (Figure 6C) varied from 10.89 \(\pm\) 0.85 to 11.72 \(\pm\) 0.58 MPa depending on the concentration of GO, while the strength of the samples without GO was 9.74 \(\pm\) 0.43 MPa. An increase in the GO content increases the strength of all ceramic-filled filaments; therefore, the tensile load is effectively transferred from the polymer with ceramics to the graphene oxide sheets. It is also noticeable that the increase in strength due to an increase in the concentration of graphene oxide is not linear. Adding 0.50 vol.% GO practically does not increase the strength of the filament compared to the addition of 0.25 vol.%. The values of the elastic modulus (Figure 6D) are also nonlinear: 486 \(\pm\) 57 MPa for 0 vol.% GO, 441 \(\pm\) 49 MPa for 0.10 vol.% GO, 528 \(\pm\) 34 MPa for 0.25 vol.% GO and 636 \(\pm\) 43 MPa for 0.50 vol.% GO. Thus, a polymer filament filled with ceramics at 74 wt.% without the addition of GO behaves like a brittle and low-strength material, but the addition of 0.10-0.25 vol.% graphene oxide increases its strength and, at the same time, gives it plasticity. In rheological studies, the sample with 0.10% GO also showed the greatest plasticity. A further increase in the GO content to 0.50 vol.% has a slight effect on the increase in strength characteristics but reduces the ductile behavior of the material. The results obtained are most likely due to the fact that, in this study, graphene oxide behaves, among other things, as a dispersant of ceramic particles [24] at a content of 0.10-0.25 vol.%.