## 5 SEM Microscopic Analyses
### Grinded Surface Tests  
The view of the exemplary internal structure of the base materials subjected to fatigue testing is presented in Figure 18. Preliminary studies of the material structures revealed that both the NextDent Denture 3D and NextDent C&B MFH Bleach materials consist, throughout their volume, of a matrix containing uniformly distributed filler particles. These filler particles are fine-dispersed, with sizes ranging from 0.1 mm to approximately 5.0 mm. Figure 18: View of sample output structures of materials tested: (**a**) NextDent Denture 3D; (**b**) NextDent C&B MFH Bleach. Studies conducted in [17] indicate that the filler components include compounds such as SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and TiO\({}_{2}\). In contrast, no filler components were detected in the matrix of the Graphy TC-80 DP material. Below, in Figure 19, a general view of the exemplary flat grinding surfaces subjected to fatigue testing is presented. Observations with the naked eye revealed that in all the examined cases, after 10,000 loading cycles, permanent dimensional changes or destruction of the samples were visible. The smallest degree of deformation/destruction was observed in the NextDent Dentrure 3D samples, which mostly exhibited visible permanent deformation in the form of compression, resulting in the sample acquiring a barrel-shaped form (Figure 19b). In the NextDent C&B MFH Bleach samples, the compression was so extensive that it caused significant delamination along the axis of the sample (Figure 19b). The greatest degree of destruction was observed in the Graphy TC-80 DP material samples, which underwent complete destruction under cyclic loading (Figure 19c). Based on the observations with the naked eye, areas (marked in the figure) were selected for further SEM analysis to provide a more detailed microscopic examination. In the areas marked in the figure above (Figure 19), on the flat grinding surfaces of both the NextDent C&B MFH Bleach and NextDent Denture 3D samples, numerous interconnected longitudinal and transverse microcracks were observed. From the longitudinal microcracks aligned with the direction of compressive forces, transverse microcracks branch out at a 90\({}^{\circ}\) angle, forming a network of microcracks on the grinding surface. The structure of the grinding surfaces, along with the revealed crack network, is shown in Figures 20 and 21. Analysis revealed that the key difference in the destruction mechanism of the discussed materials lies in the crack front's behaviour. In the NextDent Denture 3D material (Figure 20b-d), the crack front passes through the filler particles, while in the NextDent C&B MFH Bleach material (Figure 21b), it progresses along the matrix-particle boundaries. This mechanism of fracture formation results from differences in the primary fraction structure of the fillers used. Figure 19: View of example grindings of samples subjected to SEM examination with markings of areas selected for analysis: (**a**) NextDent Denture 3D, (**b**) NextDent C&B MFH Bleach, and (**c**) Graphy TC-80 DP. In the NextDent Denture 3D material, the filler elements visible in the microstructure, which form the primary fraction, typically consist of smaller particles or flakes that are packed or agglomerated. In contrast, the filler elements in NextDent C&B MFH Bleach are usually monolithic. Consequently, when the crack front encounters greater resistance from a monolithic particle, it bypasses it and progresses along the boundary. Numerous matrix-particle separations indicate weak adhesive bonding between the material's components. For a clearer depiction of the microcrack network, red lines were added to the images near the cracks, and the cracks themselves are indicated with arrows. On the grinding surfaces of the samples made from the NextDent C&B MFH Bleach material, numerous delaminations and separations of material sections were observed, concentrated in the areas marked in Figure 22a.