## 3 Results and Discussion
### Results of the Characterization of Raw and Sintered Samples
#### 3.2.5. Results of the Mechanical Properties of the Sintered Samples  
As can be seen from Figure 8, the lowest values of flexural and compressive strength were recorded for the sample with the greatest share of incorporated waste sludge, GC\({}_{20}\). Moreover, it can be observed that both the flexural and compressive strength of the solid samples decreased uniformly at the higher waste sludge content. Indeed, the reduction in the flexural and compressive strength of the solid samples could be possibly related to the lower final density of the samples along with the higher concurrent matrix microcracking at the higher magnitude of waste sludge particles. Expectedly, the results of the Vickers hardness test show that the sintered samples displayed much higher values compared to the raw ones. The lower Vickers microhardness (HV) of the raw samples could be related to their lower density. Moreover, the HV of the samples improved in concordance with the increase in the waste content share in the samples, which indicates that waste addition had a positive impact on the microhardness of the obtained samples in this case. The HV values for both the raw and sintered solid GC samples are presented in Table 5. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV). A graphical representation of the obtained values expressed in Si units (GPa) is shown in Figure 9.  
Figure 7: Dilatometric curves of percentage linear changes in all GC samples heated from RT to 800 \({}^{\circ}\)C as a function of temperature (**a**) and as a function of heating time (**b**). Even though there are no particular required normative values which have been suggested in previous research for the mechanical properties of high-temperature industrial adhesives or IT-SOFC sealant materials, it is clear that certain mechanical capabilities must be provided for the sake of preventing gas leakages and the adhesion of the device components during operation, as well as for surviving harsh conditions and stresses that  
Figure 8: Graphical representation of values of flexural (**a**) and compressive (**b**) strength of solid GC samples. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & **No.** & **HV\({}_{0.025}\), HV** & **[HV\({}_{0.025}\)]sr, HV** & **Sample** & **No.** & **HV\({}_{0.05}\), HV** & **[HV\({}_{0.05}\)]sr, HV** \\ \hline GC\({}_{0}\) & 1 & 19.5 & & & GC\({}_{0}\) & 1 & 56.8 & \\ raw & 2 & 19.6 & 19.4 & sintered & 2 & 59.7 & 59.57 \\  & 3 & 19.1 & & & 3 & 62.2 & \\ \hline GC\({}_{5}\) & 1 & 25.2 & & & GC\({}_{5}\) & 1 & 66.6 & \\ raw & 2 & 25.3 & 25.2 & sintered & 2 & 71.9 & 69.37 \\  & 3 & 25.2 & & & 3 & 69.6 & \\ \hline GC\({}_{10}\) & 1 & 26.2 & & & 1 & 77.0 & \\ raw & 2 & 25.8 & 25.83 & sintered & 2 & 79.3 & 77.2 \\  & 3 & 25.5 & & & 3 & 75.3 & \\ \hline GC\({}_{15}\) & 1 & 25.8 & & & 1 & 77 & \\ raw & 2 & 26.8 & 26.27 & sintered & 2 & 79.3 & 79.0 \\  & 3 & 26.2 & & & 3 & 80.7 & \\ \hline GC\({}_{20}\) & 1 & 28.4 & & & 1 & 91.7 & \\ raw & 2 & 28.8 & 28.63 & GC\({}_{20}\) & 2 & 90.0 & 91.9 \\ raw & 3 & 28.7 & & sintered & 3 & 94.0 & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Microhardness of samples before and after thermal treatment for 8 h at 800 \({}^{\circ}\)C.  
Figure 9: Microhardness of raw and sintered glass–ceramic samples. might occur at the high operating temperatures of these devices. Basically, these materials should be able to provide satisfying robustness and reliability to the overall system. According to the extensive literature review, it can be noticed that the compressive and flexural strength as well as the microhardness of the studied samples were significantly lower than those reported in the literature for different SOFC ceramic sealant materials [31; 32; 39; 40; 41]. Therefore, the presently studied materials would probably not be such good candidates for application in SOFC technology, except if some additives were added to improve these properties.