## 3 Results and Discussion
### Critical Roughness Parameters for Leakage Detection  
Summarizing the results of the tightness of samples with surface parameters from the code group for a compressive force equal to 2280 N, the relationship between the leakage values and the values of the parameters _Sq_, _Sp_, _Sa_, _Sz_, and _Spc_ was determined. Among the parameters mentioned, Figure 11 shows the dependencies for the parameter _Sa_. These parameters correspond to the profile parameter _Ra_ used in the industry. The drawings show the decrease in the leakage value depending on the values of these parameters. A similar tendency was observed for the parameters _Sz_, _Sq_, _Sp_, and _Spc_. The demonstrated correlation of the parameters _Sq_, _Sp_, _Sa_, _Sz_, and _Spc_ with the value of the obtained leakage at the common pressing force proves a strong influence in the peaks/hills of the surface roughness profile on the obtained leakage. As the height of the peaks of the roughness profile increases, the resulting leakage increases. The second feature of the hills that affect the leakage is the curvature of the hills itself. With an increase in the contact surface of the peaks (rounded shape of the peaks), the leakage value decreases. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Force [N]**} & \multicolumn{3}{c}{**Leakage [cm\({}^{3}\)/min]**} \\ \cline{2-4}  & **(21-37) (D–E)** & **(28–42) (D–F)** & **(35-39) (E–F)** \\ \hline
260 \(\pm\) 70 & 5.610 \(\pm\) 0.070 & 5.910 \(\pm\) 0.073 & X \\ \hline
415 \(\pm\) 70 & X & X & 0.907 \(\pm\) 0.023 \\ \hline
1035 \(\pm\) 70 & 2.200 \(\pm\) 0.036 & 2.500 \(\pm\) 0.039 & 0.077 \(\pm\) 0.008 \\ \hline
1550 \(\pm\) 70 & 0.244 \(\pm\) 0.010 & 0.184 \(\pm\) 0.009 & 0.050 \(\pm\) 0.007 \\ \hline
2280 \(\pm\) 70 & 0.130 \(\pm\) 0.008 & 0.071 \(\pm\) 0.008 & 0.043 \(\pm\) 0.007 \\ \hline
3105 \(\pm\) 70 & 0.071 \(\pm\) 0.008 & X & X \\ \hline \end{tabular}
\end{table} 
Table 10: Forces and leakage for a pair of samples with surface topography parameters from code groups D, E, and F.  
Tightness is obtained due to the plastic deformation between the two surfaces. These surfaces fit with each other due to the material flow under the high force between them. This feature was dependent on the tested samples due to the roughness parameters _Sq_, _Sp_, _Sa_, _Sz_, and _Spc_. According to the parameters _Sa_, _Sq_, and _Sz_, the lower the scope of inequality in tested surfaces roughness, the easier it is to reach tightness. The particular meaning of the leakage value in the distribution of roughness inequalities indicates the height of the surface peaks (_Sp_) and the geometry of the peaks indicates the radius of curvature of the apex (_Spc_), which has an influence on the size of contact area between the samples. The surfaces characterized by sharp peaks due to the applied compressive force do not obtain a large contact area, while in the case with gentle peaks, it is possible to obtain a great surface area. When analyzing the sets of surfaces in which one of the samples was characterized by surface parameters from code group A, while the other part of the pair belonged to one of the other groups, a relationship was observed between the surface topography parameters. As in the previous analysis, for the compressive force equal to 2280 N, a relationship was found between the leakage values and the differences in the values of the parameters _Sq_, _Sp_, _Sa_, _Sz_, and _Spc_ between the constituent parts of pairs of samples. This relationship for the Sa parameter is shown in Figure 12. It was observed that with the increase in the difference in parameter values between the surfaces, there was an increase in the leakage value. Meanwhile, the other surface parameters mentioned, i.e., _Sz_, _Sq_, _Sp_, and _Spc_, had an analogous effect on the values obtained in the tightness tests.