## 3 Results and Discussion
Their surfaces had a reduced peak height value (_Spk_), which was lower by more than 50%, indicating a potentially better abrasion resistance. Differences were also found in the core liquid retention (_Sci_), the values of which were better for slide diamond-burnished surfaces. A similar finding was made for void liquid retention (_Svi_). \begin{table}
\begin{tabular}{c c  
Both indicated that slide diamond burnishing resulted in good levelling of the surface irregularities. Autocorrelation functions and the charts plotted for the unidimensional power spectrum density illustrate the distribution of surface irregularity deviations as a function of frequency, and (according to work [37]) suggest that the surface component values were random both in the slide diamond-burnished surfaces and the standard surfaces. An analysis of the results of surface hybrid parameter measurements (Table 5) indicated that the slide diamond-burnished valve stem surfaces had lower values of _Ssc_ and _Sdq_, which indicates their larger rounding radii and lower inclination slopes than those of standard stems. These surface properties resulting from slide diamond burnishing are more favourable for the mating of couples with sliding friction and lubrication. \begin{table}
\begin{tabular}{c c c} \hline Amplitude parameters & 3D parameters & Functional parameters \\ \hline \(S\alpha=0.191\)\(\mu\)m & & \(Sbi=0.158\) \\ \(S\alpha=0.255\)\(\mu\)m & \(Spc=96.6\)\(1/\)mm & \(Sci=1.8\) \\ \(Sp=2.09\)\(\mu\)m & \(Sds=1799\)\(1/\)mm\({}^{2}\) & \(Svi=0.113\) \\ \(Sv=1.03\)\(\mu\)m & \(Str=0.203\) & \(Sk=0.506\)\(\mu\)m \\ \(St=3.12\)\(\mu\)m & \(Sal=0.0666\) mm & \(Spk=0.396\)\(\mu\)m \\ \(Ssk=0.486\) & \(Std=44.5^{\circ}\) & \(Swk=0.255\)\(\mu\)m \\ \(Sku=4.14\) & \(Sfd=2.53\) & \(Sr1=15.4\%\) \\ \(Sz=2.17\)\(\mu\)m & & \(Sr2=88.2\%\) \\ \hline \end{tabular}
\end{table} 
Table 4: Surface texture test results for standard valve stems. The microhardness and stress levels of the slide-diamond-burnished and standard surfaces of the valve stems are listed in Table 6. The sliding diamond burnishing operation was found to increase the superficial microhardness, which should reduce the rate of wear during operation with friction. All the tested stems were found to feature compressive stresses, which, however, were much higher (in absolute value) after slide burnishing. Compressive stresses are important features of the superficial layer and improve abrasion resistance and the mating of coupled parts that are exposed to corrosion-promotingfactors. High compressive stresses inhibit the penetration of corrosive agents through the surface [38] and may reduce the rate of corrosion processes. Hence, the stress state produced by slide diamond burnishing is deemed beneficial. The valve stems analysed in this paper were subjected to durability tests and, in order to better assess the impact of burnishing parameters and tribological surface texture parameters, the durability of the stems burnished with slightly different conditions was also tested (Table 7). These conditions were selected so as to obtain a similar surface roughness Sa and different values of the other texture parameters. From the data presented in Table 7, it can be concluded that there is a clear convergence of the values of surface roughness parameters with the durability of the tested valve stems and that optimization would be useful in future research. The found differences in durability prove that the slide diamond burnishing of the working surface of the valve stems may produce a beneficial condition for the surface layer, in which all parameters, without exception, are conducive to the improvement of tribological properties, which can multiply the MTBF of tested friction couples.