## 3 Results and Discussion  
The test results of the surface texture (in the form of contour line maps, surface photographs, 3D surface views, curves of material shares and load-bearing capacities, distributions of localised elevations, autocorrelation functions, power spectrum density distributions, and standard and non-standard parameter values of the surfaces) of stems after both finishing methods are listed in Tables 3 and 4. The designations of the parameters complied with EN ISO 4287:1999/A1:2010. Imaging (contour line maps, surface photography, and 3D surface views) demonstrated that the surfaces of the slide diamond-burnished specimens were more homogeneous than those of the standard stems. They also featured fewer irregularities and a lower \(Sa\) value. It could be seen that the slide diamond-burnished surfaces featured symptoms of random isotropy, as confirmed by the appearance of the autocorrelation functions with a rather defined peak and the isotropy level value of \(Str\) = 0.545. The density of the irregularity peaks was higher in the surfaces finished by slide diamond burnishing (\(Sds\) = 2225 1/mm\({}^{2}\)). It is known that, in the initial operating stage of the mating machine and equipment parts,  
Figure 3: Control valve stem test rig: 1—tested control valve, 2—pressure medium supply hose for the control valve, 3—pressure medium outlet hose for the control valve, \(\downarrow\)\(\uparrow\)—valve stem’s direction of motion. localised elevations of the surface have an unfavourable effect on running in. The slide diamond-burnished surfaces featured a distribution of surface irregularity peaks with a coefficient of concentration (_Sku_) of 158 and were dominated by elevations of approximately 7.5 \(\upmu\)m, whereas the standard surfaces had a coefficient of concentration of 4.14 and were dominated by elevations of 1.2-1.4 \(\upmu\)m. The arithmetic mean curvature of the peaks of the roughness (_Spc_) of the surface of burnished stems is about three times greater than that of standard stems. Comparing the values of roughness parameters (especially _Str_, _Sds_, and _Sku_), significant changes were found after sliding burnishing. Changes such as these are believed to have a beneficial effect on the tribological properties of frictionally cooperating machine parts [17; 18; 19; 22; 23]. \begin{table}
\begin{tabular}{c c c} \hline \hline Amplitude parameters & 3D parameters & Functional parameters \\ \hline \(Sa=0.12\)\(\upmu\)m & & _Shi_ = 0.193 \\ \(Sq=0.189\)\(\upmu\)m & \(Spc=288\)\(1/\)mm & _Sci_ = 1.2 \\ \(Sp=1.23\)\(\upmu\)m & \(Sds=2225\)\(1/\)mm\({}^{2}\) & _Spi_ = 0.121 \\ \(Sv=7.44\)\(\upmu\)m & \(Str=0.545\) & _Sk_ = 0.347 \(\upmu\)m \\ \(St=8.67\)\(\upmu\)m & \(Sal=0.115\) mm & _Spk_ = 0.159 \(\upmu\)m \\ \(Ssk=-7.17\) & \(Std=45^{\circ}\) & _Swk_ = 0.239 \(\upmu\)m \\ \(Sku=158\) & _Sfd_ = 2.12 & _Sr1_ = 10.2\% \\ \(Sz=4.58\)\(\upmu\)m & & _Sr2_ = 88.4\% \\ \hline Surface and volumetric parameters & Hybrid parameters \\ \hline \(STp=53.2\%\) & & _Sdq_ = 0.0213 \\ \(Smmtr=0.00744\) mm\({}^{3}/\)mm\({}^{2}\) & & _Ssc_ = 0.00444 \(1/\upmu\)m \\ \(Smmtr=0.00123\) mm\({}^{3}/\)mm\({}^{2}\) & & _Sdr_ = 0.0226\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Surface geometrical structure test results for slide diamond burnished valve stems. The distribution of surface ordinates in the slide diamond-burnished valve stems, which represented the changes in the material-to-void ratio as a function of height, corresponded to the surfaces with negative skewness (_Ssk_ = \(-7.17\)), which were surfaces with planar-peak elevations. A comparison between the coefficients of material volume (_Smmr_) and void volume (_Smvr_) showed that the slide diamond-burnished surfaces had a higher mean material volume with a lower void volume than those of the surfaces of the standard valve stems. The cumulative distribution of the elevation ordinates (or the load-bearing curve) clearly showed a better trend for the valve stems finished by slide diamond burnishing.