## 4 Discussion  
For the wet sample (S_W), the average value of the highest profile height (parameter Rz) was closer to the lower limit of the range (0.27 \(\upmu\)m versus 0.51 \(\upmu\)m). The Max\(-\)Min difference (range width) was 0.78 \(\upmu\)m. For the dry (S_D) surface, the average value of the Rz parameter was located near the middle of the range. The entire width of the range of values obtained was 0.50 \(\upmu\)m, and the average was below the center of the range (0.02 \(\upmu\)m). The surface after blowing with compressed air (S_CA) similarly showed a tendency of the mean to the middle of the range (0.025 \(\upmu\)m below), with the width of the range being greater than that of sample S_D but less than that of sample S_W (0.50 \(<\) 0.69 \(<\) 0.78). The surface after blowing and bathing in isopropyl alcohol showed the lowest values of the Rz parameter. The range of values was close to the S_D sample (0.51 \(\upmu\)m for S_CA + I with 0.50 \(\upmu\)m S_D). The average value was closest to the center of the range (0.015 \(\upmu\)m below the center). The width of the results obtained for the surface after bathing in acetone was 0.99 \(\upmu\)m. This was the widest range among all the results. The average value of Rz = 2.50 \(\upmu\)m was 0.115 \(\upmu\)m below the center of the range (Figure 7 top). The total height of the profile of Rt for the entire measurement section of the S_W sample was 3.39 \(\upmu\)m, with a range of variation of 1.43 \(\upmu\)m. The mean was the only one above the middle of the range (0.155 \(\upmu\)m). Sample S_D had a mean of 3.4 \(\upmu\)m, with a range width of 1.52 \(\upmu\)m. The mean was located 0.09 \(\upmu\)m below the center of the range. The post-blow surface of S_CA had the widest range of variation, with a width equal to 2.14 \(\upmu\)m. In turn, its average value of the Rt parameter was also located as the second closest value to the center of the range (below the center by 0.04 \(\upmu\)m). The surface after blowing and bathing with isopropyl alcohol had the narrowest area of variation (1.27 \(\upmu\)m), and the position of the mean was closest to the center of the range (below by 0.015 \(\upmu\)m). The surface after bathing in acetone had the second widest range of variation of the Rt parameter (1.83 \(\upmu\)m) and was significantly different from the other samples, and the mean shifted towards the minimum (relative to the center of the range, it was as much as 0.345 \(\upmu\)m--Figure 7 bottom). This is due to the random occurrence of peaks along the measurement trace--an example is shown in Figure 8--which is caused by the reaction of acetone on the measuring tip or surface. After examining the surface under the microscope, no physical cause (dirt, abrasive residue, etc.) was found to introduce such a deviation. Figure 8: One of the measurement traces for sample S_CA + A with visible peak after the 5 th mm (**top**) and microscopic view of the measured surface. The most common method of sample preparation for measurements--the use of compressed air--gives the widest range of results for Rt (spread over 2 \(\upmu\)m) and quite a similar range to the wet surface for the Rz parameter. After observation under the microscope, it is clear that lubricant fluid residues are still present on the surface, but they are dispersed around the whole area. A comparison between wet and blown surfaces is shown in Figure 9. It should be mentioned that it is also possible, depending on the quality of the compressed air installation, that additional fractions of oil or water may be introduced to the measured surface. The narrowest range of results for Rz was obtained for the dry sample, and the second for isopropyl-cleaned samples. For Rt (also for Ra = 0.062 \(\upmu\)m), the narrowest range was observed for the S_CA + I (isopropyl cleaned) sample. This method gave the lowest values for all three roughness parameters.