## 3 Research Results and Discussion
### Roughness Measurement  
Surface roughness is one of the significant factors determining forced-in joint quality. Surface roughness considerably influences the values of the force needed for joint assembly and disassembly, as it was explained in detail in the previous point. As part of this article, the basic parameters of the roughness of the shaft axle seat and sleeve hub were measured after each completed fatigue cycle. The change in the Ra and Rz roughness parameters after the successive fatigue cycles are presented in Figure 8. Figure 9 shows the sample roughness profiles recorded on the shaft surface. Figure 8: Shaft and sleeve roughness parameters. Figure 9: Roughness profiles recorded on the shaft surface. The results of Ra and Rz roughness parameter measurements on the shaft axle seat and sleeve hub indicate an increase in the value of those parameters along with the increase in the number of fatigue cycles in comparison to roughness before the first pressing process. It should be noted, however, that the measurements performed after the third fatigue cycle, that is, after 1,000,000 cycles, indicate that the values of the roughness parameters decreased compared to the value of those parameters after each previously completed fatigue cycle. The registration of the shaft surface roughness profile demonstrated that the profile is diversified depending on the number of fatigue cycles. After the first fatigue cycle, it was noted that the surface was smoothed and the microholes filled with wear products. After the next fatigue cycle (100,000 cycles), the shaft surface profile changed its shape significantly. The change is due to wear products, which move along the joint and create microbuild-ups and, occasionally, adhesive bonds. The roughness profile image, such as the one presented in the figure for 100,000 cycles, confirms fretting wear development. The shaft surface profile after the third fatigue cycle, at 1,000,000 cycles, suggests fully developed fretting wear. That wear comprises, for example, plastic deformations on the shaft surface, chippings, and numerous build-ups of the material. The shaft surface profile after the end of the third fatigue cycle is presented in Figure 10, and the sample values of roughness parameters measured in accordance with ISO 25178 are also shown there. Both in the profile diagram and in the 3D image, a change in the surface structure as a result of fretting wear development can be noted. Contrary to the surface affected by fretting wear, uniform traces of the finish treatment and randomly spaced scratches resulting from the joint assembly process can be seen on the surface not affected by that wear. Part of the shaft surface, which is affected by fretting wear, has a diversified structure at which alternating single summit and cavity peaks can be seen. This confirms the formation of material build-ups on the shaft surface on the one hand and the presence of cavities on the other. Figure 10: Shaft surface profile after wear tests. During the analysis of the parameter values for the surface not affected by wear, it may be noticed that the maximum surface height Sz is 9.416 \(\upmu\)m, and the arithmetic mean surface height Sa = 1.021 \(\upmu\)m.  
The distribution of the data for the local summits is the normal distribution with high kurtosis Sku = 8.93 and insignificant negative skewness Ssk = 0.816, which points out to the surface with a plateau shape. The surface not affected by wear has a random isotropic structure. In the case of the surface affected by fretting wear, all the amplitude parameters even doubled in value compared to the unaffected surface. The Sa parameter, whose decrease is noted, is an exception. That surface is also distinguished by a random isotropic structure. The parameters pertaining to the distribution of local summit peak levels indicate the surface with a pointed shape.