## 3 Research Results and Discussion
### Microscopic Observations with the EDS Analysis of the Chemical Composition of Wear Products  
Following the completion of the macroscopic observations, microscopic observations of the shaft surfaces were conducted, in particular at the place of changes caused by fretting wear. Sample scanning images of shaft axle seat surfaces after the forcing process and at the place of fretting wear are presented in Figure 14.  
Figure 12: Shaft wear intensity.  
Figure 13: Microscopic image of the gap between the shaft and sleeve.  
The observations of the shaft axle seat surfaces confirmed the results of the surface roughness tests. Mainly surface smoothing caused by the sleeve forcing process is visible. It is also noted that microholes are filled with moving wear products (Figure 14a). Both the smoothed surface and the wear products in microholes create the contact area between the mating elements bigger. In turn, this may promote the formation of adhesive bonds, initiating fretting wear.  
Those we \(\backslash\)ar products which did not fill the microholes but migrate during assembly or wear tests stick to the surface and create material build-ups (Figure 14c). Those build-ups plasticise under the influence of cyclic tangential displacements between the mating surfaces, which can be seen in Figure 14b.  
Migrating wear product particles not only cause the kinds of damage mentioned above but also contribute to the formation of pits and surface abrasion in places. The initial surface roughness of the elements joined together is of significant importance in this case. Noticeable are also surface microcracks (Figure 14d) which become larger with the number of wear cycles and move deeper into the material. Thus, these microcracks contribute to the initiation and development of fatigue wear, which causes shaft fracture.  
The surface of the shafts at which wear products are located was subjected to the analysis of the chemical composition by means of the EDS technique. The results of this analysis are shown in Figure 15.  
Figure 14: Scanning image of the shaft axle seat surface (**a**) after the forcing process; (**b–d**) at the place where fretting wear occurs.  
Figure 15: Results of the analysis of the chemical composition of wear products on the shaft surfaces.  
The analysis also demonstrated the presence of iron and oxygen atoms. This confirms wear product oxidation, which would explain the grey and dark brown colour of the shaft surface in the fitting wear zone, as noted during the macroscopic observations. As a result of wear product oxidation, there forms a liquid in the form of a powder, that being iron trioxide, Fe\({}_{2}\)O\({}_{3}\). This is distinguished by lower density and, thus, by greater volume.  
Oxdised wear products are distinguished by much higher hardness compared to that of the shaft and sleeve surface. Thus, wear products migrating along the joint are responsible for the rise of damage such as scratches, plastic deformation, and surface abrasion.  
It follows from the observations that only those fragments of the shaft surface oxidised at which damage in the form of build-ups and pits has arisen and at which loose wear products have formed.