# Wear Effect on the Contact between a Metallic Pin and a Rotating Polymeric Specim
## 3 Results
### Polymeric Sheet Contact Surface  
The UHMWPE specimens undergo progressive damage according to the wearing conditions, which determine the formation of grooves or tracks on their surface, as documented by the rendering in Figure 6, where three different specimens are compared.  
The more severe dry conditions show the effects of the test time: an increment from 60 min (Figure 6a) to 240 min (Figure 6b) gives rise to a deeper groove. The radial width of the wear traces was measured by the confocal microscope, and the mean values were quite similar for the specimens tested for 60 and 240 min in dry conditions (about 1.3 mm). Instead, the wear track under lubrication is less wide (about 1.2 mm) than that obtained in dry conditions with the same test time (Figure 6c).  
The increment of test time from 60 to 240 min in dry conditions gave rise to a higher amount of pile-up (due to the plastic flow of the material under contact with the metallic pin). In wet conditions, the groove was narrow, and the number of pile-ups was lower. However, compared to the respective groove volume, the presence of annular pile-ups is more accentuated after the test in SSF as a percentage (see the evaluation carried out in the Discussion).  
In this regard, the Shore-D hardness measurements in the wear tracks [20] showed a softening after wearing in SSF (the initial value is reduced by about 2 HD in the first 15 min and by only 3 HD in the interval from 15 to 240 min, due to a saturation effect). From this, it can be deduced that the fluid can increase the plasticity of the polymer surface.  
This result agrees with the research by [PERSON] et al. [39], which shows how SFF smooths the surface of the material, spreading inside the polymer and decreasing its surface hardness. Diffusion is favored by rising temperatures. The friction of the metal tip punctually generates heat in the contact area, where the plasticizing effect is further favored.  
The results of optical observations on the polymeric surface after the pin-on-disc trials are shown in Figure 7. The first two micrographs are characterized by several annular grooves with debris mainly inside them, as single particles or in clusters. Moreover, in the case of (b), scratches are present, probably due to the metal debris acting as micro-cutting tools. In the last micrograph, the number of grooves is lower, and polymeric material flows are visible.  
Figure 6: Images obtained with the confocal microscope: (**a**) dry conditions after 60 min: (**b**) dry conditions after 240 min, (**c**) lubrication with SSF after 60 min.  
Figure 7: Optical micrographs of the specimen surface: (**a**) dry condition after 60 min, (**b**) dry conditions after 240 min, (**c**) lubrication with SSF after 60 min.  
The lubricating film is fundamental for reducing the asperities interaction between the surfaces in contact and improving wear behavior. In fact, the 3D images obtained with the confocal microscope (Figure 6) revealed that, in dry conditions, the height of the pile-ups is higher than under lubrication. Moreover, in SSF, the wear track is narrower than in dry conditions. Finally, in the case of lubrication, the reduced presence of debris (Figure 7c) demonstrates a prevalence of adhesive mechanisms.  
The adhesive wear mechanism arises if the strength of the bonds at the contact between the metallic and the polymeric surface exceeds the cohesion of the softer material [40]. In this case, a flow of the softer material occurs. An adhered layer is formed, as shown in Figure 7, and documented by various authors for tribocouples of materials characterized by a high difference in hardness values [41; 42].