# Performance of Polymer Composites Lubricated with Glycerol and Water as Green Lubricants
## 3 Results
### Digital Microscopy and SEM  
To be able to compare the wear tracks of the polymers in glycerol and water, typical wear scars are shown in Figure 7a-e. As can be seen from the figure, the wear scar is narrow and difficult to be detected for all polymers when lubricated with glycerol, except for the polymer PTFE CF25. The first three polymers (Figure 7a-c) have a comparable wear scar width, while PTFE CF25 and UHMW PE have a wear scar up to 2.8 wider. In water, the intensity and width of the wear scars increased for all polymers. However, the smallest change in wear scar width was observed for PEEK CF30, followed by POM CF30, which is consistent with the measured specific wear rate (Figure 6). The greatest change in wear scar dimensions when lubricated with water was observed for PEEK MOD. Although scratches in the sliding direction were present in glycerol, the exact edge of the wear scar was difficult to identify with the digital microscope. Based on the measurements (Figures 5 and 6) and the observed wear scars (Figure 7), glycerol was able to form a film thick enough to divide the contact areas and protect the polymers, especially PEEK CF30, PEEK MOD and POM CF30, from severe wear. In contrast, the wear scar in water is clearly defined for all polymers with clearly defined edges, but also with a distinct central part of the wear scar. In the central part of the wear scar, where the sliding velocity and the contact area are most constant, there are stronger scratches, especially for PEEK CF30, PEEK MOD and UHMW PE (Figure 7a,b,e). To get a better insight into the wear mechanism, SEM images were taken at the edge of the wear scar and in the central part of the wear scar (Figure 7a-e). At PEEK CF30, the edge of the wear scar is clearly defined and shows adherent material. There are scratches in the direction of sliding and carbon fibres exposed at the surface but well integrated into the PEEK matrix. As we move to the central part of the wear scar, the scratches become more intense and denser, indicating an abrasive wear mechanism. Although the carbon fibres are fine-defined, they are well integrated into the PEEK matrix, indicating that they are capable of bearing the load. This is consistent with the lowest increase in the specific wear rate of PEEK CF30 in water, compared to the glycerol regime, but also with the lowest specific wear rate in the water among the polymers tested. Figure 6: Specific wear rate of tested polymers in water and glycerol. Figure 7: Wear scar of tested polymers (**a**) PEEK CF30; (**b**) PEEK MOD; (**c**) POM CF30; (**d**) PTFE CF25; (**e**) UHMW PE in water and glycerol with accompanied SEM images of wear scar edge (inside wear scar on the left, outside of wear scar on the right) and central part of polymer wear scar in water. In the case of PEEK MOD, although the edge of the wear scar is pronounced, the scratches are not as intense as in PEEK CF, and the surface of the scar is smoother, probably due to the presence of PTFE and graphite, known as good solid lubricants. There is also evidence of destruction of the PEEK matrix, which means that adhesive wear is present. In the central part of the wear scar, there are signs of severe destruction of the matrix, accompanied by strong scratches in the sliding direction. The fibres are broken and partially pulled out of the material. PTFE and graphite incorporated as reinforcement in PEEK MOD are well-known solid lubricants and are, therefore, often added to high-permanence PEEK to improve tribological properties, especially under dry sliding conditions [13; 17; 46; 47; 48; 49]. In our experiments, however, PEEK MOD could not follow PEEK or POM, which were reinforced with CF, especially under harsh conditions in water. The images of SEM show that the PEEK matrix in PEEK MOD does not have good interfacial adhesion of CF (as in the stable PEEK CF30) and that there is evidence of severe damage to the graphite sheets.