# Performance of Polymer Composites Lubricated with Glycerol and Water as Green Lubricants
## 4 Discussion and Conclusions  
In this paper, we have analysed the tribological properties of five polymer composites that are considered high-performance or excellent engineering polymers [53]: PEEK CF30, PEEK MOD, POM CF30, PTFE CF25 and UHMW PE. The polymers were compared based on their tribological performance under glycerol and water-lubricated conditions as two basic sustainable lubricants. The tribological tests with the reciprocating steel-ball on polymer disc tests were performed with parameters simulating the movement of a poppet in hydraulic valves. Following the tribological tests, wear tracks were analysed, which provided a better understanding of the tribological mechanisms involved. In order to understand and compare five very different tested composites, it is possible to analyse different factors that can potentially affect COF or/and wear rates, such as intrinsic material parameters (size and orientation of reinforcement fibres/particles, the  
Figure 8: Counterface wear scar after sliding against (**a**) PEEK CF30; (**b**) PEEK MOD; (**c**) POM CF30; (**d**) PTFE CF25 in water with SEM and EDS images of marked areas. roughness of specimens, etc.) and extrinsic parameters (temperature, pressure, hydrothermal ageing, load, sliding velocity, sliding time) that are part of studies on polymers or lubricants [6; 8; 34; 53]. The results of the tribological tests performed in glycerol prove glycerol's excellent performance as a lubricant, which agrees with recent studies of steel-steel tribo-pairs in glycerol and different glycerol solutions [36; 38; 41; 42; 44]. The coefficient of friction was very low, \(\sim\)0.02 and comparable for all polymers tested. All polymers performed significantly better in glycerol, especially the high-performance PEEK composites and POM CF30. PEEK CF30, PEEK MOD and POM CF30, as polymers with higher and comparable measured microhardness, showed a lower specific wear rate compared to PTFE CF25 and UHMW PE. We assume that glycerol can form an effective lubricating film in this case. In water, under harsh lubrication conditions, the mechanical properties of the individual materials were exposed. Under water-lubricated conditions, both the coefficient of friction and the specific wear rate increased significantly for all polymers tested. The high-performance polymer PEEK CF30 showed the lowest specific wear rate, interestingly followed by the affordable engineering polymer POM CF30. Both showed stable polymer matrices with well-integrated carbon fibres and abrasion as the main wear mechanism. However, POM CF30 showed a lower measured coefficient of friction under water-lubricated conditions, probably due to the transfer film on the ball mating surface. Both PEEK MOD and PTFE CF25 showed a significantly higher specific wear rate in water, indicating cracking in the matrix and adhesion as the predominant wear mechanism. Materials with lower hardness retained a lower coefficient of friction, since both PTFE and UHMW PE are known to be good solid lubricants [20; 47]. However, they could not match the low specific wear rate of stiff high-performance materials reinforced with CF. For harsh lubrication conditions, either a different reinforcement should be considered for PTFE and UHMW PE, or they should be used for different loading conditions. For the three polymers tested with higher measured microhardness, namely PEEK CF30, PEEK MOD and POM CF30, glycerol can be used as a base lubricant to reduce both the coefficient of friction and specific wear compared to water-lubricated conditions. Glycerol was not as effective in reducing the specific wear of UHMW PE and PTFE CF25. Conceptualization, A.T., F.M. and M.K. ; methodology, A.T.; software, A.T.; validation, A.T., formal analysis, A.T.; investigation, A.T. and J.P.; data curation, A.T. and M.K. ; writing--original draft preparation, A.T.; writing--review and editing, N.N and F.M. ; visualisation, AT. and N.N. ; supervision, M.K. All authors have read and agreed to the published version of the manuscript.