# Performance of Polymer Composites Lubricated with Glycerol and Water as Green Lubricants  
[PERSON]  
1 Faculty of Mechanical Engineering, University of Ljubljana, Askerveva cesta 6, 1000 Ljubljana, Slovenia;
makes it attractive for various demanding industrial applications [9]. It also has good thermal and chemical stability and excellent tribological properties [10]. These properties can even be improved when reinforced with carbon fibres CF in seawater [11; 12; 13] and in distilled water when reinforced with CF and PTFE [14]. However, additives in the form of glass fibres (GF) and bronze powder could not improve the effect of the reinforcement CF and the PTFE lubricant additive [13]. Although tested under various conditions, PEEK and its composites showed a low COF of 0.15 down to 0.03 and a specific wear rate of up to 1.66 \(\times\) 10\({}^{-8}\) mm\({}^{3}\)/(Nm) in water [14; 15; 16]. PTFE also has a very low coefficient of friction of below 0.05 and high chemical resistance and temperature stability [17] but a higher average wear rate of 4 \(\times\) 10\({}^{-5}\) mm\({}^{3}\)/(Nm) in distilled and seawater [18]. This can be improved by reinforcing the PTFE with CF, which can reduce the wear rate to 0.2 \(\times\) 10\({}^{-5}\) mm\({}^{3}\)/(Nm) under dry conditions and even more in a water bath [19]. UHMW PE was recently studied and showed good tribological properties combined with thermal and chemical stability and corrosion resistance, especially in aquatic applications, even without various fillers typically used, due to its good self-lubricating properties at temperatures up to 80 \({}^{\circ}\)C, with coefficient of friction of 0.12 and specific wear rate down 1.45 \(\times\) 10\({}^{-6}\) mm\({}^{3}\)/(Nm) [20; 21; 22; 23; 24]. Polyoxymethylene (POM) is another commercially and mechanically acceptable substitute for the high-performance thermoplastics mentioned above. It is generally very stiff, has good impact strength and shows a coefficient of friction of less than 0.1 and a low wear rate, especially in water [25], and can be further improved when reinforced with various fillers (PTFE fibres or particles [26; 27], hexagonal boron nitride nanoparticles (h- BN) [28], especially for higher loads). There are already many different applications for the above polymers, such as gear pumps [28], cylinders, pistons, piston rods and sealings [29], hydropower bearings [2], gears [30] etc. The main requirements for modern environmentally acceptable lubricants (EALs) are to be both biodegradable and non-toxic under various operating conditions. Water alone is known as a non-flammable alternative with a strong cooling effect and is used in experiments as tap water, demineralised water or seawater. However, due to the corrosion sensitivity of the material and the almost 100 times lower kinematic viscosity of water compared to mineral oils, water affects the cost of the components involved [31]. It is usually mixed with various additives (wear protection, oxidation protection, friction modifiers) [32; 33]. A similar approach to improving the physical and chemical lubricant properties also applies to commonly used natural oils, such as rapeseed, soybean, sunflower or castor oil [34; 35]. Glycerol is another example of a non-toxic, biocompatible lubricant that is a major by-product of biodiesel production. Although it is the most abundant polyhydric alcohol in nature, it has often been neglected [36; 37]. Unlike many conventional hydraulic fluids, which are based on mineral oils or synthetic compounds, glycerol is biodegradable and can be degraded by natural processes in the environment. This property reduces the risk of environmental pollution from leaks or spills in the hydraulic system. It can potentially produce a very low coefficient of friction, low wear and reduced corrosion, although it has a very high viscosity compared to conventional mineral base oils [36].