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Properties and Applications of Graphite-Filled Polymer Composites Lubrication
Most of the polymers have typically high friction coefficients that produce high wear at high loading and/or high sliding velocity. The total tribological stress (PV) is defined by multiplying the pressure (P) by the sliding speed (V). Above the so-called PV limit, the wear and/or friction coefficient drastically increases and the polymer cannot be used under these conditions. The PV limit can be increased by improving the mechanical strength (resistance to deformation) and thermal conductivity (reduction of surface temperature) but most often by decreasing the friction coefficient (reduction of frictional heating). Graphite powders are very well-known fillers used since many decades as solid lubricant for self-lubricating polymers, either alone or in combination with other fillers like PTFE, silicon, or molybdenum disulfide powders (Xian and Zhang 2005). For applications where the plastic piece is subject to severe dynamic conditions (e.g., water meter valves, bearings, gears, bushings, and rollers), graphite is used up to 30% loading in order to reduce the coefficient of friction and wear of polymer composites. Besides graphite crystallinity and granulometry, the most important property is purity, since contamination with hard material such as silicon carbide or silicates can be very detrimental for the final application. The tribological properties of polymer compounds can be evaluated by different methods like thrust washer test (ASTM D3702) and block-on-ring test (ASTM G137). The block-on-ring test determines sliding friction and wear by pressing a plastic block against the outer circumference of a rotating ring. By varying the pressure (P) and/or velocity (V), it is possible to determine the PV limit. In order to evaluate the effect of
Fig. 7 Friction coefficient and wear rate as a function of tribological stress for virgin PS and graphite-filled PS compound
graphite on the tribological properties of polystyrene, PS compounds with 20% of primary synthetic graphite (TIMREX KS44) have been produced by twin-screw extruder. Test specimen of both virgin PS and PS-KS44 compounds has been prepared by injection molding. The sample has been then positioned on the counter-body ring made from ground (Ra = 0.1-0.2 pm) and hardened (60 HRc) bearing steel (100Cr6, 1.3505). The graphite particles are aligned during injection molding and are oriented normally to the ring. Tests have been performed on Atlas TriboTester (Tribologic GmbH, Germany) at fixed pressure (0.5 MPa) and varying velocity (0.0625-0.5 m/s). As shown in Fig. 7, the friction coefficient of virgin PS is very high at low tribological stress (0.75 at 0.03 MPa.m/s) and decreases to lower values at higher stress (0.5 at 0.25 MPa.m/s). On the other hand, the wear rate strongly increases above ca. 0.1 MPa.m/s (PV limit). With the addition of graphite, the tribological performance is drastically changed. The friction coefficient is much lower and quite stable (0.25-0.30). The wear rate is maintained low and stable (40-120 10E-6 mm3/Nm) over the tested tribological stress range. These data clearly indicate that graphite works well as a solid lubricant and can significantly increase the PV limit of thermoplastic polymers like PS. Besides polystyrene compounds, graphites are used in polyester fiberglass resin systems (SMC/BMC) that are known to be extremely abrasive for the counter-body (Wypych 2009). The ratio of graphite to glass fibers must be optimized to achieve the desired reduction in wear and friction coefficient. Graphitic carbons are also commonly used to improve wear resistance and deformation strength (creep) of polytetrafluoroethylene (PTFE)
(Kandanur et al. 2012), allowing the use of this polymer for applications like seals, O-rings, and valve housing. Filled PTFE is often not as strong as virgin PTFE, whereas the low friction coefficient is usually maintained due to the formation of a thin PTFE film at the interface between body and counter-body. Because of its low coefficient of friction, graphite can be used without drastically modifying the coefficient of friction of PTFE.
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