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Friction 3(4): 259–265 (2015) 263 Raman spectroscopy analyses (Fig. 8) performed on the observed tribo-layers revealed D (~1,360 cm−1) and G (~1,580 cm−1) bands, which are common to graphite- based structures [20, 21]. The low band intensities obtained from the samples, tested at 30 °C, indicate that the tribo-layer formed under these test conditions is thinner than the one formed at higher temperatures. This behavior is in agreement with Sheiretov et al. [2] who reported that higher temperatures enhance the formation of uniform tribo-layers. Furthermore, the higher intensity ratio between the D and G bands (ID/IG) from samples tested under atmospheric air at 80 °C (0.97 vs. 0.70 from tetrafluoroethane at 80 °C) indicates greater disorder in its graphite structure [20, 21]. The origin of these graphite-based tribo-layers remains unclear because it can be attributed to different carbon sources, such as (i) graphite fillers from polymer compositions; (ii) degraded polymers (PEEK and PTFE); and (iii) degraded carbon fibers. Figure 9(a) shows the evolution of the friction coefficient during the sliding tests, whereas Fig. 9(b) shows their average steady-state values. Three tests were performed for each condition, and an average friction coefficient was calculated from each average value within the steady-state regime. During the running-in regime, samples tested in atmospheric air exhibited friction coefficients 25% higher than those of samples tested in the tetrafluoroethane atmosphere. However, once the steady-state regime was established, the samples tested in atmospheric air and at high temperatures exhibited the lowest friction coefficients, with values of approximately 0.34. At low temperatures, the friction coefficients slightly changed from running-in to steady-state regimes, and the samples tested under the tetrafluoroethane atmosphere showed no significant temperature depen- dence, with steady state friction coefficients varying around 0.38. The observed friction behavior results from dissipated energy, attributed to the adhesive interfacial forces and plowing process on the polymeric and counter body surfaces. The adhesive interfacial forces are often affected by the formation of tribo-layers, the establishment of which has long been recognized as the reason for the gradual transition from the running- in to steady-state regime [13, 14, 17]. According to Yen et al. [22], “the presence of vapors, such as water, is required for graphite to lubricate.” Thus, the water vapor present in atmospheric air reduces the bonding energy between the hexagonal planes of the graphite structures present in the formed graphite-based tribo-layer. This behavior agrees with Wear track Raman spectra. Raman spectrum of virgin AISI 304 stainless steel is also shown. Fig. 8 Fig. 9 Friction behaviour (a) and steady state friction coefficients (b) of PEEK composites sliding against AISI 304 stainless steel discs.PDF Image | Effect of temperature and atmosphere on the tribological behavior of a polyether ether ketone composite
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