Flaky and fullerene-like WS2 nanoparticles as tribologic and toughening additives for epoxy

Just like MoS2, WS2 is known for its outstanding tribologic properties. When used as additives, both were found to considerably improve the tribologic behavior of epoxy, i.e., its coefficient of friction and wear resistance. The best improvements were obtained with WS2 or MoS2 nanoparticles, in particular if they had a fullerene-like morphology. Likewise, fullerene-like WS2 nanoparticles were shown to considerably enhance the fracture toughness of epoxy. It was thus hypothesized that the improved wear resistance could be due to the toughening effect rather than due to reduced friction. Our investigations showed that both flaky and fullerene-like WS2 nanoparticles can improve the fracture toughness of certain epoxy systems, while they can embrittle others. The beneficial effect on the epoxy's wear resistance could not be confirmed either: The coefficient of friction and wear measured in pin-on-disc tests correlated insignificantly with the type or amount of nanoparticles used or the dispersion technique applied. The fact that the fracture toughness did not correlate with the measured wear suggests that the investigated epoxy system wears by adhesion rather than by abrasion. It is thus possible that tribologic additives like WS2 are unsuited for counteracting this wear mechanism. In a nutshell, both the toughening and the wear-reducing effect of flaky and fullerene-like WS2 nanoparticles seem to depend strongly on the particular epoxy system investigated.