Under conditions of dry sliding against polished stainless steel, the steady-state wear rates of composites of polytetrafluoroethylene (PTFE) filled with graphene platelets with typical platelet thickness varying between 1.25 and 60 nm were measured. The fraction of the graphene platelets was varied between 0.02 and 30 wt%, also including 0 % unfilled PTFE. With 4 % loading of the 1.25 and 1.6 nm thick graphene platelets, the measured steady-state wear rates approached *4 9 10 -7 mm 3 /Nm levels which are roughly three orders of magnitude lower than that measured for unfilled PTFE, decreasing further to 10 -7 mm 3 /Nm at 10 % loading. In addition, among all the tested graphene platelets, the thinnest graphene platelets already imparted considerable wear resistance even at a low 0.32 % loading. The thicker graphene platelets also started providing some slight resistance to sliding wear but not until a greater filler loading of 1.1 %. For a given graphene platelet, there appears to be a lowest achievable steady-state wear rate, while the filler loading is gradually increased. For the 8 nm thick graphene platelets, this minimum was found to be about 3 9 10 -7 mm 3 /Nm at a filler loading of about 20 %. When the wear rates are plotted as a function of the filler loading on log-log axes, for each of the graphene platelets, the wear rates are found to decrease linearly beyond a threshold filler loading up to at least 10 % filler loading. Wear rates corresponding to each type of graphene platelet fall on its own line on such a plot. However, when the wear rates are instead plotted as a function of the filler surface area available per unit mass of the composite, the data (with the exception of the thickest 60 nm platelets) collapse around a master line with slope of about -1.73.
Wear rates of polytetrafluoroethylene (PTFE) filled with micrometer- and nanometer-sized particles of copper, silicon nitride, and γ-phase alumina were measured under dry sliding conditions using a pin-on-plate tribometer. In their ability to limit the wear rate, micrometer-sized copper particles were found to be better than their nanometer-sized counterparts, though by only small margins, with a 20 wt.% loading of the micrometer-sized copper particles resulting in a tenfold reduction in the wear rate over that of unfilled PTFE. With 10 wt.% loading of micrometer-sized particles of silicon nitride and γ-phase alumina, very low wear rates of ∼5 × 10−7 mm3/N·m and ∼2.5 × 10−7 mm3/N·m, respectively, were measured. Wear rate of unfilled PTFE under the same testing conditions, also measured here, was found to be about 3.6 × 10−4 mm3/N·m. In all the three cases (copper, silicon nitride, and γ-phase alumina), wear resistance was either lost fractionally or completely when the size of the filler particles was reduced from the microscale to a few tens of nanometers, with nanoscale silicon nitride filler resulting in even slightly higher wear rates and larger platelike wear debris than unfilled PTFE. Micrographs of the wear tracks and the generated wear debris seem to indicate that all three filler materials in the form of more effective larger microparticles reduce wear by a common mechanism of interrupting wear debris production and limiting wear debris size, further supporting Tanaka and Kawakami's 1982 proposal of a broad general mechanism of PTFE wear reduction by filler particles having at least a requisite microscale size. Recent reports of extreme PTFE wear resistance imparted by few limited nanofiller particles appear to be reflective of an additional wear reduction mechanism they may specifically possess, rather than a contradiction of previously proposed microparticle wear reduction mechanisms.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.