Unusually Effective Nanofiller a Contradiction of Microfiller-Specific Mechanisms of PTFE Composite Wear Resistance?

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.