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Research Papers: Micro-Nano Tribology

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

[+] Author and Article Information
Suvrat Bhargava

Department of Mechanical, Aerospace,
and Nuclear Engineering,
Rensselaer Polytechnic Institute,
Troy, NY 12180

Thierry A. Blanchet

Department of Mechanical, Aerospace,
and Nuclear Engineering,
Rensselaer Polytechnic Institute,
Troy, NY 12180
e-mail: blanct@rpi.edu

1Corresponding author.

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received June 21, 2015; final manuscript received January 13, 2016; published online July 8, 2016. Assoc. Editor: Zhong Min Jin.

J. Tribol 138(4), 042001 (Jul 08, 2016) (8 pages) Paper No: TRIB-15-1218; doi: 10.1115/1.4032818 History: Received June 21, 2015; Revised January 13, 2016

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.

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Figures

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Fig. 1

Schematic representation of subsurface delamination cracks running parallel to the wear surface in PTFE filled with (a) micrometer-sized filler particles and (b) submicrometer-sized filler particles. Broad contact stresses resultant from normal pressure σ and shear stress τ under frictional conditions μ = tanα at surface are transmitted to parallel subsurface delamination crack faces, thus driving their propagation. Adapted from Blanchet and Kennedy [4].

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Fig. 2

Schematic of the pin-on-plate type tribometer used for measuring the wear rates

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Fig. 3

Wear of PTFE filled with micro- and nano-scale copper particles: (a) volume loss as a function of the sliding distance and (b) steady-state wear rate as a function of filler loading

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Fig. 4

Wear of PTFE filled with micro- and nano-scale silicon nitride particles: (a) volume loss as a function of the sliding distance and (b) steady-state wear rate as a function of filler loading

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Fig. 5

Wear of PTFE filled with micro- and nano-scale γ-phase alumina particles: (a) volume loss as a function of the sliding distance and (b) steady-state wear rate as a function of filler loading

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Fig. 6

Vickers microhardness values for unfilled PTFE and PTFE composites filled with micrometer- and nanometer-sized particles of copper, silicon nitride, and γ-phase alumina

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Fig. 7

Coefficients of friction measured between unfilled PTFE, PTFE filled with micrometer- and nanometer-sized particles of copper, silicon nitride and γ-phase alumina, and polished SS countersurfaces

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Fig. 8

Optical micrograph of wear track and debris formed during the sliding wear of unfilled PTFE over SS 304 countersurface

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Fig. 9

Optical micrographs of wear tracks and debris formed during the sliding wear of PTFE composites of various micro- and nanoscale filler particles and weight fractions formed upon SS 304 countersurfaces

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