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Research Papers: Friction & Wear

# Study on the Transfer Film Layer in Sliding Contact Between Polymer Composites and Steel Disks Using Nanoindentation

[+] Author and Article Information
Li Chang, Lin Ye

School of Aerospace, Mechanical
and Mechatronic Engineering,
The University of Sydney,
Sydney, NSW 2006, Australia

Klaus Friedrich

Institute for Composite Materials,
University of Kaiserslautern,
67663 Kaiserslautern, Germany
CEREM,
College of Engineering,
King Saud University,

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received January 26, 2013; final manuscript received November 18, 2013; published online December 27, 2013. Assoc. Editor: Prof. C. Fred Higgs III.

J. Tribol 136(2), 021602 (Dec 27, 2013) (12 pages) Paper No: TRIB-13-1031; doi: 10.1115/1.4026174 History: Received January 26, 2013; Revised November 18, 2013

## Abstract

In the present work, nanoindentation experiments were carried out to characterize the localized transfer film layer (TFL) on a steel disk, which resulted from a sliding contact of the latter against a polymer composite pin. It was found that the hybrid nanocomposites filled with both nanoparticles and traditional tribo-fillers were more effective to form durable TFLs on the steel counterpart, associated with desirable tribological properties of the sliding system, i.e., a low friction coefficient and a low wear rate. By studying the load-displacement behavior of polymeric TFLs on metallic substrates, the thickness of TFLs could be estimated, thus, allowing the comparison of TFLs formed under different sliding conditions in a quantitative way. Based on the experimental data, the effects of TFLs on the tribological performance of polymer composites were further discussed in terms of a “transfer film efficiency factor” λ, which was calculated by the ratio of the average thickness of the TFL to the surface roughness of the steel counterpart. The factor mainly considered the relative contributions of the TFL and the metallic counterface to the wear process of the polymer-on-metal system. Accordingly, the wear rate and the friction coefficient of the sliding system could be analyzed as a function of the transfer film efficiency factor, resulting in a Stribeck type diagram. The analyses provided new insight into the role of TFLs in polymer tribology.

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## Figures

Fig. 1

Schematic illustration of an indentation in a soft film on a hard substrate

Fig. 12

Variation in thickness of TFLs (over a total length of 395 μm, see Fig. 10) formed by the PA 66 based composite with and without additional nanoparticles under different loads at a constant speed of 1 m/s (a); a similar variation achieved for different speeds at a constant load of 2 MPa (b)

Fig. 11

(a) Variation in the mean values of hardness of TFL measured by different numbers of indentation tests (for all the tests, the distance between the indents was kept constant at 5 μm, see Fig. 10). (b) Variation in the hardness of TFL measured by 80 repeated indentation tests, which were carried out on three different locations of the steel disk (the average values of the hardness for three measurements were 4.70, 4.92, and 5.24 GPa, respectively). Pin: PA 66 based composite with additional nanoparticles; Sliding condition: 2 MPa, 2 m/s.

Fig. 10

Optical observation of the TFL on the steel counterpart with the indentation instrument. The inserted line shows schematically the repeated indention tests, which were carried out to “scan” the TFL with a certain length.

Fig. 9

AFM images (error signal) of a 3 × 3 indentation array on the steel ring after testing against SCF + Gr/PA 66 under 2 MPa and 2 m/s (a), and (b) shows the cross-sectional measurement of the indent 4#; (c) gives the corresponding load-displacement curves of nine nanoindentations

Fig. 8

AFM images (error signal) of a 3 × 3 indentation array on the steel ring after testing against nano-TiO2 + SCF + Gr/PA 66 under 2 MPa and 2 m/s (a) and (b) shows the cross-sectional measurements of the indents 6# and 7#; (c) gives the corresponding load-displacement curves of nine nanoindentations

Fig. 7

Fig. 6

The surface structure of the original steel counterpart examined by (a) SEM (left of white line: some loose wear debris, next to the wear track (right of white line), as indicated by TFL) and (b) by white-light interferometric profilometry

Fig. 5

Comparisons of the worn surfaces and counterfaces for the composite of nano-TiO2/SCF/Gr/PA 66 (i.e., traditional and nano-fillers) tested under different sliding conditions: (a) the worn polymer surface and (b) the surface profile of the corresponding steel counterface examined by SEM and (c) by white-light interferometric profilometry (testing conditions: 1 MPa, 1 m/s). The other figures refer to (d) the worn polymer surface and (e) the surface profile of the corresponding steel counterface examined by SEM and (f) by white-light interferometric profilometry (testing conditions: 2 MPa, 2 m/s). CWD = compacted wear debris. The sliding directions were indicated by the arrows.

Fig. 4

Comparisons of the worn surfaces and counterfaces for the composite of SCF/Gr/PA 66 (i.e., traditional fillers only) tested under different sliding conditions: (a) the worn polymer surface and (b) the surface profile of the corresponding steel counterface examined by SEM and (c) by white-light interferometric profilometry (testing conditions: 1 MPa, 1 m/s). The other figures refer to (d) the worn polymer surface and (e) the surface profile of the corresponding steel counterface examined by SEM and (f) by white-light interferometric profilometry (testing conditions: 2 MPa, 2 m/s). CWD = compacted wear debris. The sliding directions were indicated by the arrows.

Fig. 3

Comparison of friction coefficient and temperature development of two PA 66-based composites filled with and without additional nanoparticles. The specific wear rates of the two composites are 0.63 × 10−6 mm3/Nm and 4.88 × 10−6 mm3/Nm, respectively. Testing conditions: p = 2 MPa, v = 2 m/s.

Fig. 2

Wear and friction results of four polymeric composites and of two polymer matrices tested under different sliding conditions. In cases where no error bars are visible, the scatter is smaller than the size of the symbols used in the diagram.

Fig. 13

Specific wear rate and friction coefficient as a function of the transfer film efficiency factor λ as defined by Eq. (6)

Fig. 14

Schematic illustration of the contact modes for the sliding wear of short-fiber-reinforced polymer composites against metallic counterparts covered with TFLs

Fig. 15

(a) Thickness distribution of the TFL developed by a PA 66-based nanocomposite and (b) its worn surface. Sliding conditions: 8 MPa, 1 m/s, which refers to the data in regime III, as shown in Fig. 13. CWD = compacted wear debris.

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