Effects of Surface Roughness on Sliding Friction in Lubricated-Point Contacts: Experimental and Numerical Studies

[+] Author and Article Information
Shun Wang1

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, Chinawangshun04@mails.tsinghua.edu.cn

Yuan-zhong Hu, Hui Wang

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

Wen-zhong Wang

School of Mechanical and Vehicular Engineering, Beijing Institute of Technology, Beijing 100081, China


Corresponding author.

J. Tribol 129(4), 809-817 (Mar 30, 2007) (9 pages) doi:10.1115/1.2768081 History: Received November 20, 2006; Revised March 30, 2007

The objective of the present work is to investigate experimentally and numerically the influences of surface roughness, produced by typical machining processes, on friction performances in lubricated-point contacts. Prior to the full experimental investigation, a series of tests had been conducted to examine the experimental errors, resulting from repeated tests on the same specimen but at different tracks, with different amounts of lubricant supply, or after the sample reinstallation. Then, the effects of amplitude and texture of surface roughness on friction behavior are investigated in rotational and reciprocal-mode tests, respectively. The measured friction, averaged over the repeated tests and plotted as a function of sliding speed, shows Stribeck-type curves, which manifest the transition from full-film, mixed, to boundary lubrication. Results show that the roughness amplitude imposes a strong influence on the magnificence of friction and the route of lubrication transition. It is also observed that transverse roughness would give rise to a smaller friction coefficient than the longitudinal one under the same operating conditions. Moreover, the deterministic numerical solution of mixed lubrication has been extended to evaluate friction between rough surfaces over a wide range of lubrication regimes. The numerical simulation results are compared and agree very well with experiments.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Outlook of test rig: (a) rotational mode and (b) reciprocal mode

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Figure 2

Samples of disk: (a) side grinding and (b) plane grinding

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Figure 3

Results for error analysis of experiment: (a) relative errors of measured friction data from repeated tests in rotational mode; (b) relative errors of measured friction data from repeated tests in reciprocal mode; (c) relative errors of measured friction data from repeated tests in rotational mode, but with different lubricant supply rate; and (d) relative errors of measured friction data from repeated tests in reciprocal mode, but with each test conducted after disassembling and reassembling the sample

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Figure 4

Variation of friction coefficient with sliding speeds for three specimens with 1.6μm, 0.8μm, and 0.04μm Ra in rotational mode tests (load: 50g)

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Figure 5

Plane-ground surface with two marked sliding directions in which the tests are to be conducted

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Figure 6

Effects of roughness texture on friction from the reciprocal-mode tests on the same specimen with 1.6μm Ra (load: 200g) but along two perpendicular directions: (a) decreasing-speed sequence and (b) increasing-speed sequence

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Figure 7

Worn surfaces measured after tests at 200g load: (a) longitudinal sliding and (b) transverse sliding

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Figure 8

Relative deviations of friction between the longitudinal and the transverse slidings, reproduced based on the measured results in Fig. 6 but plotted as a function of speed. The relative increment is defined as ∣flongitudinal−ftransverse∣∕ftransverse.

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Figure 9

Comparison between simulation and experiment (load: 50g). The hollow circle stands for simulation results. (a) 0.8μm Ra surface. (b) 0.04μm Ra surface. (c) Variation of predicted contact area ratio for 0.8μm and 0.04μm Ra surfaces with sliding speed.

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Figure 10

Comparison of friction coefficient between experiment and numerical simulation, obtained from the sliding in two perpendicular directions and under the same conditions as in Fig. 6. The hollow rectangle stands for experimental results. (a) Longitudinal roughness. (b) Transverse roughness. (c) Variation of predicted contact area ratio for longitudinal and transverse roughnesses with sliding speed.



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