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RESEARCH PAPERS

Tribological Study of Microbearings for MEMS Applications

[+] Author and Article Information
Daejong Kim

Mechanical Engineering,  Texas A&M University, College Station, TX 77843

Dongmei Cao, Wenjin Meng

Mechanical Engineering,  Louisiana State University, Baton Rouge, LA 70803

Michael D. Bryant, Frederick F. Ling

Mechanical Engineering,  University of Texas at Austin, Austin, TX 78712

J. Tribol 127(3), 537-547 (Jan 08, 2005) (11 pages) doi:10.1115/1.1924428 History: Received February 24, 2004; Revised January 08, 2005

Microsleeve bearings intended for microrotational machinery were fabricated by X-ray lithography and Ni electroplating. Coated to the working surfaces of the bearings was a 900nm thick uniform tungsten hydrocarbon (W–C:H) coating using an inductively coupled plasma (ICP) assisted, hybrid chemical vapor deposition (CVD)/physical vapor deposition (PVD) tool. Tribological characteristics and mechanical properties of as-electrodeposited Ni microbearings, annealed Ni microbearings at 800°C, and W–C:H coated microbearings were investigated. Potential applications of the microbearings may involve very light contact pressure (530MPa) and high sliding speed, such as micromotors and microturbines. Conventional pin-on disk test methods on top flat surfaces, (001) planes, cannot effectively predict tribological characteristics because these microbearings use the sidewall (110 plane) as a working surface. A special micro wear tester and friction tester were developed. Surface morphologies of new and worn bearing surfaces were studied using SEM. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) characterized the W–C:H coated microbearings. Test results of the W–C:H coated microbearings (wear characteristics and friction) are also presented. W–C:H coated microbearings had much lower wear rate than uncoated bearings. During the wear test, a transfer layer formed on the counter steel shaft even under very small contact pressure, leading to low steady state friction and high wear resistance.

Copyright © 2005 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

SEM image of the Ni microbearing surface before coating. The lower arc is the inner diameter. (a) Low magnification (×400), (b) high magnification (×30k).

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Figure 2

Fixture to coat W–C:H coatings on the microbearings: (a) 1mm thick stainless steel sheet with holes to hold microbearings; (b) conformal coating process of bearing surfaces

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Figure 3

Raman spectrum of new W–C:H

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Figure 4

Uniform conformal coating thickness on the bearing surface

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Figure 5

SEM images of W-DLC2 coated bearing surface. (a) Low magnification (×400); (b) high magnification (×30k).

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Figure 8

Wear characteristics of as-deposited Ni bearings: (a) Top view, worn materials moved along the axial direction and accumulated at the thrust surface of the bearing (b) Inner bearing surface after test

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Figure 10

SEM images of shaft tested against (a) As-deposited Ni bearing (b) Annealed Ni bearings. No noticeable wear was observed.

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Figure 11

RPM-time relation of the W–C:H coated microbearings

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Figure 12

SEM images of W–C:H coated microbearing surfaces after wear test: (a) W-DLC1 coated microbearing (b) W-DLC2 coated microbearing

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Figure 14

Evolution of friction coefficient of W–C:H coated microbearing

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Figure 15

Raman spectrum on the wear scar on the steel shaft tested with W-DLC2 coated bearing

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Figure 16

SEM images of shaft tested against W-DLC2 coated bearing after 2h of continuous wear test

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Figure 13

Voltage signal from capacitance sensor and converted rotational angle of horizontal bar attached to bearing holder with W-DLC1 coated bearing

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Figure 9

Wear characteristics of annealed (at 800°C for 1h) Ni bearings: (a) Top view, worn materials moved along the axial direction and accumulated at the thrust surface of the bearing as multiple layers (b) Inner bearing surface after test

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Figure 7

Microfriction tester: (a) Schematic diagram; (b) Principle of friction measurement; (c) Photo of friction tester

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Figure 6

Micro wear tester: (a) Overview (b) Detailed view within dotted circle

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