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Research Papers: Coatings & Solid Lubricants

Load Capacity and Durability of H-DLC Coated Hydrodynamic Thrust Bearings

[+] Author and Article Information
Said Jahanmir, Andrew Z. Hunsberger, Hooshang Heshmat

 Mohawk Innovative Technology, Inc., 1037 Watervliet-Shaker Road, Albany, NY 12205

J. Tribol 133(3), 031301 (Jul 21, 2011) (10 pages) doi:10.1115/1.4003997 History: Received September 08, 2010; Revised March 21, 2011; Published July 21, 2011; Online July 21, 2011

Hydrogenated diamondlike carbon (H-DLC) coatings provide excellent wear resistance and low friction for bearing applications. However, the use of such coatings with aqueous lubricants could pose some difficulties due to the hydrophobic nature of the surface. A thrust bearing tribometer was used to compare performance of hydrophilic and hydrophobic surfaces in hydrodynamic lubrication with a mixture of water and glycerol as the lubricant. Hydrophobic surfaces on both runner and bearing were achieved with the deposition of H-DLC films on titanium alloy surfaces. Hydrophilic surfaces were created through modification of H-DLC surface with covalently bonded heparin. Several possible combinations of hydrophobic and hydrophilic surface conditions were used on the bearing and runner surfaces to provide full-wetting, partial-wetting, and half-wetting conditions. The experimental results confirmed that load support is still possible, when the bearing is half-wetted or partially wetted. However, the full-wetted bearing combination (i.e., Reynolds no-slip boundary condition) provided the highest load support. Introduction of slip at the surface resulted in a lower measured torque. Heparin treatment resulted in a lower than expected static friction and friction in full lubrication regime. The durability of coated surfaces was evaluated in a series of start–stop tests and in impact tests. The results confirmed that the coatings are stable and survive the test regiment that exceeded 50 test cycles; whereas the uncoated titanium alloy bearing surfaces were damaged after ten test cycles.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Thrust bearing (a) and runner counterface (b) used for the tests

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Figure 2

Schematic diagram of the thrust bearing tribometer. The counterweight system used for durability and impact testing is also shown.

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Figure 3

Bearing performance with a water-based lubricant during loading for self-mated surfaces: (a) normal load and (b) torque

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Figure 4

Bearing performance with a water-based lubricant during loading for three bearing surfaces with different degrees of wettability against H-DLC coated runner: (a) normal load and (b) torque

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Figure 5

Coefficient of friction (traction) with a water-based lubricant in full film lubrication regime: (a) self-mated surfaces and (b) bearing surfaces with different degrees of wettability against H-DLC coated runner

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Figure 6

Typical bearing performance data with a water-based lubricant in start–stop tests under 8.9 N load

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Figure 7

Film thickness, torque, and speed with a water-based lubricant for self-mated start–stop tests at 8.9 N load for (a) titanium alloy (cycle no. 5), (b) H-DLC coating (cycle no. 25), and (c) heparin treatment (cycle no. 5)

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Figure 8

Static coefficient of friction corresponding to the initial torque spike measured with a water-based lubricant for self-mated start–stop tests at 8.9 N

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Figure 9

Coefficient of friction (traction) and film thickness in hydrodynamic lubrication regime with a water-based lubricant for self-mated start–stop tests

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Figure 10

Typical load-drop test results with a water-based lubricant: (a) H-DLC self-mated test at 8.9 N, (b) H-DLC self-mated test at 13.35 N, and (c) heparin treated bearing against H-DLC coated runner at 13.35 N

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Figure 11

Calculated velocity profiles: (a) no-slip Reynolds boundary, condition 1; (b) slip at bearing, condition 2; (c) slip at runner condition 3, and (d) slip at both surfaces, condition 4

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Figure 12

Effect of surface condition on calculated hydrodynamic power loss

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Figure 13

Comparison between bearing performance of a fully wetted (titanium alloy bearing and runner) and half-wetted bearing combinations (wax coated bearing and titanium alloy runner): (a) normal load, (b) torque, and (c) coefficient of friction (traction) in full film lubrication regime with a water-based lubricant

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