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Research Papers: Lubricants

Performance of Graphite and Boron-Nitride-Silicone Based Lubricants and Associated Lubrication Mechanisms in Warm Forging of Aluminum

[+] Author and Article Information
Gracious Ngaile

Department of Mechanical and Aerospace Engineering, North Carolina State University, Campus Box 7910, Raleigh, NC 27695gngaile@ncsu.edu

Frank Botz

 Diversified Chemical Technologies, 15477 Woodrow Wilson, Detroit, MI 48238

J. Tribol 130(2), 021801 (Apr 07, 2008) (7 pages) doi:10.1115/1.2805432 History: Received December 06, 2006; Revised July 18, 2007; Published April 07, 2008

Although water/oil-graphite emulsions are widely used in warm forging processes, they carry environmental concerns. In an attempt to replace graphite-based lubricants in warm forging of aluminum alloys, two variants of boron-nitride-silicone lubricants were formulated. The two variants were made by dispersing boron nitride powder in polydimethyl siloxane oil at concentrations of 1% and 8%. The formulated lubricants were initially tested for their thermal degradation characteristics using a thermogravimetric analyzer and compared to the thermal degradation behavior of graphite and silicone oil lubricants. Ring compression tests were then carried out at 260°C and 370°C. Boron-nitride-silicone lubricant variants did not show significant difference in performance as die temperature was increased from 260°Cto370°C. This is in contrast to graphite, which performed much better at 260°C than at 370°C, due to thermal oxidation. On the other hand, silicone oil exhibited the worst performance at 260°C and the best performance at 370°C. In both boron nitride lubricant variants, the polydimethyl siloxane facilitated hydrostatic/hydrodynamic lubrication at 260°C, with boron nitride acting as a barrier film that reduced friction. However, the lubrication mechanisms changed at 370°C, where the depolymerization of polydimethyl siloxane led to formation of silica due to thermal oxidation. Silica, together with boron nitride, acted as a film barrier with low shear strength. The dual lubrication mechanisms make boron-nitride-silicone lubricants suitable for a wide range of aluminum forging temperatures.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Structure of hexagonal boron nitride structure

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Figure 2

General structure of silicone

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Figure 3

Thermogravimetric analysis curves of water graphite, silicon oil, boron-nitride-silicone-1, and boron-nitride-silicone-2

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Figure 4

(a) Experimental setup and (b) scheme of top and bottom dies

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Figure 5

Specimens before experiment

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Figure 6

Deformed samples with graphite, silicone oil, boron-nitride-silicone-1, and boron-nitride-silicone-2 (Re=60%)

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Figure 7

Friction factors for four lubricants tested at a die temperature of 260°C

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Figure 8

Friction factors for four lubricants tested at a die temperature of 370°C

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Figure 9

Influence of temperature on water graphite, boron-nitride-silicone-1, boron-nitride-silicone-2, and silicone oil

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Figure 10

Spectral analysis and location where micrographs were taken

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Figure 11

Influence of temperature on lubricant film morphology for water graphite boron-nitride-silicone-1, boron-nitride-silicone-2, and silicone oil (Re=60%)

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Figure 12

Lubricant chemical elements for graphite (a) at 260°C and (b) at 370°C

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Figure 13

Lubricant chemical element for boron-nitride-silicone-2 (a) at 260°C and (b) at 370°C

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Figure 14

Lubricant chemical element for silicone oil (a) at 260°C and (b) at 370°C

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