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Research Papers: Friction & Wear

# Does Thermal Conductivity Play a Role in Sliding Wear of Metals in Cryogenic Environment?

[+] Author and Article Information
Bikramjit Basu1

Department of Materials Science and Engineering, Indian Institute of Technology, IIT Kanpur, Kanpur 208016, Indiabikram@iitk.ac.in

Department of Materials Science and Engineering, Indian Institute of Technology, IIT Kanpur, Kanpur 208016, India

J. Sarkar

Praxair Electronics, 560 Route 303, Orangeburg, New York, NY 10962

1

Corresponding author.

J. Tribol 132(4), 041604 (Oct 07, 2010) (5 pages) doi:10.1115/1.4002503 History: Received February 25, 2010; Revised August 21, 2010; Published October 07, 2010; Online October 07, 2010

## Abstract

The thermal conductivity of a metallic test piece is one of the principal parameters that influence the temperature buildup at tribocontacts and this normally plays an important role in the unlubricated dry sliding wear of metallic materials. It is, however, not clear whether thermal conductivity is an equally important parameter in the case of wear of metals at cryogenic temperatures, in particular, at liquid nitrogen temperature $(LN2)$ of $−196°C$. In order to assess the influence of such a physical property of selected nonferrous metals on their tribological behavior in the $LN2$ environment, we have studied the friction and wear properties of high purity copper (Cu) and titanium (Ti) against the bearing grade steel. These two materials have been processed to produce samples of comparable hardness that have widely different thermal conductivities at room temperature and at test temperature. Wear tests were conducted at three different sliding speeds (0.89 m/s, 1.11 m/s, and 1.34 m/s) under 10 N load, and the friction and wear data were compared. Ti exhibited an order of magnitude higher wear rate $(∼10−3 mm3/N m)$ as compared with Cu in identical test conditions. While evidences of abrasive wear and adhesive wear, without any oxidative wear, were found in worn Cu surfaces, worn Ti surfaces showed evidences of significant oxidative wear and mechanical damage of tribolayers. Higher wear rate in Ti appeared to be a result of oxidative wear of Ti, which seemed to be driven by the depletion of $LN2$ blanket at the tribocontacts under the influence of high flash temperature $(14–76°C)$ as compared with the boiling temperature of $LN2$$(−196°C)$. These results demonstrate that the materials with similar hardness subjected to identical $LN2$ wear test conditions can have significantly different wear rates because of the difference in the flash temperatures, which depend on the thermal conductivity of the test pieces.

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## Figures

Figure 1

Variations of the steady state coefficient of friction (μF) with the sliding speed during LN2 sliding wear tests of high purity Ti and high purity Cu against the bearing steel counterbody at 10 N load

Figure 2

Comparison of the effect of sliding speed on the wear rates of high purity Ti and high purity Cu after sliding against the bearing steel counterbody for 5 min under a constant load of 10 N in the LN2 environment

Figure 3

SEM micrographs showing the worn surface of Cu samples after sliding against the bearing steel counterbody for 5 min under a constant load of 10 N in the LN2 environment at (a) the lowest sliding speed of 0.89 m/s and (b) the highest sliding speed of 1.34 m/s. While evidences of abrasive wear can been seen at both sliding speed conditions, the Cu sample tested at 1.34 m/s showed delamination at the subsurfaces. The EDS spectrum obtained from the surface tested at the higher sliding speed confirms the absence of oxidative wear in Cu. Double headed arrows indicate the sliding directions.

Figure 4

SEM micrographs showing the worn surfaces of Ti samples after sliding against the bearing steel counterbody for 5 min under a constant load of 10 N in the LN2 environment at (a) the lowest sliding speed of 0.89 m/s and (b) the highest sliding speed of 1.34 m/s. The EDS spectrum obtained from the surface tested at the higher sliding speed confirms the presence of oxide wear debris in the form of particles in Ti. Double headed arrows indicate the sliding directions.

Figure 5

An optical micrograph of the chemically etched worn surface of a Ti sample tested at 10 N load and 1.34 m/s showing the formation of deformation twins in the tribolayers because of the plastic deformation of Ti at LN2 temperature. Arrows show the deformation twins.

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