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

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 $(∼10−3mm3/Nm)$ 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.