The friction of clean metals immersed in liquid sodium

An apparatus for the measurement of friction is described, in which specimens are heated in a high vacuum (similar 10 -5 torr) to remove surface contamination such as oxide films and then immersed in liquid sodium. The friction between the specimens was measured both when they were dry and after immersion at temperatures from 100-190° C. In general, for specimens in liquid sodium, the frictional force was no longer proportional to load. Above a certain critical load, 10-40 g, depending on the specimen, the coefficient of friction increased and asymptotically approached a higher constant value, about two-thirds of the dry friction. These results are consistent with the formation of a thin layer of a double oxide on the surface of the specimen by a chemical reaction between the specimen, the liquid sodium and sodium oxide dissolved in the latter.