BEHAVIOR OF RUBBING MOLYBDENUM SURFACE IN SODIUM ENVIRONMENTS

A study has been made of the wear and friction phenomena associated with sliding molybdenum surfaces lubricated by liquid sodium. Operating conditions included sodium temperatures from 250 to 1300 deg F, sliding speeds of from 0.22 to 2.5 nan per sec, and contact stresses of 80,000 psi. Sliding specimens consisted of a ball and a flat rectangle. Sodium was found to influence the sliding behavior of molybdenum by modifying reacted films which provided boundary lubrication. These effects were also found to be temperature dependent. Addition of clean sodium between dry rubbing molybdenum surfaces in an argon atmosphere of high purity produced a marked drop in friction coefficient, and the character of the wear scar changed from galling to superficial scratching. Furthermore, these effects persisted after removal of sodium by evaperation. Studies of the films formed on molybdenum surfaces during sliding in liquid sodium and under controlled atmospheres were carried out using electron and x-ray diffraction, electron microscope, and differential thermal-analysis techniques. It was established that a sodium molybdate film on the molybdenum surfaces can be associated with low friction and diminished surface damage. Studies in vacuums up to 10/sup -6/ mm of mercury showed that native oxides on molybdenum also influence its sliding behavior. (auth