Oxygen—An “Extreme-Pressure Agent”

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Abstract
Tests using a geared-roller-test machine with steel rollers lubricated by plain mineral oil indicate scuffing resistance is strongly dependent on the presence of oxygen. Tests imply protection against scuffing is provided by formation of oxides in the contact zone. The protective layer breaks down thermally so that scuffing occurs at a “critical conjunction temperature” ranging between 390 and 450 F for the sliding speed range investigated (25 to 250 in/sec). If oxygen is present, the protective layer may be replenished so that conjunction temperatures exceeding the critical may occasionally be imposed before scuffing occurs. This is true particularly at low sliding speeds. When the steel rollers are lubricated by a diester-based lubricant which contains oxygen in the lubricant molecule (di-2-ethyl-hexyl-sebacate), scuffing resistance is provided without atmospheric oxygen. With this lubricant the critical conjunction temperature was higher with a nitrogen atmosphere than with an air atmosphere. The critical conjunction temperature depends on the surface finish of the test rollers. In general, the critical temperature increases as surface roughness decreases.