Radiotracers Reveal Activity of Extreme-Pressure Additives in Lubrication

Abstract

The mechanism of extreme-pressure (EP) lubrication has been studied with various approaches in a number of laboratories over the past years. An investigation in this area was initiated recently in this laboratory with a modified four-ball tester [Manteuffel and Wolfram, ASLE Trans. 3, 157] wherein the lubricant temperature leaving the ball interfaces is recorded as the load is continuously increased. The patterns of the temperature-load curves generated under these conditions are dependent on the lubricant compositions. This investigation has now been extended with radiotraced phosphorus and sulfur to study the comparative reactivities of these elements in EP lubrication and as functions of temperature and load. A procedure was devised to measure the contents of the radiotraced elements in the ball scar area surfaces. The results indicated that phosphorus is much more reactive than sulfur in the EP lubrication. The data also confirmed that the lubrication reaction is very temperature-dependent. The phosphorus lubricant component was more reactive at lower temperatures than any of the three sulfur components tested. Phosphorus was tested in an organic dithiophosphoric acid component form. The sulfur forms tested were the thiophosphoric, an arylalkyl disulfide, and an organic sulfide. The general sulfur reactivities decreased in the order listed here, thiophosphoric being most reactive.