Lubrication from the Vapor Phase at High Temperatures

Abstract

Vapor phase lubrication has been demonstrated using a conventional four-ball wear Lester run at 370°C using M50 hot hardness steel ball bearings. The four-ball wear tester was modified to control both the temperature and the environment surrounding the ball pot and spindle. The lubricant in all cases comprised a homogeneous gas phase mixture of nitrogen and phosphate ester vapors. The two phosphate esters used were tributyl phosphate (TBP) and tricresyl phosphate (TCP). The volume percent phosphate ester vapor in nitrogen ranged from 0.10 to 6.0 percent. The minimum temperature of the four balls was maintained at 370°C which was always at least 50°C higher than the temperature of the homogeneous vapors delivered so that there was no possibility of vapor condensation on the wear contacts. Vapor phase lubrication by phosphate esters appears to be as effective at a bulk temperature of 370°C as typical nonadditive mineral oils are in the liquid state at room temperature to 75°C. For example, TCP delivered from the vapor phase could support loads up to 50 kg at a lubricant concentration of only 0.5 percent. Scanning electron microscopic (SEM) pictures of a steel surface at 600°C delivered with 1.2 percent TBP in a nitrogen carrier gas show large amorphous like structures. This suggests that the deposited lubricating film exists as a polymer of the original TBP in a liquidlike state at this temperature. On cooling to room temperature, section of the film apparently coalesced giving he coaling observed. Such fluidity of the lubricating film at high temperatures would be particularly valuable in metal-forming operation.