Nanotribology of a Vapor Phase Lubricant: Quartz Crystal Microbalance Study of Tricresylphosphate (TCP) Uptake on Iron and Chrome

Abstract

TCP is a high-temperature vapor-phase lubricant that is known for its demonstrated anti-wear properties in macroscopic systems. Although this lubricant has been the subject of much research for over 40 years, the atomic-scale details of its lubrication mechanisms are far from being satisfactorily understood. One common macroscopic observation is that TCP acts as a lubricant on iron, but not on chrome surfaces.In this study, we have evaporated iron and chrome films in UHV conditions onto Quartz Crystal Microbalances (QCM) and then monitored the uptake rates and nanoscale dynamics of TCP on these substrates at different temperatures. Using an in-situ Auger spectrometer, the samples' surface chemistries were investigated in parallel with uptake rates at the various temperatures. TCP uptake on oxide surfaces as well as oxygen uptake on TCP exposed surfaces was also investigated, shedding light on the TCP-oxygen interaction in the presence of iron and chrome substrates and the effect of temperature on such reactions.We have found that TCP is physisorbed on iron at room temperature. Below 2000C, two to four TCP monolayers are adsorbed, and slip times of 2 ns are observed. At temperatures of 2000C and above, TCP molecules break up and intermix with the iron substrate, and no slippage is in evidence. At high temperature, chrome behaves similarly. But at room temperature its behavior is quite distinct. Exposure of TCP-coated iron surfaces to oxygen results in the formation of a viscoelastic polymer film. Such a film is not observed to form on chrome surfaces. This may well account for the difference in the observed behavior at the microscopic scale.