lubricate the contact with a friction coefficient close to that exhibited by a fully flooded contact [14].  However, there are probably few enough so that an appreciable fraction of them are in direct contact with  the substrate, where they are eventually consumed by tribochemical attack, indicating a failure by  elevated friction (Figure 3).  In this study, the friction coefficient is independent of the degradation rate. Evidently, the process  responsible for the establishment of the friction force (possibly of rheological origin) and the process  responsible for the degradation (tribochemical origin) proceed independently of each other. The  tribochemical events may be considered second order effects - events that occur relatively seldom  compared with the rheological friction that is continuous in the contact. If the tribochemical rate of attack  was higher, then the coefficient of friction might well be different for different test environments that  determine different lifetimes.  Krvtox 143ACTM  The lifetime increase is evident for even very low concentrations of water vapor in the test environment. A  concentration of 0.03 ppm in an atmosphere of nitrogen corresponds to a partial pressure of water vapor  of 0.02 milliTorr and lower concentrations may be difficult to achieve. Indeed, the somewhat longer lives  for the tests in atmospheres of “dry” nitrogen (atmospheres in which the hygrometer maintained its base  reading of Td = -99.9’C) could be due to water vapor concentrations too low to be registered. The  extrapolation of the data in Figure 2 to small orbit numbers indicates that there is some lifetime-extending  water vapor present even for the relatively short lifetime exhibited by this PFPE. This would indicate that  the lifetime tests conducted in this manner are more sensitive to water vapor than the hygrometer itself.  The tests conducted with Krytox 143ACTM in pure water vapor support the assertion that water vapor  really is the cause for extended lifetimes in nitrogen atmospheres and not some other tribochemically  active molecule that was introduced into the chamber along with the nitrogen. These results also indicate  that life testing in “soft” vacuums of about Torr or less (for which water vapor constitutes a majority of  the residual gas) can be as valid as testing in a better vacuum.  The null result of testing in 0.21 3 kPa (1.6 Torr) oxygen establishes that such trace amounts of oxygen in  nitrogen cannot protect the PFPE lubricant against tribochemical attack. This result relates to a current  idea that tribological stress exposes clean metal by removing the native oxide, allowing the clean metal to  initiate tribochemical attack on the lubricant. However, such clean metal would be oxidized by exposure to  0.21 3 kPa (1.6 Torr) oxygen thus quenching any tribochemical attack by clean metal. If the oxidation is  fast enough to oxidize clean metal in the contact, then these results imply that the “exposure” mechanism  is not operative here and that the ball is probably rolling on the native oxide of the steel during the test.  Pennzane 2001 A@  The friction traces in Figure 5 indicate that water vapor in the test environment also extends the lifetime of  the MAC. It would bedesirable to determine if there was any effect on the lifetime by testing in an  atmosphere of a truly inert gas or oxygen. However, the MAC’S long intrinsic lifetime allows the  accumulation of water vapor in the test environment: - as presently constructed - that would mask the  possible effects of these species. A study of the effects of oxygen or an inert gas must be conducted in  the absence of water vapor and remains to be done.  Mechanisms  A consideration of the mechanism by which water vapor retards the tribochemical degradation of the  lubricant molecules begins with the observation that the effect is present for both the PFPE and the MAC.  The mechanism is not specific to the particular lubricant chemistry and attention is thus directed to the  substrate. Substrate sites (probably on the steel’s native oxide) are evidently passivated by the water  vapor. Water molecules might adsorb on the sites physically, that is without chemical reaction, and simply  sterically block the interaction of the site with the lubricant molecule. Although this physical adsorption is  well known, appreciable coverage at room temperature occurs at much higher partial pressures of water  vapor [15], rendering this physisorption approach unlikely. An approach that considers the passivating  chemical reaction of water with the active surface sites may be more fruitful. Lewis acid sites have  received much attention in the context of PFPE degradation [16]. This approach may also work out for the  sites that can attack both the PFPE and the MAC. The purely tribological results presented here offer  limited chemical insight.  288