Effect of Test Environment on Lifetime of Two Vacuum Lubricants  Determined by Spiral Orbit Tribometry  Stephen V. Peppe;  Abstract  The destruction rates of a perfluoropolyether (PFPE) lubricant, Krytox 1 43ACTM, subjected to rolling  contact with 440C steel in a spiral orbit tribometer at room temperature have been evaluated as a function  of test environment. The rates in ultrahigh vacuum, 0.21 3 kPa (1.6 Torr) oxygen and one atmosphere of  dry nitrogen were about the same. Water vapor in the test environment - a few ppm in one atmosphere of  nitrogen - reduced the destruction rate by up to an order of magnitude. A similar effect of water vapor  was found for the destruction rate of Pennzane@ 2001 A, an unformulated multiply alkylated cyclopentane  (MAC) hydrocarbon oil.  introduction  The destruction of liquid lubricant molecules by tribochemical attack (tribochemistry) is well-recognized,  especially for perfluoropolyethers (PFPEs). The destruction or degradation can be manifested by the finite  lifetime of a mechanism with a limited supply of lubricant. It is also indicated by the observation of  chemical reaction films on the bearing surface, friction polymer and molecular fragments emitted into the  environment. Destruction of PFPEs, in particular, has been observed in ball bearings for vacuum service  in spacecraft and in eccentric bearing tests that operate in the starved or boundary lubrication regime,  depleting the lubricant supply and causing bearing failure [l , 21. Destruction of PFPEs is also a matter of  concern in hard disk magnetic storage media [3]  The destruction rate of the lubricant molecules is a function of both the lubricant’s chemical structure [4,  51 and bearing substrate chemistry [6, 71. It can also depend on the test environment [&lo].  Understanding environmental effects on lubricant’s destruction rate is vital to ensure that earth-based life  tests of vacuum hardware provide meaningful results. Some tests are performed in ‘inert’ gas, such as dry  nitrogen, because of significant cost and convenience advantages versus vacuum testing. However, the  test chamber may still contain trace impurities that compromise the inertness of the test environment,  even though the chamber may have been backfilled or purged with an inert gas such as nitrogen.  Impurities such as oxygen or water vapor may be tribochemically active and change the lubricant’s  lifetime compared to what would be exhibited in vacuum.  In this paper, the destruction rates of Krytox 143ACTM, a popular PFPE vacuum lubricant, in rolling  contact with 440C stainless steel in a spiral orbit tribometer are studied for test environments of ultrahigh  vacuum (UHV), dry oxygen, dry nitrogen, nitrogen containing water vapor and pure water vapor. The  destruction rate is expressed as the lifetime of a finite charge of lubricant in a test and the results give a  ranking of the lifetimes as a function of test environments. Some results are also presented for the effect  of water vapor on the destruction rate of Pennzane@ 2001A, an unformulated multiply alkylated  cyclopentane (MAC) hydrocarbon oil.  Experimental  The test instrument is a spiral orbit tribometer (SOT) depicted in Fig. 1. This rolling contact tribometer is a  retainerless thrust bearing with one ball and flat races whose elements and kinematics have been  described [l 11. The SOT was used to observe the tribochemical destruction of lubricants in vacuum and  to give a ranking of the lifetimes, or degradation rate, of two different PFPEs (Krytox 143ACTM and  Fomblin Z-2EiTM) and the unformulated Pennzaneo 2001A on 52100 steel [4}.  NASA Glenn Research Center, Cleveland, OH  Proceedings of the 38’ Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006  283