contai ning fil ms, is comm on. Ho weve r, aryl pho sphate esters can also react with iron metal [ 10,11], agai n  leaving pho sphate-co ntain ing films on  the surfa ce. The relative importan ce of these two re actio n  pathways in  an op erating  bea ring i s unknown. If the re actio n with iron metal  is mo re important tha n  reaction with oxide, there will be more rea ction in vacu um than nitroge n gas sin ce the ad ditive will not  have to com pete with oxygen for metal  sites. This would lead to relatively less a dditive in the oil, and  more p hosphate film on the su rface tha n in nitrog en. If the reactio n with metal oxides i s more importa nt,  however, there will be m ore reactio n in nitrogen  gas than vacu um since the contamin ant oxygen  will  prepare the expose d metal  for reactio n with the addit ive. This wou ld lead to rel atively less a dditive in the   oil, and more pho sphate film on th e surfa ce than  in vacuum. The wear life itself cann ot be u sed to  disting uish b etwee n the t wo pathways sin ce the  test pro cess is too complex f or straig htforward a nalysi s.  The test s are defined to fail  at a particul ar increa sed level of torque. High to rque could be d ue to adhe sive  wear due to metal on met al contact (i.e ., ineffective addi tive funct ion or exhau stion of additive in the oil),   or to thick a dditive film buildup (over efficient addit ive function ). The effica cy of the particular rea ction  product in extendin g life and the amount of ad ditive p resent in th e oil are  also inextricably conv oluted i nto  the wea r life. In future expe riment s, we intend to halt operation of b earin gs before failure, and analyze the  oil and surfa ces to discove r the relative i mportance of the metal or o xide rea ction pathways. 

Conclu sions  

The results of this study in dicate  that th e tribol ogical perfo rman ce of MMAs lubricated  with aryl phosphat e  ester-formulated oil s will be different i n vacuum  and nitrogen at atmo spheric pressure.  Apparently,  reaction bet ween the ad ditive and the metal su rfaces of ball be aring p arts is retard ed in nitroge n ga s  comp ared to vacuum  cond itions. Th us, a nitro gen environm ent is not a conservative choi ce for life te sting,   despite the p otential co st saving s of a voiding vacu um testing. In addition, th e pre sent st udy involved  specific conditions of sli ding:rolling ratio,  conta ct stress, and lu bricant formulation ; other conditi ons could  show diffe rences with respect to relative  nitrog en/vac uum b ehavio r, so the life  extension fa ctor of at l east  5 in nitroge n cannot be  used predictively for othe r tests. It is even  possibl e that  som e systems could hav e  better pe rformance occu rring du ring testing in va cuum. Th ese uncertainti es indi cate t hat on-o rbit  perfo rman ce would be difficult to pre dict from the re sults of o ur study. Therefo re, we conclude that it is  prudent to test an MMA under va cuum conditions when that is the environme nt to which the MMA will be  subjected in the appli cation.  

Ackn owledgments 

The a uthors than k S. A. Jackson fo r prepari ng th e test articles and J. J. Kirsch for pe rformi ng the tests.  This wo rk was supp orted by the U.S. Space  and Missil e Syste ms Cente r, Contract No.  F047 01-00 -C0009. 

Referen ces 

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