We then conducted a serie s of experime nts un der vacuum  and nitrog en ga s co nditions u sing  hydro carbon oils with le ad naphth enate additive un der several  load condition s (loads resultin g in Hertzian  stresses of 0.36, 0.97, and  1.52 Gpa) a nd spe eds [7]. Under the hi ghest load co ndition s, the nitroge n gas  tests l asted at least 10x lo nger with out failure than  the vacuum te sts, whi ch faile d. Un der the  lowe r loads,  there were significa nt differen ces in ad ditive chemi stry in nitroge n gas relative to vacuum [7]. We were  intere sted in wheth er aryl phosphate esters would  show simil ar behavior since they are commonly used  as anti -wear additives in  spa cecraft appli cation s. We un dertook a serie s of wea r-test experime nts to  determi ne wh ether the pe rforma nce of hydrocarbon oi l, formulated with ar yl pho sphate este r additives, is  different i n nitrogen  and vacuum  environm ents un der the same load and speed conditions a s our  previou s experiments with  lead nap hthenate additive.  

Experimenta l 

Lubri cant    Nye 20 01, a t rialkylate d cyclopentan e synthetic hydroca rbon oil with 2% Syn-O -Ad 84 78 phosphate e ster  additive, wa s use d in the experim ents reporte d here. The oil was purcha sed from Nye L ubricants, and i s  used in cu rrent spacecraft systems. 

Testing   Wear testing was performed using our i n-house, eccentri c-bearing w ear test facility,  which has been  previou sly describe d [8]. A  thrust ball  bearing  in which one of th e race ways can be repla ced by a flat di sk  comp rises the interactin g comp onent of the test fixt ure. The  remaining raceway can  be mounted sli ghtly  off the axis of  rotation of the disk (“eccentric m ode”), resulting in increased ball slidin g on th e disk surfa ce  and in creasing the severit y of the test conditions [8]. There was a ~17% sli ding com ponent in the ball  motion for th e tests reported he rein. It is the sli ding compon ent tha t accelerates failure in th ese tests. Ball   beari ng temp eratu re was measured u sing a therm ocouple tou ching the beari ng race. 

The b all bearing raceways and balls, and the disks, we re composed of 44 0C stainle ss steel. The th rust  beari ngs have a raceway diamete r of 2.29 cm  and 12 balls of 0.476-cm diamet er. Th e ave rage raceway  surfa ce (R A) was ≤0.25 µm, and the R A of the disks was ~0.01 5 µm. The balls  were ABEC grade 5, with  an R A of ~2 µin (~50  nm). The ball retainer was th e ste el ribbon retai ner furnish ed with t he thrust  ball  beari ng. The  loads on th e retaine r were lo w, so t hat the tests would not b e impacte d by reactio ns  involving the retainer surfaces.  

The metal compone nts were cleaned using Brulin 8 15GD d eterge nt (diluted with H 2O), ri nsed in distilled  H2O, and rinsed in heptane befo re testing. The te st sp eeds were ch osen to f all within th e speed rang e  typical of M MAs, an d the loa d was chosen to  produce a Hertzian stress at the high end of the typi cal  range in orde r to result in test failure in a reas onable test time.  

External cooling was p rovided fo r the t esting i n vacuum to keep the ball -bearing temp erature relatively   low du e to th e lack of con vective co oling. Cold wate r (20°C) was circulate d through a heat sin k in good  thermal co ntact with the ball bearing hou sing. The tempe rature  was measured u sing a th ermo couple i n  conta ct with  the out side of the ra ceway. The tem perature was mai ntained at 24°C, similar to  that of the   nitroge n tests.  

The te sts we re carried  out in the ecce ntric mo de with flat disk, eith er in vacuum  (pressu re less tha n 1.3 x  10-4 Pa) or g ettered nitrog en ga s. The spe ed wa s 1800 rp m, the Hert zian stress was 1.52 Gpa, and the   lubricant q uantity was 10 µL. The tribological re gime  was expe cted to be mixed  or bou ndary [8]. The test  was considered to  fail when the nomi nal running  torque in creased by a fa ctor of  1.7. Fail ures in thi s test  facility are abrupt, so that the ar bitrary choi ce of t he torque incr easing by the 1.7 factor does not  signif icantly affect the test results. The tests were either ru n to failure, or run until a reasonable time  perio d had el apsed witho ut failure occu rring. Six tests we re ru n in vacuum and  four unde r nitrogen g as  conditions. 

 358