225 Long term s torage life test data for sputter deposited MoS 2 coatings are uncommon. The data available  generally involve pure MoS 2 coatings . For example, r esults reported in the 1980’s in Ref. 14 showed that  storage of pure sputter -deposited MoS 2 coatings in humid atmospheres (i.e., 84% relative humidity [RH])  resulted in considerably greater oxidation than in dry atmospheres for stor age times of 2 weeks to 1 year.  Some oxidation increase was even seen for storage in 52% RH compared to dry atmosphere.    Panitz et al .  evaluated the friction and wear properties of  RF-sputtered MoS 2 coatings after being stored for  150 days in 35% -45% relative humidity followed by over 75 days in either a 2% or 98% relative humidity  environment.19 Two types of coatings were studied, one that was deposited on a heated substrate and  subsequently annealed, and another that was deposited on a substrate at ambient temperature and not annealed. The heat -treated coating was more crystalline than the ambient -grown coating as measured by  Raman peak intensity (this agrees with X -ray Diffraction results on RF-sputtered MoS 2 coatings that had  similar heat -treatment  in our laboratory20). The coefficient of friction for the MoS 2 coating having a more  crystalline st ructure increased from its initial air tested level of 0.085 to 0.10 after being stored in 98% RH.  Its wear life decreased by a factor of 3 when stored the 2% dry environment but dropped by a factor of 10 when stored in the wet environment. The authors poi nt out that this result was consistent with the  observation of others that water vapor promotes the conversion of MoS 2 to MoO 3. On the other hand the  less crystalline MoS 2 sample’s wear life increased when stored in the wet environment , so microstructure  clearly plays a role in oxidation .    Storage t ests were conducted by Iwaki  et al. on sputter deposited MoS 2 at 40%, 60% and 80% relative  humidity for up to 24 months of storage.21 Wear lives of samples stored for 6 months were relatively  unchanged within th e scatter of the data except for samples stored at 60% RH which showed improv ement.  However tests at 1, 1.5 and 2 years showed mixed results with some 60% RH readings showing lower life while others showing essentially no change.    For use in current spacecraft mechanisms, it has been found that greatly improved tribological properties are obtained from cosputtered coatings that consist of MoS 2 mixed with species like Ni and Sb 2O3 to form  nanocomposites. The higher densities and altered crystalline structure of these cosputtered coatings imply that their sensitivity to storage in humid air should be different also.  Storage sensitivity of these mixed  coatings is the focus of our study.  

Experimental Approach  

Ni-MoS 2, Au-Sb2O3-MoS 2, and Sb 2O3-MoS 2 coatings were deposited onto a series of polished 440C steel  disks  to a thickness of 400±100 nm . Several of each type of coating were stored in a closed container  containing air with humidity controlled to 59% RH by using a saturated solution of NaBr. Samples were  periodically removed for testing. Samples not being exposed to humid air were stored in dry, flowing N 2  gas.   Friction/wear testing was conducted using  a CSEM Pin -on-Disk Tribometer. The upper specimens for the  tests were uncoated 6- mm 440C steel balls (grade 3, R a=0.01 μm), cleaned with IPA prior to testing. For  all tests, a 3N load was used, and the sliding speed was 20 cm/s. The nominal maximum Hertzian contact stress was calculated as  938 MPa (136 ksi) based on the properties of 440C steel  substrate.    Prior to starting the friction/wear tests, the tribometer enclosure was purged with nitrogen gas (99.999% nominal purity, passed through an Aeronex Gatekeeper ® getter, which is advertised to reduce oxygen,  water, and other impurities  to less than 1 ppb) for four hours, and until the relative humidity was reduced to  below 0.08% RH. The tests were started, and were run until failure, which is defined as when the COF rose above 0.5. Three tests were conducted on each sample to determine the data uncertainty, but different radii were chosen to ensure that each test was conducted on virgin material; new balls were used for each test.    X-ray Photoelectron Spectroscopy (XPS) was used to aid in understand ing the chemical changes occurring  in the coatings that gave rise to changes in friction and endurance.  Our main interest was in the oxidation