Tribological properties of MoS2 coatings in vacuum, low relative humidity, and high relative humidity environments

We have rf-sputter deposited MoS2 coatings onto stainless-steel plates using two extreme combinations of deposition conditions. Raman spectroscopy indicates that there is a small but significant difference in the number of sulfur--sulfur bonds and the amount of amorphous material present in the two types of coatings. The friction and wear properties were evaluated within five days of deposition and after being stored for 150 days in 35%--45% relative humidity followed by over 75 days in (1) 2% or (2) 98% relative humidity environments. The more crystalline material had a coefficient of friction of 0.085 +- 0.017 in lab air and 0.06 +- 0.005 in ultrahigh vacuum. For lab air testing the coefficient of friction increased with wet storage to 0.10 +- 0.01, and the wear life decreased by a factor of 3 for dry storage and by a factor of 10 for wet storage. The MoS2 material with the larger amount of amorphous material has a coefficient of friction of 0.10 +- 0.013 which increased to 0.14 +- 0.04 for storage in the wet environment. The wear life of the MoS2 coating with the greater amount of amorphous material improved significantly (by 100%) for storage in the wet environment. These results are consistent with the observations of others who noted that water vapor promotes the conversion of MoS2 to MoO3