The purpose of our study is to further understand what governs dry lubrication efficiency for long life pure sliding space applications where lubrication must be sustained in several atmospheres. An experimental approach coupling both mechanics and physic-chemistry has been developed to understand how contamination influences the mechanisms taking part in low friction. The antagonistic tribological behaviours of MoS2 and MoS2+Ti coatings (both deposited on AISI440C) allowed highlighting and explaining the influence of the contamination. 4 environments were used: Air 50%HR, dry N2, high and ultrahigh vacuum. Results show that mechanisms taking part in sliding speeds accommodation pass from the volume of the 3rd body in ultrahigh vacuum to its top surface in air. In that range of environmental pressure, a beneficial mechano-physico-chemical segregation phenomenon is encountered. All those modifications influence the 3rd body rheology which is the switch between good or bad lubrication.