Super-low friction of MoS_2 coatings in various environments

摘要

The ultra-low friction coefficient (typically in the 10~(-2) range) of MoS_2-based coatings is generally associated with the friction-induced orientation of 'easy-shear' planes of the lamellar structure parallel to the sliding direction, particularly in the absence of environmental reactive gases and with moderate normal loads. We used an AES/XPS ultra-high vacuum tribometer coupled to a preparation chamber, thus allowing the deposition of oxygen-free MoS_2 PVD coatings and the performance of friction tests in various controlled atmospheres. Friction of oxygen-free stoichiometric MoS_2 coatings deposited on AISI 52100 steel was studied in ultra-high vacuum (UHV: 5 x 10~(-8) Pa), high vacuum (HV: 10~(-3) Pa), dry nitrogen (10~5 Pa) and ambient air (10~5 Pa). 'Super-low' friction coefficients below 0.004 were recorded in UHV and dry nitrogen, corresponding to a calculated interfacial shear strength in the range of 1 MPa, about ten times lower than for standard coatings. Low friction coefficients of about 0.013-0.015 were recorded in HV, with interfacial shear strength in the range of 5 MPa. Friction in ambient air leads to higher friction coefficients in the range of 0.2. Surface analysis performed inside the wear scars by Auger electron spectroscopy shows no trace of contaminant, except after friction in ambient air where oxygen and carbon contaminants are observed. In the light of already published results, the 'super-low' friction behaviour (10~(-3) range) can be attributed to superlubricity, obtained for a particular combination of cystallographic orientation and the absence of contaminants, leading to a considerable decrease in the interfacial shear strength.