Tribochemistry of MoS3 Nanoparticle Coatings

The tribology of nanoparticles based on transition metal dichalcogenides has been studied extensively. However, evaluation of metal chalcogenides with other stoichiometries has been lacking. We have studied the friction, endurance, and tribochemistry of bonded molybdenum trisulfide (MoS3) nanoparticle-based coatings for the first time. A facile aqueous chemistry method was used to fabricate the MoS3 nanoparticles. Pin-on-disk tribometry of an MoS3 coating using phenolic resin as the binder was conducted in a dry N2 atmosphere (0.06 % RH, using normal loads of 5 N and 10 N). The results were compared with two types of commercial bonded coatings based on the solid lubricant molybdenum disulfide (MoS2), as well as a bonded coating we formulated with commercial MoS2 nanoparticles. Surprisingly, the MoS3 coating showed similar lubricating ability to the MoS2-based coatings, exhibiting average μ k < 0.05 and endurance greater than a million cycles. To evaluate the tribochemistry occurring in the contact region, tribotesting of an MoS3 coating was halted when steady-state low friction was achieved (i.e., prefailure). Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction on the surface of this wear track showed that the MoS3 had undergone a tribochemical reaction to form the solid lubricant MoS2, which explains the excellent lubricity of the coating. This result opens up the possibility of developing MoS3 nanoparticle-based solid lubricant coatings and MoS3 nanoparticle additives for oils and greases that are synthetically easier and lower cost than formulations based on MoS2 nanoparticles.