Chemical Understanding of Friction Polymer Based Tribo-Chemical Films Derived From Nanolubricant

Abstract

MoS2 multi-component nanolubrication system showed significant friction and wear reduction (more than 30% in friction reduction and 50% in wear reduction) in sliding steel surfaces, especially under mixed and boundary lubrication conditions [1–3]. It is believed that the formation of tribofilms in MoS2 multi-component nanolubrication system under different lubrication regimes is the primary reason for reduced friction and wear. To investigate the in-depth science of the tribo-chemical interface formed by MoS2 multicomponent nanolubrication system, it is necessary to study the chemical states of tribofilm during its evolution (generation ↔ regeneration) process at tribo-interfaces. Tribofilms from various lubrication regimes were characterized by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), Raman microscope, and X-ray photoelectron spectroscopy (XPS) techniques to study the morphology, chemical composition, elemental distribution, and chemical bonding of tribo-chemical surface, respectively. Besides the evolution process, the characterization of tribofilms also reveals the possibility of forming new meta-stable phases (chemical compounds) after tribological testing. Patchy tribofilms and progressive tribofilms have been observed from the SEM analysis and the EDX results showed existence of Mo-S-P as the composition of tribo-chemical films. The Raman spectroscopy analysis of tribofilms showed significant difference (such as formation of poly-molybdates) in chemical information of nanolubricants and tribofilms, which is an indication of the formation of friction polymer [4–5]. Additionally, phosphates and oxides, acting as components of surface protecting layer of tribofilms, have been found on surface by XPS technique. Moreover, MoS2 nanoparticles are found to navigate into surface asperities to protect the contacting surfaces. The results (information about the chemical states of the tribofilm) obtained from different characterization techniques can be used to explain the mechanism of friction and wear reduction associated with MoS2 multi-component nanolubrication system that has been reported in the literature.