Atomic force microscopy and Raman spectroscopy investigation of additive interactions responsible for anti-wear film formation in a lubricated contact

Rational formulation of lubricants requires all understanding of additive interactions that impact antiwear film qualities such as thickness, topography, and friction. In all effort to understand the complex additive interactions responsible formation of anti-wear and friction-reducing films, atomic force microscopy (AFM) in conjunction with Raman microscopy has been used to conduct a nanoscale investigation of the wear tracks formed by a high-frequency reciprocating rig (HFRR) in the presence of various commercial lubricant additives combinations. Of the additives examined, zinc dithiophosphate (ZnDTP)-based additives are found to be solely, responsible,for the formation of a thick (hundreds of nm) film that exhibits a pitted topography. Addition of a molybdenum-based friction modifier to the lubricant blend reduces the film thickness considerably and reacts to produce MoS2 on the surface, suggesting an interaction with the zinc dithiophosphate-based additive that prevents anti-wear formation. Formation of MoS2, found only in the wear track, is consistent with a dramatic reduction of friction coefficient measured in the HFRR. Subsequent addition of borated dispersants to the lubricant reveals a further reduction in friction coefficient and a modest return of anti-wear film. However, addition of detergents to the formulation increases the friction coefficient and also promotes the formation of all anti-wear film. Nanoindentation measurements oil the bulk properties of the anti-wear films determined that all of the anti-wear films had similar modulus and hardness measurements which were lower than that of the parent steel material, bill did not correlate with the friction measurements obtained from the HFRR. This indicates that nanoscale measurements oil material properties of the film are necessary to elucidate friction properties of the interface, and that these properties cannot be determined from macroscale measurements oil the bulk film..