Nanomechanical properties of films derived from zinc dialkyldithiophosphate

We have applied scanning force microscopes to the problem of determining the nanomechanical properties and topography of films derived from the antioxidant/antiwear oil additive zinc dialkyldithiophosphate (ZDDP). The enhanced capabilities of the interfacial force microscope (IFM) are exploited to obtain quantitative force-displacement data and pre- and post-indent images, and the atomic force microscope (AFM) enables us to survey topography up to much larger spatial dimensions. A film which has been generated by static immersion of a steel coupon in heated ZDDP/oil solution displays a nanoindentation response similar to that of lightly loaded regions of a film which has been prepared by conventional wear testing using the same starting materials. More interestingly, highly loaded regions of the same wear specimen exhibit a much different nanomechanical response, which signals the presence of at least two different film materials. The film material in highly loaded regions displays an amazing capacity to deform in a predominantly elastic mode, which is in sharp contrast with the elastic-plastic properties of lightly loaded regions, and of the film generated by static immersion. Our results are not inconsistent with the possibility of friction-induced glass as a proper description for highly loaded regions of the wear specimen.