Relationship between mechanical properties and structures of zinc dithiophosphate anti-wear films

The structure and the rheological properties of the anti-wear films from a zinc dialkyldithiophosphate (ZDTP) solution generated in a rolling/sliding contact simulating engine valve train conditions have been studied with analytical and surface-force tools. The anti-wear film has a complex structure that has been determined by extensive use of surface analytical techniques that include SEM/EPMA, XPS, ToFSIMS and Auger spectroscopy. The film consists of at least three non-homogeneous layers: on the steel surface there is a sulphide-oxide layer, which is almost completely covered by a protective phosphate layer, with the addition of an overlayer of ZDTP degradation precipitates. This latter layer is removed when the film is washed with an alkane solvent. Therefore, the properties of the ZDTP film layers have been studied both before and after solvent washing with n-heptane. A surface-force apparatus gives, first, the rheological properties of the film, obtained from sphere-plane squeeze experiments, and, second, some mechanical properties obtained from nanoindentation experiments coupled with topographic imaging procedures performed after replacing the sphere by a diamond tip. From the indentation experiments, the properties of the solvent-washed film were determined from the normal stiffness measurements and through the application of a rheological film model. The solvent-washed specimen, comprising sulphide and phosphate layers, exhibits an elastoplastic behaviour. In particular, the mechanical properties of the phosphate layer increase during the loading stage of the indentation to accommodate the increasing applied pressure. From these observations and previous film analyses, a schematic picture of the 'full' anti-wear ZDTP film structure and its mechanical properties was obtained. A highly viscous layer of alkyl phosphate covers islands of solid polyphosphates. These islands are partly adherent to the steel surface through a sulphide layer and accommodate the real contact pressure in the tribotest. These results show that the exceptional wear resisting properties of ZDTP films arise from their ability to respond to a wide range of imposed conditions. As the severity of loading increases, so too do the resistive forces within the film. ZDTP anti-wear films can truly be described as SMART materials.