The effect of steel hardness on the performance of ZDDP antiwear films: A multi-technique approach

X-ray absorption near-edge structure (XANES) analysis has been used to characterize the chemistry of antiwear films formed in a mineral base oil containing a zinc dialkyl dithiophosphate (ZDDP) additive. These films were formed by rubbing the AISI 1095 steel samples under a reciprocating boundary contact. The steel samples were tempered to produce different Vickers hardness values. The phosphorus L-edge XANES spectra show that these films differ slightly in their chemical nature, with longer chain polyphosphates being formed on samples with higher hardness value. The surface morphology of the films was investigated using Atomic force microscopy (AFM) and the film thickness was probed by Focussed ion beam and Scanning electron microscopy (FIB/SEM) techniques. Furthermore, the nanomechanical properties of these antiwear films were investigated by nanoindentation methods. Tribological measurements of the coefficient of friction (mu) and wear scar width (WSW) indicate that the poorest antiwear film was formed on the softest substrate, which exhibited the largest WSW and the highest average mu. FIB/SEM images show that the thicknesses of the antiwear pads and the degree of damage on the substrates both change with the hardness value of the samples.