The Effect of Steel Hardness on the Performance of ZDDP Antiwear Films: A Multi-Technique Approach

Abstract

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 (l) 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 l. 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.

The impact of organic friction modifiers on engine oil tribofilms

“…ZDDPs have been used in engine oils for more than 70 years and are probably the most successful antiwear additives ever discovered. In addition to being good antioxidants [1], they reduce wear by rapidly forming relatively thick, sacrificial boundary tribofilms with hardness characteristic to soft polymeric materials [2,3].…”

Phosphorus L 2,3-edge XANES: overview of reference compounds

“…However, in some cases the speciation of P forms, particularly organic P, was difficult or even impossible at the P K-edge XANES because of the lack of distinguishable features in the selected reference compounds (Beauchemin et al, 2003;Ajiboye et al, 2008;. XANES at the L 2,3 -edge instead of the K-edge seemed to be more promising because the L 2,3 -edge spectra showed more distinguishable features (Fuller et al, 1998;Ajiboye et al, 2006;Li et al, 2008). Previously, P L-edge XANES was used mainly in material science to specify P compounds in wear films (e.g.…”

The correlation between ZDDP tribofilm morphology and the microstructure of steel

“…Tribofilms developed by ashless AW additives in some cases exhibit resemblance to those formed by ZDDPs and patch-like structures are also sometimes observed [9,10,4]. Such patchiness is generally undesirable since it significantly increases surface roughness, especially for initially smooth surfaces [11,12,13,14,15,1,16,17,18,19] and consequently leads to higher asperity stresses with the associated risk of surface damage. AW additives have been observed to cause an increase in coe cient of friction in mixed lubrication regime [5,20,17,16].…”