Spectromicroscopy of tribological films from engine oil additives. Part I. Films from ZDDP's

Abstract

Antiwear films formed from pure neutral alkyl‐ and aryl‐ZDDP's, and a commercial ZDDP, have been studied with high resolution synchrotron‐based photoemission spectromicroscopy with a new instrument, MEPHISTO. Good P L‐edge XANES spectra have been taken on areas between 12 and 400 μm2, and good images of phosphates and ZDDP have been obtained at ∼1 μm resolution on both smooth and rough steel. These spectra, and corresponding images, show immediately that both the chemistry and the morphology of the alkyl and aryl films are very different. The alkyl film contains a range of smaller and larger protective polyphosphate pads from a few to ∼25 μm2 in area. We have shown that the chemistry of small and large pads are different. The large pads contain very long chain polyphosphate; while the smaller pads contain short chain polyphosphate. The aryl films contain ortho‐ or pyro‐phosphates, are much thinner and more uniform, with obviously more streaking from initial wear, and no obvious protective pad formation. Antiwear films generated from the commercial ZDDP, rubbed in base oil, show that the long chain polyphosphate is converted to ortho‐ or pyro‐phosphate, but the amount and distribution of phosphate does not change noticeably. The antiwear films are remarkably stable physically.