The Influence of Molecular Oxygen on Wear Protection by Surface-Active Compounds

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Abstract
Four-ball wear experiments were performed with zinc di-n-oc-tyldithiophosphate (ZDP), bis [N,N′-dihexyl(dithiocarbamato-s,s′)] nickel (II) (NiDTC) and tetraoctylthioperoxydiphosphate (TPDP) dissolved in a synthetic hydrocarbon base stock. The experiments were conducted both under air and under nitrogen atmospheres. The reactivities of these compounds towards peroxy radicals were evaluated by measuring the uptake of oxygen during peroxy radical titration experiments. Atmospheric oxygen profoundly influenced metal wear protection by ZDP. This was reflected in the wear rate, wear asymmetry, production of soluble iron, and in the composition of the surface layers produced on the metal. Oxygen had little effect on results with TPDP. The NiDTC exhibited oxygen sensitivity intermediate to tile other additives. The influence of oxygen on the wear behavior of these compounds was paralleled by their relative reactivities toward peroxy radicals. Ancillary experiments showed that the activation of ZDP as a highly effective antiwear agent cannot be unambiguously ascribed to peroxy radical reactions. An initial electron transfer step is suggested as an alternate working hypothesis to account for the experimental results.