Zinc content of ZDDP films formed thermally and mechanically

Reaction films of zinc dialkyl dithiophosphate (ZDDP) were formed on flats of polished steel, and the Zn content of the films was determined. Films formed thermally were compared to those formed mechanically in reciprocating wear; both types were formed by using lubricants with a range of ZDDP concentrations. Grazing-incidence X-ray fluorescence was used to quantify the areal density of Zn and to verify that the measured fluorescence was due to a reaction film and not from debris or the bulk oil. The amount of Zn in the films generally rose with increasing ZDDP concentration, but films formed thermally were found to contain more Zn than those formed mechanically. Several possible explanations for this difference have been discussed.