The Preparation and Nature of Surface Films from Zinc Dialkyldithiophosphate

An Amsler wear-testing machine has been modified to accept mechanisms by which the nominal line-contact between a rotating disk and a stationary flat plane can be traversed slowly along the plane. Lubricant reaction films formed at the contact are thus made available over an area of about 9 × 6 mm2. It has been found that the most uniform films are produced by maintaining a constant coefficient of friction during the traverse. Running-in of the disk is also beneficial. More extensive coverage is obtained by repeating the traverse over a given block. At least two types of film are produced with zinc dialkyldithiophosphates (ZDTPs): a thiophosphate film and a ferrous sulfide film. The latter is associated with localised high-wear score marks. Most films appear to be patchy. Thiophosphate film formed from s-butyl ZDTP additive comprises patches separated by almost bare metal whereas the film from n-butyl ZDTP is more continuous. The results are discussed in relation to the interpretation of analytical and ph...