Cam and Tappet Lubrication. IV–Radioactive Study of Sulfur in the EP Film

Abstract

A radioactive tracer, sulfur-35, synthesized into zinc dialkyl dithiophosphate molecules, was used to study the EP film formed on cast iron cams and tappets run in motor oils containing this additive. The sulfur content of static films increased with immersion time and temperature, and the presence of phosphate-coated metal surfaces. The bound sulfur of films formed during dynamic tests increased with running time, load, and with the use of phosphate-coated surfaces. These conditions also influenced the ratio of Zn:P:S contained in both static and dynamic films. Zinc, and particularly phosphorus, in the films increased more rapidly than the sulfur with increased temperature and/or pressure. The dynamic films are not easily worn off by running in nonadditive oil. The mechanism of action of zinc dithiophosphates appears to be related to chemical reactions of additive decomposition products with the metal surfaces to form tightly-bound solid films which reduce damage under extreme pressure conditions. Radioactive counting and X-ray spectroscopy were used to obtain the amount of sulfur and zinc on tappets. Densitometer traces of the autoradiographs were utilized to determine the distribution of the sulfur on cam and tappet surfaces.