Film Formation by an Antiwear Additive in an Automotive Engine

Abstract

Experiments carried out in a laboratory multicylinder gasoline engine with mineral oil lubricants containing radioactive (P ³² -labeled) zinc di(C ₆ )alkyl dithiophosphate show that strongly attached films are formed by this compound on many of the rubbing metal surfaces of the engine. Results are presented on the effects of chemical treatments on the radioactivity of the metal surfaces and on a technique by which the films are removed and assayed with a Geiger counter. On the basis of radioactivity per unit of geometrical contact area, most of the active parts were the highly stressed components of the valve train mechanism. These included the cams, valve lifter faces, push rod ends, rocker arm bearing areas and valve stem ends. The activity of the valve lifter faces was found to be influenced by (a) lifter metallurgy, (b) degree of break-in and (c) the presence of a detergent-inhibitor additive in the lubricant. Autoradiographs show that practically all the activity on a cam surface is localized at the cam nose—the region of greatest stress, of greatest surface damage and where metal transfer is known to occur. It is estimated that the films found on the cams and valve lifters are several molecular layers thick. The data support the idea that the action of zinc dialkyl dithiophosphate in reducing valve train wear in automotive engines involves the formation of phosphorus-containing protective films by chemical reaction with the rubbing metal surfaces. In addition, the results of this study suggest that this radiotracer method may be useful for locating critical areas of boundary lubrication in other mechanical devices.