A Reaction Mechanism for Producing Low-Friction Iron Phosphate Coatings

Abstract

Engine friction reduction through lubricant modification is an appealing approach for improving vehicle fuel economy because the benefits realized can be readily applied to all vehicles. Since tribological processes are influenced by electrochemical interactions between a lubricant and the lubricated sliding surface, electrochemical techniques should be valuable in reducing engine friction. An iron phosphate reaction film has been coated on cast iron electrodes by means of electrolysis of a lubricant containing 2.5 mass percent dilauryl hydrogen phosphate (DHP or “acid phosphate”). Friction tests showed a 20 percent lower friction coefficient for iron phosphate-coated piston ring specimens in comparison to uncoated ones. Laboratory reaction studies indicated that the electrolysis process accelerated a spontaneous corrosion reaction between the electrode and the acid phosphate. Increasing either the electric field or the agitation of the lubricant also accelerated the reaction rate for iron phosphate film formation. A mechanism for the electrolysis reaction was proposed, which involves the formation of ferrous ions at the anode, and hydrogen evolution at the cathode. The consumption of hydrogen ion in producing hydrogen at the cathode enhances the dissociation of acid phosphate to form more acid phosphate anion. The phosphate anion, in turn, chemically reacts with ferrous ion to generate phosphate films on the electrodes.

Presented at the 45th Annual Meeting in Denver, Colorado May 7–10, 1990

Notes

Presented at the 45th Annual Meeting in Denver, Colorado May 7–10, 1990