Friction-Enhancing Properties of ZDDP Antiwear Additive: Part II—Influence of ZDDP Reaction Films on EHD Lubrication

In a companion paper, Part I, it was shown that the friction enhancement produced in mixed lubrication conditions by ZDDP reaction films is not due to surface roughening as has previously been supposed. Instead, optical measurement of elastohydrodynamic film thickness shows that the presence of a ZDDP reaction layer inhibits the entrainment of liquid lubricant into rolling/sliding contacts. As a result, these contacts operate in boundary lubrication, with consequently high friction, up to higher speeds than would otherwise be the case and the attainment of full film elastohydrodynamic lubrication is also postponed. Possible mechanisms by which ZDDP reaction films might inhibit fluid entrainment are discussed.