Wear Behavior and Chemical Friction Modification in Binary-Additives System under Boundary-Lubrication Conditions

Abstract

The present paper is primarily concerned with the synergistic effects of polar and organic sulfur compounds on the wear and surface roughness under boundary-lubrication (BL) conditions. The effects of lubricants containing the above two types of additives on the wear and the roughness of worn surface in a steel/steel sliding system have been investigated using a four-ball wear tester in air and argon atmospheres. In air, the combination of a fatty acid and a sulfur compound gives less wear and a smoother surface than would the sulfur compound alone in test run on worn surfaces. This surface smoothing is concerned with the friction modifications of surface caused by supplying binary additives and promoting elastohydrodynamic lubrication (EHL). In argon, on the contrary, it seems that the chemical reaction of sulfidized film with fatty acid causes the excessive wear. To control the evaporation of organic sulfur compounds and the effect of oxygen, a high-pressure-differential thermal analysis (HPDTA) was applied to measure the reactivity of various sulfur compounds with iron in inert gas. The results obtained by HPDTA may support those in the wear test. Also in such system, the wear and the roughness of worn surface are not only affected significantly by the reactivity and combination of additives, but also by oxygen.