Molecular Structures, Tribological Properties, and Working Mechanisms of Sulfur- and Phosphorus-Free Organic Molybdenum as Additives in Lubricants: A Short Review

Abstract

Traditional oil-soluble organic molybdenum (OM) as friction modifiers (FMs) in engine oils contain sulfur and/or phosphorus. Both sulfur and phosphorus are detrimental to the automotive exhaust gas catalysts. Consequently, sulfur and phosphorus in commercial engine oils are limited seriously by laws. Recently, oil-soluble sulfur- and phosphorus-free organic molybdenum (SPFMo) has been developed and measured intensively. This article reviews the molecular structures, tribological properties, and working mechanisms of SPFMo as FM in oils. Some bottlenecks that constrain the wide applications of SPFMo in engine oils are also summarized. In addition, some routes for overcoming the bottlenecks are suggested. Finally, some potential developments of SPFMo in the future are proposed. This review will provide a comprehensive understanding of SPFMo to the researchers in the field of oil additives.